FIELD STUDIES OF PHOSPHORUS AND CLADOPHORA IN LAKE
ONTARIO ALONG THE AJAX, ONTARIO WATERFRONT
Martin T. Auer, PhD, 2014
FIELD STUDIES OF PHOSPHORUS AND CLADOPHORA
IN LAKE ONTARIO ALONG THE AJAX, ONTARIO WATERFRONT
Martin T. Auer, Ph.D.
Submitted to the Town of Ajax, Ontario 23 January 2014
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Summary Based on the experience gained through decades of scientific, peer-reviewed research about attached algae in the Great Lakes, with particular attention to Cladophora, I was asked by the Town of Ajax to assess whether the phosphorus discharged to the Ajax nearshore from the Duffin Creek Water Pollution Control Plant (“Duffin Creek WPCP”) was causing or contributing to the nuisance algae problem occurring along the Ajax waterfront.
Cladophora requires four conditions in order to grow: sufficient light, a suitable substrate (or bottom) to attach to, the right water temperature, and a food source (phosphorus). In Lake Ontario, the optimal water temperatures occur from late-May until mid-June (the Cladophora “growing season”). Algal growth will occur over this interval, and continue at sub-optimal rates for several months, provided that the alga’s needs for substrate for attachment, light and a supply of bioavailable phosphorus are met.
During the summer of 2013, field work was carried out to examine each of these three variables in the Ajax nearshore. Sonar surveys of the lake bottom were undertaken by a team from Michigan Technological University to characterize the suitability of the substrate for Cladophora growth. Measurements of the underwater light field were made by Ecometrix to determine the extent of light penetration.
The results of this field work indicate that there is a band of lake bottom along the Ajax nearshore, extending lakeward to a distance of ~1 km, where both suitable substrate and sufficient light exist to support Cladophora growth (“Cladophora Habitat Zone”).
The Cladophora Habitat Zone (green) at Ajax, Ontario and adjacent lake bottom where the alga is either irregularly present (green patches) or absent (tan). The yellow triangle points to the location of the Duffin Creek WPCP outfall.
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Underwater video surveys of the Ajax nearshore, performed by Michigan Technological University, confirmed the presence of Cladophora, often covering 100% of the bottom of this Cladophora Habitat Zone. The existence of habitat (substrate, light, temperature) alone, however, will not create a nuisance algae problem. Algae can exist in nearshore areas without causing nuisance conditions. The difference between the mere presence of algae and nuisance conditions is productivity, i.e, how fast algal biomass is being generated.
Still image of algal biomass at a depth of 5 m derived from video obtained during a survey of the Cladophora Habitat Zone in August of 2013.
The supply of phosphorus, specifically bioavailable phosphorus, is the factor that controls Cladophora productivity. Soluble reactive phosphorus (“SRP”) is 100% bioavailable and is commonly found present in WPCP effluents and nonpoint source runoff. The effluent plume of the Duffin Creek WPCP was delineated and sampled on two occasions by a team from Upstate Freshwater Institute to quantify the impact of that discharge on SRP concentrations in the Cladophora Habitat Zone.
The highest concentrations of SRP were observed immediately adjacent to the Duffin Creek WPCP outfall with dramatic elevations above baseline SRP levels occurring throughout the Cladophora Habitat Zone. Currents traversing the Ajax nearshore have the potential to carry the plume and its SRP-enriched water to the east and to west across the Cladophora Habitat Zone.
A direct connection between the SRP-enriched plume and stimulation of growth and production within the Cladophora Habitat Zone was established by making measurements of algal tissue phosphorus across the Ajax nearshore and at a control site near Cobourg, Ontario.
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The rate of Cladophora growth and thus the potential for production is directly related to its tissue phosphorus content, i.e. the amount of food stored by the alga. The tissue P metric also helps to smooth out the variability in SRP concentrations (difficult to track by water sampling) attending movement of the plume across the Cladophora Habitat Zone. This provides a more accurate picture of SRP availability and algae productivity over the course of the growing season.
The highest tissue phosphorus concentration was observed in the immediate vicinity of the Duffin Creek WPCP outfall with levels decreasing with distance from that location. Cladophora at Ajax is phosphorus-enriched to the extent that maximum rates of growth and production are approached and, proximate to the outfall, are achieved. This pattern of tissue phosphorus distribution is consistent with the conclusion that the Duffin WPCP outfall is the driving force for Cladophora growth and production at Ajax. The results also confirm that Cladophora at Ajax is among the most phosphorus-enriched, and therefore most productive, in Lake Ontario.
Based on the results of the field investigations and analysis summarized above, there is no doubt that the effluent being discharged to Lake Ontario from the Duffin Creek WPCP is the primary contributor to nuisance conditions of Cladophora growth at Ajax. The Duffin Creek WPCP is not yet operating at its maximum approved flow rate and is, therefore, not delivering its maximum allowable phosphorus load. The amount of phosphorus being discharged to the Ajax nearshore has the potential to increase by almost a factor of 3 under the EAC approved for the Duffin Creek WPCP. This increase in phosphorus loading has the potential to worsen the nuisance conditions presently experienced at Ajax and potentially extend the area affected along the regional shoreline.
The field program described here has conclusively demonstrated that the more than 100 kg of bioavailable phosphorus discharged daily to Lake Ontario by the Duffin Creek WPCP are received directly within the Cladophora Habitat Zone along the Ajax waterfront. Analysis of the results of the field program as set out above lead to several primary conclusions:
• The Duffin Creek WPCP discharge to Lake Ontario at Ajax establishes an effluent plume of bioavailable phosphorus that overlies lake bottom having physical conditions capable of supporting the growth of attached algae and stimulates the growth of the alga Cladophora glomerata (“Cladophora”) across the Ajax nearshore.
• The bioavailable phosphorus supplied by the Duffin Creek WPCP plume leads to enrichment of the stored phosphorus (tissue P) content of Cladophora, achieving levels supporting nuisance conditions. The degree of phosphorus enrichment, and thus Cladophora growth and productivity, is highest in proximity to the Duffin Creek WPCP outfall and decreases with distance from that nutrient source.
• The level of phosphorus enrichment, and the resulting levels of Cladophora biomass and productivity, associated with the Duffin Creek WPCP outfall are well above those characteristic of sites on Lake Ontario less impacted by urban activity.
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• Based on all of the foregoing, and given that the Duffin Creek WPCP effluent is by far the largest phosphorus input to the Ajax nearshore, that source may be considered the proximal cause of the Cladophora problem and the appropriate focus for efforts to remediate conditions of nuisance algal growth.
• The Duffin Creek WPCP is slated to substantially increase the amount of phosphorus that it discharges to Lake Ontario. The impact of this increase, absent a reduction in effluent phosphorus concentrations and/or relocation of the outfall, will be greater stimulation of Cladophora growth and exacerbation of the nuisance algae problem.
The position of the Duffin Creek WPCP effluent plume (red and pink) on 8/9/2013 overlain on the Cladophora Habitat Zone (green) at Ajax, Ontario. There was no area unimpacted by plume SRP within the survey limits. The yellow triangle points to the location of the Duffin Creek WPCP outfall.
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1. Motivation and Problem Definition
The filamentous, green alga Cladophora glomerata grows attached to solid substrate in
the Great Lakes nearshore and is widely distributed in Lake Ontario (Michigan Tech Research
Institute; http://geodjango.mtri.org/static/sav/). However, Cladophora biomass and the
occurrence of bloom conditions are highly variable around the lake and phosphorus loading from
local watersheds has been identified as the underlying driver for this spatial variability (Higgins
et al. 2012). A particularly robust presence of Cladophora has been observed in waters adjacent
to urban centers, i.e. those having elevated levels of dissolved salts, measured as electrical
conductivity (Higgins et al. 2012). Among seven stations in Lake Ontario representing a range
of urban impacts, levels of conductivity and Cladophora biomass were highest at Ajax, Ontario
(Higgins et al. 2012).
The field program described in this report was designed to clarify the provenance of
phosphorus in the nearshore waters at Ajax, Ontario and to explore the degree to which those
sources may contribute to Cladophora blooms.
2. Study Site and Description of the Field Program
The study site at Ajax, Ontario consisted of three transects extending lakeward to
distances of 4.5 km, 3.0 km and 3.0 km (Figure 1). Each transect included seven stations having
water depths ranging from 2.5 to 30 m. The sampling program (Table 1) characterizes:
• Cladophora growth habitat; • Cladophora distribution and nutritional status; and • Sources of phosphorus supporting Cladophora growth
Table 1. Date, objectives and the nature of measurements made during 2013 field surveys.
Date Survey Focus Measurements
8/9 Phosphorus sources Nitrate plume surveys
8/20-23 Cladophora growth habitat, distribution and nutritional status
Side scan sonar; remotely operated vehicle video surveys;
8/29 Phosphorus sources Nitrate plume surveys; nitrate – soluble reactive phosphorus correlation
10/9 Light environment Depth profiles of light intensity
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Duffin CreekWPCP
DuffinCreekWPCP
City ofPickering
Town of Ajax
Figure 1. The study site at Ajax, Ontario with water depth and station locations.
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At Ajax, Ontario, lake bottom conditions well suited for supporting Cladophora beds are present lakeward from the shore to depths of 10-15 m, a distance of ~0.75-1.5 km. A sandy bottom type, also suitable for support of Cladophora beds when colonized by mussels, is observed at depths of 25-30 m, a distance of ~2.5-4.0 km offshore. However, the amount of light available at this offshore location is insufficient to support Cladophora growth. Thus the area physically capable of supporting the development of Cladophora beds at Ajax is limited to waters having the solid substrate to which the alga attaches at depths less than ~10 m. Given the necessary physical conditions (light and hard substrate for attachment), the presence and level of production of Cladophora depends on phosphorus availability. Cladophora beds are not a common feature of Great Lakes ecosystems featuring SRP concentrations < 0.5 µgP/L. Above this level, growth is stimulated and production increases with increasing SRP concentration, approaching its maximum growth rate at ~2 µgP/L. Above this level, Cladophora growth is largely insensitive to additional increases in SRP. Thus, SRP concentrations from 0.5 – 2.0 µgP/L, defines the range of interest in managing Cladophora blooms and attendant nuisance conditions.
Analysis of survey results drew upon additional resources including the TRCA Peer Review,
data obtained from Environment Canada and the U.S. National Oceanic and Atmospheric
Administration, interpreted satellite imagery from the Michigan Tech Research Institute, the
peer-reviewed primary scientific literature, agency and university reports, stakeholder images of
nearshore conditions and Dr. Auer’s 30 years of experience in the study of phosphorus and
Cladophora in the Great Lakes.
3. Fundamental Requirements for Cladophora Growth
As with most plants and animals, Cladophora colonizes habitat providing a particular set
of environmental conditions, e.g. hard substrate for attachment, adequate light at the lake bottom
and seasonally optimum temperatures. These features determine the suitability of a site to host a
Cladophora bed. Whether or not a suitable site will be colonized, and the degree to which the
bed will develop, depends on the availability of the limiting nutrient, in this case phosphorus.
Here we describe the physical environment and nutrient conditions that mediate Cladophora
growth, explore the importance of distinguishing algal standing crop and production in assessing
that growth and describe the various forms of phosphorus and their utility in understanding the
Cladophora growth dynamic.
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3.1 Physical Conditions
The fundamental requirement for establishment of a Cladophora bed is the presence of
solid substrate for attachment, e.g. bedrock, boulders and cobbles (Figure 2). Substrate of human
origin (e.g. piers and breakwalls), and beds of invasive mussels (Higgins et al. 2005), may also
serve this purpose. Sand, silt and clay bottoms are not sufficiently stable to support attachment.
At Ajax, the nature and suitability of the lake bottom for supporting Cladophora beds varies in
both the longshore and offshore directions. In the longshore direction, Cladophora beds are
widely distributed (Figure 3; http://geodjango.mtri.org/static/sav/), with some patchiness
reflecting the presence of unsuitable bottom types (primarily sand). A side scan sonar and high
definition video survey of the Ajax waterfront (Figure 4) served to confirm satellite-derived
information and to characterize the distribution of Cladophora beds with distance offshore and
thus increasing depth. The lake bottom at Ajax consists of boulders, cobbles, sand and mussel
beds. Cobbles and boulders are dominant from the shore to a depth of 10-15 m, a distance of
0.75-1.5 km. This bottom type becomes mixed with sand at depths of 15-20 m (~1.25-2.0 km
offshore), with cobbles and boulders being less prominently featured than in shallower
environments. At depths of 25-30 m (offshore distances of 2.5-4.0 km and beyond), the lake
bottom is predominately sand, colonized by mussel beds (Figure 5). The presence of patches of
sandy bottom in waters <10 m deep, and the transition from cobble-bounder bottom to sand as
depth increases, results from differences in water turbulence and the ability of wave action to
keep cobbles and boulders washed free of sand. Based on the side scan sonar and high definition
videos, lake bottom types suitable for supporting Cladophora beds at Ajax are present from the
shore to depths of 30 m (and likely beyond). The true suitability of these locations is influenced
as well by light availability, the second physical condition examined here.
As for all plants, light plays a critical role in mediating the growth of Cladophora. Only
light with wavelengths between 400 and 700 nanometers can support photosynthesis. Thus, in
applications relating to Cladophora, light is quantified only over that wavelength range and is
termed photosynthetically available radiation (PAR), expressed as a photon flux, i.e. moles of
photons (elementary particles of light) incident on a unit surface area per unit time. One mole
of photons is termed an Einstein and a microEinstein (µE) is one millionth of a mole, leading to
units for PAR of µE·m-2·s-1. Three metrics of PAR are important here: incident light on a cloud-
free day (~2000 µE·m-2·s-1), an optimum intensity (~550 µE·m-2·s-1; calculated from Tomlinson
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Figure 2. Cobble and boulder lake bottoms from Lake Huron (left) and Lake Erie (right). Both have the required substrate for attachment and light environment and both experience seasonal temperature regimes that include the optimum for Cladophora growth. The absence/presence of Cladophora beds at the two sites results from differences in phosphorus supply. Note that the Cladophora bed at right would be submerged below a meter or more of water under typical conditions, but has been exposed here by seiche activity.
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Figure 3. The distribution of Cladophora beds in the Ajax/Pickering/Whitby, Ontario nearshore as determined from satellite sensing data ( http://geodjango.mtri.org/static/sav/). Cladophora beds are indicated by a green color with the darkness reflecting bed density as sensed by satellite instrumentation. Areas not supporting Cladophora beds, typically sand bottom, are indicated by a tan color. Satellite sensing penetrates to the optical depth of the water, here ~8 m. Thus, Cladophora beds may extend further out in the lake, as confirmed by ground truth monitoring.
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(a) (b)
(c) (d)
Figure 4. Remotely operated vehicle (ROV) survey of bottom type and the nature and distribution of Cladophora beds in the Ajax nearshore of Lake Ontario: (a) deployment, (b) submersal, (c) descent and (d) retrieval. Images by Dave Dean, Michigan Tech Research Institute.
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cobbles and boulders
cobbles and boulders
cobbles and boulders
cobbles and boulders
2.5 m
5 m
10 m
15 m
Figure 5. High definition video and side scan sonar tracks of Cladophora bed development (left) and lake bottom type (right) at various depths in the Ajax nearshore.
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cobbles and boulders
mottled – mussel beds
mottled – mussel beds
20 m
25 m
30 m
Figure 5. Continued. High definition video and side scan sonar tracks of Cladophora bed development (left) and lake bottom type (right) at various depths in the Ajax nearshore.
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et al. 2010) and the compensation point (25-35 µE·m-2·s-1; based on Graham et al. 1982), where
positive net production by Cladophora ceases.
The amount of light that reaches a Cladophora bed depends on depth and the
transparency of the water column at a particular site. As light moves through the water, some is
reflected back to the surface and the balance is absorbed, resulting in a cumulative attenuation of
the light received at the surface (Figure 6a). Changes in light with depth are well described by a
first order decay with an attenuation coefficient, ke (m-1). The magnitude of ke increases as
levels of dissolved color, chlorophyll (phytoplankton) and sediment increase. Light attenuation
is typically greatest at shallower depths where waves and wind mixing resuspend bottom
sediment. For the sites visited in the Ajax nearshore, light levels at the lake bottom were highest
at depths of 2.5 and 5 m, about 60% of the optimum intensity (Figure 6b). As depth increased
with increasing distance from shore, light at the lake bottom decreased in an essentially
exponential manner (Figure 6b). Application of the Great Lakes Cladophora Model (Tomlinson
et al. 2010; Auer et al. 2010) demonstrated that, at depths >10 m (offshore distance of ~1 km),
net production (growth minus loss to respiration) was negligible (Figure 6c).
With respect to temperature, Cladophora is considered to be primarily a spring species,
exhibiting optimal growth at temperatures between 13 and 17 °C (Graham et al. 1982). These
temperatures typically occur from late May to mid-June in the Lake Ontario nearshore at Ajax
(NOAA Coastwatch, Great Lakes Surface Environmental Analysis, 2009-2012). The alga is
physically robust and able to maintain attachment in the turbulent nearshore environment under
optimal growth conditions. However, later in the summer, the physical integrity of Cladophora
declines due to self-shading (Higgins et al. 2008a) and/or a transition to sub-optimal light and
water temperature conditions (Tomlinson et al. 2010). At this point, and continuing forward into
the fall, wind-driven detachment (sloughing) may occur and lead to the accumulation of
windrows of algal material on beaches. Deposits of sloughed Cladophora are known to host the
bacteria responsible for avian botulism, contribute to the persistence of human pathogens
(Verhougstraete and Rose 2014) and create conditions (e.g. odor) that interfere with human use
of the ecosystem. When entrained in cooling water intakes at power plants, algal biomass may
clog screens and filters and require suspension of operations at a cost of as much as $1 million
per day.
Taken together then, Cladophora will colonize solid bottom types to depths determined
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Figure 6. The underwater light field in Lake Ontario at Ajax: (a) vertical profiles at depths of 2 m, 15 m and 30 m; dashed line is compensation point, (b) light at the lake bottom for various depths across a nearshore – offshore transect and (c) model-calculated net production of Cladophora at various depths. The optimum light intensity for Cladophora is ~550 µE·m-2·s-1.
by the transparency of the water column, exhibiting maximum growth at depths of optimum light
intensity over periods of optimum water temperature. In the Ajax nearshore of Lake Ontario,
this means that Cladophora beds may develop broadly along a shore-parallel axis wherever solid
bottom types exist and extend offshore to the depth at which the requirement for light is no
longer met; for this species, at Ajax ~10 m (~1 km offshore). These characterizations of the
physical environment help to define the region potentially supporting the development of
Cladophora beds and thus area vulnerable to stimulation of growth by phosphorus addition.
3.2 Phosphorus
Phosphorus is the nutrient limiting Cladophora growth in the Great Lakes (i.e., the alga
requires a certain concentration of phosphorus in lake water in order to grow). Cladophora beds
may thus be expected to develop at sites offering favorable physical conditions (Section 3.1)
where the alga’s minimum demand for phosphorus is met. As phosphorus supplies increase at
these sites, the annual production of algal biomass increases, as does the potential to produce
nuisance conditions. Development of an understanding of the phosphorus – Cladophora
dynamic sufficient to support efficient phosphorus management decisions requires attention to
the various forms of phosphorus present and their relative abilities to support Cladophora growth
(i.e. their bioavailability).
Analytically, the entire complement of phosphorus present in the water (total phosphorus,
TP; Figure 7) consists of three fractions: soluble reactive phosphorus (SRP), dissolved organic
phosphorus (DOP) and particulate phosphorus (PP); all three forms are found in tributary and
point source discharges and the lakes and rivers that receive them. SRP is directly and
completely bioavailable and includes dissolved inorganic P (the orthophosphate ion, −34PO ) and
that portion of the DOP analyte that is easily converted (hydrolyzed) to SRP. In addition to
direct discharges, DOP may be produced through in-lake processes (e.g. excretion by plankton
and decomposition of organic detritus). DOP is not directly available to algae, but a fraction of it
(~67%; Lambert 2012) is bioavailable and may be converted to SRP through enzymatic
hydrolysis. PP is discharged directly and produced in the lake through uptake and incorporation
of SRP by the phytoplankton. PP is not directly available to Cladophora, a portion (~36%;
Lambert 2012) may be converted to SRP through physical (desorption of P from Fe/Al-rich
solids) and biological (solubilization and hydrolysis of organic-P) processes. PP may also be
transformed to SRP through the metabolic activity of mussels. Here, the potential for PP to
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Total PhosphorusParticulate Phosphorus
Dissolved OrganicPhosphorusSoluble Reactive
Phosphorusfbio
fbiofassim
microbialtransformation
microbialtransformation
transformationby mussels
Figure 7. The total phosphorus analyte, its components and their transformation. Soluble reactive phosphorus (SRP) is the form directly available to Cladophora. A fraction of the dissolved organic P (fbio,DOP) may be made bioavailable through microbial conversion to soluble reactive P. A fraction of the particulate P (fbio,PP) may also be made bioavailable through microbial conversion to SRP. Mussels filter particulate P from the water, rejecting some as pseudofeces prior to processing and assimilating the balance (fassim) and converting it to soluble reactive phosphorus.
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become bioavailable is determined by the degree of mussel selection (e.g. phytoplankton) or
rejection (e.g. terrigenous solids) of particulate matter (MacIsaac 1996).
When absorbed from the water by Cladophora, SRP is incorporated into biomass (tissue-
P or cell quota), partitioned for use providing physical structure and in supporting plant growth.
The fraction of tissue-P providing structure (also referred to as the minimum cell quota) has been
shown to remain constant at ~0.035% of the dry weight (DW) of the alga (Tomlinson et al. 2012;
see also Table 2). Growth rate and production increase as tissue-P increases above that
minimum, eventually approaching the maximum growth rate. Tissue-P maxima of 0.230 and
0.136 %DW have been reported for Lakes Erie and Ontario (Table 2).
Table 2. Cladophora tissue-P in Lakes Erie and Ontario.
Lake Year(s) Min (P, %DW)
Max (P, %DW)
Mean (P, %DW)
Reference
Erie 1995-2002 0.028 0.230 0.066 Higgins et al. 2005
Ontario 2008 0.044 0.136 0.072 Higgins et al. 2012
Relationships between phosphorus dynamics and Cladophora growth become
increasingly reliable as the analyte of choice becomes more closely related to the mechanisms of
growth. For example, the total phosphorus analyte, commonly applied to set trophic state
objectives (i.e. relationship between nutrients and algal abundance) in open lake environments, is
of little or no use with respect to Cladophora. This is the case because its bioavailability, and the
transformations of its component parts contributing to that bioavailability, is poorly defined.
SRP, the completely bioavailable component, serves more effectively in this regard (Figure 8a)
when concentrations are evaluated in a pseudo-steady state context, i.e. ignoring short term,
temporal dynamics. Given that SRP concentration drives algal uptake, it is not surprising that
long term exposure to SRP is well related to tissue-P content (Figure 8b). Thus the relationship
between tissue-P and growth provides the best representation of nutrient conditions driving
growth within a Cladophora bed (Figure 8c). Compared with the dynamic nature of SRP in the
water column, especially at locations proximate to point source discharges, tissue-P levels are
quite stable and thus serve well in providing an integrated representation of the nutrient
environment to which Cladophora is exposed. Both the SRP- and tissue-P growth (production)
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ue-P
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Figure 8. Phosphorus nutrition relationships in Cladophora as determined using the the Great Lakes Cladophora Model (Tomlinson et al. 2010): (a) SRP and growth, (b) SRP and tissue-P and (c) tissue-P and growth.
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Cladophora is widely distributed in the Lake Ontario nearshore, with its abundance ranging from a simple presence to levels capable of creating blooms and attendant nuisance conditions. Physical conditions at the Ajax site (i.e. solid bottom for attachment, penetration of sufficient light to the lake bottom) are such that 100% coverage by Cladophora is common over a depth range of 2.5 – 10 m. In contrast to many locations on Lake Ontario, the Ajax nearshore is impacted by urban activity, and thus supports some of the highest levels of phosphorus enrichment and Cladophora biomass and production observed.
curves take the form of a rectangular hyperbola (Figures 8a and c) with the growth rate
approaching its maximum as SRP (2 µgP/L) and tissue-P (0.15 %DW) concentrations reach
saturating levels.
4. Cladophora Occurrence, Distribution and Abundance
Cladophora is a member of the community of attached algae which occupy physically-
suitable nearshore habitat across the Great Lakes. Ulothrix is common in the phosphorus-poor
waters of Lake Superior, Chara in extreme northern Lake Michigan and Lake Huron and
Cladophora in parts of Lake Huron, the balance of Lake Michigan and in Lakes Erie and
Ontario. Cladophora is broadly distributed across the Lake Ontario nearshore, with its presence
clearly evident in interpreted satellite images (Figure 9). However, presence alone provides little
information regarding Cladophora abundance (standing crop, gDW·m-2) or production (gDW·m-
2·yr-1) and it is these, not presence, that fouls beaches and clogs water intakes.
The presence and distribution of Cladophora in the Great Lakes may be driven by local
phosphorus sources, by whole lake levels of phosphorus or by combinations of the two. In the
1970s and 1980s, Cladophora growth was driven by local sources in Lakes Huron and Michigan
(i.e. open water phosphorus concentrations were insufficient to support development of
Cladophora beds) and by whole lake conditions in Lakes Erie and Ontario (i.e. phosphorus
concentrations in the open lake water were sufficient to support development of Cladophora
beds). Today, management efforts have reduced SRP concentrations in Lake Ontario to levels
where the system is in transition from whole lake to local source forcing. This change is
particularly evident in differences in the mean and maximum abundance of Cladophora in Lakes
Erie and Ontario (Table 3), and in the strong correlation between Cladophora abundance and
urban influences (as represented by conductivity levels) recently documented for Lake Ontario
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Oak Orchard, NY
Ajax, Ontario
Mexico Bay, NY
Figure 9. Distribution of Cladophora at selected sites on Lake Ontario (light and dark green = Cladophora beds present; tan = Cladophora beds absent. Images obtained from Michigan Tech Research Institute: http://geodjango.mtri.org/static/sav/.
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(Figure 10, Higgins et al. 2012). The strength of this relationship (r2 = 0.8964, n=7; Higgins et
al. 2012) clearly points to the importance of local sources of phosphorus in stimulating the
development of Cladophora beds in certain nearshore waters.
Table 3. Standing crop of Cladophora (gDW·m-2) in Lakes Erie and Ontario.
Lake Year(s) Min Max Mean Reference
Erie 1995-2002 1 600 197 Higgins et al. 2005
Ontario 2008 19 93 47 Higgins et al. 2012
Of the seven sites on Lake Ontario surveyed by Higgins et al. (2012; Figure 5), the Ajax
nearshore ranked 1st in urban influence (conductivity), 1st in Cladophora biomass and 2nd in
phosphorus nutritional status (tissue P; behind Toronto).
The spatial distribution of Cladophora beds in the Ajax nearshore is consistent with what would
be predicted based on satellite imagery (Figure 3), bottom type (side scan sonar surveys; Figure
5) and levels of light penetration (Figure 6). High definition video acquired through remotely
operated vehicle surveys (Figure 4) show that Cladophora blankets the lake bottom along the
Ajax nearshore, achieving ~100% coverage to a depth of between 10 and 15 m (1.0 - 1.4 km
offshore) wherever solid substrate is available. Cladophora beds are sparsely populated at 15
and 20 m and the alga is absent at depths of 25 m and beyond despite the presence of extensive
mussel beds. Here, Cladophora has become light limited (Figure 6). The occurrence of
nuisance conditions, anticipated given the work of Higgins et al. (2012) and the more spatially
extensive monitoring results presented here, is well documented (Figure 11).
Thus, it may be concluded that Cladophora is widely distributed in the Ajax nearshore,
achieving 100% bottom coverage over a depth range of 2.5 – 10 m wherever solid substrate is
present and resulting in the occurrence of nuisance conditions on the beaches. Field observations
reported here are consistent with the local source, urban phosphorus enrichment paradigm of
Higgins et al. (2012). Management of these conditions suggests quantification of the provenance
of that phosphorus enrichment as an appropriate next step.
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R2 = 0.8964
Tissue-P (%DW)
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omas
s (gD
W·m
-2)
minimumcell
quota
Figure 10. The relationship between Cladophora biomass and tissue-P for samples collected from Lake Ontario by Higgins et al. (2012).
25
(a)
(c)
(b)
(d)
Figure 11. Sloughed, decomposing Cladophora at the Ajax waterfront. Images (a-c) by Paul Wealleans, Ajax; image (d) by Jamey Anderson, Michigan Tech.
26
As the nutrient limiting the growth of Cladophora in the Great Lakes, phosphorus (particularly bioavailable phosphorus, is the appropriate focus for management of nuisance. Identification of the provenance of the phosphorus stimulating growth in the Ajax nearshore is the necessary first step in supporting management. The Duffin WPCP discharges 85% of the total phosphorus (TP) and 98% of the SRP to the Ajax nearshore and does so at a location where physical conditions (solid bottom type, light environment) are as required to support development of Cladophora beds.
Tributary inputs make up most of the balance of phosphorus loads, but are minor in their contribution and would be expected to have a lesser bioavailability than WPCP effluent. Phosphorus levels in the open waters of Lake Ontario have decreased markedly since 1980 and today support the presence, but not nuisance growth, of the alga. There is no evidence that phosphorus sources in adjacent communities impact conditions at Ajax.
Mussels certainly contribute to the areal extent and vigor of Cladophora growth, but they do so through modification of physical conditions and through transformation of phosphorus from various sources; mussels are not, in and of themselves, a source of phosphorus.
The Duffin Creek WPCP effluent is, therefore, the appropriate focus for remediation of nuisance conditions of Cladophora growth in the Ajax nearshore.
5. Phosphorus Provenance
Satellite mapping of Cladophora beds, near bottom ROV imaging of algal abundance and
stakeholder reports of deposits of malodorous, decomposing algal biomass confirm the existence
of nuisance conditions at Ajax, Ontario predicted from the observations of Higgins et al. (2012).
Where the required physical conditions are present (Section 3.1), the presence of Cladophora,
the density of algal biomass within Cladophora beds and the amount of Cladophora biomass
produced annually is governed by the availability of phosphorus, the growth limiting nutrient in
the Great Lakes. Management of nuisance conditions of Cladophora growth requires that the
sources of phosphorus, i.e. their provenance, be identified. For Lake Ontario at Ajax, Ontario,
these sources potentially include exchange with offshore waters, delivery by longshore transport
and inputs from point and nonpoint discharges (Figure 12).
27
Figure 12. Examples of phosphorus sources to the Ajax, Ontario nearshore.
offshore exchange
nonpointsource(tributary,Duffins Creek)
nonpointsource(stormwater)
point source(Duffin CreekWPCP outfall)
Duffin CreekWPCP
City ofPickering
Town of Ajax
Lake Ontario
28
5.1 Offshore Boundary
Where concentrations of SRP are high lakewide, such as Lake Erie, nuisance levels of
Cladophora growth are considered to be whole lake forced, i.e. such conditions occur wherever
suitable physical conditions are present (Section 3.1). The whole lake forcing of Cladophora
growth characteristic of the 1970s and 1980s is now in transition to local source forcing as a
result of successful efforts to manage phosphorus loads to Lake Ontario (Figure 13). SRP
concentrations at an open lake Environment Canada station 22 km offshore of Ajax, Ontario
have fallen from 9.6 µgP/L in 1980 to as low as 1.0 µgP/L in 2001-2003 (Figure 13a) and are
averaging 1.5 µgP/L since 2001. While additional sampling is necessary to more clearly
define SRP boundary conditions at Ajax, it is clear that offshore concentrations are now in the
region of P-limitation (Figure 8a) and Lake Ontario has shifted from being driven by whole lake
phosphorus levels to being driven by local and regional inputs.
Successful phosphorus management achieved at a whole lake level for Lake Ontario is
evident at sites in the nearshore not impacted by urban influences. For example, Cladophora
beds at Oak Orchard and Mexico Bay, New York exhibit low algal biomass and P-stressed
physiologies (Higgins et al. 2012). Even at Ajax, Ontario, a location with urban impacts, low
SRP levels have been reported: 50-75% of the SRP values measured in the Ajax nearshore by
Leon et al. (2008) fell within the P-limiting concentration range characteristic of offshore waters
(Figure 14).
Thus, while SRP levels in the offshore waters of Lake Ontario are sufficient to support
the presence of Cladophora, they are no longer able to support nuisance growth. Higgins et al.
(2012) recognized this, proposing that “effective management of Cladophora blooms in Lake
Ontario should occur through managing P loading at local scales while ensuring lake-wide P
concentrations do not increase.”
5.2 Longshore Boundary
Substances such as phosphorus, discharged at one point along the Lake Ontario shoreline,
may be moved in a shore-parallel (longshore) manner by the action of currents and diffusive
mixing (mass transport). Currents in the Ajax, Ontario portion of the Lake Ontario nearshore are
move most often west → east (35-40% of the time) and east → west (25-30% of the time; Figure
15). Candidate sources for phosphorus exchange would include the Corbett Creek WPCP (13
km east of Ajax; annual TP load 0.12 times that of the Duffin Creek WPCP) and the Highland
29
Figure 13. Long term variation in the spring (Mar-Apr) SRP offshore boundary condition in Lake Ontario. Solid and dashed lines are the mean ± S.D. for the 2001-2010 interval. Environment Canada data provided by Alice Dove.
0
2
4
6
8
10
solu
ble
reac
tive
P (µ
gP/L
)
1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010
30
0
1
2
3
4
5
6
7
8
0
1
2
3
4
5
6
7
8
solu
ble
reac
tive
P (µ
gP/L
)so
lubl
e re
activ
e P
(µgP
/L)
2007
2008
Figure 14. Spring soluble reactive phosphorus concentrations across the Ajax nearshore of Lake Ontario (data of Leon et al. 2008). Lines represent the mean ± 1 S.D. offshore SRP concentration for 2001-2010.
31
N
NE
E
SE
S
SW
W
NW
40%
30%
20%
10%
Figure 15. Direction of transport by currents in the Ajax nearshore of Lake Ontario, May-Aug 2010. Compass direction indicates the heading of the current, e.g. E = current heading west to east.
32
Creek WPCP (10 km west of Ajax; annual TP load 1.69 times that of the Duffin Creek WPCP).
Auer (2011) reviewed a conservative substance tracer database (conductivity, 23 dates in 2007-
2008) developed by the Toronto Region Conservation Authority and partners and found no clear
evidence of longshore exchange. A similar review of a soluble reactive phosphorus database
developed by Leon et al. (2008; 8 dates in 2007) noted that SRP levels at a single station on the
eastern boundary of the Ajax nearshore exceeded concentrations meaningful for Cladophora
management (0.5-2 µgP/L) on six of eight occasions and were saturating for Cladophora growth
(>2 µgP/L) on two occasions. Given the magnitude of the Corbett Creek WPCP TP load (8%
that of the Duffin Creek WPCP) and its distance from the Ajax nearshore, it is unlikely that
longshore transport would be responsible for elevated SRP Levels near Ajax; it is more likely
that the SRP elevation in the Ajax nearshore was due to a local source, e.g. the Duffin Creek
WPCP discharge. No elevations in SRP concentration were observed at the western boundary,
suggesting that the Highland Creek WPCP had no effect on SRP conditions in the Ajax
nearshore.
5.3 Nonpoint Source Inputs
Nonpoint source inputs to the Ajax nearshore include direct runoff, stormwater outfalls
and discharges from Duffins and Carruthers Creeks. At the recommendation of TRCA and its
partners, Auer (2011) focused on Duffins and Carruthers Creeks, calculating TP loads during
periods favorable for Cladophora growth in the summer (Jun → Sep) of 2007-2009. The
resulting TP loads were 34.3 and 3.7 kgP/d for Duffins and Carruthers Creeks, respectively.
These values compare favorably with the annual TP loads in 2008 calculated for Duffins (40.5
kgP/d) and Carruthers (3.0 kgP/d) Creeks by Makarewicz et al. (2012) and for Duffins Creek
(45.2 kgP/yr) calculated for the 1990-2010 interval by Malkin et al. (2010). Annual loads would
be expected to be higher than summer loads, as the annual timeframe would include more wet
weather events (see Malkin et al., 2010, Figure 7b).
Booty et al. (2013) used an event-based data set to calculate annual TP loads for Duffins
Creek. The annual estimate for TP loads in a normal to dry year (41 kgP/d, 2007) was similar to
loads described above, however, loads for a wet year (2008, 178 kgP/d) and a wet year with an
extreme event (2009, 221 kgP/d) were much higher. While an annual, event-based approach
may improve the quality of loading estimates for Lake Ontario as a whole, their application may
be misleading when applied to understanding phosphorus provenance related to Cladophora. In
33
particular, the wet weather events that contribute to high annual loads occur most frequently
during months where conditions are unfavorable for Cladophora growth (e.g. Dec-Apr, Figure
16) and the phosphorus so delivered is transported from the nearshore within a few days of its
receipt at current velocities characteristic of the region (NOAA Great Lakes Coastal Forecasting
System). It is concluded that the summer TP load of 34.3 kgP/d (Auer 2011) most appropriately
characterizes the contribution of Duffins Creek during the period of active Cladophora growth.
Summer SRP loads for 2007-2009 were calculated by Auer (2011) to be 2.09 and 0.35
kgP/d for Duffins and Carruthers Creeks, respectively. Malkin et al. (2010) estimated annual
SRP loads for Duffins Creek to be 4.7 kgP/d (1990-2010); again, higher because they were
calculated over an interval that would include more wet weather events. The TP and SRP loads
from Duffins Creek have declined dramatically (55% and 92%, for TP and SRP, respectively)
since the 1980s. Decommissioning of two WPCPs previously discharging to Duffins Creek
(flow transferred to Duffin Creek WPCP in 1980) and implementation of nonpoint source
controls are likely reasons for the documented reductions.
Based on the estimates of Auer (2011), loads from Duffins and Carruthers Creeks account
for 16% and <3% of the TP and SRP loads to the Ajax nearshore, with the balance contributed
by the Duffin Creek WPCP’s contemporary discharge (see Section 5.4 following). Using the
approved ECA loading limit for the Duffin Creek WPCP (311 kgP/d; CH2M Hill 2013), the
tributary contribution to phosphorus loads would drop to 10% for TP and 2% for SRP, assuming
a constant SRP:TP ratio). Of a relatively small magnitude on a mass basis, the tributary loads
were also calculated to be negligible in their impact on an areal basis (two dimensional model;
Auer 2011).
5.4 Point Source Input
The Ajax nearshore receives input from a single point source, the Duffin Creek WPCP.
This facility discharges 358,805 m3/d (average, Jan-Jun 2013) of treated effluent to Lake Ontario
via an outfall located ~1 km offshore of the WPCP at a depth of 9 m. Average effluent TP and
SRP concentrations for the Jan-Jun 2013 interval were 303 and 156 µgP/L, respectively.
5.4.1 Loading to the Ajax nearshore
The summer and annual average TP loads for the Duffin Creek WPCP are 217 (Auer
2011) and 249 (Makarewicz et al. 2012) kgP/d, respectively. The summer SRP load is 107
kgP/d (Auer 2011). Including Duffins and Carruthers Creeks as contributors to the Ajax
34
0
10
20
30
40
50
60
0
10
20
30
40
50
60
0
10
20
30
40
50
60
Flow
(m3 /
s)Fl
ow (m
3 /s)
Flow
(m3 /
s)
J F M A M J J A S O N D
J F M A M J J A S O N D
J F M A M J J A S O N D
2007
2008
2009
Figure 16. Duffins Creek hydrologic record for 2007-2009 illustrating reduced frequency of wet weather events contributing to annual phosphorus loads during the Cladophora growing season (green overlay). Source: Environment Canada, Water Survey of Canada.
35
nearshore, the Duffin Creek WPCP accounts for 85% of the TP and 98% of the SRP load. With
the approved ECA TP loading limit (311 kgP/d; CH2M Hill 2013), the Duffin Creek WPCP
contribution would be 89% for TP and 98% for SRP (assuming a constant SRP:TP ratio). No
calculations were performed for the 2010-2012 interval (a period of reduced phosphorus
discharge at the Duffin Creek WPCP) because the required information was not included in the
ESR.
5.4.2 Mass transport
The Duffin Creek WPCP is, by an overwhelming margin, the largest contributor of TP
and SRP to the Ajax nearshore. The offshore outfall discharges effluent directly to habitat with
the solid substrate and light environment required to support development of Cladophora beds
(Figure 17). Beyond this immediate point discharge, the impact of the Duffin Creek WPCP is
determined by the extent to which phosphorus is transported along the lakeshore through
diffusive (dilution) and advective (currents) mass transport.
The role of mass transport is examined here through the results of mapping studies of the
Duffin Creek WPCP plume conducted in August of 2013. Three dimensional mapping was
performed by acquiring spatially intensive vertical profiles at 117 (9 August 2013) and 55 (29
August 2013) stations, yielding a database of 1891 and 1031 observations for the two dates,
respectively. The target analyte for the mapping surveys was nitrate (Satlantic ISUS sensor), a
substance considered conservative in this application and thus useful in tracking the transport
and dilution of this nitrate-rich discharge. Measurements were made along eight transects
extending outward from the buoy marking the Duffin Creek WPCP discharge. The contribution
of the effluent to water column nitrate concentrations (termed % effluent) was calculated by,
100%min,max,
min,
33
33 ⋅−
−=
//
//
ONON
ONON
CCCC
where 3NOC is the nitrate concentration (µgN/L) measured at a particular site, max,3NOC is the
maximum nitrate concentration (µgN/L) measured in proximity to the Duffin Creek WPCP
outfall and min,3NOC (µgN/L) is the minimum nitrate concentration measured lakeward of the
Duffin Creek WPCP outfall. Values for max,3NOC and min,3NOC varied between surveys: 2454 and
266 µgN/L on 8/9/2013 and 1345 and 203 µgN/L on 8/29/2013.
36
Figure 17. Cladophora habitat in the Ajax nearshore as categorized by depth and offshore distance. From the shore lakeward to a depth of 10-15 m, the lake bottom is largely populated by cobbles and boulders with 100% colonization by Cladophora Some patches of sand exist in this region (see Figure 3). Continuing lakeward to a depth of ~25 m, less solid bottom material is observed (more sand), light at bottom becomes sub-optimal and Cladophora beds are sparse. From a depth of 25 m offshore, the bottom is sand with mussel beds and no Cladophora is present due to light limitation. The yellow triangle indicates the position of the Duffin Creek WPCP outfall.
Duffin Creek WPCP outfall
Duffin CreekWPCP
Town of Ajax
37
In addition, paired samples were collected at 11 stations/depths during the 8/29/2013 survey for
SRP. These results were regressed against the nitrate concentration measured at that
station/depth (Figure 18). The equation describing that regression was then applied to the nitrate
data base from 8/9 and 8/29 to yield companion estimated SRP data sets. Those data were
interpolated in space and assigned to bins representing (1) conditions unimpacted by the plume,
i.e. those associated with offshore waters, (b) conditions where the plume stimulated Cladophora
growth, i.e. increased growth rates above those of the unimpacted condition and (c) conditions
where the phosphorus nutrition of Cladophora is saturated and growth can be increased no
further.
Survey results are presented here as discrete station/depth measurements (spider plots,
Figures 19 and 20), as interpolated nitrate plots describing the distribution of the effluent plume
(Figures 21 and 22) and as interpolated estimated SRP plots describing the impact of the plume
of phosphorus nutrition and Cladophora growth (Figures 23 and 24). Four findings have
emerged from analysis of these measurements. Examination of survey results presented as
‘spider plots’ (8/9, Figure 19; 8/29, Figure 20), demonstrates that mass transport conditions
during the two surveys were similar and revealed that,
Finding 1. The location and shape of the plume on these survey dates reflect east → west mass transport of the effluent discharge along the Ajax waterfront; and
Finding 2. On these dates, the Duffin Creek WPCP plume is most prominently positioned at depths of 5.5 – 7 m, a phenomenon termed “interflow’.
Interpolation of the nitrate data set (Figures 21 and 22) makes it further evident that,
Finding 3. Plume-enriched lake water (5-15% effluent) is, during these surveys, resident at locations having bottom types and a light climate that support the development of Cladophora beds. Further, this plume extends completely to the shoreline.
Finally, based on interpolations of estimated SRP levels (Figures 23 and 24), it is apparent that,
Finding 4. Phosphorus delivered by the plume stimulates Cladophora growth above that supported by contact with offshore waters and, at some locations, supports nutrient-saturated growth.
38
R² = 0.7634
0
2
4
6
8
10
0 200 400 600 800Nitrate (µgN/L)
Solu
ble
Reac
tive
P (µ
gP/L
)
Figure 18. The relationship between measured values of nitrate and soluble reactive phosphorus used to develop estimates of SRP for plume mapping.
39
(a) 0.5 – 1.0 m
(b) 5.5 – 6.0 m
Figure 19. Spider plots, nitrate plume survey, 8/9/2013: (a) surface water, (b) mid-depth and (c) all depths.
40
% Effluent: 0-2% 2-5% 5-10% 10-15% 15-25% >25%
0.5-1.0 m 1.5-2.0 m 2.5-3.0 m
3.5-4.0 m 4.5-5.0 m 5.5-6.0 m
6.5-7.0 m 7.5-8.0 m 8.5-9.0 m
Figure 19. Spider plots, nitrate plume survey, 8/9/2013 - continued: (a) surface water, (b) mid-depth and (c) all depths.
41
(a) 0.5 – 1.0 m
(b) 5.5 – 6.0 m
Figure 20. Spider plots, nitrate plume survey, 8/29/2013: (a) surface water, (b) mid-depth and (c) all depths (next page).
42
Figure 20. Spider plots, nitrate plume survey, 8/29/2013 - continued: (a) surface water, (b) mid-depth and (c) all depths.
0.5-1.0 m 1.5-2.0 m 2.5-3.0 m
3.5-4.0 m 4.5-5.0 m 5.5-6.0 m
6.5-7.0 m 7.5-8.0 m 8.5-9.0 m
43
(a) 0.5 – 1.0 m
(b) 4.5 – 5.0 m
Figure 21. 2D plots, nitrate plume survey, 8/9/2013: (a) surface water, (b) mid-depth and (c) all depths.
44
Figure 21. 2D plots, nitrate plume survey, 8/9/2013 - continued: (a) surface water, (b) mid-depth and (c) all depths.
0.5-1.0 m 1.5-2.0 m 2.5-3.0 m
3.5-4.0 m 4.5-5.0 m 5.5-6.0 m
6.5-7.0 m 7.5-8.0 m 8.5-9.0 m
45
(a) 0.5 – 1.0 m
(b) 6.5 – 7.0 m
Figure 22. 2D plots, nitrate plume survey, 8/29/2013: (a) surface water, (b) mid-depth and (c) all depths.
46
Figure 22. 2D plots, nitrate plume survey, 8/29/2013 - continued: (a) surface water, (b) mid-depth and (c) all depths.
0.5-1.0 m 1.5-2.0 m 2.5-3.0 m
3.5-4.0 m 4.5-5.0 m 5.5-6.0 m
6.5-7.0 m 7.5-8.0 m 8.5-9.0 m
47
(a) 0.5 – 1.0 m
(b) 5.5 – 6.0 m
Figure 23. Impact of the Duffin Creek WPCP plume on soluble reactive phosphorus concentrations in the Ajax nearshore on 8/9/2013. Three levels of impact are identified: unimpacted (SRP levels are those of offshore waters); stimulated (SRP levels are increased above those of offshore waters within the range sensitive to elevation of phosphorus concentration); and saturated (SRP levels are increased above those of offshore waters to concentrations supporting the maximum possible rate of Cladophora growth).
48
0.5-1.0 m 1.5-2.0 m 2.5-3.0 m
3.5-4.0 m 4.5-5.0 m 5.5-6.0 m
6.5-7.0 m 7.5-8.0 m 8.5-9.0 m
Figure 23. Continued. Impact of the Duffin Creek WPCP plume on soluble reactive phosphorus concentrations in the Ajax nearshore on 8/9/2013. Three levels of impact are identified: unimpacted (SRP levels are those of offshore waters); stimulated (SRP levels are increased above those of offshore waters within the range sensitive to elevation of phosphorus concentration); and saturated (SRP levels are increased above those of offshore waters to concentrations supporting the maximum possible rate of Cladophora growth). Legend as in Panels (a) and (b) of this figure.
49
(a) 0.5 – 1.0 m
(b) 6.5 – 7.0 m
Figure 24. Impact of the Duffin Creek WPCP plume on soluble reactive phosphorus concentrations in the Ajax nearshore on 8/29/2013. Three levels of impact are identified: unimpacted (SRP levels are those of offshore waters); stimulated (SRP levels are increased above those of offshore waters within the range sensitive to elevation of phosphorus concentration); and saturated (SRP levels are increased above those of offshore waters to concentrations supporting the maximum possible rate of Cladophora growth).
50
0.5-1.0 m 1.5-2.0 m 2.5-3.0 m
3.5-4.0 m 4.5-5.0 m 5.5-6.0 m
6.5-7.0 m 7.5-8.0 m 8.5-9.0 m
Figure 24. Continued. Impact of the Duffin Creek WPCP plume on soluble reactive phosphorus concentrations in the Ajax nearshore on 8/29/2013. Three levels of impact are identified: unimpacted (SRP levels are those of offshore waters); stimulated (SRP levels are increased above those of offshore waters within the range sensitive to elevation of phosphorus concentration); and saturated (SRP levels are increased above those of offshore waters to concentrations supporting the maximum possible rate of Cladophora growth). Legend as in Panels (a) and (b) of this figure.
51
It is concluded from these findings that,
Conclusion 1. The effluent plume from the Duffin Creek WPCP discharge stimulates Cladophora growth in the Ajax nearshore and, at certain times and locations, saturates the phosphorus reserves of the alga, resulting in maximum rates of growth and production.
Less well defined is the shore-parallel (east-west) extent of phosphorus stimulation of
Cladophora growth associated with the Duffin Creek WPCP plume. The definition of the plume
provided here is for two dates, however, the distribution of the effluent in the nearshore is acted
upon by diffusive and advective mass transport over longer (e.g. weekly, monthly) time frames.
In addition, phosphorus-enriched effluent is distributed vertically in a non-homogenous manner
(see cross-section, Figure 25) due to differences in density between the effluent plume and
ambient lake water (the interflow phenomenon). The potential for distribution of phosphorus
across the Ajax nearshore may then be visualized as being analogous to that of a lawn sprinkler
turning on its axis (Figure 25) to distribute phosphorus across Cladophora beds in the onshore,
shore-parallel and offshore directions (Figure 26). Thus the ability to directly observe and
document the Duffin Creek WPCP plume’s presence along the Ajax nearshore requires water
sampling conducted for the full range of current direction scenarios. However, a memory of
plume presence is retained in, and reflected by, the stored phosphorus (tissue-P) content of the
alga.
5.4.3 Tissue-P distribution
While plume surveys provide a direct and definitive characterization of effluent impact,
the position and distribution of the plume is dynamic, potentially changing on a daily basis as the
winds driving mass transport shift. Here, the integrative capacity of tissue-P (Section 3.2) can
provide valuable insight. Cladophora has the capacity for luxury uptake, i.e. it can take up and
store phosphorus beyond its immediate requirement for growth. Because the rate of phosphorus
uptake (minutes to hours) greatly exceeds that for depletion due to growth (days to weeks), the
tissue-P content of the alga provides an integration of the ambient nutrient environment, which is
difficult if not impossible to obtain through conventional water column monitoring.
Higgins et al. (2012) successfully utilized tissue-P as a metric of the phosphorus nutrition
of Cladophora at sites on Lake Ontario having different degrees of urban impact. A tissue-P
52
% Effluent
shore open lake →
Duffin WPCP discharge(1 km offshore, 9 m depth)
Figure 25. Cross-sectional representation of the plume from the Duffin WPCP on 8/9/2013. Differences in temperature (density) dictate the vertical position of the plume: at the surface when the lake water is cool (overflow) and at mid- or bottom-depths when the lake water is warm (interflow, underflow). The case presented here illustrates the interflow condition.
53
currents moving east → west currents moving west → east
currents moving onshore
currents moving offshore
Figure 26. Illustration of how the plume from an offshore discharge point such as a WPCP outfall may migrate in response to changes in current direction. The red oval identifies the discharge locations; warmer colors (orange, yellow, green) indicate higher concentrations.
54
survey was conducted in August 2013 along the Ajax nearshore and at a control site near
Cobourg, Ontario, a location characterized by Higgins et al. (2012) as having low urban impact.
Cladophora samples were collected at a depth of 5 m by Remotely Operated Vehicle and by
hand at a buoy marking the Duffin Creek WPCP outfall. Tissue-P content at the outfall buoy
exceeded that of other locations in the Ajax nearshore by a factor of 4-8 (Figure 27a) and
exceeded maximum levels reported for Lakes Erie and Ontario (see Table 2) by factors of
approximately 2 and 4, respectively (Table 2). When viewed in the absence of the outfall buoy
measurement (Figure 27b), a clearer pattern of the distribution of tissue-P emerges. The highest
tissue-P concentration is observed in the immediate vicinity of the outfall and concentrations
then fall with distance from the outfall. The fact that tissue-P levels decrease, and do so
systematically, with distance from the outfall, calls into question the potential impact of sources
to the east and west of Ajax (the longshore boundary condition). The tissue-P content of
Cladophora remains above the minimum cell quota (see Section 3.2) at the east and west limits
of the Ajax nearshore, with a surplus phosphorus content (i.e. elevation above the minimum cell
quota) 1.4 – 1.8 times that of the Cobourg control. The pattern of the tissue-P distribution is
consistent with a conclusion that the Duffin Creek WPCP outfall is the driving force for
Cladophora growth and production in the Ajax nearshore.
Conclusion 2. Phosphorus contained in the effluent discharged by the Duffin WPCP increases the stored P content of Cladophora, stimulating growth potential across the entire Ajax nearshore and, at certain locations, saturating phosphorus reserves and supporting maximum algal growth.
Results from the Ajax tissue-P survey are examined within the context of similar
measurements made at various sites on Lake Ontario by Higgins et al. (2012) in Figure 28. It
was the conclusion of that study that higher levels of phosphorus reserves and a larger amount of
algal biomass would be associated with sites having a greater degree of urban impact. The
comparison offered in Figure 28 affirms the conclusion of Higgins et al. (2012) that phosphorus
reserves at Ajax are among the most enriched in Lake Ontario, far exceeding those at sites less
impacted by urban activity and thus representing a response to local P sources rather than whole-
lake nutrient conditions. Noting that Cladophora growth is proportional to tissue-P content
(Figure 8c), this elevation of phosphorus reserves is seen as the factor stimulating production and
the attendant potential for beach accumulation of algal biomass presently observed at Ajax.
55
Figure 27. Distribution of Cladophora tissue-P along the Ajax nearshore and at Cobourg, ON: (a) including and (b) excluding the measurement at the Duffin Creek WPCP outfall. Dashed line indicates minimum cell quota. Inset at center identifies sampling locations; the Duffin Creek WPCP outfall is located offshore of and between the W and WM stations.
0
0.1
0.2
0.3
0.4
0.5
W WPCP WM M EM E COB
Tiss
ue P
hosp
horu
s (P
as %
DW
)Ti
ssue
Pho
spho
rus (
P as
% D
W)
(a)
(b)
Tiss
ue P
hosp
horu
s (P
as %
DW
)
0.00
0.05
0.10
0.15
W WM M EM E COB
(a)
(b)
W5M WM5M
MID5MEM5M
E5MDuffin Creek
WPCP
Town of Ajax
56
AjaxDuffin WPCP Buoy
Ajax – WM, Toronto
Ajax - EM
Ajax - W
Oak Orchard
Ajax - E
Ajax - M
Cobourg
Mexico Bay
Ajax
Grimsby
Cobourg, Rochester
0.0
0.2
0.4
0.6
0.8
1.0
0 0.2 0.4 0.6
Nor
mal
ized
Prod
uctio
n
Tissue-P (P as %DW)
Figure 28. Tissue-P content of Cladophora in Lake Ontario as measured by Higgins et al. (2012; yellow) and in the August 2013 survey of the Ajax, ON nearshore (white) identified on the tissue-P / production potential relationship originally presented as Figure 8c.
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Conclusion 3. Phosphorus nutrition along the Ajax nearshore is enriched well above levels characteristic of sites on Lake Ontario less impacted by urban activity.
At present, the Duffin Creek WPCP plume overlays suitable habitat in a manner that stimulates,
and in some cases, P-saturates Cladophora growth (Figure 29). For the Jan-Jun interval of 2013,
the Duffin Creek WPCP discharge volume was 359 MLD, carrying a TP load of 109 kgP/d.
Under the approved EAC, the discharge volume can increase by 75% to 630 MLD and the TP
load by a factor of 2.85 to 311 kgP/d. A commensurate increase in SRP would be anticipated.
The impact of this would be to move levels of phosphorus nutrition at some presently impacted
sites from stimulated to saturated and to increase the extent over which Cladophora growth in
the Ajax lakefront will be influenced by the Duffin Creek WPCP discharge.
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Figure 29. Cladophora habitat in the Ajax nearshore overlain with the Duffin Creek WPCP plume; 8/9/2013 at a depth of 4.5-5.0 m.
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Literature Cited Auer, M.T. 2011. Monitoring, modeling and management of nearshore water quality in the
Ajax-Pickering region of Lake Ontario. Submitted to the Toronto Region Conservation Authority, 93 pp.
Auer, M.T., Tomlinson, L.M., Higgins, S.N., Malkin, S.Y., Howell, E.T. and H.A. Bootsma. 2010. Great Lakes Cladophora in the 21st Century: Same alga – different ecosystem. Journal of Great Lakes Research, 36: 248-255.
CH2M Hill. 2013. Class Environmental Assessment to Address Outfall Capacity Limitations at the Duffin Creek Water Pollution Control Plant. Prepared for the Municipalities of Durham and York, 318 pp.
Graham, J.M., Auer, M.T., Canale, R.P., and J.P. Hoffman. 1982. Ecological studies and mathematical modeling of Cladophora in Lake Huron: 4. Photosynthesis and respiration as function of light and temperature. Journal of Great Lakes Research, 8(1):100-111.
Higgins, S.N., Howell, E.T., Hecky, R.E., Guildford, S.J. and R.E. Smith. 2005. The Wall of Green: The status of Cladophora glomerata on the northern shores of Lake Erie’s eastern basin, 1995–2002. Journal of Great Lakes Research, 31: 547-563.
Higgins, S.N., Hecky, R.E. and Guildford, S.J. 2005. The collapse of benthic macroalgal blooms in response to self-shading. Freshwater Biology, 53(12): 2557-2572.
Higgins, S.N., Pennuto, C.M., Howell, E.T., Lewis, T.W. and Makarewicz, J.C. 2012. Urban influences on Cladophora blooms in Lake Ontario. Journal of Great Lakes Research, 38 (Supplement 4): 116-123.
Lambert, R.S. 2012. Great Lakes tributary phosphorus bioavailability. M.S. Thesis, Department of Civil and Environmental Engineering, Michigan Technological University, Houghton, MI, 39 pp.
Leon, L.F., Smith, R. and Hecky, R.E. 2008. 3D Hydrodynamic and Ecological Modelling. Report submitted to Ontario Power Generation and the Regions of York and Durham. University of Waterloo, Waterloo, Ontario.
MacIsaac, H.J. 1996. Potential abiotic and biotic impacts of zebra mussels on the inland waters of North America. American Zoologist, 36: 287-299.
Makarewicz, J.C., Booty, W.G. and Bowen, G.S. 2012. Tributary phosphorus loading to Lake Ontario. Journal of Great Lakes Research, 38 (Supplement 4): 14-20.
Malkin, S.Y., Dove, A., Depew, D., Smith, R.E., Guildford, S.J. and Hecky, R.E. 2010. Spatiotemporal patterns of water quality in Lake Ontario and their implications for nuisance growth of Cladophora. Journal of Great Lakes Research, 36: 477-489.
Tomlinson, L.M., Auer, M.T. and H.A. Bootsma. 2010. The Great Lakes Cladophora Model: Development and application to Lake Michigan. Journal of Great Lakes Research, 36: 287-297.
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Verhougstraete, M.P. and Rose, J.B. 2014. Microbial investigations of water, sediment, and algal mats in the mixed use watershed of Saginaw Bay, Michigan. In Press. Journal of Great Lakes Research.