112

Analysis of soluble reactive phosphorus in pore waters of Otsego Lake bottom sediments

Darcy R. Williams I

INTRODUCTION

Of life's essential nutrients, phosphorus has been the most limiting in Otsego Lake (Harman et al., 1997). It is only needed in small quantities, yet its scarcity has served control growth of primary producers. Recently Otsego Lake has experienced a significant increase in phosphorus concentrations in the water due to human presence (Harman et al., 1997). The increase in phosphorus comes from three major external sources: septic leachate, development, and agriculture (Rosen, 1998). Water runoff from these sources carries a high concentration of phosphorus, which eventually reaches the lake. This has triggered excessive growth, particularly in phytoplankton populations.

Phosphorus is consumed by organisms in its inorganic form, as phosphates. As they pass through the body some are converted to organic phosphorus; when waste material is excreted approximately half is in an organic form and half is inorganic. The majority of phosphates in lakes are orthophosphates (P04

3') (Goldman and Horne, 1983), which are also

known as soluble reactive phosphorus (SRP) (Hooper, 1973). Phosphorus present in the water column in organic form is dissolved organic phosphorus (DOP). DOP can form as a result of decomposition of organic material. To be converted to phosphates, DOP must be exposed to an enzyme such as alkaline phosphatase (Goldman and Horne, 1983) before it can be consumed by organisms.

Both organic and inorganic phosphorus that is present in the water column will eventually sediment to the bottom of the lake and will be incorporated into the substrate. According to Bostrom et al. (1982), a "major proportion" of phosphorus input into oligotrophic lakes is deposited in the sediments. Otsego Lake is a meso-oligotrophic lake (Hannan et al., 1997). If sediment conditions are aerobic, then SRP is quickly immobilized by iron and aluminum. These compounds typically form at the sedimenUwater interface where they are exposed to oxygen in the water column. The sediments below the surface are anaerobic and therefore contain free SRP in the interstitial waters (Hesse, 1973). The layer formed at the sediment surface prevents free SRP in the sediment from entering the water column. This generally creates a natural SRP deficiency in the lake, limiting growth. Any artificial phosphorus source disrupts this balance. In addition to increased external loading, a large quantity of phosphates enters the water column in Otsego Lake through regeneration, paI1icuiarly by alewives and zooplankton (Warner et al., 1996). Following the introduction of alewives in 1986, their population has grown exponentially, which has continually increased the amount of phosphates they regenerate. They have also decreased the mean size of zooplankton in the lake by grazing on them, which may increase the rate of phosphorus

I Robert C. MacWatters Internship in the aquatic sciences, summer 1998. Biological Field Station, Cooperstown, NY

113

In addition to the direct addition of phosphates to the lake, certain processes may liberate SRP to the water column. One cause of the release of SRP is the decrease in oxygen concentrations at the sediment/water interface of the lake. The large phytoplankton populations in Otsego Lake stimulate an oxygen deficiency in the hypolimnion. This is created by the decomposition of phytoplankton; when they die they fall to the hypolimnion where they are broken down by bacteria. The bacteria consume oxygen from the water, which decreases dissolved oxygen concentrations (Harman et al., 1997). The lack of oxygen reduces the iron and aluminum in the sediment and eliminates the compound barrier at the sediment/water interface. This makes SRP available for consumption (Hesse, 1973). Another cause of SRP release into the water column is by physical disturbances of the sediment, such as those created by bioturbation, natural waves and motorboat activity; boats moving in shallow water often disturb the sediments and stir SRP into the water (Rosen, 1998).

The goal of this study is to estimate the quantity of free SRP in the sediments of Otsego Lake that potentially could be released into the water column. In order to make such a measurement, interstitial water must be extracted without being exposed to oxygen. Many studies have been done to attempt similar measurements. Both Mayer (1976) and Hesslein (1976) did sediment pore water studies using dialysis membrane as a filter through which soluble nutrients could pass. Mayer (1976) submerged dialysis bags in the sediments within a perforated tube. The bags were filled with distilled water and then were allowed to sit in the sediment to equilibrate. Hesslein (1976) created samplers of acrylic plastic with compartments that could be filled with distilled water and then sealed with dialysis membrane. Other studies have followed, including Bottomley and Bayly (1984) and Carignan (1984). Summaries of their methods are published in books by Mudroch and Azcue (1995) and Muffle and De Vitre (1994).

Vertucci et al. (1981) studied phosphorus content in the sediments of Rat Cove in Otsego Lake by taking sediment core samples at various depths up to 20 cm. To prevent the pore water from being exposed to oxygen he packed sediment in beakers. Samples were suction filtered to separate sediment from pore water. His data provide comparison material for Otsego Lake despite the different methods used. Lord (1998) did a study most closely following the methods of Bottomley and Bayly (1984) at the Biological Field Station. My study has been a continuation of Lord's, with minor changes in methods.

Concurrent to my research on phosphorus, a study was done at the Biological Field Station on water seepage rates from the sediments into the water column (Donnelly et al., in prep.). Seepage meters were placed in various locations in the lake for various lengths of time. It is possible that seepage rates are representative of a quantity of SRP entering the lake from the sediments. To be able to compare results, some of my samplers were placed in the same locations as the seepage meters.

METHODS

To determine the quantity of soluble phosphorus in the sediments, two-part collectors were constructed, including external shells called inserters and internal collectors called peepers

114

(Figure 1). The inserters are 2" (5.1 cm) polyvinyl chloride (PVC) pipe with 0.5" (1.3 cm) diameter holes drilled approximately every 1.5 in (3.8 cm) along the pipe to allow water passage. To create a blunt yet tapering bottom end, a cap was created using a 2" (5.1 cm) to 1.5" (3.8 cm) PVC fitting, ending with a 1.5" (3.8 cm) PVC "trap" fitting. These pieces were glued in place using PVC cement. The top end of the inserter was capped using a 2" (5.1 cm) PVC cap with a 1116" (0.16 cm) hole drilled in its center. The inserters were placed in both shallow-water sediments close to shore, and in deep-water sediments. The shallow-water inserters were 18" (45.7 cm) in length, of which the top 0.75" (1.9 cm) were exposed when placed in the sediments. The cap on each shallow-water inserter was painted bright orange and attached to a small foam floater that hung approximately 6-12" (15-30) above the inserter so that the inserter could be easily found again. Fishing line attached the floater to the cap through a 1132" (0.08 cm) hole drilled in the cap and a hole punched through the foam. The deep-water inserters were 5' (1.5 m) long, and when placed were pushed 4' (1.2 m) into the sediments so that the top 12" (30.5 cm) were exposed. The exposed section was painted bright yellow so that it could be easily found. A floater was not attached to the cap of the deep-water inserters because the yellow paint was easier to see at depth than was the floater. Six inserters were created in total: four shallow-water inseliers and two deep-water inserters. Placement and removal of deep inserters was accomplished using SCUBA.

The inner collecters, called "peepers," were constructed of2.75" (7 cm) long segments of 1.5" (3.8 cm) PVC pipe, sealed at the ends with plexiglass squares that were glued on with PVC cement. The plexiglass was sanded down to the diameter of the pipe, and the edges were rounded smooth. Six holes were drilled with a 0.5" (1.3 cm) drill bit in each peeper to allow ion exchange. Two grooves were cut in each peeper using a rotary table saw and placed approximately 0.1" (.25 cm) deep and 0.25" (.64 cm)from each end to hold size 12 Rubber bands in place (see below). Before use the peepers were soaked in 1 M HN03 for 30 days to prevent possible contamination by phosphorus that may have been present in the lab. The peepers were again soaked in 1 M HN03 for 7 days before subsequent use.

After soaking for 30 days in an acid bath the peepers were rinsed three times with hot tap water and twice with glass distilled water and then were placed in an aluminum foil tent to dry. The peepers were wrapped in SpectraJPor® Membrane Dialysis Flat Sheets with a molecular weight cut off (MWCO) of 6-8,000. The membrane was cut with acid-washed scissors to approximately 3.5" (8.9 cm) in width (to extend just beyond the ends of the peeper), and 9.5" (24.1 cm) in length, to allow for an inch (2.5 cm) or more of overlap. To hold the membrane in place, one end of the membrane was glued to the peeper using Super Glue®, the main ingredient of which is cyanoacrylate, and left to dry overnight. The following day the peepers were placed in a container of glass distilled water that was at least 4" (I 0 cm) deep (to allow the peepers to be completely submerged), and the dialysis membrane was wrapped tightly around the peeper, devoid of air bubbles. Size 12 Rubber bands were wrapped around the peepers over the grooves to hold the dialysis membrane in place. Containers were acid-washed to prevent contamination. The water was then bubbled with nitrogen gas for 24 hours to eliminate oxygen, and the peepers were stored in the oxygen-free water in 16 oz. (473 ml) glass jars until just prior to placement in the sediment.

-----------

(a).

Waler surface

Sediment surface

Ipeeper](b).

Plexiglass top and bottom -7

3"

115

~ow-waterInserterJ

Slyrofoam submerged buoy

18"

~ groove for rubber band

~ groove for rubber band

Figure 1. Diagram of a shallow-water inserter (a) and a peeper (b).

116

The peepers were suspended within the inserters at specific intervals. The short inserters were built to hold peepers at 3" (7.6 cm) and 12" (30.5 cm) deep. The long inserters each held four peepers, at 3"(7.6 cm), 12" (30.5 cm), 24" (61.0 cm) and 36" (91.5 cm) deep. The peepers were held in place with 1.5" (J..8 cm) PVC pipe spacers, cut to the appropriate lengths.

The inserters were pulled out of the sediment by twisting and pulling, as gently as possible to prevent the dialysis membrane from ripping. They were brought quickly to just below the surface and, while still in the water, sediment was washed from the inserters to find matching holes on the peeper and inserter. A syringe was used to draw two 10 ml samples of water immediately out of each peeper. The syringe used was a 5 ml glass syringe with a size 16, 3/4" (1.9 cm) needle. Each sample was kept in a separate centrifuge tube to which a composite reagent (modi Ged from Griesbach and Peters, 1991) was added immediately following sampling in a 10: I sample-to-reagent ratio. The composite reagent was made according to the ascorbic acid method to find soluble reacti ve phosphorus (APHA et at., 1992), although in smaller quantities. SRP content was measured in flg/L using a spectrophotometer at 885nm. Standards were read with phosphorus concentrations of 0, 2, 5, 10 and 50 flg/L and used to create a standard curve.

To determine the speed of equilibration bet\veen sediment pore water and the peepers, 3 shallow-water inserters were placed together in Rat Cove of Otsego Lake in 8 feet of water (Figure 2) on July 1, 1998, each with one peeper 12" (30.5 cm) deep. They were removed after 7, 15, and 28 days. On the same day one deep-water inselier was placed at the same site with peepers at 12" (30.5 em), 24" (61.0 em), and 36" (91.5 em) to determine the depth of the highest concentrations of SRP. These were removed after 28 days. This initial study was used to determine the time and depth that the peepers should be in the sediments in order to find the greatest quantities of phosphorus.

A second set of inserters was placed at the same site on July 16, 1998. This set included 4 shallow-water inserters, each with one peeper at l' deep. These were removed after 13, 21, and 34 days. Two were removed after 34 days.

On August 26, 1998 I placed two long inserters off of Clarke point in Otsego Lake (Figure 2), under 39' (11.9 m) of water. Each had 3 peepers at l' (30.5 em), 2' (61.0 em), and 3' 91.5 em). The sediment has a high clay content at this site, meaning that it probably has a high concentration of aluminum. If the interstitial waters were exposed to oxygen, then it is likely that any phosphorus would be bonded to the aluminum and would not be free in the water.

To compare SRP content in the shallow-water sediments of Rat Cove I, placed 2 short inserters just south of the BFS dock (Figure 2), in approximately 2.5' (0.75 m) of water, and approximately 5' (1.5 m) apart on August 27, 1998. I also placed 2 short inserters in the northwest corner of the Cove (Figure 2), in 2.5' (0.75 m) of water and 5' (1.5 m) apart. According to Vertucci et at. (1981) the northern end of the Cove has significantly more SRP in the interstitial waters than the southern end.

117

Clarke Point sample site

Point 'udith Kingfisher Tower

BFS Dock sample site

Figure 2. Bathymetric map of otsego Lake and the pore water sample sites studied June-September, 1998.

Susquehanna River

118

RESULTS

Of the two sets of inserters that were placed in Rat Cove to survey equilibration rates and phosphorus at different depths, two patterns emerged (Tables I and 2). Both sets of inserters showed that the largest quantity of SRP was measured at 21 days, followed by that at 15 days (Figure 3). The SRP concentration dropped in each set of inserters sometime after the 21 st day. The deep-water inserter measuring phosphorus at various depths showed the peeper at 2' to have more SRP than at either I' (30.5 cm) or 3' (91.5 cm) (Figure 4). These data helped determine the placement of peepers and inserters later in the summer. The regression (R2

) values on the phosphorus standards throughout the study (0.480-0.999; mean = 0.893) were not quite high enough to consider readings to be certifiably precise for SRP content. This should be considered while analyzing the data.

Set I, Placed 7/1/98

Peeper Depth (ft)

Date pulled Time under (days)

Sample I (ppb)

Sample 2 (ppb)

[P] mean (ppb)

IA I 7/8/98 7 1.03 0.89 0.96 IB I 7/16/98 15 14.67 15.09 14.88 IC I 7/29/98 28 -4.93 -7.40 -6.16 ID-I I 7/29/98 28 -4.10 -6.57 -5.34 ID-2 2 7/29/98 28 141.25 77.01 109.13 ID-3 3 7/29/98 28 25.54 46.13 35.84

Table I. Phosphorus concentrations in peepers of shallow- and deep-water inserters, placed 1 July, 1998.

Set 2, Placed 7/16/98

Peeper Depth (ft)

Date pulled Time under (days)

Sample I (ppb)

Sample 2 (ppb)

Sample 3 (ppb)

[P] mean (ppb)

2A I 7/29/98 13 3.72 7.02 NA 5.37 2B I 8/6/98 21 6.00 45.17 NA 25.58 2C 1 8/19/98 34 12.32 9.16 6.46 9.30 2D 1 8/19/98 34 4.66 4.21 4.66 4.52

Table 2. Phosphorus concentrations in shallow-water peepers, placed 16 July, 1998.

At Clarke Point the pore waters showed significant quantities of SRP (Table 3). The concentrations were greater at I' (30.5 cm) and 2' (61.0 cm) below the sediment surface than at 3" (91.5 cm) (Figure 5).

-----------

-----

-------------------

119

30

;J­21 days -+-Placed 7/1

---.-Placed 7/16

25

~ 20

15

..c c.. c.. 10

34 days [

5 34 days

7 days 0

-5

-10 ­

0 5 10 15 20 25 30 35 40 time (days)

'zero value represents no detection of phosphorous «2 ppb)

13 days

28 days'

28 days'

Figure 3. Phosphorous equilibration was measured over time with 2 series of four inserters. T_h_e_Y___ _I

were placed under 8 ft of water on july 1, 1998 and July 16, 1998. Peepers were all located 1 ft below the sediment-water interface.

,---------------------- --_._-----­

I o -------------- -------------------.-,_.-­

sediment-water interface

-0_5

-1

:E. -1.5 J::. +' c.. Gl c

-2

-3

-3.5 -'---------------------------------------'

o 20 40 60 80 100 120 I [P] ppb

'zero value represents no detection of phosphorous «2 ppb)

I Figure 4. Phosphorous quantities in peepers at depths of 1, 2, and 3 ft below the sediment-water interface. The inserter was allowed to equilibrate for 28 days. Values represent an average of 2 10 ml samples taken from each peeper. I

3

120

I ..c: 12E. CI> Cl

24

I

pO"" [ 0'","" 1

12I1nserter 2 -_. -­ - -- ­

11639.23

-90.82 ~

i

~ 3879.94

I

-500 o 500 1000 1500 2000 2500 3000 3500 4000 4500 [P] (ppb)

'zero value represents no detection of phosphorous «2 ppb)

Figure 5. Phosphorus concentration in peepers of deep-water inserters, Clarke Point, 21 September, 1998.

. ...._-- .. -- ­ .----.--.-.. ·····--·------1 6546 I

i65

55

45

I-;;;~~e~-

b~" 25

15

5 -2.64 -0.07

I I -5

BFS Dock NW Rat Cove

'zero value represents no detection of phosphorous «2 ppb)

Figure 6. Phosphorus concentration in peepers of shallow-water inserters, Rat Cove, 17 September, 1998.

121

Inserter Depth in sediment (inches)

Depth under water (ft)

Sample 1 (ppb)

Sample 2 (ppb)

[P] mean (ppb)

A 3 40 109.78 642.59 376.18

A 12 40 1637.33 1641.13 1639.23

A 24 40 1356.37 1219.69 1288.03

B 12 40 -77.53 -104.10 -90.82

B 24 40 3879.94 3879.94 3879.94

Table 3. Phosphorus concentrations in peepers of deep-water inserters, Clarke Point, 26 August to 21 September, 1998.

In Rat Cove, SRP concentrations were significantly higher in the northwest corner than they were south of the BFS dock (Table 4, Figure 6). This is consistent with the work of Vertucci et al (1981). At the dock there was no measurable SRP in the pore water, yet in the northwest corner quantities exceeded 60 ppb. Data was limited to one peeper in the northwest corner because one of the peepers was destroyed and the other was lost.

Site Depth in sediment (inches)

Depth under water (inches)

Sample 1 (ppb)

Sample 2 (ppb)

[P] mean (ppb)

BFS Dock Surface 25.5 -1.74 -3.54 -2.64 BFS Dock 9 25.5 1.09 1.61 1.35 BFS Dock 12 25 -0.97 -2.00 -1.48

NW Cove 3 NA NA NA NA NW Cove 12 I I 54.65 76.28 65.46 NW Cove 12 NA NA NA NA

Table 4. Phosphorus concentrations in peepers of shallow-water inserters, Rat Cove, 27 August to 17 September, 1998.

DISCUSSION

It was expected that SRP concentrations would increase with time and then would level off as SRP in the peepers and the interstitial waters equilibrated. A decrease in SRP after three weeks was unexpected, yet it occurred in both sets of peepers and so was used as a reference for determining the length of time that peepers were left in the sediments. The greatest concentration of SRP was found at 21 days; for this reason peepers were kept in the sediments for as close to 21 days as possible. Other similar studies have found equilibration to take anywhere between 6 and 30 days, and often close to 15 days if water temperatures are 20-25°C (Mudroch and Azcue, 1995). The depth of peepers varied according to thickness of the sediments. Because the highest quantity of SRP found in the depth survey study was at 2' (61 em), peepers were placed up to 2' (61 em) deep at Clarke Point, but not deeper. The shallow­

122

water sediments often are less than two feet thick so peepers were placed at l' (30.5 cm) and just below the surface at 3" (7.6 cm). The survey study was used as a guide, but not as a set of rules because repetitions had been limited.

The large quantity of SRP found at Clarke Point may be due to the clay sediments. Clays tend to have high concentrations of iron and aluminum which bind to phosphates (Bostrom et a/~ 1982). In addition, sediment and associated phosphorus falls to the bottom of the lake which may cause the higher concentration of SRP in the bottom sediments. In Rat Cove, the greater quantities of SRP in the northwest corner may be due to the amount of organic debris present there in the form of leaf litter, woody plant material, aquatic macrophytes, and waste from the migrating bird populations that gather there.

The purpose of this study is to refine a protocol for studying interstitial waters at the Biological Field Station. Although it is time consuming and requires SCUBA divers, this protocol avoids many of the problems encountered in previous studies of phosphorus in pore waters such as filtration and exposure to oxygen. The insel1ers and peepers are inexpensive, simple to build and most materials are easy to find.

The results of a concurrent study on seepage in Otsego Lake (Donnelly et al., in prep.) indicate that there is a negligible amount of seepage at Clarke Point. Therefore, at that site, any relationship between SRP in the pore waters and seepage would be irrelevant.

REFERENCES

APHA, AWWA, WPCF. 1989. Standard methods for the examination of water and wastewater, 1i h ed. American Public Health Association. Washington, DC.

Bostrom, Bengt, Mats Jansson, and Curt Forsberg. 1982. Phosphorus release from lake sediments. Arch. Hydrobio!. Limno!., 18:5-59.

Bottomly, E. Z. and I. L. Bayly.1984. A sediment porewater sampler used in root zone studies of the submerged macrophyte, J\1yriophyllum .spicatum. Limno!. Oceanogr., 29(3):671-673.

Buffle, Jacques and Richard R. De Vitre. 1994. Chemical and Biological Regulation of Aquatic Systems. Lewis Publishers. Boca Raton, Florida.

Carignan, R. 1982. An empirical model to estimate the relative importance of roots in phosphorus uptake by aquatic macrophytes. Can. 1. Fish. Aquat. Sci. 39:243-247.

Donnelly, D., D. R. Williams and P. H. Lord. In preparation. Seepage Meters: Protocols for the construction, installation, utilization and removal of seepage meters on the bottom of Otsego Lake for the determination of groundwater flux through bottom sediments. SUNY Oneonta Bio!. Fld. Sta., SUNY Oneonta, Oneonta, N.Y.

Goldman, C. R. and A. 1. Horne. 1983. Limnology. McGraw-Hill, Inc., New York.

123

Griesbach, S. J. and R. H. Peters. 1991. The 'effects of analytical variations on estimates of phosphorus concentration in surface waters. Lake and Reserv. Manage. 7(1 ):97-106.

Hannan, W. N., L. P. Sohacki, M. F. Albright, and D. L. Rosen. 1997. The State of Otsego Lake, 1936-1996. Occasional Paper #30. SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta, Oneonta, N. Y.

Hesse, P. R. 1973. Phosphorus in Lake Sediments, In Griffith, E. J, A Beeton, J M. Spencer and D. T. Mitchell (eds.). Environmental Phosphorus Handbook. John Wiley & Sons, New York.

Hesslein, R. H. 1976. An in situ sampler for close interval pore water studies. Limnol. Oceanogr. 21:912-914.

Hooper, Frank F. 1973. Origin and Fate of Organic Phosphorus Compounds in Aquatic Systems, In Griffith, E. J, A Beeton, J M. Spencer and D. T. Mitchell (eds.). Environmental Phosphorus Handbook. John Wiley & Sons, New York.

Lord, Paul H. 1998. Sampling interstitial phosphorus levels in Otsego Lake. Unpublished term paper in BIOL 367, SUNY Oneonta, Oneonta, N.Y.

Mayer, L. M. 1976. Chemical water sampling in lakes and sediments with dialysis bags. Limnol. Oceanogr. 21:909-912.

Mudroch, Alena, .los M. Azcue, and Paul Mudroch. 1997. Manual of Physico-Chemical Analysis of Aquatic Sediments. Lewis Publishers. Boca Raton, Florida.

Mudroch, Alena and .los M. Azcue. 1995. Manual of Aquatic Sediment Sampling. Lewis Publishers. Boca Raton, Florida.

Rosen, D. L. 1998. Otsego Lake: A Guide to its Ecology and Management. SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta, Oneonta, N.Y.

Vertucci, F. A, W. N. Harman, and J H. Peverly. 1981. The ecology of the aquatic macrophytes of Rat Cove, Otsego Lake, N.Y. Occasional Paper #8. SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta, Oneonta, N.Y.

Warner, D., L. Rudstam, and W. N. Harman. 1996. An estimation of the density, abundance, biomass and species composition of the Otsego Lake pelagic fish community and zooplankton and alewife phosphorus regeneration. In 29th Ann. Rept. (1996), SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta, Oneonta, N.Y.