Reduced primary productivity, macroinvertebrate declines and

collapse of overwintering diving duck populations in a large

eutrophic lake

IRENA TOMÁNKOVÁ1, CHRIS HARROD2,3*, ANTHONY D. FOX4 and NEIL

REID1

1 Quercus, School of Biological Sciences, Queen’s University Belfast, MBC, 97

Lisburn Road, Belfast, BT9 7BL, Northern Ireland, UK2 School of Biological Sciences, Queen’s University Belfast, MBC, 97 Lisburn Road,

Belfast, BT9 7BL, Northern Ireland, UK3 Instituto de Investigaciones Oceanológicas, Universidad de Antofagasta,Avenida

Angamos 601, Antofagasta, Chile

*Correspondence author. E-mail address: c.harrod@qub.ac.uk4 Department of Bioscience, Aarhus University, Kalø, Grenåvej 14, DK-8410 Rønde,

Denmark

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Summary

1. Lough Neagh is one of the most important non-estuarine sites in British Isles

for overwintering wildfowl. A state-shift in the waterbird community

following the winter of 2000/01 was mostly driven by rapid declines in

overwintering diving duck populations. The sudden and discrete changes in

the waterbird community on Lough Neagh suggest that they might have been

the result of an intrinsic cause.

2. We investigated changes in density and biomass of benthic

macroinvertebrates, the food source for wintering diving ducks and fishes,

during 1997/98 (prior to the diving duck population decline) and 2010 (post

decline).

3. The mean total density of macroinvertebrates declined significantly from 15

300 ± 9 487 S.D. individuals m-2 during 1997/98 to 5 136 ± 4 038 individuals

m-2 during 2010. Mean total macroinvertebrate biomass declined from 15 667

± 8 799 S.D. mg m -2 to 5 112 ± 3 759 mg m -2. In terms of taxa composition,

the relative contribution of Tanypodinae, Glyptotendipes spp. and Tanytarsini

declined while the relative contribution of Chironomus spp. increased.

4. Primary productivity, as measured by chlorophyll a concentration, was

positively correlated with the index for diving duck abundance as well as the

total catch of European eels Anquilla anquilla within the system with a lag of

one year (Nt-1). Diving duck abundance and the total catch of eels were

positively correlated within the same year (Nt). Chlorophyll a concentrations

were high during the 1990s but declined throughout the 2000s.

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5. We describe a shift in productivity of the largest freshwater lake in the British

Isles which resulted in reduced macroinvertebrate density and biomass with

potential implications for ecosystem function and ecologically and

economically important consumer populations (diving ducks and fish).

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Introduction

Benthic macroinvertebrates are key components of any aquatic ecosystem, providing

nutrients and energy to a range of predators, including other invertebrates, fish and

birds. Many waterbird species act as predators in freshwater ecosystems, for example,

diving ducks feed on benthic macroinvertebrates (Snow & Perrins, 1998).

Chironomidae larvae are an important constituent of the diet of lacustrine diving

ducks (e.g. Bengtson, 1971; Nilsson, 1972; Winfield & Winfield, 1994b).

As well as their key role in ecosystem function by providing energy and nutrients

for consumers, many benthic macroinvertebrate taxa are useful sentinels of

environmental change, reflecting their sensitivity to changes in physiochemical

conditions or system productivity (Krieger & Ross, 1993; Schloesser, Reynoldson &

Manny, 1995; Carter, Nalepa & Rediske, 2006). As indicators of water quality in lake

systems, any change in macroinvertebrate abundance or community structure may

have profound consequences for the entire ecosystem, including fishes and

waterbirds. Thus, since 2000, the Water Framework Directive (Directive 2000/60/EC)

has enforced compliance from European Union member states to introduce

monitoring and surveillance of inland and coastal water quality including the

implementation of measures to maintain ‘favourable conservation status’ at

designated sites. Moreover, the EC Birds Directive (2009/147/EC) provides a

framework for the conservation and management of wild birds in Europe and confers

protection on some waterbirds populations. To fulfil this objective, Member States

invariably have site-based waterbird monitoring programmes in place (e.g. Calbrade

et al., 2010) to generate population trends in waterbird species.

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Lough Neagh (383 km2) is the largest freshwater lake in the British Isles in terms

of surface area (Carter, 1993a). It is a Ramsar wetland of international importance, a

Special Protection Area (SPA) and an Area of Special Scientific Interest (ASSI).

Historically, eutrophication has been a major concern at Lough Neagh (Foy, Lennox

& Gibson, 2003), most recently reflecting agricultural run-off (Bunting et al., 2007).

Following a large bloom of the blue-green algae Anabaena flosaquae in 1967, human

use of the lough was affected by disruption to water treatment plants, fishing industry

and recreational use (Wood & Gibson, 1973). Subsequently, Lough Neagh has

received considerable scientific attention (e.g. Wood & Smith, 1993) and has been

subject to long-term regular water quality monitoring and monthly counts of

waterbirds during the winter as part of the national Wetland Bird Survey (WeBS).

However, other components of the ecosystem have been less well studied, and there

has been no regular monitoring of the benthic macroinvertebrate community beyond

the littoral zone, apart from one major survey conducted in 1997/98 (Bigsby, 2000).

Chironomidae larvae are a major component of the lough’s benthic fauna (Carter

1978) and show seasonal variation, with Carter (1976) reporting larval densities of 4

000 individuals m-2 in autumn and 2 000 individuals m-2 in spring. Winfield (1991)

gave slightly higher figures, with an average Chironomidae larvae density of 6 250

individuals m-2 in the autumn and 3 275 individuals m-2 in the spring. In 2005, zebra

mussels were recorded in Lough Neagh for the first time (McLean et al., 2010); a

species that has significantly impacted ecosystem function in lakes throughout Europe

and North America (Ward & Ricciardi, 2007). Their impact on Lough Neagh

ecosystem has not yet been established as zebra mussel numbers are still low and the

species presence localised (McLean et al. 2010).

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Previous studies have highlighted the potential for trophic competition between

diving ducks, particularly tufted duck, and fish, especially the invasive roach (Rutilus

rutilus) for common benthic macroinvertebrate prey (Winfield, Winfield & Tobin,

1992; Winfield & Winfield, 1994). Bigsby (2000) showed some degree of trophic

overlap between diving ducks and various fish species but was unable to find any

evidence that benthic macroinvertebrate biomass was limiting in terms of availability

for predator populations.

Any changes in the macroinvertebrate community due to changes in water quality

are likely to have an impact on consumers from higher trophic levels throughout the

system, most notably waterbirds and fish. Birds often represent useful indicators of

ecosystem change because of statutory monitoring driven by their legislative

protection. Indeed, long-term monitoring of waterbirds at Lough Neagh has shown a

marked decline in wintering diving ducks, particularly pochard (Aythya ferina), tufted

duck (Aythya fuligula) and goldeneye (Bucephala clangula), since the winter of

2000/01 (Tománková et al., 2013). Between the winters of 2000/01 and 2008/09, the

overall population of diving ducks on Lough Neagh declined by 63% (Tománková et

al., 2013). Different explanations for the causes of these declines have been

suggested, including site related factors (Maclean, Burton & Austin, 2006) and

migratory short-stopping (Allen & Mellon, 2006; Lehikoinen et al., 2013). Recent

research suggests that local factors are likely to be at least partially responsible

(Tománková et al., 2013).

Here, we focus on changes in the availability and type of benthic prey available to

diving ducks before and after their decline, while simultaneously assessing system

productivity through an analysis of chlorophyll a concentrations and eel catches.

Specifically, we aimed to: i) describe the contemporary macroinvertebrate community

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in Lough Neagh in terms of composition and abundance in order to assess food

resource availability for top consumers, including diving ducks and fishes, and ii) to

assess temporal changes in the composition and abundance of benthic

macroinvertebrate community between the winters of 1997/98 (Bigsby, 2000) and

2010 (this study).

Methods

Study site

Despite its large surface area, Lough Neagh (54˚35'N, 6˚25'W) is relatively shallow,

with a mean depth of 9 m; as a consequence it usually remains well-mixed and the

water column is typically well oxygenated (Carter, 1993a). Lough Neagh drains a

large part of the north of the island of Ireland, with six inflowing rivers and a single

outflow river that drains into the Atlantic Ocean. The catchment is primarily

agricultural, dominated by dairy and beef farming (Carter, 1993b).

Macroinvertebrate surveys

In the absence of routine macroinvertebrate surveys, we compared densities and

biomass with the last most detailed study of the lough made prior to the decline in

diving duck populations (Bigsby, 2000). This involved core sampling

macroinvertebrate communities inhabiting soft sediments in 6 bays (Washing Bay,

Kinnego Bay, Bartin’s Bay, Lennymore Bay, Antrim Bay and Ballyronan Bay) at

depths ranging from 3 to 12m (Fig. 1) during October and January. These months

were selected to represent the period reflecting the use of the lough by the

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overwintering diving ducks. The previous survey covered the winter of 1997/98,

while the current study was conducted in January and October 2010. Repeated

samples (n = 5) of sediment were taken at each depth with a Kajak corer (21.23cm2)

attached to either a rope or a hand pole. This method was unlikely to effectively

sample more mobile macroinvertebrates, such as Gammarus spp., Asellus aquaticus

and Mysis spp. due to their rapid escape behaviours and therefore their numbers are

likely to have been underestimated. However, since the main purpose was the

comparison of the macroinvertebrate fauna of Lough Neagh between two time

periods, this issue was deemed not relevant. Samples were washed through a 250 µm

sieve to remove sediment. Macroinvertebrates were then picked out, counted and

identified. In most cases, Chironomidae larvae head capsules were mounted with

either Euparal or Hydromatrix solutions (Brooks, Langdon & Heiri, 2007) to allow

identification (Wiederholm 1983; Brooks et al., 2007). Not all macroinvertebrates

were identified to species; instead, operational taxonomic units were used. This was

particularly the case for Chironomidae larvae which were generally identified to

genus level, and Trichoptera larvae which were identified to family level. Although

relatively abundant, oligochaeta were excluded from the survey following Bigsby

(2000). Rare taxa were grouped in the category “other”.

All macroinvertebrates were measured to allow biomass to be estimated. Total

length ± 0.001 mm was typically recorded but occasionally, head capsule dimensions

were taken for Chironomidae larvae, if only part of the body was found in the sample.

Mass-length regressions were then used to convert body dimensions into biomass.

The regressions for Chironomidae larvae, Asellus aquaticus, Gammarus spp. and

leeches were taken from Bigsby (2000), while regressions for all other

macroinvertebrate taxa were obtained from Benke et al. (1999). Mysis were weighed

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due to the low number of individuals sampled. Both macroinvertebrate counts and dry

mass were converted into estimates of density (individuals m-2) and biomass (mg m-2).

Data from the 1997/98 survey (Bigsby 2000) were extracted from graphs using

Plot Digitizer (version 2.5.1) software, as the original raw data could not be retrieved

in any other form. These values were then converted to density (individuals m-2) and

biomass as dry mass (mg m-2) to permit comparison.

Productivity and eel data

Levels of chlorophyll a (µg L-1) at Lough Neagh were measured from 1995 to 2011.

Ten metre composite water samples were collected fortnightly using a flexible tube

from a central location in Lough Neagh (54o37’N, 6o24’W) and averaged for each

year. Chlorophyll a was measured after extraction into hot (55C) methanol (Talling,

1969).

The total annual catch of eels (both resident yellow and migratory silver phases)

were obtained for the period 1995 to 2011 based on census techniques described by

Rosell, Evans & Allen (2005). Both productivity and fish data were obtained from the

Agri-Food and Biosciences Institute (AFBI), Northern Ireland.

Statistical analysis

Variation in total macroinvertebrate abundance and dry mass was examined using

Generalized Linear Mixed Models (GLMMs), using a negative binomial distribution

with a log link function to account for the large variance of data relative to the mean.

Transect ID was fitted as a random factor to account for multiple observations at each

sampling site. Survey period, Depth and Season were fitted as fixed factors. Statistics

were conducted using SPSS v20 and graphs plotted using Sigmaplot v10.

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In order to characterise how the overall macroinvertebrate community changed

between the two survey periods, we undertook a parallel multivariate analysis. Here,

the total abundance of each macroinvertebrate taxon was summed for each survey,

segregating data by depth (3, 6, 9, and 12 m) and survey transect (Antrim, Ballyronan,

Bartin's, Kinnego, Lennymore and Washing Bays). Abundances were square-root

transformed and standardised before a Bray-Curtis dissimilarity matrix was

constructed using PRIMER 6.1.13 and PERMANOVA 1.0.3 (Anderson, Gorley &

Clarke, 2008). Spatial (Transect, Depth) and temporal (Season, Survey period)

influences on the relative abundance of macroinvertebrate taxa were examined using a

four-way permutational (n permutations = 9 999) multiple analysis of variance

(PERMANOVA). Transect ID was fitted as a random factor and Depth, Survey and

Season were fitted as fixed factors. Following the PERMANOVA analysis, we

examined those taxa driving the key differences in survey periods using the SIMPER

in PRIMER.

Variation in community structure associated with survey period and sample depth

was graphically visualised using a Principal Coordinates Analysis (PCO); an

unconstrained ordination method also known as metric multi-dimensional scaling

(Anderson & Willis, 2003; Anderson et al., 2008). We included vectors based on

multiple correlation (correlation coefficients >0.4) to highlight those taxa driving the

distribution of data along the first two PCO axes, whilst accounting for variation in

the other taxa.

Concentrations of chlorophyll a were tested before and after the known change

point in the temporal trend for diving duck abundance (Tománková et al., 2013) using

a t-test (i.e. pre- and post-2001).

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Relationships between mean chlorophyll a, the total annual catch of eels and the

diving duck index were examined using cross-correlations (Pearson’s r) at various

temporal lags.

Results

Total macroinvertebrate density decreased by two thirds between 1997/98 and 2010

(Table 1 and Fig. 2a-d), with an estimated mean ± SD density of 15 300 ± 9 487

individuals m-2 during 1997/98, compared to 5 115 ± 3 944 individuals m-2 during

2010. Total macroinvertebrate dry mass also decreased by an order of magnitude

between the survey periods (Table 1 and Fig. 2e-h), from a mean ± SD dry mass of 15

667 ± 8 799 mg m-2 in 1997/98 to 5 275 ± 3 751 mg m-2 in 2010. Both measures of

total density and biomass were higher in October than in January (Table 1 and Fig. 2).

Total density of macroinvertebrates also varied significantly with depth (Table 1).

Zebra mussels were not recorded in the current survey; however, it is likely that the

species would be under-sampled on soft sediments.

PERMANOVA revealed considerable variation in the relative contribution of

different taxa to the macroinvertebrate community associated with three of the main

factors examined: Survey (Pseudo-F1,12 = 6.44, P = 0.009), Depth (Pseudo-F3,12 =

12.89, P = 0.0001) and Transect (Pseudo-F5,12 = 3.3, P = 0.001). There was little

evidence of any effect of Season on variation in community structure (Pseudo-F1,12 =

1.41, P = 0.288). The relative influence of Survey and Depth on invertebrate

community structure is shown in the PCO (Fig. 3).

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Comparison of first order interactions suggested strong interactions between

Survey*Season (Pseudo-F1,12 = 10.32, P = 0.0049) and Depth*Transect (Pseudo-F12,12

= 2.16, P = 0.005), and a less marked interaction between Survey*Transect (Pseudo-

F5,12 = 2.28, P = 0.025). There was no obvious interaction between Survey*Depth

(Pseudo-F3,12 = 1.65, P = 0.132) or Depth*Season (Pseudo-F3,12 = 1.00, P = 0.476). Of

the second order interactions, only Survey*Depth*Transect was significant at 95%

levels (Pseudo-F12,12 = 1.82, P = 0.03).

As the principal question of interest in the current study was whether major shifts

had occurred in the community structure of the Lough Neagh benthic

macroinvertebrate community over the study period, we focused on examining which

taxa were responsible for the significant shift in community structure shown between

the two Survey periods by our PERMANOVA analysis (see above). A SIMPER

analysis (Table 2) suggested that the relative contribution of Tanypodinae,

Glyptotendipes spp., Tanytarsini and Dicrotendipes spp. to the macroinvertebrate

community fell between 1997/98 and 2010, whilst the relative contribution of

Chironomus spp., and various other taxa increased (Supplementary Fig. 1 and 2).

Concentrations of chlorophyll a differed significantly between pre- and post-2001

(Fig. 4); taken as the change point in the temporal trend for diving duck abundance

(t414 = -6.06, P < 0.001; Fig. 5).

Chlorophyll a (Fig. 4a) was positively correlated with the diving duck index (r =

0.617, p = 0.025; Fig. 4b) and the total catch of eels (r = 0.776, p < 0.001; Fig. 4c) at

a lag of 1 year (Nt-1), while the total catch of eels and diving duck index were

positively correlated in the same year (Nt ; r = 0.857, p = 0.007).

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Discussion

This study demonstrates marked changes in the macroinvertebrate community at

Lough Neagh, the largest freshwater lake in the British Isles, between 1997/98 and

2010. The macroinvertebrate fauna decreased notably in abundance and biomass.

Density of macroinvertebrates declined by two thirds from 15 300 individuals m-2

in1997/98 to 5 115 individuals m-2 in 2010, with concomitant declines in biomass.

These changes were coincident with a sustained decline in productivity, as measured

by chlorophyll a concentrations, and a sudden decline in the overwintering numbers

of diving ducks, principally pochard, tufted duck and goldeneye (Tománková et al.,

2013), as well as reductions in eel catches, another key consumer of

macroinvertebrates in the system. Such wholesale changes in the lough ecosystem

appear indicative of a state change or regime shift in ecosystem function, with

implications for economically and ecologically important consumer populations.

Chironomidae larvae densities in 2010 were similar to those reported in the

1969/70 and the late 1980s. In 2010, Chironomidae larvae densities were 31.5% lower

than in the 1969/70 and 9.5% higher than in late 1980s (Carter, 1976; Winfield 1991),

highlighting the fact that the densities in the late 1990s appear to have been

exceptionally high. Bigsby (2000) estimated the combined overwinter consumption of

macroinvertebrates by fish and diving ducks at 13% of the total available

macroinvertebrate biomass and was thus unable to find any significant effect of fish or

diving duck predation on the abundant macroinvertebrate community. However, the

reduction in macroinvertebrate prey by 2010 may have been sufficient to reduce the

quality of the food resource available to overwintering diving ducks, inducing a re-

distribution to more suitable overwintering sites. Moreover, Lough Neagh supports

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dense populations of numerous fish species (Harrod et al. 2001; Inger et al. 2010),

including roach (Rutilus rutilus), and species which are important commercially and

for their conservation significance, such as the European eel (Anguilla anguilla),

pollan (Coregonus autumnalis) and perch (Perca fluviatilis); all of which feed on

macroinvertebrates, including Chironomidae larvae (Winfield & Winfield, 1994a).

The reduced availability of such important prey items might also have a negative

effect on the populations of these fish species, all of which are exploited by

commercial fisheries. There are no reliable quantitative estimates of fish abundance

over the same period; however, both semi-quantitative (C. Harrod, unpublished data)

and quantitative (Inger et al., 2010) data indicate recent shifts in fish community

structure, with an increase in the relative abundance of perch (C. Harrod & W.

Campbell, unpublished data). Such changes are characteristic of the Lough Neagh

fish community which has historically been extremely dynamic both in terms of the

species present (Thompson, 1856; Kennedy & Vickers, 1993; Winfield, Tobin &

Montgomery, 1993), as well as the relative abundance and size structure of different

fishes (Menzies, 1924; Harrod et al., 2002).

Here we demonstrate a decline in total biomass and total abundance of

macroinvertebrates between autumn (October) and winter (January). Carter (1976)

found that Chironomidae larvae at Lough Neagh decreased in abundance between

autumn and the beginning of spring, indicating significant overwintering mortality.

Chironomidae generally emerge in the spring or the beginning of the summer (Carter,

1975) and diving ducks feed on macroinvertebrates throughout the winter, so a

depletion of resources would be expected.

The decline of macroinvertebrate abundance and biomass at Lough Neagh was

associated with long-term changes in levels of primary productivity as indicated by

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measured levels of chlorophyll a. During the 20th century, Lough Neagh underwent

cultural eutrophication and by the 1970s it was considered one of the most eutrophic

lakes in the world (Wood & Gibson, 1973). In an effort to control eutrophication,

tertiary sewage treatment works were installed in 1981 at major sewage treatment

works in the Lough Neagh catchment (Foy et al., 2003). Initially, total phosphorus

concentrations decreased (Heaney et al., 2001) but the impact was only temporary and

by late 1990s, total phosphorus levels were in excess of those prior to control efforts,

mostly due to non-point source pollution (Heaney et al., 2001). Efforts to control

eutrophication were more recently bolstered by the implementation of the Water

Framework Directive during 2000. Thus, the decline of macroinvertebrates at Lough

Neagh and concomitant changes in overwintering duck populations and eel catches

may well be the unintended consequence of attempts to improve water quality.

In other lakes, improvements in water quality have led to shifts in the

macroinvertebrate community structure (Carter et al., 2006; Schloesser et al., 1995),

decreased total macroinvertebrate abundances (Köhler et al., 2005) and reductions in

the abundance of planktivorous fish (Ney, 1996; Jeppesen, Jensen & Søndergaard,

2002). In Lake Constance in Germany, whitefish (Coregonus lavaretus) fecundity

was positively related to the trophic state of the lake (Hartmann & Quoss, 1993). The

fish community of Lough Neagh has historically been dynamic, with temporal shifts

in the relative abundance of different species, but there have been no reported

dramatic changes in abundance or growth that might be expected if benthic prey

availability had changed so markedly. For instance, the eel fishery has remained

productive over time; although there was a slow decline in the catch of both yellow

and silver eels (Rosell et al., 2005) there was no evidence for dramatic changes in

output that might be expected following such a drastic putative change in food

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availability. However, there has been a gradual 52% decline in the total yield of eels

over the period of 1995-2011. This decline was significantly correlated with an index

of diving duck abundance on Lough Neagh in the same year and chlorophyll a levels

at a lag of 1 year, indicating a possible link with reduced productivity. However, in

such a heavily managed and regulated fishery, it is likely that other factors, e.g.

fishing effort, stock inputs and variable quotas, have also influenced the total catch of

eels. In particular, the eel fishery has suffered from lower recruitment even with

artificial stocking (Allen, Rosell & Evans, 2006) which would negatively affect the

output of eels from the system.

The main period of benthic invertebrate consumption and associated growth by

fishes of Lough Neagh is during the summer months (May-September), with only the

cold-adapted pollan feeding to any large degree during winter months (Harrod, 2001).

This may mean that fish are able to continue to consume sufficient benthic prey to

maintain numbers and growth over the summer, but limit the availability of prey for

diving ducks (Winfield et al., 1992; Winfield & Winfield, 1994). Unfortunately,

suitable data are not currently available to test the long-term interactions between

diving ducks, fish and their common macroinvertebrate prey, as statutory monitoring

only extends to overwintering bird counts at Lough Neagh. Clearly, there is a need to

extend monitoring to different components of the ecosystem beyond birds, e.g. at least

to benthic macroinvertebrate and fish, in order to identify future ecological shifts in

the largest freshwater lake in the British Isles.

Acknowledgements

This project was funded by the Natural Heritage Research Partnership (NHRP)

between the Northern Ireland Environment Agency (NIEA) and Quercus, Queen’s

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University Belfast (QUB). We are grateful to the Bob Foy, Derek Evans and Chris

Barry of the Agri-Food and Biosciences Institute (AFBI) who provided the

chlorophyll a and eel data. Thanks to Clare Carter for help with chironomid

identification, Garin Cael for help with Plot Digitizer and Kendrew Colhoun (RSPB

NI) for his support.

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Table 1 GLMM results describing changes in a) total density and b) total dry mass of benthic macroinvertebrates in Lough Neagh.

Model F df p

a) Total abundance (F7,78= 22.703, P < 0.001)Survey 111.047 1,78 <0.001Depth 7.819 5,78 <0.001Season 6.236 1,78 0.015

b) Dry mass (F7,78= 9.656, P < 0.001)Survey 60.430 1,78 <0.001Depth 0.371 5,78 0.867Season 5.902 1,78 0.017

23

511512513

514515

Table 2 Results of the SIMPER analysis examining the relative contribution of different taxa to the Lough Neagh macroinvertebrate community in the period 1997/98 and 2010. Also shown for each taxon is the mean dissimilarity between survey dates, and the percentage contribution (individual and cumulative) to the overall dissimilarity between survey periods.

Species

Mean contribution97/98

Mean contribution2010

Mean (± SD) dissimilarity

% contribution to dissimilarity

Cumulative % dissimilarity

Tanypodinae 31 21 12.7 (± 0.1) 21 21Chironomus spp. 15 27 12.3 (± 1.2) 20 41Glyptotendipes spp. 21 14 9.7 (± 1.2) 16 57Tanytarsini 17 10 9.5 (± 1.0) 16 73Other Chironomus spp. 6 8 4.1 (± 0.9) 7 80Others 2 8 3.6 (± 0.9) 6 86Molluscs 2 3 2.0 (± 0.3) 3 89Dicrotendipes spp. 3 2 1.8 (± 1.0) 3 92

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Fig. 1 Lough Neagh showing the location of macroinvertebrate sampling at 3m (triangle), 6m (inverted triangle), 9m (square), 12m (diamond).

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542543

Abundance

a) Oct 1997 b) Oct 2010

c) Jan 1998 d) Jan 2010

Dry mass

e) Oct 1997 f) Oct 2010

g) Jan 1998 h) Jan 2010

Fig. 2 Densities (individuals m-2) of total macroinvertebrates during a) Oct 1997 b) Oct 2010, c) Jan 1998 and d) Jan 2010. Dry mass (mg m-2), excluding molluscs, during e) Oct 1997 f) Oct 2010, g) Jan 1998 and h) Jan 2010.

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544545546547548549550551552553554555556557558559560561562563564565566567568569570571572573574575576577578579580581582583584585586587588589590591592593594595

* sampling was not possible due to the hard nature of the substrate

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Fig. 3 Principle Coordinates Analysis (PCO) ordination showing variation in macroinvertebrate community structure (data square-root transformed, standardised, Bray Curtis dissimilarity matrix) associated with Depth and Survey. Arrows reflect multiple correlations (r > 0.4) between taxa and the two primary PCO axes.

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a)

b)

c)

Fig. 4 a) Productivity of Lough Neagh as measured by chlorophyll a concentrations (µg L-1 ± SE). The periods sampled for benthic macroinvertebrates are shown as asterisks during 1997/98 (Bigsby, 2000) and 2010 (this study). b) Diving ducks index (±SE) for pochard, tufted duck, scaup and goldeneye [extracted and modified from Tománková et al., 2013]. c) Total eel catch (yellow plus silver eels) at Lough Neagh.

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