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Efficient zooplankton grazing is a prerequisite for establishing a cascading food web control over phytoplankton in a lake. We studied if the top-down impact of fish could reach phytoplankton in a lake where the grazing pressure of small-sized zooplankton on filamentous phytoplankton is considered weak. We analysed >30 years of data on plankton, fish catches, hydrochemistry, hydrology, and meteorology from Võrtsjärv, a large and shallow eutrophic lake in Estonia with intensive commercial fisheries. The lake’s unregulated water level has been considered the strongest factor affecting the ecosystem through modifying sediment resuspension, internal loading of nutrients, and underwater light conditions and spawning conditions for fish. We found a negative relationship between phytoplankton biomass and pikeperch biomass indicating a potential top-down cascading effect in the food web. Top-down control of phytoplankton by zooplankton was reflected in a negative relationship between phyto- and zooplankton biomasses. A decrease of the individual weight of crustacean zooplankton with increasing biomass of small fish suggested top-down control of zooplankton by planktivorous fish. In contrast, we could not demonstrate a direct linkage between piscivorous fish and small fish. The top-down food web impact of piscivores, however, was manifested at zooplankton level in a positive correlation of pikeperch biomass with the biomass of dominating cladoceran species Bosmina coregoni and the individual weight of copepods. At high biomasses of small fish, ciliate domination over metazooplankton increased and thus enhanced the strength of the microbial food web. According to our results, fishery management measures that increase small plankti- and benthivorous fish biomass have to be avoided as they have a cascading negative effect on the ecosystem health.
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Is fish able to regulate filamentous blue-green dominated
Tiina No
˜ges .Ain Ja
¨rvalt .Juta Haberman .
Priit Zingel .Peeter No
Received: 11 November 2015 / Revised: 24 May 2016 / Accepted: 29 May 2016 / Published online: 13 June 2016
ÓSpringer International Publishing Switzerland 2016
Abstract Efficient zooplankton grazing is a prereq-
uisite for establishing a cascading food web control
over phytoplankton in a lake. We studied if the top-
down impact of fish could reach phytoplankton in a
lake where the grazing pressure of small-sized
zooplankton on filamentous phytoplankton is consid-
ered weak. We analysed [30 years of data on plank-
ton, fish catches, hydrochemistry, hydrology, and
meteorology from Vo
¨rv, a large and shallow
eutrophic lake in Estonia with intensive commercial
fisheries. The lake’s unregulated water level has been
considered the strongest factor affecting the ecosys-
tem through modifying sediment resuspension, inter-
nal loading of nutrients, and underwater light
conditions and spawning conditions for fish. We
found a negative relationship between phytoplankton
biomass and pikeperch biomass indicating a potential
top-down cascading effect in the food web. Top-down
control of phytoplankton by zooplankton was reflected
in a negative relationship between phyto- and zoo-
plankton biomasses. A decrease of the individual
weight of crustacean zooplankton with increasing
biomass of small fish suggested top-down control of
zooplankton by planktivorous fish. In contrast, we
could not demonstrate a direct linkage between
piscivorous fish and small fish. The top-down food
web impact of piscivores, however, was manifested at
zooplankton level in a positive correlation of pike-
perch biomass with the biomass of dominating clado-
ceran species Bosmina coregoni and the individual
weight of copepods. At high biomasses of small fish,
ciliate domination over metazooplankton increased
and thus enhanced the strength of the microbial food
web. According to our results, fishery management
measures that increase small plankti- and benthivo-
rous fish biomass have to be avoided as they have a
cascading negative effect on the ecosystem health.
Keywords Ecosystem-based fishery management
Large shallow lake Food web interactions Fish
Phytoplankton Zooplankton Ciliates
In large lakes, fish yield is one of the most important
provisioning ecosystem services, whose value depends
Guest editors: Paula Kankaala, Tiina No
˜ges, Martti Rask,
Dietmar Straile & Arkady Yu. Terzhevik / European Large
Lakes IV. Ecosystem Services and Management in a Changing
Electronic supplementary material The online version of
this article (doi:10.1007/s10750-016-2849-9) contains supple-
mentary material, which is available to authorized users.
T. No
˜ges (&)A. Ja
¨rvalt J. Haberman
P. Zingel P. No
Centre for Limnology, Institute of Agricultural and
Environmental Research, Estonian University of Life
Sciences, Rannu, 61117 Tartu County, Estonia
Hydrobiologia (2016) 780:59–69
DOI 10.1007/s10750-016-2849-9
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... Despite a reduction of external nutrient loadings since the mid-1980s, the cyanobacteria abundance in the lake has kept growing (Nõges et al. 2010). Several reasons have been invoked for explaining this discrepancy: relative stability of the in-lake P and N concentrations (Nõges et al. 2008), high shade tolerance of the dominating species (Nõges et al. 2010) in the context of possible "brownification" of water, concomitant rise of temperature, or topdown control of grazer populations Nõges, Järvalt et al., 2016b); release of nutrients from the sediments and turbid-state resilience (Jeppesen et al. 2014), so that constructing a unified basin management policy has been out of reach to date. Additionally, the long-term monitoring on the lake has resulted in a vast amount of limnological data which defies a descriptive modelling approach (Nõges et al. 2010;Cremona et al. 2014). ...
... Ecological monitoring on Lake Võrtsjärv dates back to the early 1960s and the study methods are described thoroughly by Nõges et al. (2010), Nõges, Cremona et al. (2016a), and Nõges, Järvalt et al. (2016b). Unless stated otherwise, all biomasses were given in wet weights (ww). ...
... Higher temperatures increase carbonate weathering rates and hence in-lake alkalinity, pH, and dissolved inorganic carbon concentration, all of which enhance photosynthesis of cyanobacteria (Paerl et al. 2001). Very alkaline waters lead to CO 2 supersaturation which reduces the limitation of primary production that might occur after strong algal growth Nõges, Järvalt et al., 2016b, Cremona et al. 2016b. ...
We aimed at quantifying the importance of limnological variables in the decadal rise of cyanobacteria biomass in shallow hemiboreal lakes. We constructed estimates of cyanobacteria (blue-green algae) biomass in a large, eutrophic lake (Estonia, Northeastern Europe) from a database comprising 28 limnological variables and spanning more than 50 years of monitoring. Using a dual-model approach consisting in a boosted regression trees (BRT) followed by a generalized least squares (GLS) model, our results revealed that six variables were most influential for assessing the variance of cyanobacteria biomass. Cyanobacteria response to nitrate concentration and rotifer abundance was negative, whereas it was positive to pH, temperature, cladoceran and copepod biomass. Response to total phosphorus (TP) and total phosphorus to total nitrogen ratio was very weak, which suggests that actual in-lake TP concentration is still above limiting values. The most efficient GLS model, which explained nearly two thirds (r2 = 0.65) of the variance of cyanobacteria biomass included nitrate concentration, water temperature and pH. The very high number of observations (maximum n = 525) supports the robustness of the models. Our results suggest that the decadal rise of blue-green algae in shallow lakes lies in the interaction between cultural eutrophication and global warming which bring in-lake physical and chemical conditions closer to cyanobacteria optima.
... Fishing activities and cultural eutrophication due to nutrient loads from catchments constitute two of the oldest and main anthropogenic stressors in lakes. Commercial fisheries in particular modify both the standing stock and community structure of fish and other functional groups (Nõges et al., 2016a). Biomanipulation experiments have demonstrated that the removal of piscivorous fish could provoke top-down trophic cascades leading successively to an increase of planktivorous fish biomass, a decrease of large zooplankton herbivores and a rise of phytoplankton producers (Carpenter et al., 2001). ...
... As cyanobacteria constitute relatively poor food prospects for metazooplankton, the detrital trophic pathway is well developed in Lake Võrtsjärv and protozooplankton biomass regularly exceeds twice the metazooplankton biomass (Nõges et al., 2016a). The main protozooplankton taxa are small-bodied ciliates-oligotrichs from Rimostrombidium, Limnostrombidium, Pelagostrombidium, and Halteria genera. ...
... Quantitative protozooplankton sampling started only in 1995. Group sampling and biomass measurement methods are described in Nõges et al. (2016a) for fish, phytoplankton, and zooplankton; and in Cremona et al. (2014b) and Kangur (2015) for macroinvertebrates. The detritus biomass was calculated from the organic carbon burial values of Nõges et al. (2016b) and then converted to wet mass with Brey's (2001) empirical relationship. ...
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We modeled energy transfer and trophic position of fish, plankton, and macroinvertebrates and the relative importance of top-down versus bottom-up processes in Lake Võrtsjärv, a large shallow eutrophic lake in Estonia (northeastern Europe). We employed input values based on 37 years of biomass and fishing activity monitoring for calibrating the Ecopath with Ecosim (EwE) model. Energy flows from primary producers and detritus, represented by total system throughput, were nearly equal (51 and 49%, respectively). Simulation revealed that top-down and bottom-up forces were at play, metazooplankton was not efficiently grazing phytoplankton production, and a trophic cascade proceeded through macroinvertebrates rather than through zooplankton. Detritivory was responsible for the relatively low trophic position of Võrtsjärv fish compared to other lakes. Bottom-up processes were the main drivers for the dual, primary production- and detritus-based pathways in energy flow. Our findings suggest that the predicted biomass increase of cyanobacteria in shallow lakes in the future will strengthen the reliance of consumers on the detrital pathway at the expense of the primary production pathway.
... Ruffe (Gymnocephalus cernuus (L.)), bleak (Alburnus alburnus (L.)) and lake smelt (Osmerus eperlanus m. spirinchus Pallas) have lost their commercial importance following prohibition of the use of fine meshed trawls since the 1970s (Järvalt et al., 2004). The lake has an intensive commercial fishery with well documented yearly catches for all commercial fish species since 1971, and at a lower resolution since 1935 (Nõges et al., 2016). In years when a severe winter coincides with a low water level, serious fish kills may occur (Nõges and Nõges, 2012). ...
... Strong impacts of fisheries management measures on fish community composition and the balance between predatory and non-predatory fish species have been demonstrated in Võrtsjärv, where the banning of small-meshed fishing gear in the 1970s caused a major change in the age and size structure of fishes and contributed to the establishment of predatory fish control over previously dominant ruffe and roach populations (Nõges et al., 2016). Neither of the fish feeding groups' abundances were correlated with Daphnia or TP and only temperature was significantly negatively correlated with the main piscivore abundance (Fig. 4). ...
... In Võrtsjärv, the present correlative analysis and a recent study by Nõges et al. (2016) demonstrated that the main predator (pikeperch) could exert control over phytoplankton, reflected by a significant negative correlation between phytoplankton and pikeperch biomasses ( Fig. 4) most likely caused by a cascading top-down effect through the food web. Supporting this, Nõges et al. (2016) found negative correlations between phyto-and zooplankton biomasses in this lake and a shift in zooplankton size structure relative to pikeperch biomass: higher pikeperch abundances were associated with smaller rotifers and larger copepods. ...
Technical Report
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In the previous report (D4.1, September 2016) we have developed predictive linkages between indicators of environmental quality and ecosystem services, and different types of pressures, single or multiple, across river basins from all over Europe, in a latitudinal and a west-east gradient, and having very different conditions of climate and land use drivers. Using such predictive linkages resulting both from empirical data treatment and process based modelling, and following a common approach for climatic scenario changes, downscaled to region level, we have studied the future evolution of indicators and services. Furthermore, we have looked into the programs of measures that are being implemented in each country and we attempted to understand how will be the responses of the indicators of quality and services, according to such implementation. The studies performed in these 16 basins have a great potential for common applications and studies, and these are being developed in the moment. Although the hydrological and nutrient process-based models that were used and calibrated at each basin scale mostly contain liaisons between the different water compartments, these studies were mostly focused in surface waters and in the indicators of quality that were developed for the Water Framework Directive, as also the information from monitoring taking pace for the last decade, or in previous times. Many issues remain to be studied, related with interfaces between water compartments and also with the biotic aspects of the pressures, also important for some communities and in some cases. This Deliverable (D4.2) is composed of five reports, dealing with stressor effects at the river basin level. These reports cover special aspects of the linkage between ecosystem compartments, multiple pressures and the responses of particular elements, notably the interface between terrestrial and aquatic environments represented by the riparian transitional ecosystems (D4.2-1), stressor propagation between surface waters and groundwaters (D4.2-2) and estuarine waters (D4.2-3), or deal with particular biological stressor effects when such are added to the more common chemical and physical stressors (D4.2-4 and D4.2-5). Most of these reports use different case studies (D4.2-1, D4.2-2 and D4.2-4) from MARS to pool data and experiences for a common target. The other two focus on the stressor propagation in transitional waters in the example of the estuary of Nervion (D4.2-3) and finally two other are related with biotic pressures, i.e. fisheries management and pathogens (D4.2-4 and D4.2-5). All of these studies will be adapted/are being prepared for publication submission. MARS Deliverable 4.2: Manuscripts on stressor effects at the river basin level The Deliverable 4.2. is therefore composed of five reports, with the non-technical summaries following this introduction: D4.2-1: Riparian-to-catchment management options for stressor reduction and service enhancement D4.2-2: Stressor propagation through surface-groundwater linkages and its effect on aquatic systems D4.2-3: Stressor propagation through inland-transitional linkages and management consequences D4.2-4: Fisheries as a source and target of multiple stressors D4.2-5: Multiple-stressor risks for pathogens Summary of D4.2-1 Riparian management is considered a key management option to improve lotic ecosystem status, functioning and services. Vegetated riparian buffer strips can retain nitrogen from sub-surface runoff, and phosphorous and fine sediments from surface runoff. Thus, they can reduce and mitigate the effects of diffuse pollution by agricultural and other land uses. However, the effectiveness of riparian buffers, to a large extent, depends on the location within the stream continuum, as well as on the land use conditions further upstream of the buffered stream sections. Catchment-scale effects can counteract riparian management effects. In the manuscript, we synthesise the evidence of riparian management options in light of catchment-scale pressures. We reviewed 53 management studies addressing both scales and developed a conceptual model to highlight management options with and without conflicts among management scales. Summary of D4.2-2 The good ecological status of Europe's freshwaters is still lacking. This paper reviews the role of groundwater in these systems and demonstrates that it is an important factor to include in surface water management. Groundwater influences streamflow, water chemistry and water temperature and connects rivers and streams with their catchment and thus functions as a pathway for stressors to reach the surface water. A new 'Groundwater DPS' framework is proposed which shows how groundwater fits in the system of a stressed aquatic ecosystem. The functioning of this framework is demonstrated using examples from four different European lowland catchments: the Thames, Odense, Regge and Dinkel catchments. The importance of groundwater varies between scales, between catchments and within catchments.
... Pikeperch (Sander lucioperca) is a common pelagic predatory fish species that is found in many large lakes in Europe (Lammens et al., 1992;Wysujack et al., 2002;Argillier et al., 2003;Kopp et al., 2009;Nõges et al., 2016). It is also highly valued in the recreational and commercial fishery (Johnston et al., 2013;Heikinheimo et al., 2014;Mustamäki et al., 2014;Ginter et al., 2015). ...
Full-text available
The density and mean size of 0 + pikeperch (Sander lucioperca) were studied in late summer in 2004–2017 in clay-turbid and eutrophic Lake Tuusulanjärvi in southern Finland. Hydroacoustics and simultaneous experimental trawling were used to estimate the density of 0 + pikeperch. In some autumns, 0 + pikeperch was the most abundant species in the pelagic fish assemblage. However, the annual amplitude of pikeperch density was extremely high (1,300 - 19,900 individuals ha⁻¹). The density and size variations of 0 + pikeperch were analysed in relation to air temperature sums and degree days. Unexpectedly, the density of 0 + pikeperch showed no correlation with the air temperature sums or degree days, but both the mean length and weight of 0 + pikeperch correlated positively with these temperature variables. The highest correlation coefficients between mean size and temperature were found with temperature sums over 10 °C and degree days over 5 °C. The correlation between density and condition of 0 + pikeperch was negative but non-significant. The high density of 0 + pikeperch in some autumns suggests that pikeperch may play a central role in the pelagic food web in eutrophic lakes – not only as a predator of planktivores but also by its own planktivory.
... The following variables were measured on a monthly basis: biomass and abundance of total metazooplankton (B meta , A meta ), rotifers (B roti , A roti ), copepods (B cope , A cope ), cladocerans (B clad , A clad ), protozooplankton (ciliates B cili , A cili ), biomass of phytoplankton (B phyto ), water and air temperature (T wat , T air ), water level (WL), pH, dissolved oxygen (O 2 ), chemical oxygen demand by permanganate (CODmn), alkalinity (HCO 3 ), total nitrogen (TN) and phosphorus (TP), nitrate (NO 3 ), ammonium (NH 4 ) and phosphate concentrations (PO 4 ). The sampling and analysis methods for most of the variables are described in Nõges et al. (2010Nõges et al. ( , 2016 whereas air temperature time series were obtained from Estonian Environment Agency (https://www. ...
Full-text available
We aimed to investigate the influence of environmental factors and predict zooplankton biomass and abundance in shallow eutrophic lakes. We employed time series of zooplankton and environmental parameters that were measured monthly during 38 years in a large, shallow eutrophic lake in Estonia to build estimates of zooplankton community metrics (cladocerans, copepods, rotifers, ciliates). The analysis of historical time series revealed that air temperature was by far the most important variable for explaining zooplankton biomass and abundance, followed, in decreasing order of importance, by pH, phytoplankton biomass and nitrate concentration. Models constructed with the best predicting variables explained up to 71% of zooplankton biomass variance. Most of the predictive variables had opposing or antagonistic interactions, often mitigating the effect of temperature. In the second part of the study, three future climate scenarios were developed following different Intergovernmental Panel on Climate Change (IPCC) temperature projections and entered into an empirical model. Simulation results showed that only a scenario in which air temperature stabilizes would curb total metazooplankton biomass and abundance. In other scenarios, metazooplankton biomass and abundance would likely exceed historical ranges whereas ciliates would not expand. Within the metazooplankton community, copepods would increase in biomass and abundance, whereas cladocerans would lose in biomass but not in abundance. These changes in the zooplankton community will have important consequences for lake trophic structure and ecosystem functioning.
... Although there are many studies investigating the cascading effects of fish predation on zooplankton and phytoplankton communities [31][32][33][34], there are only a few focusing on zooplankton communities' resilience to predation [30,[35][36][37][38]. These former studies investigated the zooplankton communities in several lakes in North America, which had a history of fish stocking but experienced gradual fish removal or disappearance due to unsuitable spawning grounds. ...
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Planktivorous fish predation directly affects zooplankton biomass, community and size structure, and may indirectly induce a trophic cascade to phytoplankton. However, it is not clear how quickly the zooplankton community structure and the cascading effects on phytoplankton recover to the unaffected state (i.e. resilience) once short-term predation by fish stops. The resilience has implications for the ecological quality and restoration measures in aquatic ecosystems. To assess the short-term zooplankton resilience against fish predation, we conducted a mesocosm experiment consisting of 10 enclosures, 6 with fish and 4 without fish. Plankton communities from a natural lake were used to establish phytoplankton and zooplankton in the mesocosms. High biomasses (about 20 g wet mass m ⁻³ ) of juvenile planktivorous fish (perch, Perca fluviatilis) were allowed to feed on zooplankton in fish enclosures for four days. Thereafter, we removed fish and observed the recovery of the zooplankton community and its cascading effect on trophic interactions in comparison with no fish enclosures for four weeks. Short-term fish predation impaired resilience in zooplankton community by modifying community composition, as large zooplankton, such as calanoids, decreased just after fish predation and did not re-appear afterwards, whereas small cladocerans and rotifers proliferated. Total zooplankton biomass increased quickly within two weeks after fish removal, and at the end even exceeded the biomass measured before fish addition. Despite high biomass, the dominance of small zooplankton released phytoplankton from grazer control in fish enclosures. Accordingly, the zooplankton community did not recover from the effect of fish predation, indicating low short-term resilience. In contrast, in no fish enclosures without predation disturbance, a high zooplankton:phytoplankton biomass ratio accompanied by low phytoplankton yield (Chlorophyll-a:Total phosphorus ratio) reflected phytoplankton control by zooplankton over the experimental period. Comprehensive views on short and long-term resilience of zooplankton communities are essential for restoration and management strategies of aquatic ecosystems to better predict responses to global warming, such as higher densities of planktivorous fish. © 2019 Ersoy et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
... [61,62]. In the studied lakes, water level is acknowledged to define the trajectory of ecosystem development through modifying sediment resuspension, internal loading of nutrients, and underwater light conditions and spawning conditions for fish [63]. Thus, shifts between phases of high and low water levels are behind many of the observed RSs in biotic time series in the early 1980s and the mid 1990s. ...
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Climate change in recent decades has been identified as a significant threat to natural environments and human wellbeing. This is because some of the contemporary changes to climate are abrupt and result in persistent changes in the state of natural systems; so called regime shifts (RS). This study aimed to detect and analyse the timing and strength of RS in Estonian climate at the half-century scale (1966−2013). We demonstrate that the extensive winter warming of the Northern Hemisphere in the late 1980s was represented in atmospheric, terrestrial, freshwater and marine systems to an extent not observed before or after the event within the studied time series. In 1989, abiotic variables displayed statistically significant regime shifts in atmospheric, river and marine systems, but not in lake and bog systems. This was followed by regime shifts in the biotic time series of bogs and marine ecosystems in 1990. However, many biotic time series lacked regime shifts, or the shifts were uncoupled from large-scale atmospheric circulation. We suggest that the latter is possibly due to complex and temporally variable interactions between abiotic and biotic elements with ecosystem properties buffering biotic responses to climate change signals, as well as being affected by concurrent anthropogenic impacts on natural environments.
Ecosystem models that measure the impact of quantitative interactions between trophic levels are widely used tools in ecosystem studies and fishery management. We constructed a mass-balance trophic model using an Ecopath with Ecosim (EwE) modelling suite for large shallow Lake Võrtsjärv, Estonia. The model was calibrated for 36 years (1983–2018) and included 23 functional groups. We examined trophic relationships, functional group interactions, energy fluxes, and keystone groups having a high impact on the ecosystem relative to their biomass. We tested 6 hypothetical scenarios based on future biomass changes for the major functional groups (phytoplankton, zooplankton, macrozoobenthos, piscivorous fish, and bream) for 20 years. The output of the predictive scenarios showed that the biomass changes of planktonic groups would affect the whole food web. Among consumers, macrozoobenthos was crucial for the food web balance because a reduction of their biomass would also reduce the biomass of the fish community. Changes in fish catches would cause minimal biomass difference in other groups. While increased fishing pressure on large piscivores would have a marked effect on the rest of the food web, the reduction of nonpiscivorous fish like bream would have little effect. The results suggested a positive relationship between the biomass of small phytoplankton and fish, alluding to the prevalence of bottom-up trophic processes. These outcomes could be helpful for assessing trophic dynamics in shallow lakes and important aspects for fisheries and ecosystem management.
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We studied the responses of a food web, especially fish and zooplankton, to summertime aeration, pumping of oxygen-rich epilimnetic water to the hypolimnion in Lake Vesijärvi, southern Finland. The aim of hypolimnetic aeration was to reduce internal loading of phosphorus from sediment. The population of smelt (Osmerus eperlanus L.), the main planktivore of the pelagial area, collapsed during the two 1st years of aeration due to increased temperature and low oxygen concentrations in the hypolimnion. The population recovered after the 4th year of hypolimnetic aeration, when oxygen conditions were improved. Despite elevated hypolimnetic temperature, smelt reached exceptionally high abundance, which led to a significant reduction in cladoceran body size. The density of perch (Perca fluviatilis L.) increased at first, but then decreased when the proportion of smelt and cyprinids increased. Biomasses of Daphnia decreased probably as a result of the disappearance of dark, low-oxygen deep-water refuge against fish predation and low availability of nutritionally high-quality algae. Occasionally filamentous cyanobacteria, such as turbulence tolerant Planktothrix agardhii (Gomont), were abundant, suggesting deteriorated food resources for zooplankton. The responses of food web were controversial with respect to the aim of the management, which was to prevent the occurrence of harmful algal blooms.
Relationships between biomass and ecological factors including trophic interactions were examined to understand the dynamics of six fish species in Lake Võrtsjärv, a large shallow eutrophic lake located in Estonia (north-eastern Europe). The database contained initially 31 predictive variables that were monitored in situ for nearly forty years. The strongest predictive variables were selected by three parallel approaches: single correlation (Pearson), a multivariate method (Co-inertia analyses), and a machine learning algorithm (Random Forests), followed by a Generalized Least Squares model to determine meaningful relationships with fish biomass. Models with both additive and interactive effects were constructed. The results revealed that the indicators of degraded ecological conditions (high cyanobacteria biomass and their proportion in total phytoplankton, high summer temperature, high nutrient concentration) were negatively correlated to fish biomass. Benthic macroinvertebrates and other biotic predictors (biomass of specific fish prey and predators) were also important contributors to fish biomass dynamics. Together, abiotic and biotic factors explained 40–60% of the variance of fish biomass, depending of the species. Our findings suggest that both abiotic and biotic factors control fish biomass changes in this eutrophic lake.
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Small-bodied cladocerans and cyclopoid copepods are becoming increasingly dominant over large crustacean zooplankton in eutrophic waters where they often coexist with cyanobacterial blooms. However, relatively little is known about their algal diet preferences. We studied grazing selectivity of small crustaceans (the cyclopoid copepods Mesocyclops leuckarti, Thermocyclops oithonoides, Cyclops kolensis, and the cladocerans Daphnia cucullata, Chydorus sphaericus, Bosmina spp.) by liquid chromatographic analyses of phytoplankton marker pigments in the shallow, highly eutrophic Lake Võrtsjärv (Estonia) during a seasonal cycle. Copepods (mainly C. kolensis) preferably consumed cryptophytes (identified by the marker pigment alloxanthin in gut contents) during colder periods, while they preferred small non-filamentous diatoms and green algae (identified mainly by diatoxanthin and lutein, respectively) from May to September. All studied cladoceran species showed highest selectivity towards colonial cyanobacteria (identified by canthaxanthin). For small C. sphaericus, commonly occuring in the pelagic zone of eutrophic lakes, colonial cyanobacteria can be their major food source, supporting their coexistence with cyanobacterial blooms. Pigments characteristic of filamentous cyanobacteria and diatoms (zeaxanthin and fucoxanthin, respectively), algae dominating in Võrtsjärv, were also found in the grazers' diet but were generally avoided by the crustaceans commonly dominating the zooplankton assemblage. Together these results suggest that the co-occurring small-bodied cyclopoid and cladoceran species have markedly different algal diets and that the cladocera represent the main trophic link transferring cyanobacterial carbon to the food web in a highly eutrophic lake.
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Virtual Populations Analysis (VPA) of the data on catch number by age and year, is used to estimate fishing mortality, abundance and biomass of the fishable stock of pikeperch in Lake Võrtsjärv (270 km2, mean depth 2.8 m), Estonia. The catch rate of the fishery is rather low, finishing mortality (F) of the dominant age group (4-10 years) was 0.21-0.63 during 1978-1990. Natural mortality (M) was 0.13. During the last 14 years there were no major fluctuations of abundance, owing to the strict regulation of fishery. However, changes in pikeperch catches have been 80-fold during the history of the fishery. The causes of such abundance fluctuations are discussed. Optimum annual catch ought to constitute 40-50% of fishable stock (F = 0.65).
L. Võrtsjärv is a large shallow eutrophic pike-perch lake of rather low biological productivity. In the 1950s and 1960s the bulk of the fish caught here consisted of ruffe, young perch and roach. Attempts to reduce the abundance of these undesirable fishes by intensive trawling were unsuccessful. Trawl catch damaged the stocks of commercially important big fishes (first of all, pike-perch), and therefore their numbers became small (except for bream). Subsequently, trawling was stopped, elvers were regularly introduced into the lake and the protection of valuable fishes was improved. As a result of these measures, the total catch of fish decreased temporarily, while the stocks and catches of valuable fishes (above all those of pike-perch and eel) began to grow rapidly. The increasing pressure of predatory fishes (mainly pike-perch and pike) led to a sudden fall in the abundance of less valuable small fish. Recently, fishery has reached an optimum level in the lake. In comparison with the 1950s and 1960s the total catch of fish has now risen more than 1.5 times and the value of the catch as much as 6-8 times (first and foremost on the account of eel).
The metazooplankton of the large (270 km2), shallow (mean depth 2.8 m) and strongly eutrophic (total N 2 g m-3, total P 53 mg m-3) Lake Võrtsjärv (Estonia) has been studied monthly during 1991-1993. In connection with increasing trophy, several species (Asplanchna herricki, Bipalpus hudsoni, Conochilus unifornis, Kellicottia longispina, Bosmina berolinensis, B. obtusirostris, Bythotrephes longimanus, Eudiaptomus gracilis, Cyclops kolensis) have disappeared or nearly disappeared from the zooplankton. The abundance of the indicators of eutrophy (Anuraeopsis fissa, Keratella cochlearis tecta, Bosmina longirostris) has increased. 71% of all zooplankton species and 64% of the dominating species are rotifers. Since small-bodied rotifers and cladocerans dominate, the mean weight of zooplankter is small (0.4-2 μg). Abundance is high (mean for the year 1181 thous. ind m-3, mean for the vegetation period 2072 thous. ind m-3), whereas biomass (mean for the year 0.969 g m-3, mean for the vegetation period 1.681 g m-3) and production (herbivores 5.5 g C m-2 for the vegetation period, predatory zooplankton 0.5 g C m-2) are modest. The production of herbivorous zooplankton constitutes 2.3% of primary production; the detrital type of food chain prevails in the lake. The food consumption (uptake, ration) of herbivores makes up 12% of phytoplankton production; zooplankton feeds mostly on detritus and bacteria. Zooplankton biomass forms only 13% of phytoplankton biomass. Predatory zooplankton consumes 68% of herbivores production; about 25% of zooplankton production reaches fishes. Zooplankton accounts on average for 8.5% of decomposition of organic matter during vegetation period. The efficiency of the zooplankton community is rather low in L. Võrtsjärv.
Seasonal population dynamics and community composition of planktonic ciliates of shallow eutrophic Lake Vortsjarv were studied from January 1995 to December 1996. The abundances (1995 mean = 39.0 x 103 cells/l, 1996 mean = 60.1 x 103 cells/l) and biomasses (1995 mean = 0.68 mg/l, 1996 mean = 2.91 mg/l) were high and showed an increasing trend with the decrease in water level. Ciliate abundances peaked in spring (May) and in late summer (July, August) reaching values up to 191.1 x 103 cells/l. The biomass maxima did not coincide with maximum abundances and were tied with the mass occurrences of large-sized carnivorous species. Ciliates accounted for 50 to 64% of the zooplankton biomass (in 1995 and 1996, respectively). Oligotrichs, haptorids, scuticociliates and prostomatids dominated the community of ciliates.