Reaction of large and shallow lakes Peipsi and Võrtsjärv to the changes of nutrient loading

More than 20-year monitoring of Estonian rivers reveals that the loading of nitrogen to large shallow lakes Peipsi (3,555 km², mean depth 7.1 m) and Võrtsjärv (270 km², mean depth 2.8 m) decreased substantially in the 1990s. Phosphorus loading decreased to a much smaller extent than nitrogen loading. In L. Võrtsjärv both N and P concentrations followed the decreasing trends of loading, which show the high sensitivity of large shallow lakes to catchment processes. Our study showed a positive relationship between P content in sediments and the relative depth of the lake. Assumingly the resilience of a lake in responding to the reduction of nutrient loading decreases together with the decrease of its relative depth. In L. Peipsi the concentration of P has not decreased since the 1990s. Our data show indirectly that P loading from Russia to L. Peipsi may have increased. The N/P ratio has decreased in both lakes. Cyanobacterial blooms have been common in both lakes already at the beginning of the 20th century. The blooms disappeared during heavy nitrogen loading in the 1980s but started again in L. Peipsi in recent years together with the drop of the N/P ratio. In L. Võrtsjärv the N/P ratio is higher and the ecosystem is more stable although the share of N₂-fixing cyanobacteria increased from the 1990s. Reappearing cyanobacterial blooms in L. Peipsi have caused fish-kills in recent years. In L. Peipsi summer/autumn fish-kills during water-blooms are a straightforward consequence of reduced nitrogen level at remaining high phosphorus level while in L. Võrtsjärv the climatic factors affecting water level are more critical––at low water level winter fish-kills may occur. In L. Võrtsjärv nutrient loading has decreased and water quality has improved, present ecological status seems to be mostly controlled by climatic factors through changes of water level. The most important measure to improve water quality in L. Peipsi would be the reduction of phosphorus loading from both Estonian and Russian subcatchments.     

The role of cladocerans reflecting the trophic status of two large and shallow Estonian lakes

The role of pelagic cladoceran communities is discussed on the basis of a comparative study conducted in two Estonian lakes, the moderately eutrophic Lake Peipsi (N_tot 700, P_tot 40 μg l^?1 as average of ice-free period of 1997–2003) and in a strongly eutrophic Lake Võrtsjärv (N_tot 1600, P_tot 54 μg l^?1). The cladoceran community was found to reflect the differences in the trophic state of these lakes. In L. Peipsi, characteristic species of oligo-mesotrophic and eutrophic waters co-dominated (making up 20% or more of total zooplankton abundance or biomass), whereas in L. Võrtsjärv only species of eutrophic waters occurred. In L. Peipsi, the dominant cladocerans were Bosmina berolinensis and Daphnia galeata, while Chydorus sphaericus was the most abundant cladoceran in L. Võrtsjärv. The cladocerans of L. Peipsi (mean individual wet weight 25 μg) were significantly (threefold) larger than those of L. Võrtsjärv (8 μg). The mean wet biomass of cladocerans was higher and total cladoceran abundance was lower in L. Peipsi compared to L. Võrtsjärv (biomass varied from 0.133 to 1.570 g m^?3; mean value 0.800 g m^?3 in L. Peipsi and from 0.201 to 0.706 g m^?3, mean 0.400 g m^?3 in L. Võrtsjärv; the corresponding data for abundances were: 8,000–43,000 ind. m^?3, mean 30,000 ind. m^?3 for L. Peipsi, 50,000–100,000, mean 52,000 ind. m^?3 for L. Võrtsjärv). Based upon differences in body size, cladocerans were more effective transporters of energy in L. Peipsi than in L. Võrtsjärv. Cladocerans proved to be informative indicators of the trophic status and of the efficiency of the food web in studied lakes.     

A comparison of zooplankton densities and biomass in Lakes Peipsi and Võrtsjärv (Estonia): rotifers and crustaceans versus ciliates

The abundance and biomass of ciliates, rotifers, cladocerans and copepods were studied in Lake Peipsi and Lake Võrtsjärv, both of which are shallow, turbid and large. Our hypothesis was that in a large shallow eutrophic lake, the ciliates could be the most important zooplankton group. The mean metazooplankton biomass was higher in Peipsi than in Võrtsjärv (mean values and SD, 1.8 ± 0.7 and 1.3 ± 0.6 mg WM l^?1). In Peipsi, the metazooplankton biomass was dominated by filtrators that feed on large-sized phytoplankton and are characteristic of oligo-mesotrophic waters. In Võrtsjärv, the metazooplankton was dominated by species characteristic of eutrophic waters. The planktonic ciliates in both lakes were dominated by oligotrichs. The biomass of ciliates was much greater in Võrtsjärv (mean 2.3 ± 1.4 mg WM l^?1) than in Peipsi (0.1 ± 0.08 mg WM l^?1). Ciliates formed about 60% of the total zooplankton biomass in Võrtsjärv but only 6% in Peipsi. Thus, the food chains in the two lakes differ: a grazing food chain in Peipsi and a detrital food-chain in Võrtsjärv. Consequently, top-down control of phytoplankton can be assumed to be much more important in Peipsi than in Võrtsjärv. When the detrital food chain prevails, the planktonic ciliates become the most important zooplankton group in shallow, eutrophic and large lake. Neglecting protozooplankton can result in serious underestimates of total zooplankton biomass since two-thirds of the zooplankton biomass in Võrtsjärv comprises ciliates.     

Critical N:P ratio for cyanobacteria and N2-fixing species in the large shallow temperate lakes Peipsi and Võrtsjärv, North-East Europe

In the 1990s a sharp decrease in nitrogen loading occurred in Estonian rivers, bringing about a reduction of the nitrogen-to-phosphorus ratio (N:P ratio) in the large shallow lakes, Peipsi (3,555 km², mean depth 7.1 m) and Võrtsjärv (270 km², 2.8 m). The average mass ratio of total nitrogen (TN) and total phosphorus (TP) in Võrtsjärv (45) was about twice as high as that in Peipsi (22). In Peipsi, the N₂-fixing Gloeotrichia echinulata, Aphanizomenon flos-aquae and Anabaena species prevailed in the summer phytoplankton, while in Võrtsjärv the dominant cyanobacteria were Limnothrix planktonica, L. redekei and Planktolyngbya limnetica, which cannot fix N₂; the main N₂-fixing taxa Aphanizomenon skujae and Anabaena sp. seldom gained dominance. In May–October the critical TN:TP mass ratio, below which N₂-fixing cyanobacteria (Nfix) achieved high biomasses, was ～40 in Võrtsjärv and ～30 in Peipsi. The percentages of both total cyanobacteria (CY) and Nfix (CY% and Nfix%) in Peipsi achieved their maximum values at an N:P mass ratio at or below 20 for both TN:TP and Nmin:SRP. In Võrtsjärv, the TN:TP supporting a high Nfix% was between 30 and 40 and the N_min:SRP supporting this high percentage was in the same range as that in Peipsi (<20), though the maximum Nfix% values in Võrtsjärv (69%) were much lower than in Peipsi (96%). The Nmin:SRP ratio explained 77% of the variability in Nfix% in May–October. The temperature dependence of Nfix% approximated to the maximum function type, with an upper limiting value at a certain water temperature, and this was most distinct in May–October. The critical TN:TP ratios obtained from our study (roughly 30 for Peipsi and 40 for Võrtsjärv) are much higher than the Redfield N:P mass ratio routinely considered (7). Our results represent valuable guidelines for creating effective management strategies for large shallow lakes. They provide a basis for stressing the urgent need to decrease phosphorus loading and to keep the in-lake P concentration low, and not to implement nitrogen reduction measures without a simultaneous decrease of phosphorus concentration.     

A comparison of the palaeolimnology of Peipsi and Võrtsjärv: connected shallow lakes in north-eastern Europe for the twentieth century, especially in relation to eutrophication progression and water-level fluctuations

We applied a multi-proxy palaeolimnological approach to provide insights into the natural variability and human-mediated trends of two interconnected temperate large shallow lakes, Peipsi and Võrtsjärv, during the twentieth century. The history of the lakes was assessed on the basis of age-related changes in the sediment main constituents (water, organic matter and carbonate), sub-fossil pigments, diatom assemblages and organic matter dissolved in pore water. The temporal changes in the palaeodata indicate an increase of the in-lake biological production in both lakes from about the 1960s, suggesting enhanced nutrient inputs. In subsequent decades, the gradual increase of autochthonous organic matter becomes more obvious, indicating progressive eutrophication of the lakes. Palaeolimnological indicators from the sediment record of Lake Peipsi indicate a slight recession of the lake’s eutrophication in the 1990s but not for Lake Võrtsjärv. The results of the study also suggest that after the lakes became eutrophied, the climatically induced water-level fluctuations ceased to be the main driver determining the abundance of phytoplankton. Responses of the lakes to human-induced impacts are better recorded in the sediments of Lake Peipsi than in those of Lake Võrtsjärv, which is shallower of the two and where the wave-induced resuspension of deposits markedly smooths or erases the signals of environmental changes. The results of the investigation expand the knowledge on how large shallow lakes respond to human-mediated and natural perturbations, including those in the lake catchment areas and the capability of the lakes to store the chronology and sequence of these changes.     

Diet patterns and ontogenetic diet shift of pikeperch, <Emphasis Type="Italic">Sander lucioperca</Emphasis> (L.) fry in lakes Peipsi and Võrtsjärv (Estonia)

Pikeperch is a major predator in the pelagic zone of eutrophic waters, such as the large north-temperate lowland lakes Võrtsjärv (Estonia) and Peipsi (Estonia/Russia). The size and structure of the pikeperch population is strongly influenced by their success at the juvenile stage. Therefore, we investigated the diet and prey selection of pikeperch fry caught in the ice-free period in lakes Peipsi and Võrtsjärv in 2007 and 2008. We analysed the stomach contents of 635 pikeperch from Lake Peipsi and 202 pikeperch from Lake Võrtsjärv, and compared our findings with similar data from the 1950s (Erm, About Biological and Morphological Differences of Pikeperch. Hydrobiological Researches II (in Estonian), 1961). Analysing 4–20 cm long fry, we studied differences in prey size, seasonal diet patterns and the ontogenetic diet shift. In both lakes, 0+ pikeperch feed mostly on large predatory zooplankters. However, in Lake Peipsi the stomach content weight and the average number of food items in stomach were higher, and the food spectrum was wider than in Lake Võrtsjärv. There was also a difference in the type of food that dominated fry’s stomach content (calculated by weight) in the two lakes. In Lake Peipsi, chironomids larvae, as well as zooplankters Daphnia galeata and Bythotrephes longimanus dominated, while in Lake Võrtsjärv zooplankters Mesocyclops leuckarti and Leptodora kindti. Seasonal analysis showed that cladocerans dominated in pikeperch fry stomach content in summer and at the beginning of September, but copepods were dominant in autumn and spring. In contrast to the studies carried out from 1952 to 1958 (Erm, About Biological and Morphological Differences of Pikeperch. Hydrobiological Researches II (in Estonian), 1961), the shift from planktivory to piscivory at the end of the first growing season was hardly ever observed during our investigation. We believe this is due to the lack of suitable prey fish as there was a collapse of the smelt, Osmerus eperlanus (L.), population in both lakes. The transition of pikeperch from planktivory to piscivory was delayed till the next summer.     

Condition and growth of ruffe Gymnocephalus cernuus (L.) in two large shallow lakes with different fish fauna and food recourse

Hydrobiologia - Ruffe is one of the most numerous fishes in Estonian large Lakes Peipsi (area 3555 km2, mean depth 7.1 m) and Võrtsjärv (270 km2, 2.8 m). Owing to its small size ruffe has...     

Primary production of aquatic macrophytes and their epiphytes in two shallow lakes (Peipsi and Võrtsjärv) in Estonia

In shallow lakes with large littoral zones, epiphytes and submerged macrophytes can make an important contribution to the total annual primary production. We investigated the primary production (PP) of phytoplankton, submerged macrophytes, and their epiphytes, from June to August 2005, in two large shallow lakes. The production of pelagic and littoral phytoplankton and of the dominant submerged macrophytes in the littoral zone (Potamogeton perfoliatus in Lake Peipsi and P. perfoliatus and Myriopyllum spicatum in Lake Võrtsjärv) and of their epiphytes was measured using a modified ¹⁴C method. The total PP of the submerged macrophyte area was similar in both lakes: 12.4 g C m^?2 day^?1 in Peipsi and 12.0 g C m^?2 day^?1 in Võrtsjärv. In Peipsi, 84.2% of this production was accounted for by macrophytes, while the shares of phytoplankton and epiphytes were low (15.6 and 0.16%, respectively). In Võrtsjärv, macrophytes contributed 58%, phytoplankton 41.9% and epiphytes 0.1% of the PP in the submerged macrophyte area. Epiphyte production in both lakes was very low in comparison with that of phytoplankton and macrophytes: 0.01, 5.04, and 6.97 g C m^?2 day^?1, respectively, in Võrtsjärv, and 0.02, 1.93, and 10.5 g C m^?2 day^?1, respectively, in Peipsi. The PP of the littoral area contributed 10% of the total summer PP of Lake Peipsi sensu stricto and 35.5% of the total summer PP of Lake Võrtsjärv.     

Variability of bio-optical parameters in two North-European large lakes

The bio-optical properties of some North-European large lakes were examined during 1995–2005 using field data and laboratory measurements. The key variables were optically active substances (OAS: chlorophyll, total suspended matter and dissolved organic matter), Secchi depth, and the “spectrometric” and diffuse light attenuation coefficients. Our main study sites were Lake Peipsi and Lake Võrtsjärv in Estonia, both eutrophic with mean Secchi depth below 3 m. The measured water parameters were compared with those obtained from two clear-water Swedish lakes, Lake Vänern and Lake Vättern. This comparison describes the bio-optical differences of the water in eutrophic and oligotrophic lakes. The variability of water parameters in the turbid Estonian lakes was rather high, e.g. the chlorophyll content varied from 1.8 to 102 mg m^?3 and the diffuse light attenuation coefficient from 0.92 to 6.5 m^?1. The change in water properties depends on the season and the biological activity of phytoplankton. We found no apparent long-time trend in water properties. Regression analysis showed that in the turbid Estonian lakes the optical properties were well correlated with chlorophyll and suspended matter, but not with dissolved organic matter. The highest determination coefficients (between 0.73 and 0.89) were obtained when the optical parameters were correlated with all three OAS together (multiple regressions). Our results concerning the variability and interconnections among bio-optical parameters in two Estonian large lakes illustrate the effect of OAS and light field on the ecological conditions of lakes in general.     

Live labeling technique reveals contrasting role of crustacean predation on microbial loop in two large shallow lakes

We tested the hypotheses that the ciliate assemblages in moderately eutrophic lake are controlled by the effective crustacean predation, and the high abundances of planktonic ciliates in highly eutrophic and turbid lake are due to insufficient regulation by crustacean zooplankton. A food tracer method coupled with natural assemblage of microciliates labeled with fluorescent microparticles was used to measure the cladoceran and copepod predation rates on planktonic ciliates and to estimate the carbon flow between the ciliate–crustacean trophic links. The results revealed that the microciliates (15–40 μm) were consumed by all dominant cladoceran and copepod species in both the lakes studied, mainly by Chydorus sphaericus and cyclopoid copepods in Lake Võrtsjärv, and by Daphnia spp. and Bosmina spp. in Lake Peipsi. The grazing loss in moderately eutrophic Peipsi indicated strong top-down control of ciliates mainly by cladocerans. The extraordinary abundant population of planktonic ciliates having a predominant role in the food web in highly eutrophic and turbid Võrtsjärv is explained by the measured low crustacean predation rates on ciliates. The estimated carbon flow from the ciliates to crustaceans suggest that in eutrophic lakes majority of the organic matter channeled via metazooplankton to higher trophic levels may originate from the microbial loop.     

Dynamic carbon budget of a large shallow lake assessed by a mass balance approach

To study the role of large and shallow hemiboreal lakes in carbon processing, we calculated a 3-year carbon mass balance for Lake Võrtsjärv (Estonia) based on in situ measurements. This balance took into account hydrological and biogeochemical processes affecting dissolved inorganic (DIC), dissolved organic (DOC) and particulate organic (POC) carbon species. Accumulation varied greatly on a seasonal and yearly basis. The lake exported carbon during most of the year except during spring floods and in late autumn. In-lake processes were responsible for exporting POC and storing DOC while DIC switched between storage and export. The carbon cycle was alternatively dominated in 2009 by biogeochemical processes and in 2011 by riverine fluxes, whereas in 2010 the two process types were of the same magnitude. These results suggest that the role of large shallow lakes like Võrtsjärv in the global C cycle is equally driven by hydrological factors, in particular seasonal water level changes, and by biogeochemical in-lake reactions.     

Contributions of DOC from surface and groundflow into Lake Võrtsjärv (Estonia)

Hydrological changes have the greatest impact on shallow lakes where they alter the water volume and lake depth noticeably. Dissolved organic carbon (DOC), which is markedly affected by hydrological factors, has an important role in many biogeochemical processes. The DOC load supplied to Lake Võrtsjärv, the second largest lake in Estonia, was studied on the scale of the subcatchments discharging into the lake. Daily discharges and biweekly or monthly DOC concentrations were measured close to the river outlets over the years 1990–2002. The stream flow data were separated into groundflow and surface flow by applying local minimum and recursive digital filtering methods. Constituent load estimation software, LOADEST, was used to estimate DOC concentrations and load. LOADEST performed well for three of the four rivers. The total estimated DOC load to Võrtsjärv from all four main rivers varied from 1,320 to 4,934 t year^?1. The average annual load over the 13-year period was 3,265 t year^?1 or 1.18 g C m^?2 year^?1. Baseflow separation analysis indicated that the DOC load originating from groundflow contributed 79% and 69% of the total load according to the digital filter and local minimum methods, respectively. The results of our study demonstrate the utility of linking the rating-curve method and baseflow separation to assess the allochthonous DOC load to Võrtsjärv both currently and under changing climatic conditions.     

Water level as the mediator between climate change and phytoplankton composition in a large shallow temperate lake

Hydrobiologia - We studied the effect of water level changes in Lake Võrtsjärv (270 km2, mean depth 2.8 m) on the abundance and composition of phytoplankton based on a 35-year database....     

Water level changes in a large shallow lake as reflected by the plankton:periphyton-ratio of sedimentary diatoms

Biostratigraphic diatom analyses were carried out on a short sediment core from the large shallow-water Lake Võrtsjärv, Estonia, in order to relate the diatom composition to the instrumental water level record. We dated the sediment core by radiometric methods (²¹⁰Pb, ¹³⁷Cs, ²⁴¹Am) and spheroidal fly-ash particle abundance chronology and evaluated the statistical significance of the relationships between the percentage of planktonic diatoms and the water level continuously monitored since 1871. Before the 1960s, the percentage of planktonic diatoms in the sediment showed quite strong positive relationship to water level. The impact of eutrophication after the 1960s presumably masked the influence of water level changes on the diatom community. In addition, statistical analysis of the upper part of the sediment core (1970—present day) together with measured limnological parameters of the lake showed that water transparency had the strongest influence on diatoms, while temperature, pH and alkalinity had lesser impacts. Our study shows that the planktonic:periphytic diatom ratio in the sediment can be used to track overall trends of the lake-level changes in Lake Võrtsjärv before the onset of cultural eutrophication; however, the results have to be interpreted carefully, taking into consideration other possible limnological factors such as water transparency, nutrients and wind.     

Ciliate communities of a large shallow lake: Association with macrophyte beds

We investigated the influence of macrophyte composition on ciliate community structure in a large, shallow, eutrophic Lake Võrtsjärv. We hypothesized that macrophyte composition must have strong influence on the dispersal of ecologically different ciliate groups in a shallow lake and that more diverse macrophyte stands cause also a greater diversity in the ciliate community. In Võrtsjärv macrophyte distribution is spatially strongly polarized both in east–west and north–south directions in relation to abiotic factors. Phragmites australis and Myriophyllum spicatum were the most widespread species occurring in most parts of the lake. Correlation of environmental, macrophyte and planktonic ciliate variables confirmed the suggested spatial gradients. More diverse macrophyte stands supported a high species richness and abundance of epiplanktonic community but showed negative influence on the number and abundance of euplanktonic ciliate taxa. Opposite trends were found relative to the abundance of P. australis. Benthic ciliates showed a similar distribution pattern to euplanktonic taxa being most abundant in sites were the Shannon–Weaver index for macrophytes was low. Strong polarizing effect of the lake's vegetation on planktonic ciliate diversity was reflected in correlations of the number of ciliate taxa as well as the numbers of eu- and epiplanktonic taxa with geographic co-ordinates.     

Diversity and structure of the winter rotifer assemblage in a shallow eutrophic northern temperate Lake Võrtsjärv

In the eutrophic Lake Võrtsjärv (Central Estonia, area 270 km², mean depth 2.8 m) rotifers form ca. 90% of total abundance and 80% of biomass in winter zooplankton community. The winter rotifer assemblage was dominated by Polyarthra dolichoptera, both in abundance and in biomass. Synchaeta verrucosa and Keratella quadrata were the sub-dominants. Thus, winter rotifer community had low diversity and high dominance of a few species. This pattern probably refers to the period of extreme environmental conditions where the rotifer assemblage is composed of few well-adapted species, and the low diversity here was not indicating instability of community structure, but the scarcity of suitable niches. These community structure indices indicate that the winter rotifer assemblage of L. Võrtsjärv was very similar to autumn assemblage, but very different from the spring one. In winter, small raptors were the most important functional group. The second place is occupied by larger raptors. Marginal role of fine particle sedimentators, absence of suckers and high proportion of large raptors were contrasting features of the winter trophic structure in comparison with the other seasons. Changes have taken place in the winter rotifer assemblage in L. Võrtsjärv in 1990–2007. Against the background of diminishing rotifer abundance, the dominant species has become even more prevalent, and the diversity of the winter rotifer assemblage has decreased. Shifts in the community trophic structure were also observed.     

Water temperatures and ice cover in lakes of the Tatra Mountains

In 2000 and 2001, miniature thermistors with integrated data loggers were employed to measure lake surface water temperatures (LSWTs) and temperature profiles in high-altitude mountain lakes lying between 1580 and 2145 m a.s.l. on both the Slovak and Polish sides of the Tatra Mountains. This allowed the annual cycle of water temperatures and ice cover in these lakes to be described quantitatively, and their dependence on lake altitude above sea level to be investigated. LSWTs in the Tatra Mountains are found to decrease approximately linearly with increasing altitude from late spring to autumn. LSWT in summer can be modelled well in terms of exponentially smoothed ambient air temperature. Although the timing of ice-off is dependent on altitude, the timing of ice-on is not; the dependence of the duration of ice cover on altitude is therefore wholly due to the altitudinal dependence of the timing of ice-off. The temperature profile measurements allow quantitative characterization of summer and winter stagnation, and spring and autumn turnover.     

Reconstructed long-term time series of phytoplankton primary production of a large shallow temperate lake: the basis to assess the carbon balance and its climate sensitivity

Aiming at building the carbon budget for further climate change impact research in the large and shallow northern temperate Lake Võrtsjärv, the present paper focuses on reconstructing the full phytoplankton primary production (PP) data series for the lake for the period of 1982–2009 covered by disconnected measurements, and testing the uncertainties involved both in the PP measurements and bio-optical modelling. During this 28-year period, in situ PP was measured in Võrtsjärv in 18 years with ¹⁴C-assimilation technique. We reconstructed the full time series using a semi-empirical PP simulation model based on continuously measured PAR irradiance and interpolated values of monthly measured chlorophyll a (C _chl). The modelling results, which proved highly reliable during the calibration phase, correlated rather weakly with the annual PP estimates for the 18 years, which were based on 2-h incubations at midday, 1–2 times per month. Being based on continuous irradiance data, the modelled PP can be considered more reliable than the sparse measurements, especially for short to medium term studies. We demonstrate that in the long-term, the bio-optical method can be biased if changes in water colour or water level alter the light climate causing adaptive responses in the cellular chlorophyll content of light-limited phytoplankton.     

Migration patterns of a potamodromous piscivore,asp (Leuciscus aspius), in a river–lake system

Potamodromous fishes require safe migration routes between spawning, feeding and wintering habitats to complete their life cycle. As knowledge on asp migrations is restricted, this work investigated the movements of adult asp tagged with acoustic transmitters for 3 years in the large Peipsi-Emajõgi-Võrtsjärv lake–river system, Estonia, which is free of migration barriers. Asp showed complex migration patterns, moving between and within different waterbodies (lakes, river, tributaries) in all seasons, but with a tendency to repeat habitat use patterns between years. Lakes were mainly used for feeding during spring and summer (after spawning 65% of the fish migrated to Lake Peipsi), and more so by large fish. The majority (80–96%) of the fish spent the winter in the rivers, mostly close to their subsequent spawning area. Spawning areas were in swift-flowing waters in tributaries and the main river. The results indicate that asp may benefit from an extensive and diverse complex of habitats, and any migration barrier during any season may restrict the natural habitat use by asp. Maintenance and restoration of habitat heterogeneity and connectivity is critical to protect behaviourally diverse fish populations and increase resilience in rivers negatively impacted by various human activities.     

Homage to Hutchinson: does inter-annual climate variability affect zooplankton density and diversity?

G. Evelyn Hutchinson proposed that external control by climate limits the fundamental productivity and the possible diversity of ecological communities. These climatic drivers are currently changing as a result of human activity, which may herald a shift in the influence of climate on global ecosystems. Long-term records reveal a reduction in ice cover on northern lakes over the last several centuries. Hence, we explore whether inter-annual climatic variability, represented by ice cover, influences the productivity and diversity of zooplankton communities in long-term datasets for five lakes in Northern Wisconsin. We used a multilevel modeling approach to test three predictions: (1) density will increase, (2) diversity will increase, and (3) community composition will be altered. We found an inverse relationship between ice-off date and annual zooplankton density. Daphnia density, for example, was inversely related to ice-off date, with 10-fold variability across the gradient of ice-off dates in Northern Wisconsin. In contrast, we did not observe a consistent shift in diversity or community structure. Thus, from ice cover records of northern lakes we found support for Hutchinson’s idea that external climatic forces may regulate aquatic productivity; however, the response was numeric and we did not find evidence that lakes moved closer to maximum diversity on an inter-annual scale.