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Invasive species constitute a threat not only to native populations but also to the structure and functioning of entire food webs. Despite being considered as a global problem, only a small number of studies have quantitatively predicted the food web‐level consequences of invasions. Here, we use an allometric trophic network model parameterized using empirical data on species body masses and feeding interactions to predict the effects of a possible invasion of Amur sleeper (Perccottus glenii), on a well‐studied lake ecosystem. We show that the modeled establishment of Amur sleeper decreased the biomasses of top predator fishes by about 10%–19%. These reductions were largely explained by increased larval competition for food and Amur sleeper predation on fish larvae. In contrast, biomasses of less valued fish of lower trophic positions increased by about 0.4%–9% owing to reduced predation pressure by top piscivores. The predicted impact of Amur sleeper establishment on the biomasses of native fish species vastly exceeded the impacts of current‐day fishing pressures. [ABSTRACT FROM AUTHOR]

Submerged macrophytes are of key importance for the structure and functioning of shallow lakes and can be decisive for maintaining them in a clear water state. The ongoing climate change affects the macrophytes through changes in temperature and precipitation, causing variations in nutrient load, water level and light availability. To investigate how these factors jointly determine macrophyte dominance and growth, we conducted a highly standardized pan‐European experiment involving the installation of mesocosms in lakes. The experimental design consisted of mesotrophic and eutrophic nutrient conditions at 1 m (shallow) and 2 m (deep) depth along a latitudinal temperature gradient with average water temperatures ranging from 14.9 to 23.9°C (Sweden to Greece) and a natural drop in water levels in the warmest countries (Greece and Turkey). We determined percent plant volume inhabited (PVI) of submerged macrophytes on a monthly basis for 5 months and dry weight at the end of the experiment. Over the temperature gradient, PVI was highest in the shallow mesotrophic mesocosms followed by intermediate levels in the shallow eutrophic and deep mesotrophic mesocosms, and lowest levels in the deep eutrophic mesocosms. We identified three pathways along which water temperature likely affected PVI, exhibiting (a) a direct positive effect if light was not limiting; (b) an indirect positive effect due to an evaporation‐driven water level reduction, causing a nonlinear increase in mean available light; and (c) an indirect negative effect through algal growth and, thus, high light attenuation under eutrophic conditions. We conclude that high temperatures combined with a temperature‐mediated water level decrease can counterbalance the negative effects of eutrophic conditions on macrophytes by enhancing the light availability. While a water level reduction can promote macrophyte dominance, an extreme reduction will likely decrease macrophyte biomass and, consequently, their capacity to function as a carbon store and food source. [ABSTRACT FROM AUTHOR]

Theoretical pelagic primary production of phytoplankton and benthic primary production of periphyton were modelled for two small lakes in Estonia (Northeast Europe). Although located only 500 m apart, the water colour and light attenuation of these two lakes differed markedly. The Secchi depth (SD) in the clear‐water lake was 4.5 m and only 0.47 m in the dark‐water lake. The total phosphorus (TP) concentrations were, respectively, 15 μg/l and 28 μg/l. An empirical model whose inputs were morphometric, light conditions and dissolved organic carbon parameters obtained from in situ measurements was employed for the present study. The model calculated primary production with a time‐step of 10 min, and a spatial resolution of 10 cm, from sunrise to sunset and from lake surface to lake bottom. The primary production of periphyton and phytoplankton was almost equal in the clear lake, whereas only phytoplankton contributed to whole‐lake primary production in the dark lake because of the stronger light attenuation in the water column. The results of the present study indicated the depth‐distribution profiles differed dramatically between the two lakes. The clear lake had a deep, U‐shaped curve, with the productive layer reaching considerable depth soon after sunrise and maintaining a similar profile throughout the light hours. In contrast, the dark lake production declined rapidly with increasing depth, whereas the profile changed over the day reaching the greatest depth at noon. [ABSTRACT FROM AUTHOR]

While 13C‐depleted carbon derived from biogenic methane can substantially contribute to the benthic secondary production in deep stratified lakes, its role in shallow lakes is less clear. We investigated the dynamics of δ13C and δ15N in the larvae of Chironomus plumosus throughout an annual cycle in two ecologically distinct basins (open‐water plankton‐dominated and sheltered macrophyte‐covered) of a large (270 km2), shallow, polymictic and eutrophic lake (Võrtsjärv, Estonia, North Europe). The larval stable isotopic compositions were linked to the presence of methane‐oxidising bacteria (MOB) in larval guts and sediments.Molecular detection of MOB revealed their presence in various sediment types, but stable isotope (SI) analysis revealed clear differences in the feeding of chironomid larvae between the plankton‐ and macrophyte‐dominated habitats.In the plankton‐dominated habitat, the mean δ13C values of larvae remained relatively constant (−38.3‰ to −35.5‰) and corresponded closely to the sediment δ13C values. Mean δ13C values of chironomid larvae were generally lower in macrophyte‐dominated habitats (−43.4‰ to −33.0‰), and both seasonal and individual variation in larval δ13C values were more pronounced. MOB presence in larval guts proved a dietary contribution from biogenic methane in macrophyte‐dominated habitats. Both the SI and molecular results indicated that MOB could help support larvae even during the cold temperature‐limited and ice‐covered periods.Our study indicates that methane‐derived carbon makes a low but steady contribution to the larval chironomids throughout an annual cycle in large shallow Võrtsjärv. However, this contribution can be substantially higher in the lake habitats with abundant macrophytes. The study provides further evidence that a carbon flow pathway from biogenic methane can contribute to the benthic food web under variable habitat conditions in a shallow polymictic lake. [ABSTRACT FROM AUTHOR]

Diatom-based assays have been successfully associated worldwide with the tropic status of lakes. Several studies have demonstrated a correlation between epiphytic diatoms and nutrient load in shallow lakes and wetlands. We examine the relative importance of environmental factors in explaining the structure of epiphytic diatom communities in a set of Western European shallow lakes. The effects of lake physical/chemical, morphometric, and geographical variables on diatom assemblages were tested using distance-based, canonical correspondence and regression analyses. Our results show that epiphytic diatom communities respond, mainly to physical/chemical variables, overriding the effects of lake depth, size, and location. The clustering of studied systems based along a total phosphorus concentration gradient concurs with previous classification of diatom taxa and nutrient state. Assessment of epiphytic diatoms provided a potential method for shallow lake classification. [ABSTRACT FROM AUTHOR]

Biogenic methane as an alternative carbon and energy source for freshwater organisms has been receiving increasing attention, but the phenomenon is still poorly understood for shallow lakes. We measured the carbon and nitrogen stable isotope signatures (δ13C, δ15N) for key groups of pelagic and benthic organisms, including crustacean zooplankton, chironomid larvae, young-of-the-year and adult fish, to assess whether biogenic methane contributes to pelagic and benthic food webs in a large, shallow lake, Lake Võrtsjärv, Estonia., In the southern part of the lake, covered by macrophytes, crustacean zooplankton showed strong seasonal variation of δ13C, with the lowest values occurring in the period of oxygen depletion. Chironomid ( Chironomus plumosus) larvae showed high isotopic variability within the lake, with strongly 13C-depleted and 15N-depleted signatures (down to δ13C −64.0‰ and δ15N −2.6‰) in the macrophyte-covered area., Our results indicate that carbon derived from biogenic methane can contribute seasonally to the benthic food web and, to some extent, also to the pelagic food web, where the lake is covered by macrophytes. Moreover, δ13C values for roach ( Rutilus rutilus), perch ( Perca fluviatilis) and pike ( Esox lucius) from the macrophyte-dominated area were on average 3.5‰ lower than those for the same fish from the plankton-dominated lake area, suggesting some carbon derived from methane is transferred up through the food web., Although no direct evidence is available, our results, together with previous studies of the lake, suggest that protozoans could be a potentially important link from methane-oxidising bacteria to animals higher in the web. [ABSTRACT FROM AUTHOR]

We traced the origin of dissolved organic matter (DOM) in the large, shallow, eutrophic Lake Võrtsjärv in Estonia. Allochthonous DOM (Al-DOM) had higher δ13C values than autochthonous DOM (Au-DOM). The δ13C of inflow DOM varied from -28.2‰ to -25.4‰ (mean -26.7‰) and in-lake DOM varied from -28.4‰ to -26.1‰ (mean -27.2‰). Low stable isotope (SI) signatures of Au-DOM were caused by relatively 13C-depleted values of its precursors (mainly phytoplankton) with mean δ13C of -28.9‰. SI signatures of dissolved inorganic carbon (DIC) in the inflows and in the lake were also relatively low (from -15.1‰ to -3.28‰). SI values of DOM were lower during the active growing season from May to September and higher from October to April, with the corresponding estimated average proportions of Al-DOM 68% and 81%. The proportion of Al-DOM decreased with increasing water temperature, chlorophyll a, and pH and increased with increasing water level and concentration of yellow substances and DIC. The high proportion of Al-DOM in Võrtsjärv shows that, even in this highly productive ecosystem, the labile Au-DOM produced is rapidly utilized and degraded by microorganisms and thus makes a relatively small contribution to the instantaneous in-lake DOM pool. [ABSTRACT FROM AUTHOR]

1. As grazers on picoplankton and nanoplankton, planktonic ciliates form an important link in pelagic food webs. Ciliate communities may be controlled by predation by metazooplankton. In eutrophic systems, however, where the number of large crustaceans is often low, the mechanisms that regulate ciliate dynamics have rarely been described. 2. We conducted an enclosure experiment with natural and screened (145 μm) summer plankton communities to investigate the effect of the small-sized crustacean zooplankton on ciliate community structure and the microbial loop in a shallow eutrophic lake. 3. The removal of the larger fraction of crustaceans initiated a decrease in total ciliate abundance. At the community level, we observed a substantial increase in large-sized predacious ciliates (>100 μm) and a simultaneous decrease in the abundance of smaller ciliates (<20-40 μm) that were mostly bacterivores and bacterio-herbivores. The compositional shift in the ciliate community, however, did not cascade down to the level of bacteria and edible phytoplankton. [ABSTRACT FROM AUTHOR]

We assessed feeding of planktivorous fish larvae on ciliate protozooplankton in shallow eutrophic Lake Võrtsjärv, Estonia to determine whether ciliates are an important part of larval fish diet. We collected larvae of the most common fish species and examined their gut contents by gut segmentation and epifluorescence microscopy. Ciliates were present in the gut of all fish larvae. Gut segmentation analysis showed clear differences in food composition between gut quarters. The hard-bodied food items were quite evenly distributed in the gut, but the soft-bodied ciliates and rotifers were present only in the first gut quarters. Neglecting differences in prey digestion rate leads to underestimation of diet amount and composition and, especially, to underestimation of the role of protists in food. In Võrtsjärv, ciliates account for ~ 60% of the total carbon biomass consumed by fish larvae. The food requirement of fish larvae during first feeding stages (May-Jun) would not be met without consumption of ciliates. [ABSTRACT FROM AUTHOR]

Dominance by cyanobacteria hampers human use of lakes and reservoirs worldwide. Previous studies indicate that excessive nutrient loading and warmer conditions promote dominance by cyanobacteria, but evidence from global scale field data has so far been scarce. Our analysis, based on a study of 143 lakes along a latitudinal transect ranging from subarctic Europe to southern South America, shows that although warmer climates do not result in higher overall phytoplankton biomass, the percentage of the total phytoplankton biovolume attributable to cyanobacteria increases steeply with temperature. Our results also reveal that the percent cyanobacteria is greater in lakes with high rates of light absorption. This points to a positive feedback because restriction of light availability is often a consequence of high phytoplankton biovolume, which in turn may be driven by nutrient loading. Our results indicate a synergistic effect of nutrients and climate. The implications are that in a future warmer climate, nutrient concentrations may have to be reduced substantially from present values in many lakes if cyanobacterial dominance is to be controlled. [ABSTRACT FROM AUTHOR]

1. Changes in cladoceran subfossils in the surface sediments of 54 shallow lakes were studied along a European latitude gradient (36–68°N). Multivariate methods, such as regression trees and ordination, were applied to explore the relationships between cladoceran taxa distribution and contemporary environmental variables, with special focus on the impact of climate. 2. Multivariate regression tree analysis showed distinct differences in cladoceran community structure and lake characteristics along the latitude gradient, identifying three groups: (i) northern lakes characterised by low annual mean temperature, conductivity, nutrient concentrations and fish abundance, (ii) southern, macrophyte rich, warm water lakes with high conductivity and high fish abundance and (iii) Mid-European lakes at intermediate latitudes with intermediate conductivities, trophic state and temperatures. 3. Large-sized, pelagic species dominated a group of seven northern lakes with low conductivity, where acid-tolerant species were also occasionally abundant. Small-sized, benthic-associated species dominated a group of five warm water lakes with high conductivity. Cladoceran communities generally showed low species-specific preferences for habitat and environmental conditions in the Mid-European group of lakes. Taxon richness was low in the southern-most, high-conductivity lakes as well as in the two northern-most sub-arctic lakes. 4. The proportion of cladoceran resting eggs relative to body shields was high in the northern lakes, and linearly (negatively) related to both temperature and Chl a, indicating that both cold climate (short growing season) and low food availability induce high ephippia production. 5. Latitude and, implicitly, temperature were strongly correlated with conductivity and nutrient concentrations, highlighting the difficulties of disentangling a direct climate signal from indirect effects of climate, such as changes in fish community structure and human-related impacts, when a latitude gradient is used as a climate proxy. Future studies should focus on the interrelationships between latitude and gradients in nutrient concentration and conductivity. [ABSTRACT FROM AUTHOR]

Recent studies have highlighted the impact of the winter North Atlantic Oscillation (NAO) on water temperature, ice conditions, and spring plankton phenology in specific lakes and regions in Europe. Here, we use meta-analysis techniques to test whether 18 lakes in northern, western, and central Europe respond coherently to winter climate forcing, and to assess the persistence of the winter climate signal in physical, chemical, and biological variables during the year. A meta-analysis approach was chosen because we wished to emphasize the overall coherence pattern rather than individual lake responses. A particular strength of our approach is that time-series from each of the 18 lakes were subjected to the same robust statistical analysis covering the same 23-year period. Although the strongest overall coherence in response to the winter NAO was exhibited by lake water temperatures, a strong, coherent response was also exhibited by concentrations of soluble reactive phosphorus and soluble reactive silicate, most likely as a result of the coherent response exhibited by the spring phytoplankton bloom. Lake nitrate concentrations showed significant coherence in winter. With the exception of the cyanobacterial biomass in summer, phytoplankton biomass in all seasons was unrelated to the winter NAO. A strong coherence in the abundance of daphnids during spring can most likely be attributed to coherence in daphnid phenology. A strong coherence in the summer abundance of the cyclopoid copepods may have been related to a coherent change in their emergence from resting stages. We discuss the complex nature of the potential mechanisms that drive the observed changes. [ABSTRACT FROM AUTHOR]

We determined relative nitrate-nitrogen (NOS3-N) loss rates in 100 north-mid-European lakes from late spring to summer by using the exponential function N3 = N1e-k>(t2 - t2), where N1 and N2 are NO3-N concentrations at the beginning (t1) and the end (t2) of the time interval, respectively, and k is the specific NO3-N loss rate. We found that k decreased with increasing lake depth. Adjusting k to the lake depth (kadj), we observed that kadj was positively related to spring NO3-N concentrations, but this relationship became insignificant at mean lake depths exceeding 12.5 m. A relationship between kadj and spring NO3-N concentrations in lakes shallower than 12.5 m implies that changes in spring NO3-N concentrations influence the NO3-N loss rate and thereby summer NO3-N concentrations. Time series from one Estonian, one German, and 14 Swedish lakes shallower than 12.5 m since 1988 revealed that May to August NO3-N concentrations have decreased over time everywhere, and the number of time periods exhibiting a NO3-N depleted condition, i.e., NO3-N levels below 10 mg L-1, in these lakes has tripled since 1988. We explained the decreasing NO3-N concentrations by a reduction in external nitrogen loading including atmospheric deposition, and by changes in climate. The observed prolongation of NO3-N depleted conditions might be one possible explanation for the increasing occurrence of nitrogen-fixing cyanobacteria in a variety of lake ecosystems [ABSTRACT FROM AUTHOR]