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Lake restoration by biomanipulation or phosphorus fixation has been commonly applied methods to improve the ecological status of lakes. However, the effects of lake restoration on food-web dynamics are still poorly understood, especially when biomanipulation and nutrient fixation are used simultaneously. This study investigated the combined effects of a 70% fish removal (mainly roach (Rutilus rutilus Linnaeus, 1758) and bream (Abramis brama Linnaeus, 1758) and Phoslock® treatment on fish trophic ecology in Lyngsø (area: 9.6 ha, mean depth: 2.6 m), Denmark. The lake restoration resulted in decreased nutrient levels, increased water clarity, and increased coverage of more structurally complex submerged macrophytes. Following lake restoration, significant changes in diets of the dominant fish species were observed. Stomach content analyses of roach and perch (Perca fluviatilis Linnaeus, 1758) revealed significantly reduced detritus utilization and increased foraging on macrophytes and macrophyte living invertebrates. Results from stable isotope mixing models indicated a shift from littoral benthic to more pelagic food resources by the dominant fish species. Our findings provide further evidence that lake restorations can lead to substantial changes in lake food webs and fish communities, thereby potentially facilitating a shift toward an ecological state resembling the pristine reference state, less influenced by anthropogenic factors. [ABSTRACT FROM AUTHOR]

Invasion of zebra mussels (Dreissena polymorpha) can have profound effects on lake ecosystems. We investigated the overall ecological effects after invasion and fast expansion of zebra mussels in eight lakes in Denmark belonging to the same watercourse systems but with different morphological and eutrophication characteristics. A few years after the invasion, most of the lakes had experienced profound changes in lake water quality, e.g. chlorophyll a had been reduced by 18–62% and Secchi depth had increased by 38–171%. Nutrient concentrations also decreased, total phosphorus to 37–64% and total nitrogen to 49–77% of pre-invasion concentrations. Seasonally, differences between both summer and winter phosphorus and nitrogen concentrations diminished, suggesting a change in internal nutrient cycling after the invasion. In most lakes, the abundance of submerged macrophytes rose substantially, and often coverage increased by a factor 10 or more, and the maximum colonisation depth increased by up to 5.4 m. Plant species numbers increased markedly in four of the six lakes with macrophyte data. The fish community changed towards higher dominance of potential piscivorous perch and an increase in their body size. The profound effects of zebra mussel invasion may affect lake management and how ecological quality should be interpreted. [ABSTRACT FROM AUTHOR]

Net ecosystem production (NEP) is an important indicator of lake ecosystem function and integrity. An earlier study, restricted to one geographical region, indicated that oxygen saturation levels (DO%) might be used to predict daily NEP in shallow lakes. To test the generality of the method, we used DO% data collected in a standardised pan-European mesocosm experiment with contrasting trophic states and water levels covering a large climate gradient (from Sweden to Turkey). We corroborated these data with process-based DO simulations. The NEP ~ DO% relation depended on factors influencing gas transfer: water depth and wind. The NEP ~ DO% relation per volume became weaker with increasing depth (1–2 m) but was independent of depth when area based. Simulations indicated that the marginalisation of the depth was sensitive to wind conditions. Trophic status, temperature and light showed no or only marginal (climate zone) effects (experimental data), while the simulations indicated influence of those factors under particular wind–depth conditions. We confirmed that when considering also wind and depth effects, midday DO% potentially provides reliable estimates of daily NEP. Therefore, historical monitoring data of DO% might be used to estimate NEP, and process-based oxygen models may be valuable tool therein. We encourage further tests. [ABSTRACT FROM AUTHOR]

We used biological and physical responses at 71 shallow waterbodies with contrasting nutrient levels undergoing recovery from eutrophication to predict potential changes in waterbird species abundance, an important component of lake ecosystems. These general predictions were tested using 28 years of breeding waterbird data from three Danish shallow eutrophic lakes, comparing species-specific responses to improved nutrient and water transparency in two lakes with a third where conditions remained constantly suitable for breeding waterbirds. We predicted positive responses to improved water quality from pursuit diving predators (three grebe species), a specialist zooplankton feeder (northern shoveler Anas clypeata) and waterbirds feeding on (common pochard Aythya ferina) or within (tufted duck A. fuligula) submerged macrophyte underwater canopies. These species were characterised by positive waterbird community composition changes (using Principal Components Analysis) associated with decreasing nutrient loading and increasing water transparency at two lakes, with no change in breeding waterbird community at the third. Secchi depth explained 73–95% of variance in both PC axes at both restored lakes, but not at the third, suggesting water transparency was the major factor driving waterbird community composition. These examples show predicting waterbird species-specific responses to management can usefully direct the use of breeding waterbirds as indicator species. [ABSTRACT FROM AUTHOR]

Using data on 23 Danish lakes, we conducted mass balances to develop total nitrogen (TN) models for predicting annual mean TN in lakes based on external TN loading and found high predictability when including lake hydraulic retention time and mean depth in the model. We further used a unique 30-year mass balance data series from 10 Danish lakes with contrasting mean depths and hydraulic retention times to elucidate the effect of external TN loading reduction and N legacy on lake TN. We found that the TN retention percentage during the 30 years was generally not sensitive to an often major reduction in the external TN loading; it overall followed the pattern of the above model predictions, suggesting a low TN legacy effect. Moreover, the TN retention percentage was not affected by changes in TP. Our results, therefore, show a fast response to TN loading reduction, indicating that we can expect an immediate effect on lake water quality in shallow lakes suffering from internal phosphorus loading during re-oligotrophication provided that inorganic N is low enough to become a growth-limiting nutrient. [ABSTRACT FROM AUTHOR]

The Water Framework Directive (WFD) requires actions from European Union Member States to reduce external nutrient loading in lakes of moderate/poor/bad ecological status and restore their good ecological status. Several poor-quality Danish lakes have been exempted from WFD criteria (and more proposed), due to suspected major imported nutrient contributions from waterbirds, without supporting evidence. Here, we estimated relative nutrient contributions from increasing numbers of roosting waterbirds compared to total nutrient loadings in four brackish lakes (43–491 ha) at an internationally important Ramsar site/EU Birds Directive Special Protection Area. Combining night- and daytime counts of roosting numbers of the most numerous species with intake and faecal output models, we estimated roosting birds (mostly geese) contributed 0.2–0.4% of all N inputs (0.3–1.2 kg N/ha/year) and 0.6–2.0% of all P inputs (0.04–0.12 kg P/ha/year) in three lakes (one of which has been proposed exempted from WFD criteria), but potentially as high as 14% and 76%, respectively, at a fourth clearwater lake without direct inflow or outlets. These results confirm the need for case-by-case assessments of avian nutrient import relative to total nutrient balance before exempting lakes from WFD conditions due to supposed natural loading from waterbirds. [ABSTRACT FROM AUTHOR]

Zooplankton grazing at similar nutrient levels is generally regarded as lower in brackish than in freshwater lakes, but experimental evidence of this is lacking. Accordingly, we conducted short-term zooplankton grazing experiments in bottles with water from 12 Danish brackish lakes covering a large gradient in salinity (0.3–17.4‰) and nutrient concentrations as well as with water from 24 mesocosms established in the same area with various salinities (0.5–12‰), two nutrient levels and low fish density. Grazing was low in 11 of the 12 lakes, even when they were dominated by edible phytoplankton and nutrient addition led to a major increase in phytoplankton biomass. By contrast, grazing was significant in most of the mesocosms, particularly at high nutrient levels and salinities of 8‰ or below where Daphnia dominated. Moreover, grazing decreased the biomass of most phytoplankton taxa, except for a few (e.g. Ankyra at 0.5–2‰ and Ochromonas and Chaetoceros at 8‰). Our results provide experimental support for potentially significant grazing by zooplankton on phytoplankton in brackish lakes up to a salinity of 8‰ at low fish density; however, grazing in summer was generally low in the majority of the lakes, which we attribute to high predation on zooplankton. [ABSTRACT FROM AUTHOR]

Temporary summer stratification is probably a common, but easily overlooked phenomenon in many shallow lakes, because short-term temperature stratification and mixing events are not easily discovered by routine samplings. We used two years of high frequency measurements and monitoring of 5-m deep and hypereutrophic Lake Ormstrup, Denmark to study the dynamics of temporary stratification and its effects on oxygen concentrations, nutrient cycling, greenhouse gas emissions and fish habitat use. Temporary stratification followed by complete mixing of the water column occurred several times each summer, but its duration was highly variable between years despite only small climatic variations. Anoxia at depths greater than 2–3 m developed within a few days after stratification. During anoxia fish accumulated in the upper 2 m and high accumulation of phosphate (up to 1.34 mg/l), ammonia (up to 2.32 mg/l) and greenhouse gases (methane up to 5 mg/l) occurred at depths greater than 2–3 m that were released at mixing. Lake Ormstrup is an example of how very dynamic, unpredictable, and variable mixing conditions develop each summer with significant implications for fish habitat use, nutrient dynamics and greenhouse gas emissions. This might adjust our view of these lakes in a management and climate change perspective. [ABSTRACT FROM AUTHOR]

We analysed long-term monitoring data on submerged macrophytes and water chemistry from 666 Danish lakes > 1 hectare and mean depth < 3 m, encompassing a total of 1447 lake years. Our aim was to describe how plant cover (COV), plant volume inhabited (PVI) and species richness related to physical and chemical and environmental variables. Boosted regression tree (BRT) analyses revealed that chlorophyll a, Secchi depth and depth were the strongest predictors of COV and PVI. Chlorophyll had a strong negative effect up to 50 µg/l, whereas the changes related to Secchi depth and depth were more gradual and covered more of the gradient. Macrophyte species richness was best predicted by lake area and alkalinity, with chlorophyll a, nutrients and colour having significant but less marked effects. For chlorophyll a, 78% of the observed variance could be explained by the BRT model, with the most powerful predictors being both phosphorus and nitrogen, but with significant additional effects of plant cover and alkalinity. Our analyses revealed limited direct effect of nutrients on macrophyte abundance, but an indirect hierarchical effect of nutrients mediated through chlorophyll a with additional interactive effects by plant cover itself, alkalinity, mean depth and colour. [ABSTRACT FROM AUTHOR]

Common pochard Aythya ferina are rapidly declining globally, partly due to water quality change at breeding habitats. Lake restoration at two southern Danish lakes (external nutrient loading reduction, bream Abramis brama and roach Rutilus rutilus removal and stocking of pike Esox lucius) improved water clarity and submerged macrophyte cover. Nesting pochard on one lake increased from 2.3 females per annum pre-treatment to 13.9 afterwards and from 22.7 females to 99.6 post-treatment at a second. Numbers fell from 27.7 to 11.3 at a third untreated lake with consistently high water clarity, but which was colonised and became dominated by holly-leaved naiad Najas marina, which provides no food resources for breeding pochard. Linear modelling (controlling for serial autocorrelation) showed statistically significant effects of annual summer measurements of suspended solids (negative) and Secchi depth (positive) on pochard abundance at both restored lakes and chlorophyll (negative) at one of those lakes, but no effects at the third. Breeding pochard numbers also correlated positively with submerged macrophyte cover at one restored lake with adequate data. Results support the hypothesis that lake restoration to improve water quality enhances conditions for locally breeding pochard, as long as restored conditions secure appropriate submerged macrophyte communities for nesting pochard. [ABSTRACT FROM AUTHOR]

Differences in trophic web structure in otherwise similar ecosystems as a consequence of direct or indirect effects of ambient temperature differences can lead to changes in ecosystem functioning. Based on nitrogen and carbon stable isotope analysis, we compared the food-web structure in a series of subtropical (Uruguay, 30-35°S) and temperate (Denmark, 55-57°N) shallow lakes. The food-web length was on average one trophic position shorter in the subtropical shallow lakes compared with their temperate counterparts. This may reflect the fact that the large majority of subtropical fish species are omnivores (i.e., feed on more than one trophic level) and have a strong degree of feeding niche overlap. The shapes of the food webs of the subtropical lakes (truncated and trapezoidal) suggest that they are fuelled by a combination of different energy pathways. In contrast, temperate lake food webs tended to be more triangular, likely as a result of more simple pathways with a top predator integrating different carbon sources. The effects of such differences on ecosystem functioning and stability, and the connection with ambient temperature as a major underlying factor, are, however, still incipiently known. [ABSTRACT FROM AUTHOR]

Lakes can be sources or sinks of carbon, depending on local conditions. Recent studies have shown that the CO efflux increases when lakes recover from eutrophication, mainly as a result of a reduction in phytoplankton biomass, leading to less uptake of CO by producers. We hypothesised that lake restoration by removal of coarse fish (biomanipulation) or invasion of mussels would have a similar effect. We studied 14-22 year time series of five temperate Danish lakes and found profound effects on the calculated CO efflux of major shifts in ecosystem structure. In two lakes, where limited colonisation of submerged macrophytes occurred after biomanipulation or invasion of zebra mussels ( Dreissena polymorpha), the efflux increased significantly with decreasing phytoplankton chlorophyll a. In three lakes with major interannual variation in macrophyte abundance, the efflux declined with increasing macrophyte abundance in two of the lakes, while no relation to macrophytes or chlorophyll a was found in the third lake, likely due to high groundwater input to this lake. We conclude that clearing water through invasive mussels or lake restoration by biomanipulation may increase the CO efflux from lakes. However, if submerged macrophytes establish and form dense beds, the CO efflux may decline again. [ABSTRACT FROM AUTHOR]

Lake Säkylän Pyhäjärvi has been an important fishing site and drinking water source for the local population for centuries. The lake has undergone significant changes: (1) the water level was lowered in the 1600s and in the 1850s; (2) planktivorous coregonid fish were successfully introduced in the early 1900s; (3) nutrient input from intensified agriculture has increased since the 1950s and (4) the effects of the current variable climate on the lake and its catchment have become more evident since the 1990s. We determined the phases of oligotrophication, eutrophication and recovery and elucidated the ecosystem changes by combining palaeolimnological records with detailed neolimnological data. The sedimentary diatom and cladoceran assemblages first showed a relatively eutrophic period followed by oligotrophic periods, linked with the artificial changes in water level and consequent shifts in macrophyte abundance. The oligotrophic period in the early 1900s is thought to represent the target trophic state for the lake. After the 1950s, introduction of vendace resulted in higher planktivory reflected by an increased relative abundance of small-bodied pelagic cladocerans. Signs of eutrophication occurred due to increased nutrient load. During the last 10 years, signs of recovery have been recorded. A complex history such as that of Lake Pyhäjärvi illustrates the difficulties in selecting management targets, and the risk of setting false targets, for lakes based solely on monitoring data-both neolimnological and palaeolimnological approach are needed. [ABSTRACT FROM AUTHOR]

The role of nitrogen (N) in the shift from a macrophyte-dominated state to a phytoplankton-dominated one at high N concentrations in shallow lakes is still debated. To elucidate possible toxic and ecological effects of high N on macrophyte growth, we conducted a short-term (40 day) study of a eutrophication-tolerant macrophyte, Vallisneria spinulosa (Hydrocharitaceae), incubated in pots in a mesocosm system subjected to different N concentrations (1, 3, and 5 mg l). Plant leaf and root length as well as growth rate decreased significantly with increased N concentrations, but most N- and P-related physiological parameters, including the soluble protein content, nitrate reductase activity, acid phosphatase activity, and tissue N and P contents, did not differ significantly among the N treatments. Only the alkaline phosphatase activity differed, being lower at high nitrogen loading, likely due to P limitation. Epiphyton and phytoplankton biomasses increased significantly with increasing N loading. Our results including a large number of physiological tests of the macrophytes, therefore, provide supporting evidence that the loss of submerged macrophytes, like V. spinulosa, seen at high N loading in shallow lakes, can be attributed to competition with phytoplankton and epiphyton rather than to toxic effects. [ABSTRACT FROM AUTHOR]