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With the retreat of glaciers, new ponds and lakes are often formed. These are gradually colonised and become more productive as vegetation develops in their catchments, creating more complex food webs. Near the Jakobshavn Isbræ in West Greenland, we studied trophic structure and food web complexity using stable isotopes in 26 lakes belonging to two different age groups (19 new lakes and 7 nearby older (> 150 years) ones). The older lakes had significantly higher total nitrogen and pelagic chlorophyll-a concentrations, as well as a higher organic matter content in the surface sediment. The biomass and richness of cladocerans, copepods and rotifers were higher in the older lakes and so was the zooplankton:phytoplankton biomass ratio. Multivariate analyses showed a marked difference between the zooplankton communities of new and older lakes. Layman food web metrics indicated higher food chain length and width of invertebrates (zooplankton and benthic macroinvertebrates) in the older lakes, being significantly higher in lakes with fish. Our findings highlight a potential sequence of succession occurring in lakes created by glacial retreat in the Arctic, implying an increase in food web complexity and higher taxonomic (and likely also functional) diversity following ageing and increased nutrient state. [ABSTRACT FROM AUTHOR]

No previous study has examined the large-scale distributional drivers of the entire global pool of 3,499 macrophyte species, despite the obvious importance of this for understanding the macroecology of these plants. To assess the hypothesis that natural rather than human-related transfer vectors act as the primary long-distance drivers of global movement of aquatic macrophytes, we analysed current macrophyte species distributions in relation to a set of human-related and natural transfer vectors. Most macrophytes (2,492 species: 71.2% of the global total) are endemic to a single ecozone, and generally lack the various functional adaptations needed for successful long-distance propagule transport. Such traits are, however, common in the 1,007 (28.8%) species native in > 1 ecozone. In total, 779 species (22.3%) are introduced, naturalised or invasive (I species) in one or more ecozones outwith their native range. The proportion of I species varies between ecozones and is best predicted by annual temperature and longitude. A migratory bird transfer vector and climatic variables strongly predict global native macrophyte species occurrence. Some native species of Miocene origin (or older) may have had their world distribution influenced by ancient vicariance events, while inter-ecozone hydrochory and Late Quaternary climate change are also relevant factors influencing a few species. [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]

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]

Cladocerans are increasingly used in palaeolimnological studies as their community composition is sensitive to both anthropogenic and natural forces in lakes. We present the results of a palaeolimnological investigation of three Turkish shallow lakes located in cold dry steppe and semi-dry Mediterranean climatic regions. The aim was to elucidate historical changes in environmental conditions by analysing sub-fossil cladocerans in Pb-dated sediment cores. Sub-fossil cladoceran remains from the surface sediment of 40 Turkish lakes were analysed to examine the environmental factors that most correlated with variation in the cladoceran assemblage. Redundancy analysis showed that salinity, macrophyte abundance, fish density, depth and total phosphorus were the most correlated with change in cladoceran assemblage composition with eigenvalues for the first and the second axes being λ = 0.312 and λ = 0.061, respectively. Sedimentary cladoceran assemblages from three cores were placed passively within the framework of the surface sediment ordination. The results reveal a prevalent impact of salinity, fish abundance and water level changes from the past to present. Thus, using cladoceran-based inferences, we traced key environmental changes related to variation in climate change, restoration and water level regulation over the last century. [ABSTRACT FROM AUTHOR]

Zooplankton community response to the combined effects of nutrients and fish (hereafter N + F) at contrasting temperatures was studied in a long-term experiment conducted in 24 shallow lake mesocosms with low and high nutrient levels. We found a positive effect of N + F on zooplankton biomass, chlorophyll- a and turbidity. In contrast, zooplankton species and size diversity decreased with added N + F, as did submerged macrophyte plant volume inhabited (PVI). The community composition of zooplankton in high N + F mesocosms was related to chlorophyll- a and turbidity and to macrophyte PVI in the low N + F mesocosms. Macrophytes can protect zooplankton from fish predation. Compared to N + F effects, temperature appeared to have little effect on the zooplankton community. Yet analysis of community heterogeneity among treatments indicated a significant temperature effect at high N + F levels. The results indicate an indirect temperature effect at high N + F levels that can be attributed to temperature-dependent variation in fish density and/or chlorophyll- a concentration. [ABSTRACT FROM AUTHOR]