Fish play a key role in the trophic dynamics of lakes, not least in shallow systems. With climate warming, complex changes in fish community structure may be expected owing to the direct and indirect effects of temperature, and indirect effects of eutrophication, water-level changes and salinisation on fish metabolism, biotic interactions and geographical distribution. We review published and new data supporting the hypotheses that, with a warming climate, there will be changes in: fish community structure (e.g. higher or lower richness depending on local conditions); life history traits (e.g. smaller body size, shorter life span, earlier and less synchronised reproduction); feeding mode (i.e. increased omnivory and herbivory); behaviour (i.e. stronger association with littoral areas and a greater proportion of benthivores); and winter survival. All these changes imply higher predation on zooplankton and macroinvertebrates with increasing temperatures, suggesting that the changes in the fish communities partly resemble, and may intensify, the effects triggered by eutrophication. Modulating factors identified in cold and temperate systems, such as the presence of submerged plants and winter ice cover, seem to be weaker or non-existent in warm(ing) lakes. Consequently, in the future lower nutrient thresholds may be needed to obtain clear-water conditions and good ecological status in the future in currently cold or temperate lakes. Although examples are still scarce and more research is needed, we foresee biomanipulation to be a less successful restoration tool in warm(ing) lakes without a strong reduction of the nutrient load.

Freshwater ecosystems and their biodiversity are presently seriously threatened by global development and population growth, leading to increases in nutrient inputs and intensification of eutrophication-induced problems in receiving fresh

Several studies have demonstrated a latitudinal gradient in the proportion of omnivorous fish species (that is, consumers of both vegetal and animal material) in marine ecosystems. To establish if this global macroecological pattern also exists in fresh and brackish waters, we compared the relative richness of omnivorous fish in freshwater, estuarine, and marine ecosystems at contrasting latitudes. Furthermore, we sought to determine the main environmental correlates of change in fish omnivory. We conducted a meta-analysis of published data focusing on change in the relative richness of omnivorous fishes in native fish communities along a broad global latitudinal gradient, ranging from 41°S to 81.5 N°including all continents except for Antarctica. Data from streams, rivers, lakes, reservoirs, estuaries, and open marine waters (ca. 90 papers covering 269 systems) were analyzed. Additionally, the relationship between the observed richness in omnivory and key factors influencing trophic structure were explored. For all ecosystems, we found a consistent increasing trend in the relative richness of omnivores with decreasing latitude. Furthermore, omnivore richness was higher in freshwaters than in marine ecosystems. Our results suggest that the observed latitudinal gradient in fish omnivory is a global ecological pattern occurring in both freshwater and marine ecosystems. We hypothesize that this macroecological pattern in fish trophic structure is, in part, explained by the higher total fish diversity at lower latitudes and by the effect of temperature on individual food intake rates; both factors ultimately increasing animal food limitation as the systems get warmer.

In some arctic areas, marine-derived nutrients (MDN) resulting from fish migrations fuel freshwater and terrestrial ecosystems, increasing primary production and biodiversity. Less is known, however, about the role of seabird-MDN in shaping ecosystems. Here, we examine how the most abundant seabird in the North Atlantic, the little auk ( Alle alle ), alters freshwater and terrestrial ecosystems around the North Water Polynya (NOW) in Greenland. We compare stable isotope ratios ( δ 15 N and δ 13 C) of freshwater and terrestrial biota, terrestrial vegetation indices and physical–chemical properties, productivity and community structure of fresh waters in catchments with and without little auk colonies. The presence of colonies profoundly alters freshwater and terrestrial ecosystems by providing nutrients and massively enhancing primary production. Based on elevated δ 15 N in MDN, we estimate that MDN fuels more than 85% of terrestrial and aquatic biomass in bird influenced systems. Furthermore, by using different proxies of bird impact (colony distance, algal δ 15 N) it is possible to identify a gradient in ecosystem response to increasing bird impact. Little auk impact acidifies the freshwater systems, reducing taxonomic richness of macroinvertebrates and truncating food webs. These results demonstrate that the little auk acts as an ecosystem engineer, transforming ecosystems across a vast region of Northwest Greenland.

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The North Water (NOW) polynya is one of the most productive marine areas of the Arctic and an important breeding area for millions of seabirds. There is, however, little information on the dynamics of the polynya or the bird populations over the long term. Here, we used sediment archives from a lake and peat deposits along the Greenland coast of the NOW polynya to track long-term patterns in the dynamics of the seabird populations. Radiocarbon dates show that the thick-billed murre (Uria lomvia) and the common eider (Somateria mollissima) have been present for at least 5500 cal. years. The first recorded arrival of the little auk (Alle alle) was around 4400 cal. years bp at Annikitsoq, with arrival at Qeqertaq (Salve Ø) colony dated to 3600 cal. years bp. Concentrations of cadmium and phosphorus (both abundant in little auk guano) in the lake and peat cores suggest that there was a period of large variation in bird numbers between 2500 and 1500 cal. years bp. The little auk arrival times show a strong accord with past periods of colder climate and with some aspects of human settlement in the area.Electronic supplementary materialThe online version of this article (10.1007/s13280-018-1031-1) contains supplementary material, which is available to authorized users.

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