| Summary: | The last decades have witnessed unprecedented changes at a global scale including, among others, an increasing human population and goods consumption rates, climate change (warming, changes in precipitation regime), species invasions and globalisation of trade, among others. Aquatic systems are exposed to several of these stressors, shallow lakes being particularly vulnerable. Shallow lakes are the most abundant lake type on Earth, hold great biodiversity and act as very important sources of ecosystem services by providing, among other things, drinking water and food. Eutrophication is one of the most studied stressors on shallow lakes so far; and it may interact with the other stressors related to global change with profound negative effects on shallow lake food webs. The aim of this thesis was to study the effects of different stressors related to global change on food web structure and the functioning of shallow lakes, with particular emphasis on enhanced air temperatures (objective 1), salinisation (objective 2) and species invasions (objective 3). A stable isotope approach and stomach content analyses of fish were used in order to characterise the food web structure, combined with structure analysis of the main communities inhabiting shallow lakes. To assess the consequences of increased air temperatures, space-for-time substitution comparisons were performed of the food webs, community structure and potential cascading effects in two sets of lakes located in remote islands with similar seasonality, nine lakes in the Azores and eleven in the Faroe Islands. The impact of salinisation on food web and community structure was studied in lakes covering a wide range of salinities located in a semiarid region in north-west China in the Xinjiang province and the Qinghai province, where 24 and 12 lakes, respectively, were sampled. The effect of exotic species invasion was analysed in 14 Tasmanian shallow lakes, with special emphasis on the trophic structure of native galaxiids and potential cascading effects. Likely as a consequence of higher temperatures, the body size of the fish community in the warmer Azorean lakes was found to be smaller than in lakes in the Faroe Islands, even after size standardisation according to the maximum size reported for each species in continental lakes (paper 1). This indicates a higher predation pressure on the zooplankton community in the warmer region and, accordingly, stronger cascading effects on the lower trophic levels could be traced in the Azorean lakes. However, in contrast to expectations, no effect on the trophic position of fish was found, but the shape of the food web structure was more triangular in the (cold) Faroese lakes that were also characterised by a wider carbon range at the base of the food web than in the Azorean lakes. In the case of salinity increases, we found that the composition of the zooplankton, macroinvertebrate and fish communities was negatively affected by enhanced salinity (paper 2). The body size of macroinvertebrates was negatively correlated with salinity, while the relationship with zooplankton body size was positive. Fish body size was not related to salinity, but size diversity was negatively affected. The height of the entire food web (trophic position and nitrogen range) declined with increased salinity, while trophic width was positively related to lake depth. Food web metrics at single community level were not related to salinity, and morphometric variables (depth and area) were more important. Exotic fishes introduced to Tasmanian lakes negatively affected the trophic niche of the native galaxiids (paper 3). This impact seemed to be less drastic in clear water lakes, but the direct effect of exotic fishes predation (consumption) on native fishes was, contrary to our expectations, stronger in the turbid lakes. However, only weak cascading effects of predator introduction were detected; the size of calanoid zooplankton increased with predator abundance, but no effects could be detected at the pelagic primary producer level. In this thesis we provide evidence that stressors related to global change can alter the community structure and food webs dynamics in shallow lakes in a variety of ways. Warming negatively affects the size structure of fish and consequently increases the predation pressure on zooplankton with cascading effects on lower trophic levels, augmenting the negative effect of eutrophication on water quality (paper 1). Warming decreases runoff in arid and semiarid regions, and water abstraction for irrigation leads to enhanced salinisation in many aquatic systems. A consequent disappearance of higher trophic positions (mostly fish) resulted in reduced food web complexity (paper 2). Human direct and indirect modifications, in addition to climate change, may alter the current biogeographic distribution of species and enhance the success of predator invasion and increase the vulnerability of native fish species (paper 3). Understanding the interactions among stressors within a global change perspective, with the aim to mitigate and minimise the potential negative effects on food webs, particularly in shallow lakes, is a major challenge in future research.
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