• Energy Research

Summary1. Shallow lakes and their ectothermic inhabitants are particularly vulnerable to the effects of climatic warming. These impacts are likely to depend on nutrient loading, especially if the combination of warming and eutrophication leads to severe hypoxia.2. To investigate effects of realistic warming and nutrient loading on a fish species with high tolerance of warming and hypoxia, we observed population changes and timing of reproduction of three‐spined sticklebacks in 24 outdoor shallow freshwater ecosystems with combinations of temperature (ambient and ambient +4 °C) and three nutrient treatments over 16 months.3. Warming reduced stickleback population biomass by 60% (population size by 76%) and nutrient‐addition reduced biomass by about 80% (population size 95%). Nutrients and warming together resulted in extinction of the stickleback populations. These losses were mainly attributed to the increased likelihood of severe hypoxia in heated and nutrient‐addition mesocosms.4. Warming of nutrient‐rich waters can thus have dire consequences for freshwater ectotherm populations. The loss even of a hardy fish suggests a precarious future for many less tolerant species in such eutrophic systems under current climate change predictions.

Summary Shallow lakes are important components of the biosphere, but they are also highly vulnerable to damage from human activities in their catchments, such as nutrient pollution. They may also be particularly vulnerable to current warming trends. Forty‐eight tanks were used to create 3‐m3 mesocosms of shallow lake communities, in which the effects of warming by 4 °C and regular nutrient loading at two levels relevant to current degrees of eutrophication were studied in the presence and absence of fish. Warming changed concentrations of soluble phosphate, total nitrogen and conductivity, increased total plant biomass and decreased the amount of phytoplankton through shading by floating plants. Nutrient additions decreased total plant biomass but increased floating plant biomass. Nitrogen increase and warming increased floating plant biomass and decreased plant species richness. The plant community remained intact and did not switch to the turbid‐water, phytoplankton‐dominated community often predicted to be a consequence of global warming and eutrophication. Synthesis and applications. Likely future temperature increase will exacerbate some, but not all symptoms of eutrophication in shallow lakes. Alone it will not cause a switch from plant‐dominated to algal‐dominated systems, but may result in nuisance growths of floating lemnids. Currently underplayed, nitrogen loading should be taken more seriously in the management of European freshwaters.

Metabolic rates are fundamental to many biological processes, and commonly scale with body size with an exponent (bR) between 2/3 and 1 for reasons still debated. According to the ‘metabolic-level boundaries hypothesis',bRdepends on the metabolic level (LR). We test this prediction and show that across cephalopod species intraspecificbRcorrelates positively with not onlyLRbut also the scaling of body surface area with body mass. Cephalopod species with highLRmaintain near constant mass-specific metabolic rates, growth and probably inner-mantle surface area for exchange of respiratory gases or wastes throughout their lives. By contrast, teleost fish show a negative correlation betweenbRandLR. We hypothesize that this striking taxonomic difference arises because both resource supply and demand scale differently in fish and cephalopods, as a result of contrasting mortality and energetic pressures, likely related to different locomotion costs and predation pressure. Cephalopods with highLRexhibit relatively steep scaling of growth, locomotion, and resource-exchange surface area, made possible by body-shape shifting. We suggest that differences in lifestyle, growth and body shape with changing water depth may be useful for predicting contrasting metabolic scaling for coexisting animals of similar sizes.This article is part of the theme issue ‘Physiological diversity, biodiversity patterns and global climate change: testing key hypotheses involving temperature and oxygen’.