SUMMARY1. Using data from 71, mainly shallow (an average mean depth of 3 m), Danish lakes with contrasting total phosphorus concentrations (summer mean 0.02-1.0 mg P L − l ), we describe how species richness, biodiversity and trophic structure change along a total phosphorus (TP) gradient divided into five TP classes (class 1-5: B 0.05, 0.05-0.1,). 2. With increasing TP, a significant decline was observed in the species richness of zooplankton and submerged macrophytes, while for fish, phytoplankton and floatingleaved macrophytes, species richness was unimodally related to TP, all peaking at 0.1 -0.4 mg P L − 1 . The Shannon-Wiener and the Hurlbert probability of inter-specific encounter (PIE) diversity indices showed significant unimodal relationships to TP for zooplankton, phytoplankton and fish. Mean depth also contributed positively to the relationship for rotifers, phytoplankton and fish. 3. At low nutrient concentrations, piscivorous fish (particularly perch, Perca fluviatilis) were abundant and the biomass ratio of piscivores to plankti-benthivorous cyprinids was high and the density of cyprinids low. Concurrently, the zooplankton was dominated by large-bodied forms and the biomass ratio of zooplankton to phytoplankton and the calculated grazing pressure on phytoplankton were high. Phytoplankton biomass was low and submerged macrophyte abundance high. 4. With increasing TP, a major shift occurred in trophic structure. Catches of cyprinids in multiple mesh size gill nets increased 10-fold from class 1 to class 5 and the weight ratio of piscivores to planktivores decreased from 0.6 in class 1 to 0.10-0.15 in classes 3 -5. In addition, the mean body weight of dominant cyprinids (roach, Rutilus rutilus, and bream, Abramis brama) decreased two-threefold. Simultaneously, small cladocerans gradually became more important, and among copepods, a shift occurred from calanoid to cyclopoids. Mean body weight of cladocerans decreased from 5.1 mg in class 1 to 1.5 mg in class 5, and the biomass ratio of zooplankton to phytoplankton from 0.46 in class 1 to 0.08 -0.15 in classes 3-5. Conversely, phytoplankton biomass and chlorophyll a increased 15-fold from class 1 to 5 and submerged macrophytes disappeared from most lakes. 5. The suggestion that fish have a significant structuring role in eutrophic lakes is supported by data from three lakes in which major changes in the abundance of planktivorous fish occurred following fish kill or fish manipulation. In these lakes, studied for 8 years, a reduction in planktivores resulted in a major increase in clado- ceran mean size and in the biomass ratio of zooplankton to phytoplankton, while chlorophyll a declined substantially. In comparison, no significant changes were observed in 33 'control' lakes studied during the same period.

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Shallow lakes are likely to be strongly impacted by climate changes and, in particular, by increased temperatures. To enable realistic experimental studies of the effects of higher temperatures on in-lake processes and dynamics, technologically advanced systems are required. This paper presents design details, operating characteristics, and background information on a currently operating experimental flow-through mesocosm system that allows investigation of the interactions between simulated climate warming and eutrophication and their impacts on biological structure and ecosystem processes in shallow lakes. We use 24 mesocosms to combine three temperature scenarios (one unheated and two heated relative to the Intergovernmental Panel on Climate Change climate scenario A2 and A2 + 50%, respectively) and two nutrient levels (enriched and nonenriched). Planktivorous fish (male sticklebacks, Gasterosteus aculeatus) are stocked in accordance with the nutrient level. The water residence time is regulated by the semicontinuous addition of water and is approximately 2.5 mo in each mesocosm. For heating, we use electrically powered heating elements. The heating system has performed well over 16 mo of continuous heating, and seasonal and diurnal temperature variations of the unheated reference mesocosms were paralleled well by the heated mesocosms. The performance of the flow-through system and the heating technique are discussed with special emphasis on strengths, limitations, and potential improvements of the system. To illustrate the performance of the system and its potential, we present data for selected periods on total phosphorus retention in the mesocosms and system primary production and respiration.

Fresh waters make a disproportionately large contribution to greenhouse gas (GHG) emissions, with shallow lakes being particular hot spots. Given their global prevalence, how GHG fluxes from shallow lakes are altered by climate change may have profound implications for the global carbon cycle. Empirical evidence for the temperature dependence of the processes controlling GHG production in natural systems is largely based on the correlation between seasonal temperature variation and seasonal change in GHG fluxes. However, ecosystem-level GHG fluxes could be influenced by factors, which while varying seasonally with temperature are actually either indirectly related (e.g. primary producer biomass) or largely unrelated to temperature, for instance nutrient loading. Here, we present results from the longest running shallow-lake mesocosm experiment which demonstrate that nutrient concentrations override temperature as a control of both the total and individual GHG flux. Furthermore, testing for temperature treatment effects at low and high nutrient levels separately showed only one, rather weak, positive effect of temperature (CH4 flux at high nutrients). In contrast, at low nutrients, the CO2 efflux was lower in the elevated temperature treatments, with no significant effect on CH4 or N2 O fluxes. Further analysis identified possible indirect effects of temperature treatment. For example, at low nutrient levels, increased macrophyte abundance was associated with significantly reduced fluxes of both CH4 and CO2 for both total annual flux and monthly observation data. As macrophyte abundance was positively related to temperature treatment, this suggests the possibility of indirect temperature effects, via macrophyte abundance, on CH4 and CO2 flux. These findings indicate that fluxes of GHGs from shallow lakes may be controlled more by factors indirectly related to temperature, in this case nutrient concentration and the abundance of primary producers. Thus, at ecosystem scale, response to climate change may not follow predictions based on the temperature dependence of metabolic processes.