1. Results are analysed from 11 experiments in which effects of fish addition and nutrient loading on shallow lakes were studied in mesocosms. The experiments, five in 1998, six in 1999, were carried out in six lakes, distributed from Finland to southern Spain, according to a standard protocol. 2. Effects of the treatments on 29 standard chemical, phytoplankton and zooplankton variables are examined to assess the relative importance of bottom-up (nutrient enrichment) and top-down (fish predation) effects. For each year, the experiments in different locations are treated as replicates in a meta-analysis. Results of individual experiments are then compared in terms of the patterns of significant influences of nutrient addition and fish predation with these overall results (the baseline), and between years in the same location. 3. The overall meta-analysis gave consistent results across the 2 years, with nutrient loading influencing all of the chemical variables, and on average 31% of primary producer and 39% of zooplankton variables. In contrast, fish influenced none of the chemical variables, 11% of the primary producer and 44% of the zooplankton variables. Nutrient effects on the system were thus about three times greater than fish effects, although fish effects were not inconsiderable. 4. The relative importance of nutrients and fish in individual experiments often differed between years at the same location and effects deviated to varying degrees from the baseline. These deviations were treated as measures of consistency (predictability) of conclusions in repeat experiments. Consistency increased southwards and this is interpreted as a consequence of more variable annual weather northwards. 5. The influence of nutrient loading was greater southwards and this was probably manifested through naturally greater annual macrophyte abundance in warmer locations in consequence of the longer plant growing-season. There was no trend in the relative importance of fish effects with latitude but this may partly be an artefact of the simple fish Correspondence: Brian Moss,

Summary 1. Shallow lake ecosystems are normally dominated by submerged and emergent plants. Biological stabilising mechanisms help preserve this dominance. The systems may switch to dominance by phytoplankton, however, with loss of submerged plants. This process usually takes place against a background of increasing nutrient loadings but also requires additional switch mechanisms, which damage the plants or interfere with their stabilising mechanisms. 2. The extent to which the details or even major features of this general model may change with geographical location are not clear. Manipulation of the fish community (biomanipulation) has often been used to clear the water of algae and restore the aquatic plants in northerly locations, but it is again not clear whether this is equally appropriate at lower latitudes. 3. Eleven parallel experiments (collectively the International Mesocosm Experiment, IME) were carried out in six lakes in Finland, Sweden, England, the Netherlands and Spain in 1998 and 1999 to investigate the between‐year and large‐scale spatial variation in relationships between nutrient loading and zooplanktivorous fish on submerged plant and plankton communities in shallow lakes. 4. Comparability of experiments in different locations was achieved to a high degree. Cross‐laboratory comparisons of chemical analyses revealed some systematic differences between laboratories. These are unlikely to lead to major misinterpretations. 5. Nutrient addition, overall, had its greatest effect on water chemistry then substantial effects on phytoplankton and zooplankton. Fish addition had its major effect on zooplankton and did not systematically change the water chemistry. There was no trend in the relative importance of fish effects with latitude, but nutrient addition affected more variables with decreasing latitude. 6. The relative importance of top‐down and bottom‐up influences on the plankton differed in different locations and between years at the same location. The outcome of the experiments in different years was more predictable with decreasing latitude and this was attributed to more variable weather at higher latitudes that created more variable starting conditions for the experiments.

Many phytoplankton species form resting stages when environmental conditions are harsh and these can survive for a long time in the sediment (Livingstone and Jaworski, 1980; Lampert, 1995). When environmental conditions are favourable again, they recruit to the water phase and continue growing (Hansson et al., 1994; Hansson, 1996a). Many species of cyanobacteria, for example Microcystis, Anabaena and Aphanizomenon, form resting stages and are, in addition, the most frequent bloom-forming cyanobacteria (Willén and Mattsson, 1997). Some studies have shown that recruitment of algae from sediment may be important for the pelagic populations (Reynolds and Rogers, 1976;

Many phytoplankton species produce resting cysts, forming 'seed banks' on lake sediments until they germinate and provide inocula (recruitment) for future pelagic populations. We have addressed the question of whether the littoral or the profundal zone provides the inoculum for planktonic populations in a eutrophic and dimictic lake (Lake Erken, Sweden). Our hypotheses were that high temperature, light, and sediment mixing would enhance recruitment. Also, we hypothesized that recruitment from littoral sediments would be greater than from profundal sediments. In situ recruitment traps were utilized to compare littoral and profundal recruitment, while laboratory experiments were performed to analyze which factors were most important. Seven common taxa were investigated: Asterionella formosa (Bacillariophyceae), Ceratium hirundinella (Dinophyceae), Microcystis botrys, M. wesenbergii, Anabaena sp., A. solitaria, and A. lemmermannii (Cyanobacteria). Our main findings were that light and sediment mixing were the most important factors in triggering and enhancing recruitment in the laboratory. Temperature and sediment origin (littoral/profundal) had a significant effect on recruitment only for A. lemmermannii and C. hirundinella, respectively. The field experiments showed that recruitment at the littoral site was much higher than at the profundal site, where little or no recruitment occurred. Together, these results strongly suggest that littoral sediments in temperate lakes provide inocula for most phytoplankton populations, likely due to favorable light conditions and high sediment mixing.

Effects of different bioturbators on recruitment of several nuisance algae, Anabaena spp. (Cyanophyta), Microcystis spp. (Cyanophyta), and Gonyostomum semen (Raphidophyta), from sediment to water were studied in a long‐term laboratory experiment. Natural sediment, where macrofauna larger than 1 mm had been removed, was added to 18 aquaria. To each of six aquaria, individuals of Asellus aquaticus (Isopoda) or Chironomus plumosus (Arthropoda) larvae were added, and six aquaria were left as bioturbation‐free controls. Recruitment of Anabaena, Microcystis, and G. semen from the sediment was detected using inverted traps that were sampled once a week during 8 weeks. The activities of the isopod A. aquaticus increased recruitment rates of all algal groups investigated, whereas chironomids had a less pronounced effect. With respect to Anabaena, increased recruitment rate was expressed as a promotion of growth in the pelagic habitat. To our knowledge, these results are the first to demonstrate that bioturbating invertebrates affect the recruitment of phytoplankton resting stages. Moreover, our results suggest that recruitment rate might be more pronounced in littoral areas, which are often dominated by A. aquaticus, rather than in profundal areas of a lake, generally dominated by chironomids. Hence, with respect to algal dynamics, the strength of the coupling between the benthic and pelagic zones might vary both spatially and temporally, depending on composition of the benthic invertebrate community and the ontogenetic development of the individuals within it.