Abundance and biomass of the microbial loop members [bacteria, heterotrophic nanoflagellates (HNF), and ciliates] were seasonally measured in the naturally eutrophic and shallow (2.8 mean depth) Lake Võrtsjärv, which has a large open surface area (average 270 km2) and highly turbid water (Secchi depth <1 m). Grazing rates (filter feeding rates) on 0.5-, 3-, and 6-microm-diameter particles were measured to estimate pico- and nanoplankton grazing (filter feeding) by micro- and metazooplankton. Among grazers, HNF had a low abundance (<50 cells mL-1) and, due to their low specific filtering rates, they only grazed a minor fraction of the bacterioplankton (

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Climate change is expected to profoundly affect both temperature and net precipitation, with implications for lake water level. We describe the design of a harmonized, simultaneous, cross-European mesocosm experiment to elucidate the effects of climate change on community structure, functioning, and metabolism in shallow lakes at low and high nutrient levels with contrasting depths (1 and 2 m). We used cylindrical (D = 1.2 m) tanks that were either 1.2 or 2.2 m high, each having a 10-cm sediment layer. We inoculated the mesocosms with a mixed sample of sediment and plankton from lakes with contrasting nutrient concentrations and added macrophytes and planktivorous fish. Sediment was pre-equilibrated to the required experimental nutrient concentration. During the experiment the water level decreased with increasing temperature (up to 90 cm in the Mediterranean mesocosms) while conductivity increased. The average chlorophyll a concentration increased with temperature in the deep mesocosms but was more variable in the shallow mesocosms. Macrophyte F. Landkildehus et al. 72abundance increased with temperature, while the oxygen data suggest that net primary production peaked at intermediate temperatures. We conclude that our experimental design has the potential for tracking the interacting effects of global warming and eutrophication in shallow lakes.

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We assessed feeding of planktivorous fish larvae on ciliate protozooplankton in shallow eutrophic Lake Võ rtsjä rv, Estonia to determine whether ciliates are an important part of larval fish diet. We collected larvae of the most common fish species and examined their gut contents by gut segmentation and epifluorescence microscopy. Ciliates were present in the gut of all fish larvae. Gut segmentation analysis showed clear differences in food composition between gut quarters. The hard-bodied food items were quite evenly distributed in the gut, but the soft-bodied ciliates and rotifers were present only in the first gut quarters. Neglecting differences in prey digestion rate leads to underestimation of diet amount and composition and, especially, to underestimation of the role of protists in food. In Võ rtsjä rv, ciliates account for , 60% of the total carbon biomass consumed by fish larvae. The food requirement of fish larvae during first feeding stages (May-Jun) would not be met without consumption of ciliates. Hjort (1914) was first to put forth the so-called 'critical period' hypothesis: the availability of suitable prey at the time when larval fish switch from endogenous to exogenous feeding determine their survival and, thus, the fish yearclass strength. This hypothesis and the further refined theories that derive from it, are all based primarily on the consumption of crustacean prey. Up to date, there are very few studies that have considered the direct protist consumption by fish larvae. Most planktonic protozoa are fragile and therefore get easily degraded in the fish gut. Thus, protist ingestion may have been neglected in the past because of difficulties describing the gut contents of fish larvae, due to a high degree of degradation of the food and the use of normal techniques of microscopy for species identification (Fukami et al. 1999).It has been suggested that most of the coastal waters do not contain enough (metazoan) prey to support larval growth and survival (MacKenzie et al. 1990). Also, some studies indicate that the guts of field-caught larvae often appear empty (van der Meeren and Naess 1993). One explanation for these paradoxes (i.e., apparent low prey levels and empty guts) is that food is sufficiently abundant, but easily digested and therefore not detected by researchers (Figueiredo et al. 2005;Montagnes et al. 2010). One such possible food source that larval fish can ingest and quickly digest is protozoa. If this is the case, the focus on metazoan prey has resulted in fisheries scientists overlooking a key trophic linkage (Montagnes et al. 2010).There have been only sparse field studies concerning protozoa as food of the fish larvae, and most of them have focused only on the hard-bodied protists (Watson and Davis 1989). Only few have tried to estimate the importance of soft-bodied protozoans in the larval diet (Fukami et al. 1999;Nagano et al. 2000;Figueiredo et al. 2005). Modern fluorescence microscopy approaches have been rarely used in this specific research field. Lessard et al...

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