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...
Summary 1. As grazers on picoplankton and nanoplankton, planktonic ciliates form an important link in pelagic food webs. Ciliate communities may be controlled by predation by metazooplankton. In eutrophic systems, however, where the number of large crustaceans is often low, the mechanisms that regulate ciliate dynamics have rarely been described. 2. We conducted an enclosure experiment with natural and screened (145 μm) summer plankton communities to investigate the effect of the small‐sized crustacean zooplankton on ciliate community structure and the microbial loop in a shallow eutrophic lake. 3. The removal of the larger fraction of crustaceans initiated a decrease in total ciliate abundance. At the community level, we observed a substantial increase in large‐sized predacious ciliates (>100 μm) and a simultaneous decrease in the abundance of smaller ciliates (<20–40 μm) that were mostly bacterivores and bacterio‐herbivores. The compositional shift in the ciliate community, however, did not cascade down to the level of bacteria and edible phytoplankton.
While 13C‐depleted carbon derived from biogenic methane can substantially contribute to the benthic secondary production in deep stratified lakes, its role in shallow lakes is less clear. We investigated the dynamics of δ13C and δ15N in the larvae of Chironomus plumosus throughout an annual cycle in two ecologically distinct basins (open‐water plankton‐dominated and sheltered macrophyte‐covered) of a large (270 km2), shallow, polymictic and eutrophic lake (Võrtsjärv, Estonia, North Europe). The larval stable isotopic compositions were linked to the presence of methane‐oxidising bacteria (MOB) in larval guts and sediments. Molecular detection of MOB revealed their presence in various sediment types, but stable isotope (SI) analysis revealed clear differences in the feeding of chironomid larvae between the plankton‐ and macrophyte‐dominated habitats. In the plankton‐dominated habitat, the mean δ13C values of larvae remained relatively constant (−38.3‰ to −35.5‰) and corresponded closely to the sediment δ13C values. Mean δ13C values of chironomid larvae were generally lower in macrophyte‐dominated habitats (−43.4‰ to −33.0‰), and both seasonal and individual variation in larval δ13C values were more pronounced. MOB presence in larval guts proved a dietary contribution from biogenic methane in macrophyte‐dominated habitats. Both the SI and molecular results indicated that MOB could help support larvae even during the cold temperature‐limited and ice‐covered periods. Our study indicates that methane‐derived carbon makes a low but steady contribution to the larval chironomids throughout an annual cycle in large shallow Võrtsjärv. However, this contribution can be substantially higher in the lake habitats with abundant macrophytes. The study provides further evidence that a carbon flow pathway from biogenic methane can contribute to the benthic food web under variable habitat conditions in a shallow polymictic lake.
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