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Summary This study investigates the adaptation of certain feeding strategies by a population of Atherina boyeri inhabiting a freshwater ecosystem (Lake Trichonis, Greece) and links feeding habits with ecosystem interactions, using stomach content analysis and food availability. The use of Ivlev's electivity index (E) revealed the selection of different prey categories in respect to size, species and even gender. A. boyeri showed positive selection for larger zooplankters such as adult copepods and the egg‐bearing females of Eudiaptomus drieschi (0.53 < E < 0.92 and 0.24 < E < 0.99, respectively) or the cladocerans Diaphanosoma orghidani and Daphnia cucullata (0.25 < E < 0.53 and 0.23 < E < 0.37, respectively) in spring and/or summer. In contrast, the fish totally avoided rotifers and copepod nauplii, while showing negative selection for smaller prey items like copepodites (−1.00 < E < −0.14). A shift to larger prey was recorded as age increased. Larger specimens showed cannibalistic behaviour by consuming larvae of A. boyeri and the endemic, threatened, fish Economidichthys trichonis. The results provide certain indications that, through selective predation, A. boyeri is able to affect zooplankton abundance and community composition, the vertical distribution and migration of crustaceans and the abundance of other fish species, However, more research on the dynamics and feeding ecology of A. boyeri and other fish species is necessary.

The investigation of the vertical distribution of the zooplankton community in the temperate Lake Trichonis during four seasons in 2005, showed the existence of vertical segregation among species, ontogenetic stages and sexes within and between the major groups. In each season, the two or three more abundant rotifer species distributed at separate depth layers, while this feature was maintained during the entire 24 h period, since no diel vertical migrations (DVM) were performed. In contrast, the crustacean community, comprised mainly by the calanoid copepod Eudiaptomus drieschi and the cladoceran Diaphanosoma orghidani, showed various patterns of DVM, being more pronounced in spring and summer. Females of E. drieschi distributed deeper than males, while the copepod nauplii were found mainly in the surface layer in all four seasons. Temperature was the most important abiotic factor affecting directly and indirectly the vertical distribution and migration of various species. During stratification, the metalimnion was the most productive layer in Lake Trichonis, having maximum values of dissolved oxygen and low transparency due to high concentration of organic matter and phytoplankton. The DVM patterns of the crustaceans indicate that the metalimnion acts probably as a daylight refuge against predation by Atherina boyeri, which is the dominant planktivorous fish in the lake.

The trophic state of a freshwater ecosystem reflects its environmental quality. This is why several trophic indicators have been developed for such water bodies based on chemical, physical and biological parameters. Apart from that, there are several biotic elements which can be used in accessing the environmental condition of a freshwater ecosystem. Zooplankton organisms are important elements of the structure and function of lakes and are considered useful indicators of alterations in their trophic dynamics and ecological state related to changes in nutrient loading and climate. In accordance to the above, the present study is an attempt to assess the trophic condition of the largest lake in Greece (Lake Trichonis) through the investigation of the physicochemical elements, along with the biotic indications provided by a three-year study of the lake's zooplankton. The present results, compared with previous studies conducted between 15 and 25 years before, showed that there was an increase in the maximum values of the concentrations of chlorophyll-α and nutrients, while there was a decrease in water transparency. The implementation of Carslon's trophic state index (TSI) revealed that Lake Trichonis still remains an oligo-to mesotrophic ecosystem as it was in the past. However, although the zooplankton investigation showed several features that are common in oligotrophic lakes, there are certain eutrophic characteristics of the zooplankton community (e.g. abundance variation pattern, indicator species, seasonal succession of cladocerans) pointing out a different state of the ecosystem in comparison to the past. In conclusion, the use of a biotic element like zooplankton revealed that Lake Trichonis is experiencing a transitional condition towards the eutrophic state and points out the necessity for constant inspection and monitoring of this ecosystem.

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