Diverse impacts of semiaquatic bird colonies on biochemical composition of seston, plankton, and fish fry in a plain reservoir

Diverse, multidirectional changes in biochemical composition of seston, plankton, and fish fry are detected in the coastal zone of a plain reservoir in the vicinity of a semiaquatic bird colony during vegetation season with extra high water level. Under these conditions, specific patterns of phyto- and zooplankton structure and quantitative characteristics, that, as was found out earlier, result from the vital activity of birds belonging to the family Ardeidae (namely, raising the share of mixotrophic phytoflagellates and copepods in plankton species composition) are leveled out. The main role in this process belongs to decomposition of immersed vegetation in the coastal zone, development of littoral macrophytes, and trophic relationships between algae and invertebrates. However, increase in atmospheric precipitation leads to increase in copepods abundance, which seems to be related to fast and intensive inflow of nitrogen-rich products of birds' vital activity. Due to this, during the whole period of observations, the following tendencies are revealed in biochemical composition of seston, plankton, and fish fry in the vicinity of the bird colony: 1) concentrations of carbon, nitrogen, and phosphorus and their ratios turn out to be auspicious for predominance of diatoms and mixotrophic phytoflagellates, rich in docosahexaenoic acid (DSA); 2) the abundance of phytoplankton, rich in polyunsaturated essential fatty acids (PEFA), favors propagation of those planktonic crustaceans which contain significantly more DSA and are more valuable food items for fish; 3) predominance of zooplankters, rich in PEFA, leads to higher concentration of PEFA in fry fish tissues, which, in certain periods, turns out to be advantageous for successful development and growth of dominant species of fish fry,such as roach, bream, white bream, and perch, by comparison with the test site.