Fish stock as a variable modifying trophic pattern of phytoplankton

Abstract

Fish stock, zooplankton, chlorophyll a concentration, algal biomass (fresh mass), phytoplankton composition, species dominance and diversity (Shannon-Weaver Index and equitability) were evaluated to demonstrate their relevance in the trophic gradient at the levels of eutrophy and hypertrophy. Two patterns of food-web connectedness between phytoplankton and fish stock can be detected. One is direct (phytoplankton is mainly consumed directly by herbivorous or omnivorous fish) another one indirect (phytoplankton is consumed by zooplankton, which is regulated by the fish stock). Release of nutrients from fecal pellets and from the bottom due to the feeding activity of fish in shallow waters provide further feedbacks to the enhancement of phytoplankton growth. Changes in the predation pressure and bioturbation, as a consequence of shallowness, may be a source of intermediate disturbance which might promote phytoplankton diversity. Results from managed fish ponds (Komarkova, 1983; Komarkova et al., 1986), from reservoirs before and after biomanipulation (Komarkova et al., 1995) and examples from a tropical lake Catemaco (Komarkova & Tavera, 1996) confirm that the composition of phytoplankton is influenced, to varying degrees, by the nature and intensity of predation. Changes in fish stock can have both positive and negative effects on the transparency, concentration of nutrients and abundance of phytoplankton. Taxonomic composition and diversity of phytoplankton are affected differently, the effect being often the opposite from that predicted. Thus, in a water body with an unbalanced fish stock, using phytoplankton characteristics as the only criteria for identification of the trophic status can give incorrect information.