Top-down and bottom-up control of plankton structure and dynamics in hypertrophic fishponds

We investigated the effects of strong top-down control by high fish stock on structure and seasonal dynamics of plankton in nine fishponds under conventional fishery management based on auxiliary feeding during two vegetation seasons. Mean concentrations of total nitrogen, phosphorus, and high densities of phytoplankton, bacteria, heterotrophic nanoflagellates, and ciliates indicated hypertrophic state of the fishponds, as well as a markedly reduced control of these microbial food web components by crustacean zooplankton. Mean seasonal densities of zooplankton varied within one order of magnitude for cladocerans, copepods, nauplii, and rotifers. Daphnia were found in most fishponds in densities up to 630 ind. l−1 (median: 53 ind. l−1). While TN and TP concentrations were high, dissolved inorganic N (median: 29 µg l−1) and reactive P (median: 11 µg l−1) indicated possible nutrient deficiency. The fish stock index (defined as the product of biomass and square root of densities) was used as a proxy for fish predation pressure. Multivariate analysis revealed that nutrients and high fish stocks (market carp, carp fry, and/or undesirable small planktivorous fishes) were the main driving forces shaping the fishpond plankton. The resulting trophic structure thus severely reduced the herbivorous zooplankton–fish link during a vegetation season.