Seasonal Development of Phytoplankton in South Bohemian Fishponds (Czechia)

Abstract

Fishponds with a relatively small water volume, high fish abundance, and wide range of nutrient concentrations serve as suitable models for ecological studies. Intensified fish production, together with increased input of nutrients from the watershed, resulted in hypertrophic conditions in the majority of fishponds, the most common type of lentic ecosystems worldwide. In order to understand the processes driving plankton succession, we analyzed eight-year data from nine fishponds in Czechia with differing trophic status. The mean concentration of phosphorus (P) was 200 µg L−1 in hypertrophic ponds, 130 µg L−1 in eutrophic, and 40 µg L−1 in mesotrophic. Correspondingly the mean concentration of phytoplankton was 14.9 mg L−1 in hypertrophic ponds, 7.3 mg L−1 in eutrophic, and 1.96 mg L−1 in mesotrophic. Although the fish stock of 200–900 kg ha−1 eliminated zooplankton in eutrophic and hypertrophic ponds the faster-growing algae did not prevail over cyanobacteria. Zooplankton grazing pressure on algae is thus not relevant in studied food webs. Due to the rapid biological denitrification in hypertrophic and eutrophic fishponds resulting in low concentration of mineral nitrogen (N), these ponds were dominated by N-fixing cyanobacteria throughout the whole season. Similarly, the faster-growing algae prevail over cyanobacteria in mesotrophic ponds until the decrease of available mineral nitrogen. The limitation by mineral N is thus the primary driver of phytoplankton composition reflected in cyanobacterial dominance, independently of the trophic status and fish density in studied fishponds.