Hydrological Regime and Fish Predation Regulate the Zooplankton Community Size Structure in a Tropical Floodplain Lake

Abstract

Floodplain ecosystems are characterised by alternating flood and drought periods that can affect the structure of the aquatic community. Dynamic changes in the hydrological regimes from flooding to dry periods influence the migration and dispersal of aquatic fauna and the exchange of particulate matter and nutrients. Riverine floodplains are among the most productive ecosystems; however, increasing pressure from anthropogenic activities has altered the hydrological regimes, threatening aquatic biodiversity. In this study, we examined the temporal patterns of zooplankton community size structure and fish density during three distinct hydrological events in a tropical floodplain lake, Lake Tempe, Indonesia. We included fish density data and three contrasting hydrological conditions, moderate-, high-, and low-water periods, as the environmental factors regulating zooplankton community structure. In high- and low-water conditions, the ecosystem heterogeneity was characterised by high chlorophyll-a, total nitrogen, and total phosphorus concentrations; high fish density; and high zooplankton abundance and biomass. During the early flood period, the ecosystem was characterised by lower concentrations of trophic indicators and significant decreases in zooplankton abundance and biomass, as well as decreased fish density. While there was no clear association between hydrological conditions and zooplankton size structure, our findings indicate that fish predation probably suppressed zooplankton size diversity in Lake Tempe, shown by the dominant contribution of small-sized zooplankton towards total abundance and biomass under all hydrological conditions. Our results indicate that the patterns of environmental variables, zooplankton community, and fish density are affected by hydrological conditions, highlighting the role of water level fluctuation as the driving factor for zooplankton community structure. Our results also indicated that fish predation led to the development of a small-sized population of zooplankton in Lake Tempe.