Zooplankton communities of small floodplain water bodies in longtime limnophase (Khoper River valley, European Russia)

Abstract

Under conditions of low floods and disturbed water exchange with the river, small floodplain water bodies gradually dry up, which leads to a decrease in biodiversity of the entire floodplain area. The zooplankton is one of the main indicators of the ecosystem status in aquatic ecosystems. Hence, this study was aimed to investigate the structure of zooplankton communities, their seasonal and interannual dynamics in an oxbow system, which has been in the limnophase stage (in isolation from the river) for a long time. The study has been carried out in the upper reaches of the River Khoper in the buffer zone of the Privolzhskaya Lesostep State Nature Reserve (Penza Region, European Russia) at the forest-steppe natural zone. A part of the studied water bodies is located in the centre of the forested area («forest» sites), while other water bodies are located along the forest edge («open» sites). In 2016–2017, from April to September, 80 zooplankton samples were taken from eight water bodies near the water body shores by filtering 10 litres of water through an Apstein net. A total of 129 species and forms of planktonic invertebrates were found in the studied water bodies. A high proportion of species, which prefer plant thickets, was observed, especially in the forested sites. Based on the used structural parameters of zooplankton communities, the studied water bodies have a high similarity being located in various parts of the oxbow system. The zooplankton communities of forested water bodies are at later stages of the successional development. The temporal and spatial differences of the zooplankton communities are indirectly determined by water temperature, with which the number of species, abundance and biomass of the zooplankton are positively correlated. The zooplankton communities of open water bodies are more spatially variable, but they are more stable in dynamics than zooplankton communities of forested water bodies, which is probably determined by differences in the time of the water level drop. A sharp decrease in the area of open water bodies is being observed in spring, while it less affects zooplankton communities at summer. The drying of forested water bodies occurs gradually throughout the whole summer. Therefore, under conditions of the limnophase and absence of flooding, the zooplankton community dynamics are determined by the intensity of the water level drop at summer, but not the water level fluctuations at spring. The obtained data emphasise the necessity of the monitoring of the whole complex of various floodplain water bodies (pondscapes) but not individual oxbows. In turn, this will allow us to assess the consequences of the prolonged isolation of these ecosystems from a river.