Limnophase versus potamophase: how hydrological connectivity affects the zooplankton community in an oxbow lake (Vistula River, Poland)

Abstract

The functioning of oxbow lakes depends primarily on the succession of two phases: potamophase (when water from the river flows into the lake) and limnophase (when water remains in the riverbed, ensuring stability in the lake). Hydrologic conditions in the river determine whether oxbows are isolated or connected with the river. They also determine the time and duration of both phases. The studied oxbow lake, located in the floodplain of the lower Vistula, was created as the result of river regulation in the 19th century. Observations were conducted during growing seasons in 2006, 2007, 2008 and 2013, during limnophase and potamophase. The study was aimed at evaluating the impact of changes in the river level on the oxbow lake. Potamophase brought greater zooplankton diversity. We recorded a higher number of species and a higher Shannon diversity index (H ′). During limnophase, higher water temperature led to greater zooplankton density. However, its diversity decreased. In recent years, probably due to climate change, the amount of precipitation has decreased. As a result, water levels in rivers are lower and potamophase is rare, while periods of isolation are longer. Low water levels in oxbows increase their primary productivity and decrease zooplankton biodiversity.