Long-term changes in phytoplankton in a humic lake in response to the water level rising: the effects of beaver engineering on a freshwater ecosystem

Although water level changes are supposed to be a key factor affecting the functioning of lake ecosystems, knowledge on this topic is scarce, particularly for humic lakes. This paper presents the results of 18 years’ research on a small humic lake exposed to hydrological change (rising of the water level), which was induced by spontaneous colonization of the lake by the European beaver (Castor fiber L.). We put forward a hypothesis that this change will be reflected in the quantity and structure of summer phytoplankton due to expected changes in the water chemistry. We noted a statistically significant decrease in total phosphorus and calcium concentrations, electrolytic conductivity, and Secchi disc transparency, and an increase in water color. The phytoplankton structure changed, with cyanoprocaryota and greens decreasing and flagellates increasing. The alteration was observed in a lake which had previously been drained by ditches, so beaver damming appeared to cause the return of the lake to its original endorheic conditions as well as to a water chemistry and phytoplankton structure more typical of undisturbed humic lakes.