Changes in Internal Phosphorus Loading and Fish Population as Possible Causes of Water Quality Decline in a Shallow, Biomanipulated Lake

Abstract

AbstractImpacts of internal nutrient loading and the role of biota in phosphorus (P) dynamics were studied in a shallow, hypertrophic, biomanipulated lake. Reduced fish stock resulted in clearing water and the development of a dense submerged vegetation by 2005. However, an abrupt shift occurred in 2007, seven years after the fish manipulation. Simultaneously, water quality deteriorated which became obvious in elevated chlorophyll‐a concentration in lake water, associated with increased biomass of fish and decay of a previously extended macrophyte cover. There were no significant differences in lake water P concentrations between the two periods (2005–2006 and 2007); however, peaks of different P forms were markedly higher in 2007 than in 2005–2006. At the same time, P content of sediment pore water declined considerably in 2007. Our mesocosm experiment, carried out in the manipulated lake, emphasize the positive role of the dominant fish species (roach) in P regeneration. We suggest that fish manipulation should be carried out every 5 year to maintain clear water conditions permanent, until the total removal of redundant nutrients accumulated in the lake ecosystem. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)