Oligotrophication of Lake Balaton over a 20-year period and its implications for the relationship between phytoplankton
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PRIMARY RESEARCH PAPER
Oligotrophication of Lake Balaton over a 20-year period and its implications for the relationship between phytoplankton and zooplankton biomass Ga´bor Berna´t . No´ra Boross . Bogla´rka Somogyi . Lajos Vo¨ro¨s . La´szlo´ G.-To´th . Gergely Boros
Received: 15 May 2020 / Revised: 6 August 2020 / Accepted: 10 August 2020 Ó The Author(s) 2020
Abstract Lake Balaton, the largest lake in Central Europe, underwent severe eutrophication from the 1960s to the 1990s, due to phosphorus loadings from external anthropogenic sources. The subsequent and complex eutrophication control and lake restoration program resulted in a significant decrease in the external phosphorus loading to the lake. Consequently, Lake Balaton has been returning to its former meso-eutrophic character. In this paper, we explore the long-term dynamics of chlorophyll a (Chl a) concentration, a proxy for phytoplankton biomass, and zooplankton biomass in Lake Balaton during its reoligotrophication period from 2001 to 2017, and attempt to draw some conclusions on the subsequent changes in the fish stock. We found a proportional decrease in zooplankton and phytoplankton biomasses at moderate phytoplankton levels. However, below a certain phytoplankton concentration (\ 10 lg l-1 Chl a), the decrease in phytoplankton biomass was not coupled with a further decline in zooplankton biomass because the fraction of small phytoplankton, edible for Handling editor: Judit Padisa´k. Lajos Vo¨ro¨s, La´szlo´ G.-To´th, and Gergely Boros: contributed equally to this work. G. Berna´t (&) N. Boross B. Somogyi L. Vo¨ro¨s L. G.-To´th G. Boros Centre for Ecological Research, Balaton Limnological Institute, Klebelsberg Kuno u. 3, 8237 Tihany, Hungary e-mail: [email protected]
zooplankton, showed a much smaller decrease in biomass compared with large non-edible phytoplankton. Thus, improvements in water quality (i.e., reduced nutrient loading), partly via concomitant changes in the phytoplankton size distribution, did not cause a large difference in the fish stock in this shallow lake. Keywords Shallow lake Trophic state Algal size distribution Inverted pyramid
Introduction Eutrophication control is one of the foremost challenges in protecting freshwater ecosystems worldwide (Vollenweider & Kerekes, 1982; Schindler, 2006). By contemporary definition, eutrophication is an excessive, mostly undesired algal (or plant) growth in aquatic ecosystems, due to the increased availability of one or more limiting factors. Phosphorus acts as an important limiting nutrient in both freshwater and marine systems, and an increase in either the external or internal phosphorus input is primarily responsible for algal blooms in most cases. Human activities contribute extensively to the increase of phosphorus load into aquatic ecosystems and accelerate the rate and extent of eutrophication through both point-source discharges and non-point loadings (Schindler, 2006). Eutrophication may have negative impa
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