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 re-oligotrophication 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 μg 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 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.
Highlights
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
We explore the long-term dynamics of chlorophyll a (Chl a) concentration, a proxy for phytoplankton biomass, and zooplankton biomass, as well as their interrelationship in Lake Balaton during its re-oligotrophication period from 1997 to 2017
Temporal trends in Chl a concentration and zooplankton biomass during oligotrophication The oligotrophication of Lake Balaton during the study period was characterized by a significant decrease in Chl a concentration
Summary
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 social and economic impacts of the water quality deterioration initiated a complex process of eutrophication control and lake restoration program in the 1980s It included (i) the reconstruction of the Kis-Balaton Water Protection Reservoir system (wetland), (ii) sewage direction from the watershed, and (iii) introduction of new wastewater treatment methods for phosphorus removal (Istvanovics et al, 2007). Future global warming, which presumably leads to elevated lake temperatures, will expectedly negatively affect water quality (Mooij et al, 2007) and, in turn, may lead to an increase in bloom intensities This vulnerability highlights the importance of a better understanding of re-oligotrophication and associated changes in shallow lakes. Many aspects of the recovery in the trophic state of Lake Balaton (as well as other shallow lakes), including its impact on food web dynamics, have not yet been investigated in sufficient detail
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