Spatiotemporal changes and drivers of trophic status over three decades in the largest shallow lake in Central Europe, Lake Balaton

Abstract

The over-enrichment of shallow lakes in nutrients has emerged as one of the main causes of water quality deterioration, and is today a major focus of water quality studies worldwide. In the present work, changes in trophic conditions over three decades (1985–2017) in the largest shallow freshwater lake in Central Europe, Lake Balaton, are assessed using the time series of 10 water quality variables measured at 4 sites, one in each basin of the lake. Using combined cluster and discriminant analyses, and assessing each of the four basins of the lake separately, it was possible to divide the history of the lake into three time intervals. Principal component and Sen's slope analyses highlight the fact that the oligotrophization of the lake took place at a different pace in each of these three major time intervals (1985–1994; 1995–2003; 2004–2017) along the lake's major axis. A significant decrease in the concentration of parameters indicating trophic conditions (e.g. chlorophyll-a, soluble reactive phosphorus) was first observed in the western basins, in the proximity of the main water input to the lake, followed by the eastward spread of this phenomenon. At the same time, the importance of external total phosphorus input to the lake was found to decrease eastwards, thereby diminishing its capacity to explain the variance of the water quality parameters in the lake. Over the time period covered by this study, various measures were taken to reduce the nutrient loads to the lake. These were, in the main, successful, as may be seen in the decade-by-decade overview of the lake's trophic state presented here. A brief review of similar cases from around the world only serves to reinforce the conclusion that a drastic reduction in external phosphorus loads arriving in similar shallow lakes will result in their oligotrophization, albeit with a time-lag of at least ten years.