Further improvements in water quality of the Dutch Borderlakes: two types of clear states at different nutrient levels

The Borderlakes are a chain of ten shallow, largely artificial, interconnected lakes in the Netherlands. The ecological recovery of the central Borderlakes (viz. lake Veluwe and Wolderwijd) has been well documented. These lakes shifted from a eutrophic, Planktothrix dominated state in the 1970s to a clear state in 1996. Around 2010, the formerly hypertrophic, southern Borderlake Eem also reached a clear state, but at considerably higher nutrient levels. In this paper, monitoring data are used to compare these changes and identify the differences in driving processes and their consequences. The 1996 shift in Lake Veluwe was linked to increased fishery for benthivorous Bream, followed and stabilized by increase in Zebra Mussels and charophytes. Nutrients in Lake Eem decreased as well and Planktothrix disappeared here too in 1996, despite relatively high TP concentrations which remained stable over time. The start of the change into the clear state in this case also involved a decrease in the Bream population, but with a stronger additional role for dreissenids, particularly of Quagga Mussels. Remaining blooms of cyanobacteria almost disappeared, but the current situation in Lake Eem represents a different type of clear water state than in the central Borderlakes. This type is characterized by the combination of a relatively high phosphorus load, intense dreissenid filtration and filamentous macro-algae instead of either blooms of cyanobacteria or dominance of charophytes. With the dominant role of the River Eem, the relatively short residence time and increasing difficulty to bring down nutrient loading any further, the stability of this clear state depends on high densities (and filtration rates) of dreissenids.