Assessment of internal and external lake restoration measures for two Berlin lakes

Two previously highly eutrophic lakes, Lake Tegel and Schlachtensee in Berlin, Germany, were subjected to similar external but different internal restoration measures during the last 20-25 years. External phosphorus (P) load was reduced in both lakes by P-stripping their main inflows using P-elimination plants; internal P load was treated by aeration in Lake Tegel and by hypolimnetic withdrawal in Schlachtensee. Loads before and after treatment are compared with the targets using the Vollenweider model and a modified One-Box model. The results indicate that external load reduction was the main cause of the pronounced lake water quality improvements. The hypolimnetic withdrawal in Schlachtensee was effective only in the initial years. No significant positive effect can be identified for the aeration of Lake Tegel.