Effects of nutrient addition, recovery thereafter and the role of macrophytes in nutrient dynamics of a Mediterranean shallow lake: a mesocosm experiment

Eutrophication in coastal wetlands has increased greatly because of human impact, reducing water quality and affecting aquatic plants. The aim of the present study was to investigate the eutrophication of Mediterranean wetlands and their possible recovery. An in situ mesocosm nutrient enrichment experiment was performed in a coastal shallow lake from eastern Spain, holding dense Chara meadows. In this lake, 36 mesocosms were installed and fertilised weekly with four levels of N and P addition over an 8-week period from June to August. Mesocosms fertilised with the two higher levels (N : P = 5 : 0.5 mg L–1) shifted to turbid status with macrophyte loss before Week 4. Saturation concentrations were similar under these two conditions, with averages ~1.0 mg L–1 total particulate P and 700 µg L–1chlorophyll-a reached in the last 3 weeks. After 5 weeks of the experiment, macrophytes were removed from the mesocosms without nutrient addition and those fertilised with 1 : 0.1 mg L–1 N : P. Macrophyte removal resulted in a release of phosphate and ammonium from sediments, enhancing eutrophication. Three months after stopping nutrient addition, high nutrient and chlorophyll concentrations persisted in the highly fertilised mesocosms, but decreased in the low fertilised mesocoms, which shows that restoration of eutrophic Mediterranean shallow lakes depends to a great extent on previous external loads.