Assessing the long-term efficacy of internal loading management to control eutrophication in Lake Rauwbraken
F. van Oosterhout, Said Yasseri, Natália Pessoa Noyma, Vera L. M. Huszar, Marcelo Manzi Marinho, Maíra Mucci, Guido Waajen, Miquel Lürling
Abstract
Lake Rauwbraken was impacted by eutrophication caused by diffuse external phosphorus (P) loads (total 1.21 mg m−2 d−1, estimated in 2008). Over 40 years, this load built up a legacy pool in the sediments, resulting in 6.82 mg m−2 d−1 PO4-P internal load (estimated in 2008), causing cyanobacterial blooms and swimming bans. To address the internal load in this lake, a low dose treatment of flocculant (polyaluminium chloride) combined with a solid phase phosphate fixative (lanthanum-modified bentonite) was applied in 2008. We examined the chemical and ecological responses to this treatment to demonstrate the efficacy of controlling internal loading without reducing external loading. Based on 2 years pre- and 10 years post-treatment monitoring, the mean Secchi disk depth (3.5–4.0 m) and the hypolimnetic oxygen concentration (0.86–4.55 mg L−1) increased while decreases occurred in turbidity (5.4 to 2.2 NTU), chlorophyll a (16.5 to 5.5 µg L−1), contribution of cyanobacteria (64% to 17% of chlorophyll a), total phosphorus (134 to 14 µg L−1), and total nitrogen (0.96 to 0.50 mg L−1). The treatment reduced the PO4-P release from sediment under anoxic conditions from 15.1 to 1.7 mg m−2 d−1 post-treatment in 2008, 2.3 mg m−2 d−1 in 2011, and 4.7 mg m−2 d−1 in 2013. Post-treatment, submerged macrophytes reached high coverage in 2008 and 2009. Longer term, post-treatment macrophyte cover was reduced. The lake is returning to a eutrophic state as a result of ongoing external P loads 10 years following the control of internal loading.