Assessing the long-term efficacy of internal loading management to control eutrophication in Lake Rauwbraken

Lake Rauwbraken was impacted by eutrophication caused 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 the sediments, resulting 6.82 d−1 PO4-P internal (estimated 2008), causing cyanobacterial blooms and swimming bans. To address lake, low dose treatment of flocculant (polyaluminium chloride) combined with solid phase phosphate fixative (lanthanum-modified bentonite) applied 2008. We examined chemical ecological responses to demonstrate efficacy controlling loading without reducing loading. Based on 2 years pre- 10 post-treatment monitoring, mean Secchi disk depth (3.5–4.0 m) hypolimnetic oxygen concentration (0.86–4.55 L−1) increased while decreases occurred turbidity (5.4 2.2 NTU), chlorophyll (16.5 5.5 µg L−1), contribution cyanobacteria (64% 17% a), total (134 14 nitrogen (0.96 0.50 L−1). The reduced release from sediment under anoxic conditions 15.1 1.7 2008, 2.3 2011, 4.7 2013. Post-treatment, submerged macrophytes reached high coverage 2008 2009. Longer term, macrophyte cover reduced. lake is returning eutrophic state as result ongoing P following control