Abstract
The release of phosphorus (P) stored in the sediment may cause long-term delay in the recovery of lakes, ponds, and lagoons from eutrophication. In this paper, we tested on a laboratory scale the efficacy of the flocculant polyaluminium chloride (PAC) and a strong P-binding agent (lanthanum-modified bentonite, LMB) on their ability to flocculate a cyanobacterial bloom and hamper P release from a hypertrophic, brackish lagoon sediment. In addition, critical P loading was estimated through PCLake. We showed that cyanobacteria could be effectively settled using a PAC dose of 2 mg Al L−1 combined with 400-mg L−1 LMB; PAC 8 mg Al L−1 alone could also remove cyanobacteria, although its performance was improved adding low concentrations of LMB. The efficacy of LMB to bind P released from the sediment was tested based on potentially available sediment P. A dose of 400 g LMB m−2 significantly reduced the P release from sediment to over-standing water (either deionized water or water from the lagoon with and without cyanobacteria). In sediment cores, LMB + PAC reduced sediment P flux from 9.9 (± 3.3) to − 4.6 (± 0.3) mg P m−2 day−1 for the experimental period of 3 months. The internal P load was 14 times higher than the estimated P critical load (0.7 mg P m−2 day−1), thus even if all the external P sources would be ceased, the water quality will not improve promptly. Hence, the combined LMB + PAC treatment seems a promising in-lake intervention to diminish internal P load bellow the critical load. Such intervention is able to speed up recovery in the brackish lagoon once external loading has been tackled and at a cost of less than 5% of the estimated dredging costs.
Highlights
Eutrophication is one of the main anthropogenic stressors leading to major degradation of coastal waters worldwide (Kennish 2002)
With the polyaluminium chloride (PAC) dose fixed at 2 mg Al L−1, the chlorophyll-a concentrations in the top of the test tubes declined with increasing lanthanum-modified bentonite (LMB) dose
LMB has been used in dozens of freshwater systems where it in general led to an improved water quality (e.g., Copetti et al 2016; Spears et al 2013, 2016; Epe et al 2017)
Summary
Eutrophication is one of the main anthropogenic stressors leading to major degradation of coastal waters worldwide (Kennish 2002). Water quality problems caused by eutrophication include fish deaths due to anoxia, loss of biodiversity, bad smells, and massive plant growth (Paerl and Huisman 2008; Conley et al 2009). Key symptom of eutrophication is a blooming of harmful algae and cyanobacteria, which pose an additional risk to wildlife and humans because of the toxins they may produce (Correl 1998; Huszar et al 2000; Paerl and Paul 2012). There is a great need to control these nuisance blooms. Since blooms are fueled by nutrients, the first step in mitigation would be reducing the nutrient discharge into the receiving waters (Cooke et al 2005; Paerl et al 2014). Some waters will clear up and recover rapidly from such
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