Application of PAC-modified kaolin to mitigate Prorocentrum donghaiense: effects on cell removal and phosphorus cycling in a laboratory setting
2017
Lu, Guangyuan | Song, Xiuxian | Yu, Zhiming | Cao, Xihua
Among various strategies for mitigating harmful algal blooms (HABs), coagulation–flocculation of algal cells by modified clay (MC) has proven to be an effective method that is widely applied in East Asia. Inorganic nutrients produced by algal detritus decomposition after an HAB are quickly released into the overlying water, which may cause second eutrophication. For environmental safety reasons, it is of critical importance to consider the fate of flocculated organic matter and clarify its effects on nutrient cycling. The present study therefore compared cell density, in vivo chlorophyll-a fluorescence, pH, and nutrient concentrations (especially phosphorus) between control (A1) and MC-treated algal bloom waters (A2) in a laboratory setting. The results showed that 0.4 g L⁻¹ of MC removed approximately 60 % of Prorocentrum donghaiense cells (original concentration = 4.63 ± 0.05 × 10⁸ cells L⁻¹) and delayed residual algal apoptosis. MC effectively scavenged 51.4 % of total phosphorus (TP) and 94.2 % of dissolved inorganic phosphorus (DIP) in seawater. Additionally, there was no excess DIP release from the MC-algae matrix into the water column, providing secondary eutrophication through algal degradation unlikely. This study newly demonstrated that MC treatment can significantly alter the velocities of phosphorus cycling (small scale) through two main pathways: (1) the direct coagulation–flocculation effect and (2) indirect blocking. This study provided fundamental insight into the quick downward sinking and slow decomposition of algal detritus in a laboratory setting and directly relates to natural processes occurring during HAB senescence/termination.
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