Effects Of HABs Mitigation By Modified Clay On Major Nutrient Cyclings And Algal Toxins

Harmful algal blooms (HABs) are one of the most serious and complex globalphenomena affecting aquatic environmental systems. The outbreak of HABs has beenaccelerating in frequency, scale, and duration over the past several decades. WhenHABs occur, they negatively affect not only water qualities and the habitats of marineorganisms but also aquaculture industries and local tourism, mostly because of thehigh biomass of cells. Among various options of mitigation strategies for harmfulalgal blooms (HABs), flocculation of algal cells by modified clay (MC) has beenproven to be an effective, low-cost, and environmentally benign method and is theonly technique that has been widely applied in the field, particularly in Japan, Korea,North America and China. Because of the enormous amounts of clay that may beapplied for a single event of HABs, however, it is of critical importance to examinethe long-term environmental effects of this treatment on sensitive aquatic systems.The study report here the investigation on the major nutrient variations and waterquality changes by comparing the natural decaying of different harmful algae withbloom levels of cell density to the flocculation of algae by modified clay application.The thesis concluded all results to summarize the following points.(1) The low dosage of modified clay could efficiently remove and control the algalcells of Prorocentrum micans (60%~89.5%removal), Prorocentrum donghaiense(>56%), Skeletonema costatum (>90%), and Alexandrium tamarense (>95%)with high cell density, particularly in diatom control. Modified clay particlesimpact the metabolism of harmful algae and inhibit the re-blooming of HABsthrough the combining effects of the “shading” effects of suspended particles,changing the chemical stoichiometry and buffer capacity in seawater. No cells were found to escape from the algae-clay aggregates and release into seawaterduring the one-month incubation period.(2) Modified clay particles also effectively eliminated the nutrients from seawaterand adsorbed both inorganic and organic phosphorus with39.28%~42.47%of TPand26.22%~29.93%of TN removal in Prorocentrum micans,51.39%of TP and17.24%of TN removal in Prorocentrum donghaiense,93.36%%~93.60%of TPand53.75%~71.95%of TN removal in Skeletonema costatum,42.86%~56.30%of TP and20.72%~35.53%of TN removal in Alexandrium tamarense.(3) The concentration, proportion and cycling of nutrient were alternated by modifiedclay flocculation. Phosphate and silicate were mainly removed by the direct effectof physical adsorption of modified clay. Dissolved inorganic nitrogen was mainlyeliminated by the indirect effect of chemical adsorption and time cumulativeeffects from modified clay suspended. The conversion rates of dissolvedinorganic macronutrients exhibited that v(NO3--N)<v(DIP)<v(NO2--N)<v(NH4+-N). Thus, the nutrient structure was changed to an unfavorableenvironment (TDN/TDP was up to>100in seawater), which was significantlydeviated from the required nutritional stoichiometry for HABs re-blooming.(4) The mechanisms underlying the effects elicited by MC on algal organicnitrogen/phosphorus are also discussed in the study. This study demonstrated thatMC particles could accumulate the algal matters to coagulate the polymericalgae-organic clay complexes settling into the surface sediment. The complexesblocked the nutrients and delayed the degradation and decomposition of algalorganic matter by preserving the recycling velocity of inorganic nutrients withoutsecondary pollution, exerting an antimicrobial activity effect of PACl in MC,organometallic chelation, and consolidation of particles, particularly within thewater-sediment environment, as the result, this part of the organic materials asfossil stored in burial sediments. On the other hand, the surface catalyticproperties of the modified clay particles provided reaction sites for the conversionof chlorophyll-a and algal toxins (PSTs) or other active organic matters.Moreover, the higher toxicity (GTX1and GTX4) changed into the low toxicity (GTX2and dcGTX3). The oxidation potential and high pH in seawater playedthe most important role in the degradation process of PSTs.The process of MC treatment forms aggregates, similar to the “marine snow” butdifferent sources, formation states and composition of aggregates. The integration andassociation of the whole interactions must be considered as well as the acute impact ofthe artificial addition process. Innovative synthesis studies investigating the long-termenvironmental effects of MC treatment should be conducted prior to the large-scaleapplication of the MC method. Although our group has had extensive experience inHABs on the Chinese coasts (in the Changjiang Estuary of Shanghai City, SouthChina Sea coast of Shenzhen City, Yellow Sea coast of Qingdao City, and the BohaiBay coast of Qinhuangdao City, among other locations), our findings suggest that areevaluation is necessary to determine the way in which algal detritus is representedin ocean biogeochemical models, and new field applications and observations arerequired to further clarify these processes. Further studies will be related to differentalgal scales, species, nutrient biogeochemical cycles, and how PSTs re-enter the foodchain to ensure the temporal environmental safety of the MC strategy.