Harmful algal blooms(HABs) can elicit several negative effects on aquatic environment(such as depleting the oxygen, blocking the sunlight, destroying the habitats of organisms) and life health(including poisoning/killing marine mammals, birds and human). Among the various control strategies for HABs(physical manipulation needs lots of manpower and expensive equipment, chemicals treatment has some toxic byproduct and high residual, microbial agents only has limited in laboratory research), the coagulation-flocculation of HAB species by modified clay(MC) has been proven to be an effective, lowcost and environmentally friendly method that has been widely applied in the field, particularly in eastern Asia. In order to examine the long-term effects of MC treatment, this study investigated the alternations in seawater of Skeletonema costatum, a high biomass dominant HAB species along the Chinese coast, by comparing the degradation of S. costatum detritus(A1) with the application of MC treatment(A2) and MC treatment in sediment condition(A3). The low dosage of 0.25 g/L MC could efficiently remove 4×108 cells/L of S. costatum cells within 3.5 h(approximately 97% removal). In addition, the results showed that both inorganic and organic nutrients were effectively reduced from seawater by MC particles. Compared to the total nitrogen(TN) and total phosphorus(TP) concentrations in A1 seawater, 44% of TN and 93% of TP in A2 seawater, as well as 72% of TN and 93% of TP in A3 seawater were removed during the onemonth incubation period. Simultaneously, 64% of DISi in A2 and 44% of DISi in A3 significantly decreased( P <0.001). This study demonstrated that MC treatment was able to significantly increase the downward flux of nutrients and delay the release velocity of inorganic nutrient from MC-algae matrix into the overlying seawater, particularly within sediment environment.
We present results on the effect of modified clay on cyst formation of Scrippsiella trochoidea in harmful algal bloom （HAB）. Modified clay （in concentration of 0, 0.1, 0.5, and 1.0 g/L） were added to cultures, and observations were made on cysts of S. trochoidea under controlled laboratory conditions. Results indicate that the removal rate of algal cells reached 97.7% at the clay concentration of 1.0 g/L. The cyst formation rate increased from 4.6% to 24.6% when the concentration of clay was increased from 0 to 1.0 g/L. Two cyst metamorphs were observed： spinal calcareous cysts and smooth noncalcareous ones. The proportion of the spinal cysts decreased from 76.9% to 24.1% when clay concentration increased from 0 to 1.0 g/L. In addition, modified clay affected cyst germination. The germination rate decreased with the increases in the clay concentrations. Non-calcareous cysts had a lower germination rate with a longer germination time. We conclude that modified clay could depress algal cell multiplication and promote formation of temporal cysts of S. trochoidea, which may help in controlling HAB outbreaks.
Prevention of annual"green tides"caused by blooms of the green macroalga Ulva prolifera,which have occurred in the Yellow Sea since 2007,has received much attention.Increasing evidence indicates that micro-propagules on the rafts used for Porphyra yezoensis aquaculture along the coastline of Jiangsu Province in China,were the primary source of these green tides.In this study,we investigated the effectiveness of a silicone antifouling coating for preventing the adhesion and germination of U.prolifera micro-propagules on bamboo and nylon ropes,which have been used in rafts for P.yezoensis aquaculture.Our results demonstrated that a coating of silicone(Sylgard 184)containing 1.0 wt.%sodium benzoate(NaB)could reduce the adhesion rate to<5%and completely inhibited the growth of U.prolifera propagules on the coated surfaces.Coating bamboo and nylon ropes with NaB-incorporated silicone thus proved a potentially effective method for controlling the initial biomass accumulation of U.prolifera.
To avoid unfavorable environmental conditions, Scrippsiella trochoidea is capable of forming a resting cyst in the process of growth. In the present study,we investigated the effects of nitrate and ammonium on the growth and encystment of S. trochoidea. We incubated S.trochoidea in modified f/2 media without nitrogen and silicate in flasks. The flasks were divided into two groups.Nitrate was added as a nitrogen source in the first group,and ammonium was added in the second group. The concentrations of the nitrogen compounds were 0, 10, 30, 60,and 90 lmol/L. The results indicate that NO3--N favors cell growth, and the cultures with a higher concentration of NO3--N were ineffective at forming cysts. In contrast,NH4--N promoted cell growth and cyst formation. At similar concentrations as NO3--N, NH4--N had a toxic effect on cell growth and increased the cyst formation ratio.Thus, the NH4--N concentration is an important factor for controlling encystment. We believe that the impact of NH4--N in inducing cyst formation may be a useful feedback mechanism in ecological systems.
