Effects of co-existing microalgae and grazers on the production of hemolytic toxins in Karenia mikimotoi

Karenia mikimotoi （Miyake ＆ Kominami ex Oda） Hansen ＆ Moestrup is associated with harmful algal blooms in temperate and subtropical zones of the world. The hemolytic substances produced by K. mikimotoi are thought to cause mortality in fishes and invertebrates. We evaluated the composition of the hemolytic toxin produced by K. mikimotoi cultured in the laboratory using thin-layer chromatography. In addition, we evaluated the effect of co-occuring algae （Prorocentrum donghaiense and Alexandrium tamarense） and the cladoceran grazer Moina mongoliea on hemolytic toxin production in K. mikimotoi. The hemolytic toxins from K. mikimotoi were a mixture of 2 liposaccharides and 1 lipid. Waterborne clues from P. donghaiense and A. tamarense inhibited the growth of K. mikimotoi but increased the production of hemolytic toxins. Conversely, K. mikimotoi strongly inhibited the growth of caged P. donghaiense and A. tamarense. In addition, the ingestion of K. mikimotoi by M. mongolica induced the production of hemolytic toxins in K. mikimotoi. Taken together, our results suggest that the presence of other microalgae and grazers may be as important as environmental factors for controlling the production of hemolytic substances. K. mikimotoi secreted allelochemicals other than unstable fatty acids with hemolytic activity. The production of hemolytic toxins in dinofiagellates was not only dependent on resource availability, but also on the risk of predation. Hemolytic toxins likely play an important role as chemical deterrents secreted by K. mikimotoi.

Effect of Salinity on the Zooplankton Community in the Pearl River Estuary

Understanding the relationship between the zooplankton distribution and salinity may provide key information to understand ecosystem function under the condition of a global mean sea level rise caused by global climate change.However,little is known about how increasing salinity level will affect the entire zooplankton community on a large scale.Here we completed 1 year of field investigations on the Pearl River Estuary and analyzed the distribution and structure of the zooplankton community.A total of 68 zooplankton species were identified during the survey.The number and diversity(richness,evenness,Shannon index,and Simpson’s index)of the zooplankton species decreased as salinity increased from 0.10 to 21.26.Salinity negatively affected the abundances of rotifers,cladocerans,and total zooplankton,while it had little effect on copepod abundance.Some salt-tolerant species,such as Keratella tropica,Polyarthra vulgaris,and Paracalanus crassirostris,survived at high-salinity sites.A pattern was observed at all sites:the peak in copepod abundance always occurred when rotifers were abundant(sites S1 and S2)or after rotifer abundance reached a maximum level(sites S3,S4,and S5).In general,salinity was the most important environmental factor shaping zooplankton biodiversity and abundance.This study provides insight into potential biodiversity and structure of the zooplankton community in response to salinity change.

Characteristics of the microzooplankton community in Jiaozhou Bay,Qingdao,China

The species composition and abundance of microzooplankton at 10 marine and five coastal stations （Hongdao, Daguhe, Haibohe, Huangdao and Hangxiao） in the Jiaozhou Bay （Qingdao, China） were studied in 2001. The microzooplankton community was found to be dominated by Tintinnopsis beroidea, Tintinnopsis urnula, Tintinnopsis brevicoUis and Codonellopsis sp. The average abundance of microzooplankton was highly variable among stations. Specifically, the abundance of microzooplankton was higher at inshore stations and lower in the center of the bay （St. 5）, bay mouth （St. 9） and outside the bay （St. 10）. The highest average annual densities （346 ind./L） was observed at St. 3, while the lowest （55 ind./L） was at St. 10. Two abundance peaks were recorded in May （324 ind./L） and February （300 ind./L）. The distribution of microzooplankton in three sampling layers at the 10 stations was relatively homogenous and the abundance decreased slightly as the water depth increased. At coastal stations, the highest average annual density was recorded at Hongdao Station （677 ind./L）, followed by Daguhe Station （616 ind./L）, Haibohe Station （400 ind./L ）, Huangdao Station （275 ind./L） and Hangxiao Station （73 ind./L）. Furthermore, a 24-h sampling analysis conducted at Hangxiao Station revealed that the microzooplankton assemblages were characterized by a bimodal diel vertical migration pattem, with the highest densities occurring at dusk （154 ind./L）, followed by dawn （146 ind./L）, noon （93 ind./L） and midnight （77 ind./L）. The density of microzooplankton in the Jiaozhou Bay was in the middle range of the densities of temperate coastal waters worldwide.