Trophic Cascading of Medusae on the Relationships between Copepods and Diatoms in a Subtropical Coastal Ecosystem


To investigate the spatial and temporal variation of phytoplankton and copepods, eight seasonal cruises (January 2005 ~ Oc to ber 2006) were conducted in a subtropical coast site that encom passed 19 sampling stations. The results in dicated that the temperature (18.4 - 29.7¢XC) varied more than 10¢XC seasonally, while nutrient concentrations (ni trate: 0.4 - 6.3 mM, phosphate: 0.1 - 0.8 mM, and silicate 2.1 - 12.1 mM) were in versely corre lated with temperature. Diatoms (2792 - 1602437 cells m-3) constituted > 90% of the phytoplankton as semblages and dinoflagellates (226 - 10029 cells m-3) represented only ~10% of the algaebiomass. Copepod abun dance (8 - 1031 ind. m-3) varied positively with that of diatom and tempeature. Therelative abundance of Temora spp. (1 - 712 ind. m-3) varied from < 2% in win ter to > 69% in summer; that of Paracalanus spp. (0.1 - 176 ind. m-3) were > 25% in spring and au tumn and Calanus spp. (0.5 - 24 ind. m-3) dom i nated in win ter and spring, but ac counted for only ~10% of the to tal copepods. Both di a tom den sity and copepod abun dance were positively correlated with temperature, indicating these two plank tongroups were left bot tom-up control. The copepod abun dance positively co-aried with diatoms in 2005, but not in 2006. Fur ther sea son-to-sea son analysis showed that the diatomvs. copepodratios changed positively only when the medusa abun dance changed greatly (> 40-fold) between consecutive sea sons. During periods when the medusa abundance varied less than 10-fold between seasons, there is no relationship between the ratios and the medusa abun dance. This study suggests that the stability of the medusa abundance from season to season could be important in regulating the ecological linkage between diatoms and their predators ¡V copepods.

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