Trophic cascading of medusae on the relationships between copepods and Diatoms in a subtropical coastal ecosystem

Abstract

To investigate the spatial and temporal variation of phytoplankton and copepods, eight seasonal cruises (January 2005 ∼ October 2006) were conducted in a subtropical coast site that encompassed 19 sampling stations. The results indicated that the temperature (18.4-29.7°C) varied more than 10°C seasonally, while nutrient concentrations (nitrate: 0.4-6.3 μM, phosphate: 0.1-0.8 μM, and silicate 2.1-12.1 μM) were inversely correlated with temperature. Diatoms (2792-1602437 cells m-3) constituted > 90% of the phytoplankton assemblages and dinoflagellates (226-10029 cells m -3) represented only ∼10% of the algae biomass. Copepod abundance (8-1031 ind. m-3) varied positively with that of diatom and temperature. The relative abundance of Temora spp. (1-712 ind. m-3) varied from < 2% in winter to > 69% in summer; that of Paracalanus spp. (0.1-176 ind. m-3) were > 25% in spring and autumn and Calanus spp. (0.5-24 ind. m-3) dominated in winter and spring, but accounted for only ∼10% of the total copepods. Both diatom density and copepod abundance were positively correlated with temperature, indicating these two plankton groups were left bottom-up control. The copepod abundance positively co-varied with diatoms in 2005, but not in 2006. Further season-to-season analysis showed that the diatom vs. copepod ratios changed positively only when the medusa abundance changed greatly (> 40-fold) between consecutive seasons. During periods when the medusa abundance varied less than 10-fold between seasons, there is no relationship between the ratios and the medusa abundance. 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 - copepods.