Seasonal and spatial patterns of picophytoplankton growth, grazing and distribution in the East China Sea
Dynamics of picophytoplankton population distribution in the East China Sea (ECS), a marginal sea in the western North Pacific Ocean, were studied during two cruises in August 2009 (summer) and January 2010 (winter). Dilution experiments were conducted during the two cruises to investigate the growth and grazing among picophytoplantkon populations. Comparisons of phytoplankton growth ( μ0) and microzooplankton grazing rates ( m) on seasonal (summer and winter), spatial (plume, transitional and Kuroshio regions) and vertical (surface and depth of chlorophyll maximum) scales were made. The three picophytoplankton populations occupied different ecological niches and showed different distribution patterns (especially in summer), which is, however, not coincident with their maximum growth rate. The distribution and population transition of picophytoplankton is therefore a result of the balance between growth and grazing mortality. Average growth rates ( μ0) for Prochlorococcus (Pro), Synechococcus (Syn) and picoeukaryotes (Peuk) were 0.36, 0.81 and 0.90 d −1 in summer, and 0.46, 0.58 and 0.56 d −1 in winter, respectively. Average grazing mortality rates ( m) were 0.46, 0.63 and 0.68 d −1 in summer, and 0.25, 0.22 and 0.23 d −1 in winter for Pro, Syn and Peuk, respectively. The spatial pattern of both growth and grazing mortality rates showed decreasing trends from the inshore to offshore region, indicating a strong influence of the nutrient gradient induced by Yangtze River input. In summer, Pro, Syn and Peuk were dominant in Kuroshio, transitional and plume regions, respectively, while in winter all the three populations tended to thrive in the offshore regions, particularly for Pro and Syn. Vertically, picophytoplankton exhibited the highest abundance at ~ 20 m in summer and at the surface in winter. Both growth rate and grazing mortality were higher at the surface than in the deep chlorophyll maximum (DCM) layer. On average, protist grazing consumed 84, 79 and 74% and 45, 47 and 57% of production for Pro, Syn and Peuk in summer and winter, respectively.