Seasonal and spatial comparisons of phytoplankton growth and mortality rates due to microzooplankton grazing in the northern South China Sea

[摘要] We conducted a comprehensive investigation on the microzooplankton herbivoryeffect on phytoplankton in the northern South China Sea (SCS) using theseawater dilution technique at surface and deep chlorophyll maximum (DCM)layers on two cruises (July–August of 2009 and January of 2010). We comparedvertical (surface vs. DCM), spatial (onshore vs. offshore), and seasonal(summer vs. winter) differences of phytoplankton growth (μ₀) andmicrozooplankton grazing rates (m). During summer, both μ₀ andm were significantly higher at the surface than at the DCM layer, whichwas below the mixed layer. During winter, surface μ₀ wassignificantly higher than at the DCM, while m was not significantly differentbetween the two layers, both of which were within the mixed layer. Surfaceμ₀ was, on average, significantly higher in summer than in winter,while average surface m was not different between the two seasons. There wereno cross-shelf gradients of μ₀ in summer or winter surfacewaters. In surface waters, μ₀ was not correlated with ambientnitrate concentrations, and the effect of nutrient enrichment onphytoplankton growth was not pronounced. There was a decreasing trend of mfrom shelf to basin surface waters in summer, but not in winter.Microzooplankton grazing effect on phytoplankton (m/μ₀) wasrelatively small in the summer basin waters, indicating a decoupling ofmicrozooplankton grazing and phytoplankton growth at this time. Onaverage, microzooplankton grazed 73% and 65% of the daily primaryproduction in summer and winter, respectively.

[学科分类] 地球化学与岩石