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

We conducted a comprehensive investigation on the microzooplankton herbivory effect on phytoplankton in the northern South China Sea (SCS) using the seawater dilution technique at surface and deep chlorophyll maximum (DCM) layers in two cruises (July- August of 2009 and January of 2010). We compared vertical (surface vs. DCM), spatial (onshore vs. offshore), and seasonal (summer vs. winter) differences of phytoplankton growth (µ₀) and microzooplankton grazing rates (m). During summer, both µ₀ and m were significantly higher at the surface than at the layer of DCM, which was below the mixed layer. During winter, surface µ₀ was significantly higher than at DCM, while m was not significantly different between the two layers, both of which were contained within the mixed layer. Surface µ₀ was, on average, significantly higher in summer than in winter; while average surface m was not different between the two seasons. There were no significant cross-shelf trends of µ₀ in summer or winter surface waters. In surface waters, µ₀ was not correlated with ambient nitrate concentrations and the effect of nutrient enrichment on phytoplankton growth was not pronounced. There was a decreasing trend of m from shelf to basin surface waters in summer, but not in winter. Microzooplankton grazing effect on phytoplankton (m/µ₀) did not increase with distance offshore, suggesting that the importance of microzooplankton as grazers of phytoplankton may not decrease in onshore waters. On average, microzooplankton grazed 73% and 65% of the daily primary production in summer and winter, respectively. [ABSTRACT FROM AUTHOR]