Abstract
Abstract. 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 on 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 DCM layer, which was below the mixed layer. During winter, surface μ0 was significantly higher than at the DCM, while m was not significantly different between the two layers, both of which were 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 cross-shelf gradients 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) was relatively small in the summer basin waters, indicating a decoupling of microzooplankton grazing and phytoplankton growth at this time. On average, microzooplankton grazed 73% and 65% of the daily primary production in summer and winter, respectively.
Highlights
Hydrology and Microzooplankton (< 200Eμamr;tihnclSudyinsgtneamnoflagellates, cilitahteesm, adjionrogflraagzeelrlsatoesf,pshayrtcoopdlianneksSt,ocann,ideacnscmcoauelnlstimngetafozroathnes)loasres of ∼ 60–80 % of daily primary production in the sea (Calbet and Landry, 2004)
Surface chlorophyll a (Chl a) concentrations were significantly lower than those at deep chlorophyll maximum (DCM) layers in slope and basin waters; this vertical difference was not observed in winter (Fig. 2)
Chl a concentrations were insignificantly different between surface and DCM waters in both summer and winter
Summary
Hydrology and Microzooplankton (< 200Eμamr;tihnclSudyinsgtneamnoflagellates, cilitahteesm, adjionrogflraagzeelrlsatoesf,pshayrtcoopdlianneksSt,ocann,ideacnscmcoauelnlstimngetafozroathnes)loasres of ∼ 60–80 % of daily primary production in the sea (Calbet and Landry, 2004). Owing to their fast reproduction rates (Banse, 1982), microzooplankton grazing is able to respond quickly to the increaseOd pcheytaopnlanSktconiegnrocweth rate upon nutrient enrichment (Landry et al, 2000). The ratio of the microzooplankton grazing rate to the phytoplankton growth rate (m/μ0), which describes how much of the primary production is consumed by microzooplankton, is related with the efficiency of the biological pump, sumed as by tmheicrpoazrot oopTflaphnrkeimtoaCnryirsyperoiotsdhueprchtciooennrsteuhmatedis not conby mesozooplankton or directly sinks out of the euphotic zone (Landry et al, 1995, 1998). The m/μ0 ratio is often reasonably believed to be greater in oligotrophic waters where phytoplankton are dominated by small-sized species which are more edible
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