[1] Two cruises were carried out in the summer and winter of 1998 to study coupled physical-chemical-biological processes in the South China Sea and their effects on phytoplankton stock and production. The results clearly show that the seasonal distributions of phytoplankton were closely related to the coupled processes driven by the East Asian Monsoon. Summer southwesterly monsoon induced upwelling along the China and Vietnam coasts. Several mesoscale cyclonic cold eddies and anticyclonic warm pools were identified in both seasons. In the summer, the upwelling and cold eddies, both associated with rich nutrients, low dissolved oxygen (DO), high chlorophyll a (Chl a) and primary production (PP), were found in the areas off the coast of central Vietnam, southeast of Hainan Island and north of the Sunda shelf, whereas in the winter they form a cold trough over the deep basin aligning from southwest to northeast. The warm pools with poor nutrients, high DO, low Chl a, and PP were found in the areas southeast of Vietnam, east of Hainan, and west of Luzon during the summer, and a northwestward warm jet from the Sulu Sea with properties similar to the warm pools was encountered during the winter. The phytoplankton stock and primary production were lower in summer due to nutrient depletion near the surface, particularly PO 4 . This phosphorus depletion resulted in phytoplankton species succession from diatoms to dinoflagellates and cyanophytes. A strong subsurface Chl a maximum, dominated by photosynthetic picoplankton, was found to contribute significantly to phytoplankton stocks and production.

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Abstract.A bell-shaped vertical profile of chlorophyll a (Chl a) concentration, conventionally referred to as a subsurface chlorophyll maximum (SCM) phenomenon, has frequently been observed in stratified oceans and lakes. This profile is assumed to be a general Gaussian distribution in this study. By substituting the general Gaussian function into ecosystem dynamical equations, the steady-state solutions for SCM characteristics (i.e., SCM layer depth, thickness, and intensity) in various scenarios are derived. These solutions indicate that (1) the maximum concentration of Chl a occurs at or below the depth of maximum growth rates of phytoplankton located at the transition from nutrient limitation to light limitation, and the depth of SCM layer deepens logarithmically with an increase in surface light intensity; (2) thickness and intensity of the SCM layer are mainly affected by nutrient supply, but independent of surface light intensity; and (3) intensity of the SCM layer is proportional to the diffusive flux of nutrients from below, which becomes stronger as a result of this layer being shrunk by a higher light attenuation coefficient or a larger sinking velocity of phytoplankton. In addition, the limitation and potential application of the analytical solutions are also presented.

[1] Increasing anthropogenic atmospheric deposition of nutrients, trace metals, and toxic substances to oceans may synergistically enhance or inhibit some specific phytoplankton growth, subsequently modulating primary productivity. In this study, onboard incubation experiments were performed in the southern Yellow Sea in the spring of 2011 to explore the responses of microphytoplankton, nanophytoplankton, and picophytoplankton to various combinations of added substances. The water samples used were collected at a lower nutrient concentration zone with an N/P ratio of 10, where satellite data showed a bloom on the eleventh day after collection. The bloom also occurred in the control experiment on the ninth to eleventh days. We simulated atmospheric input by artificially adding Asian dust, rainwater, nitrogen (dissolved inorganic N), phosphorus (P), and iron (Fe). The addition of a large amount of Asian dust increased both the maximum concentration of chlorophyll a (Chl a) and the conversion efficiency index of N into Chl a (CEI) by~40% and~30%, respectively, compared to the control, indicative of promoting growth of the phytoplankton. However, no promotion effect on phytoplankton growth was observed when the addition of Asian dust was reduced to10% of the original amount. The addition of rainwater increased the maximum concentration of Chl a bỹ 40% but decreased the CEI by~40%, indicating inhibition coexisting with promotion of some phytoplankton species. Moreover, the size-fractioned Chl a data showed that the inhibition effect pertained to nanophytoplankton and occurred following the bloom (after the eighth day).Citation: Liu, Y., T. R. Zhang, J. H. Shi, H. W. Gao, and X. H. Yao (2013), Responses of chlorophyll a to added nutrients, Asian dust, and rainwater in an oligotrophic zone of the Yellow Sea: Implications for promotion and inhibition effects in an incubation experiment,

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