Effect of wind events on phytoplankton blooms in the Pearl River estuary during summer

Abstract

The Pearl River estuary fuels phytoplankton productivity in the estuarine coastal waters in the oligotrophic South China Sea. Cruises were conducted to investigate the spatial distribution of primary productivity, and two size fractionations (o5 and 45 mm) of chlorophyll in the Pearl River estuary and across the coastal plume south of Hong Kong during the season of high river discharge (July, 1999 and 2000). During 18-19 July 1999, phytoplankton biomass and productivity were low in the freshwater dominated estuary (o20 mg chl am 2 and o100 mgCm 2 d 1, respectively), and were maximal at the edge of the estuarine coastal plume south of Hong Kong, reaching 70mg chl a m 2 and 4.2 gCm 2 d 1, respectively. Chl a was dominated by the45 mm size fractionation (450%). The Pearl River estuary displayed a typical spatial variability of phytoplankton biomass: low biomass and productivity due to rapid dilution and light limitation in turbid estuarine waters, a regional maximum of biomass and productivity at intermediate salinities in the coastal plume and low biomass due to nutrient limitation in oceanic waters. In summer when south-southwest monsoonal winds blow, part of the coastal plume moves eastwards. A regional maximum of phytoplankton usually develops in the coastal plume south of Hong Kong. However, a change in wind direction to the east or northeast can cause the following effects: pushing the coastal plume westward to the estuary, holding (retaining) the estuarine plume in the estuary (due to the Coriolis effect) and at the same time, mixing the water column vertically. Such a strong east or northeast wind event was observed during July 22-26 1999 and caused an increase in salinity at the surface in the estuary compared to before the wind event. This wind event increased the residence time of the estuarine plume inside the estuary, and as a result, a phytoplankton bloom (52-151mg chl am 2) developed during July 25-26 inside the estuary. This similar easterly wind effect was observed during July 2000. After a strong easterly wind event (speed 10ms 1) on July 11, the coastal plume was absent in waters south of Hong Kong on July 13, but it returned on July 19 after winds blew from the south and southwest direction. The results suggest that wind events not only changed the spatial movement of the coastal plume, but also the spatial distribution of phytoplankton biomass and productivity in the Pearl River estuary and adjacent coastal waters.