Controls of seasonal variability of phytoplankton blooms in the Pearl River Estuary

Abstract

Based on both physical and biological data collected from cruises in different seasons between 2010 and 2011, we found that the phytoplankton bloom was highly seasonally variable along the longitudinal axis of the Pearl River Estuary (PRE) and was confined to very limited locations because of the variation in the coupled physical–biological forcing of the phytoplankton growth and accumulation. We investigated the relative importance and joint control of the bloom′s variation by freshwater residence time, water column stability, and light limitation that were governed by river discharge, vertical mixing, and turbidity in the PRE, respectively. We found that the bloom occurred in the middle of the well/partially mixed estuary during the dry season when river discharge was relatively small. Although the water nutrient concentration and residence time were favorable for phytoplankton accumulation during this season, the bloom was absent in the upper and lower parts of the estuary likely associated with high turbidity in the upper region and strong vertical mixing in the lower region. In contrast, strong river discharge during the wet season pushed the nutrient-rich river plume farther seaward and formed a salt-wedged estuary. With a stable water column and relatively low turbidity during this season, the bloom could only occur in the lower part of the estuary where the phytoplankton growth rate was greater than the water turnover rate.