Environmental gradients regulate the spatial variations of phytoplankton biomass and community structure in surface water of the Pearl River estuary

Regulations of estuarine environmental gradient on phytoplankton communities were evaluated about the Pearl River estuary during June 25 to July 2 of 2009 (wet season) and January 11 to 16 of 2010 (dry season). Downstream increase of salinity was 0.19 and 0.23 per km in wet and dry seasons; whereas the decrease of NO3-+NO2- content was 0.57 and 5.54 μmol L−1 per km, of PO43- was 0.013 and 0.014 μmol L−1 per km and of SiO32- was 1.05 and 2.89 μmol L−1 per km. Chl a biomass decreased from 11.3 to 1.94 μg L−1 and from 12.5 to 2.04 μg L−1 in wet and dry seasons, respectively; while total phytoplankton abundance decreased from 2.42 × 106 to 3.05 × 105 cells L−1 and from 1.59 × 106 to 0.32 × 105 cells L−1, with the dominating groups changing from diatoms and chlorophytes to solely diatoms. In particular, the nutrient PO43- was found to limit the growth of phytoplankton in wet season, but not in dry season. Our results also indicate that chlorophytes are more sensitive to salinity changes than other species in this estuary.