Can Saltwater Intrusion Affect a Phytoplankton Community and Its Net Primary Production? A Study Based on Satellite and Field Observations

Abstract

Net primary production (NPP) by phytoplankton is a crucial regulator of the carbon cycle and other biological processes in estuaries. Under the impacts of rising sea levels, most estuaries are experiencing increased saltwater intrusion. To determine how these changes in intrusion affect phytoplankton communities and their NPP, a study was performed in the Pearl River Estuary (PRE), China. The results based on monthly global ocean NPP satellite products from 2003 to 2015 show that NPP generally decreased from the north (the upper outlet) to the south (the estuary mouth) in the PRE, whereas temporally, NPP in the rainy season (April to September) exhibited an inverted V-shaped curve, with a maximum value in July and was generally higher than that in the dry season (October to the following March). Field observations of salinity increased from north to south in the PRE, but phytoplankton species and cell numbers decreased from outlets toward the outer estuary. Further analysis showed that NPP and phytoplankton species were positively correlated (R2 ~ 0.5), whereas NPP and salinity were negatively correlated (R2 ~ 0.4). Overall, saltwater intrusion in the dry season increased salinity and changed the phytoplankton species abundance and composition, and it may inhibit phytoplankton growth, thereby decreasing NPP.