Real‐time monitoring of nutrients in the Changjiang Estuary reveals short‐term nutrient‐algal bloom dynamics

The Changjiang Estuary is a large‐river estuary ecosystem in the East China Sea, and its plume, the Changjiang Diluted Water (CDW), transports a large mass of nutrients ( NO3– + NO2–, PO43–, SiO32–) to the shelf sea, leading to substantial eutrophication; the CDW also supports high primary production. However, relationships between nutrient delivery and phytoplankton responses have been difficult to establish, as many nutrient delivery events and algal blooms are episodic, and the CDW may expand or become detached with changing winds. To study the relationship between nutrient delivery events, algal blooms and estuarine metabolism dynamics, a buoy system was deployed in the CDW from 9 September to 10 October 2013, with measurements of chlorophyll a and dissolved nutrients. Day‐to‐day nutrient increases covaried with salinity decreases, regulated by both the spring‐neap tidal cycle and wind events. Several specific nutrient injection periods were detected, each followed by nutrient drawdown and chlorophyll a accumulation (algal blooms). Each algal bloom had its own unique pattern of nutrient uptake based on change in nutrient ratios (ΔN:ΔP; ΔN:ΔSi) and appeared to be dominated by different algal groups. These events occurred under weak wind and stable hydrodynamic conditions. Ecosystem metabolism based on net community production (NCP) showed that the upper estuarine ecosystem was autotrophic when chlorophyll a accumulated, but heterotrophic when wind‐induced mixing strengthened, and upwelling brought organic‐rich water to the near surface. In spite of several short‐lived algal blooms, the average NCPdaily was negative during the observation period, indicating a net source of CO2 to the atmosphere. Key Points Nutrients are mainly transported by the Changjiang Diluted Water in the plume area Different types of algae likely respond to different short‐term nutrient inputs Dissolved oxygen fluxes allowed ecosystem metabolism changes to be determined