Xiaobo Ni

Picoplankton distribution in different water masses of the East China Sea in autumn and winter

Picoplankton distribution was investigated in different water masses of the East China Sea in November, 2006 and February, 2007. The autumn and winter cruises crossed three major water masses: the coastal water mass (CWM), the mixed water mass (MWM), which forms on the continental shelf, and the Kuroshio water mass (KWM). Picoplankton composition was resolved into four main groups by flow cytometry, namely Synechococcus, Prochlorococcus, picoeukaryotes, and heterotrophic bacteria. The average abundances of Synechococcus, picoeukaryotes, and heterotrophic bacteria were (0.63±10.88)×103, (1.61±1.16)×103, (3.39±1.27)×105 cells/mL in autumn and (6.45±8.60)×103, (3.23±2.63)×103, (3.76±1.37)×105 cells/mL in winter, respectively. Prochlorococcus was not found in the CWM and seldom observed in surface samples in either season. However, Prochlorococcus was observed in the MWM and KWM (approximately 103 cells/mL) in both autumn and winter. Synechococcus distribution varied considerably among water masses, with the highest levels in KWM and lowest levels in CWM. The depth-averaged integrated abundance of Synechococcus was approximately 5-fold higher in KWM than in CWM, which may be due primarily to water temperature. In the MWM, Synechococcus was resolved as two subgroups; the presence of both subgroups was more common in autumn. Picoeukaryote abundance varied less among water masses than Synechococcus, and heterotrophic bacteria depth-averaged integrated abundance exhibited the smallest seasonal variations with respect to water mass. Correlation analysis showed that relationships between picoplankton abundances and environmental factors (temperature, nutrients, and chlorophyll a) differed among the three water masses, suggesting that the three water masses have different effects on picoplankton distribution (particularly Synechococcus).

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.