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

Abstract

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.

Transient Hypoxia Extent Off Changjiang River Estuary due to Mobile Changjiang River Plume

“…Mechanisms controlling the formation and extent of bottom hypoxia have been investigated based on observations (e.g., H. Wang, Dai, et al, ; Wei et al, ; Z. Zhu et al, ; J. Zhu et al, ), as well as numerical modelings (e.g., X. Chen et al, ; J. Zheng et al, ; F. Zhou, Chai, et al, ). Seasonal hypoxia formation and its spatial extent off the Changjiang River estuary are a multifactor controlled phenomenon, which are closely linked with nutrient loads (K. Wang, Chen, et al, ), wind speed and direction (Ni et al, ; B. Wang et al, ; J. Zheng et al, ), properties of the Kuroshio subsurface water, magnitude of freshwater discharge (J. Zheng et al, ), and other qualities such as topography (D. Li et al, ) and tide (X. Chen et al, ; Z. Zhu et al, ). Combining the stratification effect with the simultaneously introduced nutrients to promote ecosystem production and give rise to hypoxia off the estuary.…”

“…Zhou, Chai, et al, 2017). Seasonal hypoxia formation and its spatial extent off the Changjiang River estuary are a multifactor controlled phenomenon, which are closely linked with nutrient loads (K. Wang, Chen, et al, 2016), wind speed and direction B. Wang et al, 2017;J.…”

High‐frequency time‐series autonomous observations of sea surface pCO2 and pH

“…5b, 6b), which was capable of blowing the nearshore water eastward and offshore. In addition, the surface ocean condition would normally favor phytoplankton blooms in several days with the supply of nutrients from subsurface waters (Chen et al 2017; Wang et al 2017 a , b ). The p CO 2 would consequently go down as observed (Figs.…”

Contribution of Biological Effects to the Carbon Sources/Sinks and the Trophic Status of the Ecosystem in the Changjiang (Yangtze) River Estuary Plume in Summer as Indicated by Net Ecosystem Production Variations

“…Changjiang-diluted water has a strong influence on the Changjiang River estuary plume by virtue of a water discharge of about 944 × 10 9 m −3 year −1 [17] that carries a large amount of nutrients and sediments [18]. The eutrophic Changjiang-diluted water enters the upper estuary area, resulting in phytoplankton blooms that can absorb a substantial quantity of atmospheric CO 2 [19]. At the same time, fluvial carbon input [20], as well as the decomposition and regeneration of organic matter in primary production, causes the estuary to release CO 2 into the atmosphere [21].…”

“…Further, ∆pCO 2mix can be calculated. The equations are (19) where (TA 2 ) CDW , (TA 2 ) KSW , (TA 2 ) KSSW and (DIC 2 ) CDW , (DIC 2 ) KSW , (DIC 2 ) KSSW denote the TA and DIC concentrations of the three end-member at time t 2 , respectively. Finally, the pCO 2 changes caused by biological processes (∆pCO 2bio ) were calculated from the other DIC changes.…”