Phytoplankton Blooms off a High Turbidity Estuary: A Case Study in the Changjiang River Estuary

Abstract

The occurrence of phytoplankton blooms in estuarine and coastal waters is codetermined by the export of fluvial nutrients and other water properties such as turbidity. In this study, the Changjiang River Estuary was used as an example to investigate the intrinsic link between river plume dynamics, riverine nutrient loading, suspended sediment concentration, and phytoplankton blooms with a coupled hydrodynamic-sediment-ecosystem numerical model. Suspended sediment concentration from the sediment module was used to parameterize light attenuation in waters, which, in association with river plume dynamics, was found to impact phytoplankton distribution. Data that was collected in situ, in addition to satellite data for chlorophyll, nutrients, and suspended sediments, were used to validate the model, which performed well. The surface high chlorophyll centers exhibited a patch-like pattern from the northeast to the south off the Changjiang River mouth. The high chlorophyll concentration within the river plume was mainly determined by riverine nutrient exports, and potential phosphate limitation occurred away from the river mouth. The chlorophyll concentration decreased dramatically shoreward of the sediment front, which was controlled by the bottom river plume front. The bottom plume front was an intrinsic feature of the Changjiang River plume and similar river plume systems, which set a boundary separating the well-mixed (turbid) nearshore waters and stratified (clear) offshore waters. Without considering the sediment shading effect, the model yielded an unrealistic chlorophyll distribution with artificially high values in the turbid area. This indicated that light attenuation was a key factor limiting phytoplankton blooms in turbid river estuaries.Plain Language Summary In estuaries and coastal waters, the aquatic environment is affected by both riverine nutrient input and water properties such as turbidity. The associated river plume may play an essential role in controlling water properties, thus influencing the aquatic environment such as the algal blooms. In the current study, a well-validated hydrodynamic-sediment-ecosystem model was used to simulate phytoplankton blooms and the dynamic link with the Changjiang River plume. The results showed that the surface suspended sediment front was of vital importance in determining the shoreward boundary of the phytoplankton bloom area. Light attenuation due to high turbidity became a major factor limiting phytoplankton blooms in turbid nearshore waters. In environments with a strong tidal influence, the surface suspended sediment front was closely related to the bottom river plume front, which gave rise to differences in water stabilization and thus resulted in turbid conditions on either side. The results of this study provided a good reference for understanding the phytoplankton distribution characteristics in estuaries with high turbidity.

Regulation of Algal Bloom Hotspots Under Mega Estuarine Constructions in the Changjiang River Estuary

“…The ecosystem module used a vertical one-dimensional N2P2ZD model which was derived from the Flexible Biological Module in Finite-Volume Coastal Ocean Model (Chen et al, 2006) by Lin (2011). The state variables had two kinds of nutrients [dissolved inorganic nitrogen (DIN) and phosphate], two phytoplankton species (dinoflagellate and diatom), one zooplankton species, and one kind of detritus (Wang et al, 2019a). The settings of the upstream nutrient flux boundary used the data reported by Gao et al (2012).…”

“…Moreover, the initial and open boundary conditions for nutrients were derived from the climatological monthly World Ocean Atlas. For a complete set of biological equations, the readers are referred to Wang et al (2019a).…”

“…The estuarine and coastal zone, which is a key area for human being inhabiting, is economically developed and densely populated. Various natural dynamic processes, including runoff, tide, wind, and shelf circulation, altogether transport nutrients and regulate the turbidity in the estuary, which is essential for algal growth, thereby controlling the aquatic ecosystem (Caffrey et al, 1998;Anderson et al, 2002;Gao and Song, 2005;Tang et al, 2006;Statham, 2012;Fan and Song, 2014;Liu et al, 2015;Wang et al, 2019a). In addition, poorly managed human activities have been recognized as another key driver to cause severe environmental issues, such as eutrophication, harmful algal blooms, hypoxia, and habitat fragmentation, among many others (May et al, 2003;Chen et al, 2017;Zhang et al, 2018;Boesch, 2019).…”

Advances on Coastal and Estuarine Circulations Around the Changjiang Estuary in the Recent Decades (2000–2020)

“…Chen et al (2017) proposed that hypoxia usually occurs in the bottom waters in the eutrophication area with the concentration of surface chlorophyll-a exceeds 3.0 mg L −1 . The formed hypoxia are destructive to the marine ecosystem (Xu et al, 2015;Zhu et al, 2016b) in ECS (Chen et al, 2003;Wang et al, 2012;Qian et al, 2017;Wei et al, 2017a;Zhou et al, 2017;Zhang et al, 2018b;Wang et al, 2019c;Wang et al, 2019b;Zhang et al, 2019c;Chen et al, 2020a) and the YS (Zhai et al, 2020). It is generally considered that during the recent decades from 1980s to 2010s, there is an accelerated anthropogenic nutrient loading, and occurrence of eutrophication Wang et al, 2016Wang et al, , 2018 and hypoxia (Wu et al, 2020) in the coastal waters (Liu et al, 2018;Chen J. et al, 2020).…”

The Effects of Suspended Particulate Matter, Nutrient, and Salinity on the Growth of Amphidinium carterae Under Estuary Environmental Conditions

“…It is clear that both nutrient and SPM play an important role in affecting the phytoplankton growth in estuarine and coastal waters. Although the separate effects of SPM and nutrient on phytoplankton growth have previously been described, the studies investigating their combined effects are still limited (Leonardos and Geider, 2004;Domingues et al, 2017;Wang et al, 2019), especially considering their substantial variability in dynamic coastal environments.…”