Interannual variabilities of nutrients and phytoplankton off the Changjiang Estuary in response to changing river inputs

Abstract

<p>Coastal ecosystems are strongly influenced by terrestrial and oceanic inputs of water, sediment and nutrients. Terrestrial nutrients in freshwater discharge are particularly important for mega-river estuaries. A remarkable increase in nutrient loads transported from the Changjiang River through the estuary to the shelf has been observed from 1999 to 2016. The Finite-Volume Community Ocean Model and the European Regional Seas Ecosystem Model were coupled to assess the interannual variability of nutrients and phytoplankton under these flux dynamics. The system exhibited a rapid ecosystem response to the changing river nutrient contribution. Singular vector decomposition (SVD) analysis demonstratedthat abundant nitrate from the river was diluted by low-nitrate water transported from the oceanic domain. In contrast, phosphate exhibited local variation, suggesting the estuarine ecosystem was phosphate-limited. The SVD results showed that there were no significant correlations between the suspended sediment and nutrients, but a significant correlation between sediment and phytoplankton. The nutrient structure of the river discharge resulted in the dominance of non-diatom species in the phytoplankton bloom from spring to autumn. The ratio of diatom and dinoflagellate populations showed a rapid feedback response to the strong oscillations in river nutrient input. High diatom primary production occurred near the sediment front, whereas dinoflagellate bloom extended significantly offshore. Both diatoms and dinoflagellates had major peaks representing spring blooms from empirical orthogonal function Mode 1 and 2, and secondary peaks from Mode 2 in the autumn, which coincided with the autumn bloom.</p>

Long-term response of an estuarine ecosystem to drastic nutrients changes in the Changjiang River during the last 59 years: A modeling perspective

“…River discharge used in this study was collected at DT (Figure 1A) (http://xy.cjh.com.cn/). Nutrient data at the DT station and nearby sites were collected from different sources, i.e., previous studies conducted from 1960 to 2019 (Dai et al, 2011;Gao et al, 2012;Zhu et al, 2014;Li et al, 2021), and our own measurements made since 2013 (Ge et al, 2020a;Ge et al, 2020b;Ge et al, 2021). A detailed description of the nutrient sources can be found in Table 1.…”

“…Different models have been developed to compensate for the lack of empirical data and study how ecosystems respond to environmental changes (Riley, 1949;Fasham et al, 1990;Sarmiento et al, 1993;Franks and Chen, 2001;Spitz, 2003;Gruber et al, 2006). In Ge et al (2020a), the Finite-Volume Community Ocean Model (FVCOM) and the European Regional Shelf Ecosystem Model (ERSEM), was used to study the effect of offshore sediment front on seasonal marine ecosystem dynamics; the interannual variability of nutrient and phytoplankton in the Changjiang Estuary and shelf from 1999 to 2017 were discussed based on this model in Ge et al (2020a). However, none of these studies involved simulations on a multi-decadal scale.…”

New Insights Into the Non‐Conservative Behaviors of Nutrients Triggering Phytoplankton Blooms in the Changjiang (Yangtze) River Estuary

“…These methods applied to studying non‐conservative behaviors include box budget calculations (Gao et al., 2008a), partial correlation analysis (Gao et al., 2008b), harmonic analysis (Gao et al., 2011), comparison among detached Changjiang diluted water (CDW) patches with the same or similar salinity values but quite different transport distances (Gao et al., 2015, 2019, 2020; Song et al., 2017), and analysis of water age of seawater masses (Gao & Zong, 2021). Previous studies also used numerical models and stable isotopes to explore the carbon and nutrient dynamics in this region (Ge, Shi, et al., 2020; Liu et al., 2009; Wang et al., 2018; Xu et al., 2020; Yu et al., 2015). In the present study, we applied another method in which we conducted in situ seawater incubation experiments to study the non‐conservative behavior of nutrients in this typical large river estuarine area.…”

“…Algal bloom events frequently occur in the spring and summer in the Changjiang River Estuary and the adjacent ECS shelf (Zhou et al., 2008). These blooms change the status and structure of marine ecosystems and also the biogeochemical cycling of substances that are transported through the environment (Ge, Shi, et al., 2020). However, the mechanisms responsible for the occurrence of algal blooms are still not well addressed in the literature.…”

Unraveling environmental drivers of chlorophyll seasonal and interannual variability in the East China Sea

“…Owing to the interactions of multiple physical, chemical, and biological processes, the timing and magnitude of phytoplankton blooms can vary remarkably (i.e., the annual primary productivity could vary up to 10-fold within ecosystems and 5-fold from year to year) (Cloern et al, 2014). For example, shallow depth, rapid turnover, and higher nutrient supplies from riverine inputs and upwelling could result in much higher primary productivity on the shelves (Chen et al, 2003), which leads to light limitation of photosynthesis in the estuary and slow incorporation of nutrients into phytoplankton biomass (Xu et al, 2013a;Ge et al, 2020); additional factors, such as different types of oceanic fronts, have chemical and biological manifestations as well (Olson et al, 1994;Belkin et al, 2009).…”