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

Abstract

The riverine nutrient inputs to the ocean reflects land-use changes and can affect the health of coastal environments over time, especially for a highly-anthropogenically influenced river-estuary-shelf system. To investigate the impact of riverine inputs on the Changjiang Estuary ecosystem at a multi-decadal time scale where long-term observations are limited, we built a three-dimensional physics-biogeochemistry-coupled model system based on the Finite-Volume Community Ocean Model (FVCOM) and the European Regional Shelf Ecosystem Model (ERSEM). Our model successfully simulated the temporal and spatial nutrient variabilities in the river-estuary-shelf con7tinuum from 1960 to 2018. The results showed increasing trends of nitrate and phosphate and fluctuating silicate variability, thereby leading to rising nitrogen (N) to phosphorus (P) ratios and decreasing silicon (Si) to N and P ratios. Such changes in the stoichiometric relationship of nutrient species also alter the community structure of the primary producers in estuaries. Our model showed a general increase of diatoms over the 59 years, corresponding to decreased proportions of micro-phytoplankton and pico- phytoplankton. With different backgrounds of light and nutrient limitations in the river and inner shelf, our model suggests that the trend of the diatom proportion in the light-limited river mouth is more associated with silicate variability, with decreased diatom proportions occurring in the 2000s. Our model relates the hydroclimate, nutrient load, and biogeochemical cycling, reproducing estuarine ecosystem variability and clarifying issues such as the causality of the ecosystem interactions.

Persistent eutrophication and hypoxia in the coastal ocean

“…Changes in the riverine nutrient inputs have significantly increased DIN and DIP concentrations in coastal surface waters since the mid-1990s (Figure S4c) (Wang et al, 2018a). In addition to nutrient enrichment, the N:P ratio increased at a rate of 0.35 yr -1 , while the Si:N decreased at a rate of 0.04 yr -1 from 1960-2018 (Dai et al, 2011;Shi et al, 2022). The altered nutrient stoichiometry in the Changjiang, superimposed over the regional warming at a fastest global warming rate in the world (Liu et al, 2016;Xiao et al, 2018), has clearly induced the algal species shift with an increasing percentage of dinoflagellates abundance up to over 25% at the cost of a decline in diatoms since the 2000s (Chen et al, 2020;Chen et al, 2021b;Shi et al, 2022) and increased HABs (e.g., dinoflagellates…”

“…A weak but statistically significant negative correlation is found between the DO minima and hypoxic areas (R=0.66, p<0.01; Figure 4c). The low DO conditions and extensive hypoxia off the Changjiang Estuary have not receded, although both the nutrient inputs and the phytoplankton biomass show declining trends recently due to a reduction in human inputs and the increasing damming upstream (Figure S4d) (Shi et al, 2022).…”

Role of phosphorus in the seasonal deoxygenation of the East China Sea shelf

“…Silicate (Si) is not included in the model as it is assumed to be non-limiting. During the simulation period, observed Si / N and Si / P were close to and above Redfield, respectively (Shi et al, 2022). The single phytoplankton group includes all phytoplankton types and therefore does not differentiate between diatoms and dinoflagellates.…”

“…Typically, primary production is limited by light in the Changjiang estuarine and sediment plume waters, whereas nutrient limitation may occur in the transition zone and further offshore (Harrison et al, 1990;Li et al, 2021;Zhu et al, 2009). As elsewhere, the unbalanced supply of DIN and DIP from the Changjiang (Liu et al, 2003(Liu et al, , 2018Shi et al, 2022) led to the development of P limitation on the shelf (Li et al, 2009;Wang et al, 2015). Based on laboratory studies and local observations, there are indications that high N : P and the onset of P limitation on the ECS shelf alter the species composition of phytoplankton blooms (Li et al, 2009;Ou et al, 2020;Xing et al, 2016;Zhou et al, 2008).…”

“…Coastal eutrophication occurs when excess nitrogen (N) and phosphorus (P) nutrients, associated mainly with the use of synthetic fertilizer in agriculture, wastewater discharge in urban areas, and livestock farming (Alexander et al, 2008;Chen et al, 2020) reach the coastal ocean where they stimulate primary production, the development of harmful algal blooms (Glibert et al, 2014;Heisler et al, 2008), and subsequently respiration and O 2 depletion in subsurface waters (Fennel and Testa, 2019). Low O 2 can have detrimental effects on the benthos (Levin et al, 2009) and the food web (Kidwell et al, 2009) and ultimately have economic consequences for fisheries (Rabalais and Turner, 2001;Smith et al, 2017).…”