Chlorophyll-a Variation Trends in Marginal Seas: Assessing the Impact of Global warming and Anthropogenic Activities Using Time Series Satellite Data (1998–2020)

Abstract. Global warming has been identified as the main cause of the decline of surface chlorophyll-a (Chl-a) concentrations in the oceans. Conversely, an increase in Chl-a concentration has been observed in a number of marginal seas over recent decades due to increasing anthropogenic input of key nutrients. However, with the intensification of global warming, its impact on Chl-a in coastal waters has been rarely studied, with the superimposed effects of human regulation of nutrients emissions. This study utilized time series of oceanic color satellite data from 1998 to 2020 to examine the spatio-temporal distribution of Chl-a in a range of marginal sea areas, and its relationship with environmental factors, particularly with sea surface temperature (SST), photosynthetically active radiation (PAR) and surface wind speed (SWS) are considered as well. The results suggested that the sea areas examined with varying mixing and water exchange characteristics and degrees of human influence have differing responses (in terms of their Chl-a trends) to increasing SST. Specifically, eutrophic closed seas with weak hydrodynamic exchange capacity, like the Bohai Sea, increasing SST did not apparent suppress Chl-a concentration, but instead continuous increase in Chl-a was observed in the central of the sea. In comparison, the open marginal seas examined show strong negative relationships between SST and Chl-a with distance offshore regardless of the degree of pressure from human activities, indicating that expected global warming effects driving reductions in Chl-a are extending to nearshore / marginal sea areas. This trend may be exacerbated due to stricter environment management policies imposed in recent years which have reduced anthropogenic nutrient inputs. Distinct from the above effect of global warming, PAR and SWS shape Chl-a in ways that are strongly modulated by geography and climate. PAR is the dominant positive control only in the Amazon estuary, where equatorial cloudiness and high turbidity create a light-limited regime, so any PAR increase directly stimulates phytoplankton. In mid-latitude open waters, PAR is secondary to SST: its seasonal rise is coupled to SST and therefore correlates negatively with Chl-a once thermal stratification reduces nutrient supply. SWS emerges as a key driver in the three open regimes (East China Sea >US East Coast> Amazon shelf), through injecting nutrient-rich cold deep water and episodically raise Chl-a. Inside the two enclosed seas (Bohai, Gulf of Mexico), correlations with both PAR and SWS are weak (│r│ < 0.2); Thus, PAR and SWS control Chl-a in a complex way, but both are more or less links to SST and nutrients input. This study highlights the complex interaction between primary production, SST, nutrient inputs and exchange, and environmental protection controls under the dual pressures of changes in human activity and coastal development, and global warming.