Chlorophyll a variation trends in marginal seas: assessing the impact of global warming and anthropogenic activities using time-series satellite data (1998–2020)

Abstract

Abstract. Global warming has been identified as the primary cause of the decline of surface chlorophyll a (Chl a) concentrations in the oceans. Conversely, increasing Chl a concentrations have been observed in a number of marginal seas over recent decades due to the increasing anthropogenic input of key nutrients. With the intensification of global warming, however, its impact on Chl a in coastal waters along with the superimposed effects of human regulation of nutrients emissions has been rarely studied. To address this research gap, in this study, we provided a comparative analysis of enclosed versus open marginal seas, revealing divergent physiological and ecological responses to rising sea-surface temperature (SST) across different nutrient regimes. We utilized time series of ocean color satellite data from 1998 to 2020 to examine the spatiotemporal distribution of Chl a in a range of marginal seas, and we considered its relationship with environmental factors, in particular with SST, photosynthetically active radiation (PAR), and surface wind speed (SWS). The results suggested that the sea areas examined, with their varying mixing and water exchange characteristics and degrees of human influence, have had different responses in terms of their Chl a trends to increasing SST. Specifically, in eutrophic enclosed seas with weak hydrodynamic exchange capacity, such as the Bohai Sea, increasing SST did not suppress Chl a concentration; instead, we observed a continuous increase in Chl a in the center of the sea. In comparison, the open marginal seas examined showed strong negative relationships between SST and Chl a with increasing distance offshore regardless of the degree of pressure from human activities. This result indicated that the expected global warming effects driving reductions in Chl a have been extending to nearshore and marginal sea areas. This trend may be exacerbated by stricter environmental management policies imposed in recent years, which have reduced anthropogenic nutrient input. Distinct from the noted effect of global warming, PAR and SWS have shaped Chl a in ways that are strongly modulated by geography and climate. PAR is the dominant positive driver only in the Amazon Estuary, where equatorial cloudiness and high turbidity create a light-limited regime, so any increase in PAR directly stimulates phytoplankton. In mid-latitude open waters, PAR is secondary to SST. Its seasonal rise is coupled to SST and therefore it is negatively correlated with Chl a after thermal stratification reduces nutrient supply. SWS has emerged as a key driver in the three open regimes (the East China Sea > the Eastern Coastal Waters of the United States > the Amazon Estuary) by injecting nutrient-rich cold deep water and episodically raising Chl a. Inside the two enclosed seas (the Bohai Sea and the Gulf of Mexico), correlations with both PAR and SWS have been weak (|r| < 0.2). Thus, the control PAR and SWS exert over Chl a is complex, but both are linked to SST and nutrient input. In this study, we highlighted the complex interactions among primary production, SST, nutrient input and hydrodynamic exchange, and environmental protection controls under the dual pressures of changes in human activity and coastal development combined with global warming.