An evolving marine environment and its driving forces of algal blooms in the Southern Yellow Sea of China

Abstract

Continuing global warming and intensification of human activities have substantially disturbed the balance of coastal marine ecosystems, potentially creating favorable conditions for algal blooms. Using serial remote sensing data and various national and provincial statistics, we investigated the spatial and temporal variations of the environmental driving forces for algal blooms in the Southern Yellow Sea between 2003 and 2017. The findings suggest that (1) Continual warming was observed in the Southern Yellow Sea. The study area evidenced more than three times the warming speed (0.41 °C/decade) of the global oceans (0.12 °C/decade) during the same period. There was an apparent warming zone in the region where macroalgal blooms tended to spread, with a heating of 1.0–1.5 °C (May–June). (2) Nutrient loadings have diverse patterns, characterized by fast-growing aquaculture activities and declining nutrients from land-based agriculture fertilizers and sewage discharge (based on published national and provincial statistics). (3) Growing expansion of algal blooms in the Southern Yellow Sea was confirmed by the relative increases in average May–June chlorophyll-a concentration of 46.7% and floating biomass area from 3.3% in 2003 to 13.4% in 2017. (4) While spatial correlation analysis showed a positive influence of the ocean surface temperature on chlorophyll-a, their relatively moderate (r = 0.40, p < 0.15) and declining correlations suggest that nutrient enrichment could be comparatively more influential on macroalgal blooms. Nutrient loading from the discharge of wastewater sourced from coastal aquaculture and organic residuals from land-sourcing sewage and industrial pollution, even though declining as reported, is still upholding a high level of nutrient enrichment in the study area. In addition, the fixed facilities for seaweed mariculture in the region provide vast breeding surfaces for algae. Consequently, the Southern Yellow Sea has become an ideal marine area for algal blooms.