Marine ecosystems rely on nutrients, but changes in land imports cause fluctuations. The study focused on the nutrient trends and phytoplankton abundance in Daya Bay, South China Sea, from 1986 to 2020, exploring the impact of natural and human-induced factors on the ecosystem. Dissolved inorganic nitrogen (DIN) concentrations consistently exceeded the optimal threshold for the growth of phytoplankton since the late 1988s. Although DIN showed an insignificant increasing trend from 1986 to 2020, the period from 2010 to 2020 witnessed lower levels. Dissolved inorganic phosphate (DIP) exhibited a significant decreasing trend throughout the studied period, falling below the threshold for phytoplankton growth from 2010 to 2020. A crucial nutrient for the growth of diatoms in coastal waters is dissolved silicate (Cao et al., 2020). The Silicate concentration in Daya Bay remains continually significant declining (p < 0.05) from 1990 to 2020. It was within the range of 1.672 to 5.216 between 2010 and 2020 and remained above the diatom growth threshold of 2.0 μmol L−1 (Brown and Button, 1979; Nelson and Brzezinski, 1990). The value (1.672 μmol L−1) was only observed to be below the threshold in 2013.Concurrently, chlorophyll-a (Ch-a) concentration increased from 1985 to 2020, indicating a rising trend in phytoplankton abundance. The findings emphasize the influence of human activities on nutrient levels and phytoplankton in the bay's ecosystem, illustrating the complex interplay between natural processes and anthropogenic impacts on marine environments.


 

long-term; Daya Bay;Nutrient