Variability and drivers of plankton community metabolism in Yazhou Bay: A spatial and seasonal assessment.

Planktonic carbon metabolism is often used as a critical indicator to assess the health of marine ecosystems and their carbon sequestration capacity. In this study, we examined the spatial variability and driving factors of plankton metabolism, including gross primary production (GPP), respiration (CR), and their balance, net community production (NCP), in Yazhou Bay, a representative tropical open bay. The results revealed that the distribution of GPP and CR in Yazhou Bay is influenced by terrestrial inputs, seasonal rainfall, and oceanic currents, resulting in pronounced seasonal and spatial variations. In winter and spring, nitrogen limitation suppressed chlorophyll a (Chl a) and GPP, whereas in summer silicon limitation-coupled with intensified CR -led to a strongly heterotrophic state with an average water depth-integrated NCP of 1808.94 ± 577.34 mg C m-2 d-1. In autumn, increased rainfall alleviated nutrient limitations and increased GPP, resulting in a strongly autotrophic water column with integrated NCP of 988.20 ± 947.17 mg C m-2 d-1. Overall, the coastal waters of Yazhou Bay function as a net source of carbon dioxide, with an annual average water depth-integrated NCP of -258.38 ± 1154.52 mg C m-2 d-1. Plankton in the main contributor of the ecosystem, while the respiration of the benthic community accounts for approximately 3.73 % of the respiratory metabolism in the pelagic-benthic system. The characteristics of carbon metabolism and the associated hydrological conditions indicate a low risk of algal blooms and hypoxia in Yazhou Bay. However, given its economic importance and increasing anthropogenic pressures, attention should be paid to potential risks of regional eutrophication and abnormal metabolic processes.