Nutrients and carbon play important roles in algal bloom and development. However, nutrients and carbon interactions in the period of the spring algal bloom are not well understood. The aim of this study is to explore the nutrients and carbon interactions in the period of the spring algal bloom covering an urban Jinsha Bay (JSB) coastal water in Zhanjiang Bay (South China Sea) using in situ multidiscipline observation. The results showed that the average concentration of total nitrogen (TN), total phosphorus (TP), and dissolved silicon (DSi) was 97.79 ± 26.31 μmol/L, 12.84 ± 4.48 μmol/L, and 16.29 ± 4.00 μmol/L in coastal water, respectively. Moreover, the average concentration of total dissolved carbon (TDC), dissolved inorganic carbon (DIC) and organic carbon (DOC) in JSB was 2187.43 ± 195.92 μmol/L, 1516.25 ± 133.24 μmol/L, and 671.13 ± 150.81 μmol/L, respectively. Furthermore, the main dominant species were Phaeocystis globosa and Nitzschia closterium during the spring algal bloom. Additionally, the correlation analysis showed salinity (S) was significantly negatively correlated with nutrients, indicating that nutrients derived from land-based sources sustained spring algal bloom development. However, as the major fraction of TDC, DIC was significantly positively correlated with S, which was mainly derived from marine sources. Besides, the algal density showed a significant positive correlation with temperature (T) (p < 0.001) and dissolved oxygen (DO) (p < 0.001), but a significant negative correlation with DIC (p < 0.05), suggesting that spring algal blooms may be simulated by water T increase, and then large amounts of DIC and nutrients were adsorbed, accompanying DO release through photosynthesis in coastal water. This study revealed nutrients and carbon interactions in the spring algal bloom of urban eutrophic coastal water, which has implications for understanding the nutrients and carbon biogeochemical cycle and algal bloom mitigation under climate change and anthropogenic pressures in the future.

Human activities have altered global nutrient cycling and have significantly changed marine systems. This is evidenced by the significant changes in nitrogen and phosphorus availability. The Maowei Sea (MWS) is the largest oyster culture bay in southwest China. From August 2018 to May 2019, the spatial and temporal nutrient concentrations and fluxes in MWS using system-wide scale seasonal data were assessed from river estuaries and adjacent coastal waters. The annual average concentrations of total nitrogen (TN) and total phosphorus (TP) in the three estuaries of Maolingjiang River (MLJR), Dalanjiang River (DLJR) and Qinjiang River (QJR) were 3.00 mg/L and 0.183 mg/L, respectively. Therein, the highest TN and TP concentrations were in DLJR, the lowest TN concentration was in MLJR, and the lowest TP concentration was in QJR. DIN and DIP were the main forms of TN and TP, accounting for 80.9% and 59.4%, respectively. The main form of DIN in MLJR and QJR was NO3−, accounting for 86.8% and 84.4%, respectively, while the main form of DIN in DLJR was NH4+, accounting for 55.9%. The annual flux of pollutants discharged into MWS from the three estuaries is 10,409.52 t for TN and 556.21 t for TP. The month with the largest contribution to the annual load was July, accounting for 29.2% and 24.2% of TN and TP, respectively, and the fluxes of TN and TP were significantly different among the three seasons (p < 0.05). The annual average concentrations in the surface waters of the MWS were 1.07 mg/L for TN and 0.129 mg/L for TP, and there were significant differences (p < 0.05) in the concentrations of TN and TP among the three seasons. The annual average N/P ratios of the river water and seawater were 43 and 18, respectively, which were higher than the Redfield ratio (N/P = 16), indicating that the growth of phytoplankton in MWS may be limited by phosphorus. Eutrophication owing to nutrient pollution in the three estuaries may be persistent in adjacent coastal waters, and land–ocean integrated mitigation measures should be taken to effectively improve the water quality in the river estuary and coastal water.

As heavy metals are easy to accumulate and have strong biological toxicity, they pose a potential threat to human health by entering the human body through the cumulative effect of marine life. Land-based input is an important source of heavy metals in the ocean, which has a great influence on coastal water quality. In this study, the spatial distribution characteristics of heavy metals (Zn, Cu, Cd, Pb, Cr, As) in the coastal waters of the desulfurization process outlet of a power plant in Zhanjiang Bay were investigated, and the enrichment behavior of heavy metals by organisms (oysters and barnacles) were also studied. The results showed that, before the seawater desulfurization system was closed, there were high concentrations of heavy metals (Cu, Zn, Cd, Pb and Cr) in the surface seawater near the drainage outlet. The concentrations of these heavy metals in the surface seawater were higher than those in the bottom seawater within 100 m of the drainage outlet. After the seawater desulfurization system was closed, the average concentrations of Cu, Cr and As in seawater at each station decreased by 17.04%, 37.52% and 29.53%, respectively, while the average concentrations of Zn, Cd and Pb increased by 17.05%, 32.87% and 48.77%, respectively. Single factor pollution index (SFI) and bio-concentration factor (BCF) showed that there was a potential high accumulation risk of Zn in oysters and barnacles near the drainage outlet of desulfurization wastewater (0.5 < SFI < 1 and BCF > 1000). The SFI and BCF of each metal in oysters and barnacles decreased with the increase in distance from the drainage outlet. Generally, the coastal water quality of desulfurization process drainage area in Zhanjiang Bay were below the class Ⅱof the “Seawater quality standard” (GB 3097-1997) of China. However, the heavy metals content in seawater and organisms near the drainage outlet is slightly higher. This suggested that if the seawater desulfurization process runs for a long time, it will have a negative impact on the coastal water and organisms.