Dissolved silicate (DSi) is an important nutrient in coastal water, which is used by planktonic diatoms for cell division and growth. In this study, surface water samples were collected in the eutrophic Zhanjiang Bay (ZJB) in 2019, covering a seasonal variation of coastal water and land-based source water discharge. The spatiotemporal DSi distribution, land-based sources flux input and behaviors in ZJB were studied and discussed. The results show that the DSi concentration had significant differences in spatiotemporal scale. The average concentration of DSi in ZJB was 38.00 ± 9.48 μmol·L⁻¹ in spring, 20.23 ± 11.27 μmol·L⁻¹ in summer, 12.48 ± 1.42 μmol·L⁻¹ in autumn and 11.96 ± 4.85 μmol·L⁻¹ in winter. The spatiotemporal DSi distribution showed a decreasing gradient from the top to the mouth of ZJB, which was affected by land source inputs and hydrodynamics. The land-based sources’ input concentration of DSi in ZJB ranged from 0.021 to 0.46 mol·L⁻¹, with an average of 0.14 mol·L⁻¹, and the total annual flux of DSi was 1.06 × 10⁹ mol, comprising up to 8.28%, 41.55% and 50.17% in dry, normal, and wet seasons, respectively. The Suixi River contributed the highest DSi flux proportion in all seasons. The DSi in land-based source water was mainly affected by water flow discharge, diatom uptake and impacts from anthropogenic activities. Compared with dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP), the DSi was the limitation nutrient in ZJB. Additionally, the DSi concentration in the coastal water was negatively correlated with salinity. The seasonal DSi/DIN and DSi/DIP ratios in land-based sources discharge water was significantly higher than that in coastal water (p < 0.05). Land-based sources of DSi input played an important role in nutrients composition that sustained diatoms as the dominant species in ZJB.

In marine environments, urea is an important component of the biogeochemical cycle of nitrogen. The autochthonous and allochthonous sources (rivers, aquaculture, waste water input, etc.) of urea play a key role in urea cycles in adjacent coastal waters. Because urea is a specific marker to trace the sewage fluxes in coastal waters, we investigated urea associated with terrestrial source input and coastal water in Zhanjiang Bay (ZJB) during the time from November 2018 to July 2019, and the spatiotemporal urea distribution and the bioavailability of dissolved organic nitrogen (DON) based on urea concentration in the ZJB were explored. The results showed that the urea enrichment in coastal water was mainly due to discharge from urban sewage systems, rivers, and coastal aquaculture. The concentration of urea ranged from 1.14 to 5.53 μmol·L⁻¹, and its mean value was 3.13 ± 1.02 μmol·L⁻¹ in the ZJB. The urea concentration showed a significantly different seasonal variation in the ZJB (p < 0.05), and the highest and lowest concentrations were found in November 2018 and April 2019, respectively. Its high value appeared in the north and northeast of the ZJB, which were polluted by coastal aquaculture and agriculture fertilizer utilization. The range of urea concentration of terrestrial source inputs in the ZJB was 1.31–10.29 μmol·L⁻¹, and the average urea concentration reached 3.22 ± 0.82 μmol·L⁻¹. Moreover, the total urea flux surrounding the ZJB was 2905 tons·year⁻¹. The seasonal terrestrial source of urea flux contributions had significant seasonal variation in wet, normal, and dry seasons (p < 0.05). The ZJB was subjected to a large flux of urea by estuaries and sewage outlet discharges. The seasonal urea concentration in all stations (>1 μmol·L⁻¹) indicated that urea in the ZJB may have a bioavailable DON source. As a bioavailable nitrogen source, the ability of terrestrial source-derived urea to increase eutrophication should not be ignored in ZJB.

The increasing riverine nutrient sources have significantly affected the ecological environment of estuaries and coastal waters, resulting in deteriorating land-sea water quality and intensified eutrophication. However, the effects of river input flux on spatiotemporal patterns of total nitrogen (TN) and total phosphorus (TP) were poorly understood in the Pearl River Estuary (PRE). In this study, the spatiotemporal patterns of TN and TP concentrations and river input flux of PRE were studied based on the seasonal nutrients monitoring data obtained for the rivers and estuary in 2019. The results showed the spatiotemporal patterns of the TN and TP concentrations in the eight rivers of the PRE were different, and the annual average concentrations of TN and TP in the rivers entering the PRE were 207.18 ± 105.13 and 3.51 ± 1.70 μmol/L, respectively. The annual river TN and TP fluxes discharged into the PRE were 8.61 × 10¹⁰ and 1.55 × 10⁹ mol/year, respectively. In addition, the significantly decreasing trends in of TN and TP concentrations from upper estuary to offshore seawater were observed in the PRE, which implied TN and TP showed conservative behaviour in all season and only the dry season, respectively. Moreover, the annual average concentrations of TN and TP in the estuary were 70.64 ± 10.10 and 1.67 ± 0.78 μmol/L, respectively. The annual average ratios of TN/TP for rivers, freshwater, mixed and seawater were 59.31 ± 5.98, 59.45 ± 3.75, 45.73 ± 21.27 and 330.94 ± 434.71 respectively, which were higher than the Redfield ratios. These results indicated that the water quality in most areas of the PRE is significantly polluted and that the TN and TP in seawater were significantly influenced by river inputs. Therefore, monitoring and management of unified TN and TP nutrient indicators discharge from rivers entering the sea can be strengthened in the PRE, and eutrophication mitigation strategy should be established and implemented across river-estuary-coast continuum systems in the Great Bay Area.