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.

Climate change and anthropogenic pressures have significantly affected coastal environments. This study obtained historical data on harmful algal blooms (HABs) and nutrient patterns over a 30-year period to explore responses to long-term climate change and anthropogenic pressure indicators. Although the surrounding area has achieved great economic success over the past 30 years, the Zhanjiang Bay (ZJB) has been seriously affected by various pollutants and is threatened by increasing eutrophication and HABs due to climate change and anthropogenic pressures. In the ZJB, HABs rarely occurred before the 1980s but have occurred periodically and frequently since the 2000s. The largest HAB covered a cumulative area of 310 km2 in 2005. Most of the HABs occurred during spring. Additionally, the dominant phytoplankton species were Skeletonema costatum and Phaeocystis globosa, accounting for 37.50 and 43.75% of the HABs observed, respectively. Anthropogenic pressures have caused the nutrient regime to significantly increased in the ZJB over the past three decades (P &lt; 0.05). Specifically, the concentration of dissolved inorganic nitrogen (DIN) increased threefold from the beginning of the 1990 to 2019 period, while the dissolved inorganic phosphorus (DIP) concentration increased 21-fold. Unsynchronized variation in nutrient patterns has led to changes in the composition of nutrients, and the ZJB ecosystem has shifted from a P-limited oligotrophic state before the 2000s to an N-limited eutrophic state. Anthropogenic pressure indicators showed a significant linear correlation with nutrients (P &lt; 0.05), but climate change indicators did not play a direct role in the eutrophication problem in the ZJB during this period (P &gt; 0.05). Therefore, integrated land-ocean environment management should be introduced to reduce land-based pollution sources, mitigate eutrophication, and curb the blooms of harmful algae in the ZJB.

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Noctiluca scintillans is an eurythermal and euryhaline dinoflagellate with large cell size (300-800 μm) (Elbrächter & Qi, 1998;Harrison et al., 2011;Piontkovski et al., 2021), which is well known to produce beautiful bioluminescence under perturbation. People call this bioluminescence phenomenon "blue tears", and therefore attracted countless tourists to watch this magical scene. However, N. scintillans is also one of the most common harmful species worldwide and has caused harmful algal blooms (HABs) in many coastal waters includ-

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