Ocean productivity is the foundation of marine food web, which continuously removes atmospheric carbon dioxide and supports life at sea and on land. Spatio-temporal variability of net primary productivity (NPP), sea surface temperature (SST), sea surface salinity (SSS), mixed layer depth (MLD), and euphotic zone depth (EZD) in the northern Bay of Bengal (BoB) during three monsoon seasons were examined in this study based on remote sensing data for the period 2005 to 2020. To compare the NPP distribution between the coastal zones and open BoB, the study area was divided into five zones (Z1−Z5). Results suggest that most productive zones Z2 and Z1 are located at the head bay area and are directly influenced by freshwater discharge together with riverine sediment and nutrient loads. Across Z1−Z5, the NPP ranges from 5315.38 mg/(m²·d) to 346.7 mg/(m²·d) (carbon, since then the same). The highest monthly average NPP of 5315.38 mg/(m²·d) in February and 5039.36 mg/(m²·d) in June were observed from Z2, while the lowest monthly average of 346.72 mg/(m²·d) was observed in March from Z4, which is an oceanic zone. EZD values vary from 6−154 m for the study area, and it has an inverse correlation with NPP concentration. EZD is deeper during the summer season and shallower during the wintertime, with a corresponding increase in productivity. Throughout the year, monthly SST shows slight fluctuation for the entire study area, and statistical analysis shows a significant correlation among NPP, and EZD, overall positive between NPP and MLD, whereas no significant correlation among SSS, and SST for the northern BoB. Long-term trends in SST and productivity were significantly positive in head bay zones but negatively productive in the open ocean. The findings in this study on the distribution of NPP, SST, SSS, MLD, and EZD and their seasonal variability in five different zones of BoB can be used to further improve the management of marine resources and overall environmental condition in response to climate changes in BoB as they are of utmost relevance to the fisheries for the three bordering countries.

Eutrophication caused by inputs of excess nitrogen (N) has become a serious environmental problem in Hangzhou Bay (China), but the sources of this nitrogen are not well understood. In this study, the August 2019 distributions of salinity, nutrients [nitrate (${{\rm {NO}}_3^-} $), nitrite, ammonium, and phosphate], and the stable isotopic composition of ${{\rm {NO}}_3^-} $ (δ¹⁵N and δ¹⁸O) were used to investigate sources of dissolved inorganic nitrogen (DIN) to Hangzhou Bay. Spatial distributions of nitrate, salinity, and nitrate δ¹⁸O indicate that the Qiantang River, the Changjiang River, and nearshore coastal waters may all contribute nitrate to the bay. Based on the isotopic compositions of nitrate in these potential source waters and conservative mixing of nitrate in our study area, we suggest that the ${{\rm {NO}}_3^-} $ in Hangzhou Bay was likely derived mainly from soils, synthetic N fertilizer, and manure and sewage. End-member modeling indicates that in the upper half of the bay, the Qiantang River was a very important DIN source, possibly contributing more than 50% of DIN in the bay head area. In the lower half of the bay, DIN was sourced mainly from strongly intruding coastal water. DIN coming directly from the Changjiang River made a relatively small contribution to Hangzhou Bay DIN in August 2019.

The sinking of diatoms is critical to the formation of oceanic biological pumps and coastal hypoxic zones. However, little is known about the effects of different nutrient restrictions on diatom sinking. In this study, we measured the sinking velocity (SV) of Thalassiosira weissflogii using a new phytoplankton video observation instrument and analyzed major biochemical components under varying nutrient conditions. Our results showed that the SV of T. weissflogii under different nutrient limitation conditions varied substantially. The highest SV of (1.77 ± 0.02) m/d was obtained under nitrate limitation, significantly surpassing that under phosphate limitation at (0.98 ± 0.13) m/d. As the nutrient limitation was released, the SV steadily decreased to (0.32 ± 0.03) m/d and (0.15 ± 0.05) m/d, respectively. Notably, under conditions with limited nitrate and phosphate concentrations, the SV values of T. weissflogii significantly positively correlated with the lipid content (P < 0.001), with R² values of 0.86 and 0.69, respectively. The change of the phytoplankton SV was primarily related to the intracellular composition, which is controlled by nutrient conditions but did not significantly correlate with transparent extracellular polymer and biosilica contents. The results of this study help to understand the regulation of the vertical sinking process of diatoms by nutrient restriction and provide new insights into phytoplankton dynamics and their relationship with the marine nutrient structure.

Hypoxia off the Changjiang River Estuary has been the subject of much attention, yet systematic observations have been lacking, resulting in a lack of knowledge regarding its long-term change and drivers. By revisiting the repeated surveys of dissolved oxygen (DO) and other relevant hydrographic parameters along the section from the Changjiang River Estuary to the Jeju-do in the summer from 1997 to 2014, rather different trends were revealed for the dual low-DO cores. The nearshore low-DO core, located close to the river mouth and relatively stable, shows that hypoxia has become more severe with the lowest DO descending at a rate of −0.07 mg/(L·a) and the thickness of low-DO zone rising at a rate of 0.43 m/a. The offshore core, centered around 40-m isobath but moving back and forth between 123.5°–125°E, shows large fluctuations in the minimum DO concentration, with the thickness of low-DO zone falling at a rate of −1.55 m/a. The probable factors affecting the minimum DO concentration in the two regions also vary. In the nearshore region, the decreasing minimum DO is driven by the increase in both stratification and primary productivity, with the enhanced extension of the Changjiang River Diluted Water (CDW) strengthening stratification. In the offshore region, the fluctuating trend of the minimum DO concentration indicates that both DO loss and DO supplement are distinct. The DO loss is primarily attributed to bottom apparent oxygen utilization caused by the organic matter decay and is also relevant to the advection of low-DO water from the nearshore region. The DO supplement is primarily due to weakened stratification. Our analysis also shows that the minimum DO concentration in the nearshore region was extremely low in 1998, 2003, 2007 and 2010, related to El Niño signal in these summers.

Petroleum hydrocarbon pollution is a global concern, particularly in coastal environments. Polycyclic aromatic hydrocarbons (PAHs) are regarded as the most toxic components of petroleum hydrocarbons. In this study, the biomonitoring and ranking effects of petroleum hydrocarbons and PAHs on the marine fish model Oryzias melastigma embryos were determined in the Jiulong River Estuary (JRE) and its adjacent waters in China. The results showed that the levels of petroleum hydrocarbons from almost all sites met the primary standard for marine seawater quality, and the concentrations of the 16 priority PAHs in the surface seawater were lower compared with those in other coastal areas worldwide. A new fish expert system based on the embryotoxicity of O. melastigma (OME-FES) was developed and applied in the field to evaluate the biological effects of petroleum hydrocarbons and PAHs. The selected physiological index and molecular indicators in OME-FES were appropriate biomarkers for indicating the harmful effects of petroleum hydrocarbons and PAHs. The outcome of OME-FES revealed that the biological effect levels of the sampling sites ranged from level Ⅰ (no stress) to level Ⅲ (medium stress), which is further corroborated by the findings of nested analysis of variance (ANOVA) models. Our results suggest that the OME-FES is an effective tool for evaluating and ranking the biological effects of marine petroleum hydrocarbons and PAHs. This method may also be applied to evaluate other marine pollutants based on its framework.

Zhanjiang Bay is a major aquaculture area in China with many types of mariculture products (such as oysters, fish, and shrimp). The culture area and shrimp output in Zhanjiang Bay are ranked first in China. We investigated the total organic carbon (TOC), total nitrogen (TN), TOC/TN ratio, and stable isotopes (δ¹³C and δ¹⁵N) of the fish and shrimp feed, fish and shrimp feces, and sedimentary organic matter (SOM) in and around different aquaculture areas of northeastern Zhanjiang Bay to study the impact of aquaculture activities on SOM. The average TOC contents of fish and shrimp feed were 39.20% ± 0.91% and 39.29% ± 0.21%, respectively. The average TOC content in the surface sediments of the oyster culture area, the mixed (fish and shrimp) culture area, and the cage fish farm area were 0.66%, 0.88% ± 0.10%, and 0.58% ± 0.19%, respectively, which may indicate that mixed culture had a greater impact on SOM. The relatively high TOC and TN contents and relatively low TOC/TN ratios, and δ¹⁵N values in the upper layer of the core sediment in the mixed culture area could also support the significant influence of mixed culture. The average δ¹³C and δ¹⁵N values of fish and shrimp feed were −20.6‰ ± 2.2‰ and 1.8‰ ± 1.2‰, respectively, which were different from the isotopic values of SOM in the study area. δ¹³C and δ¹⁵N values for SOM in different aquaculture areas were different from those of nearby reference stations, probably reflecting the influence of aquaculture. The δ¹³C and δ¹⁵N values in the oyster culture area (−25.9‰ and 6.0‰, respectively) seemed to have reduced δ¹³C and enriched δ¹⁵N relative to those of the reference station (−24.6‰ and 5.8‰, respectively). This may reflect the influence of organic matter on oyster culture. The δ¹⁵N value of the station in the mixed culture area (7.1‰ ± 0.4‰) seemed to be relatively enriched in δ¹⁵N relative to that of the reference station (6.6‰). Sedimentation and the subsequent degradation of organic matter from mixed cultures may have contributed to this phenomenon. The surface sediment at the cage fish farm area seemed to be affected by fish feces and primary production based on the indication of δ¹³C and δ¹⁵N values. The sediment core at the mixed culture region (NS6) had lower TOC/TN ratios and more positive δ¹³C and δ¹⁵N values than the sediment core at the oyster culture area, suggesting a higher proportionate contribution of marine organic matter in the mixed culture area. In summary, oyster culture, mixed culture, and cage fish culture in northeastern Zhanjiang Bay had a certain degree of impact on SOM, and mixed culture had more significant influences on SOM based on the high TOC contents and the significant vertical variations of TOC/TN ratio and δ¹⁵N value in the sediment of this area. This study provides new insights into the impact of aquaculture activities on SOM content.

Eutrophication in coastal waters has been increasing remarkably, severely impacting the water quality in mariculture bays. In this study, we conducted multiple isotopic measurements on suspended particulate nitrogen (δ¹⁵N-PN) and dissolved nitrate (δ¹⁵N-${\mathrm{NO}}_3^- $ and δ¹⁸O-${\mathrm{NO}}_3^- $) in Zhanjiang Bay, a typical mariculture bay with a high level of eutrophication in South China, to investigate the changes in nitrogen sources and their cycling between the rainy and dry seasons. During the rainy season, the study found no significant relation between δ¹⁵N-PN and δ¹⁵N-${\mathrm{NO}}_3^- $ due to the impact of heavy rainfall and terrestrial erosion. In the upper bay, a slight nitrate loss and slightly higher δ¹⁵N-${\mathrm{NO}}_3^- $ and δ¹⁸O-${\mathrm{NO}}_3^- $ values were observed , attributed to intense physical sediment-water interactions. Despite some fluctuations, nitrate concentrations in the lower bay mainly aligned with the theoretical mixing line during the rainy season, suggesting that nitrate was primarily influenced by terrestrial erosion and that nitrate isotopes resembled the source. Consequently, the isotopic values of nitrate can be used for source apportionment in the rainy season. The results indicated that soil nitrogen (36%) and manure and sewage (33%) were the predominant nitrogen sources contributing to nitrogen loads during this period. In contrast, the dry season saw a deficient ammonium concentration (<0.2 μmol/L) in the bay, due to nearly complete consumption by phytoplankton during the red tide period. Additionally, the significant loss of nitrate and simultaneous increase in the stable isotopes of dissolved and particulate nitrogen suggest a strong coupling of assimilation and mineralization during the dry season. More active biogeochemical processes during the dry season may be related to decreased runoff and increased water retention time. Overall, our study illustrated the major seasonal nitrogen sources and their dynamics in Zhanjiang Bay, providing valuable insights for formulating effective policies to mitigate eutrophication in mariculture bays.

The nutrients from the East China Sea (ECS) through the Tsushima/Korea Strait (TS) strongly impact the ecosystem of the Japan Sea (JS). The complex origins of the Tsushima Warm Current and the various nutrient sources in the ECS result in complex spatial-temporal variations in nutrients in the TS. Using a physical-biological model with a tracking technique, we studied the effects of nutrient sources from the ECS on the TS. Among all the nutrient sources, the Kuroshio has the highest nutrient concentrations in the TS. Its maximum concentration occurs at the bottom, while those of rivers and atmospheric deposition occur at the surface, and that of the Taiwan Strait occurs in the middle layer. The nutrient transport through the TS exhibits similar seasonal variations, as does the volume transport. The transport of nutrients from the Kuroshio accounts for more than 85% of the total. The transport of nutrients from the Taiwan Strait is greater during autumn and winter. The transport of dissolved inorganic nitrogen (DIN) from both rivers and atmospheric deposition through the TS peak in August. Nutrient transport cannot be equated with volume transport. The DIN in the less saline zone originates not only from rivers but also from atmospheric deposition and the Kuroshio. The transport of nutrients from the Taiwan Strait is not as significant as its volume transport in the TS.

Understanding the mechanism of harmful algal bloom formation is vital for effectively preventing algal bloom outbreaks in coastal environments. Karenia spp. blooms in the East China Sea show a significant correlation with nutrient regimes. However, the impact of key components of nutrients, especially dissolved organic nitrogen (DON), on the blooms of Karenia spp. is not clear. Quantitative research is still lacking. In this study, the cruise observations, field mesocosm-flask culture experiments, and a multinitrogen-tri-phytoplankton-detritus model (NTPD) are combined to reveal the quantitative influence of nutrient regimes on the shift of Prorocentrum donghaiense and Karenia spp. in the East China Sea. It has a synchronism rhythm of diatom-P. donghaiense-Karenia spp.-diatom loop in the field culture experiment, which is consistent with the results of the cruise observation. The results showed that the processes of terrigenous DON (TeDON) and dissolved inorganic nitrogen (DIN: $\mathrm{NO}_{3}^{-} $-N, $\mathrm{NH}_{4}^{+} $-N) absorption promoted P. donghaiense to become the dominant algae in the community; whereas the processes of DON from P. donghaiense absorption promoted Karenia spp. to become the dominant algae in ambient DIN exhaustion. In addition, the three-dimensional fluorescence components of humus C, tyrosine and fulvic acid can indicate the processes of growth and extinction of P. donghaiense and Karenia spp., respectively. This study infers that P. donghaiense and Karenia spp. regime shift mechanism associated with the nutrient regime in coastal waters, which provides a scientific basis for the environmental management of coastal ecosystem health.

Marine sediments collected from the Zhujiang (Pearl) River Estuary (ZRE) and South China Sea (SCS) were utilized to study the occurrence and spatial distribution of tetrabromobisphenol A (TBBPA) and hexabromocyclododecane (HBCDD). The levels of TBBPA and HBCDD in sediments ranged from not detected (nd) to 6.14 ng/g dry weight (dw) and nd to 0.42 ng/g dw. TBBPA concentrations in marine sediments were substantially higher than HBCDD. The concentrations of TBBPA and HBCDD in the ZRE sediments were significantly greater than those in the SCS. α-HBCDD (48.7%) and γ-HBCDD (46.2%) were the two main diastereoisomers of HBCDD in sediments from the ZRE, with minor contribution of β-HBCDD (5.1%). HBCDD were only found in one sample from the northern SCS. The enantiomeric fraction of α-HBCDD in sediments from the ZRE was obviously greater than 0.5, indicating an accumulation of (+)-α-HBCDD. The enantiomers of HBCDD were not measured in sediments from the SCS. This work highlighted the environmental behaviors of TBBPA and HBCDD in marine sediments.