Coastal ecological security assessment in Laizhou Bay, China: from the perspective of demographic-social-economic-natural complex ecosystem.

Assessment of ecological security is essential for understanding the status of bay ecosystem and developing appropriate management strategy. Based on the driving force-pressure-state-impact-response (DPSIR) model, the demographic, economic, social, and ecological data of Laizhou Bay and its three neighboring counties were selected for the period from 2015 to 2021. An ecological security evaluation index system of Laizhou Bay containing 26 indicators was established, and the weights of each indicator were determined by the methods of AHP and EWM, and a comprehensive evaluation of the ecological security of Laizhou Bay was carried out by ESI. Correlations between indicators were analyzed by the Spearman's rank coefficient of correlation. The results showed that there were significant correlations between marine conditions and indicators such as population size in the surrounding area, mariculture area, industrial and domestic wastewater discharge, and treatment rate. Overall, from 2015 to 2021, the ecological security of Laizhou Bay showed a favorable trend, from a relatively unsafe level to a generally safe level, and then to a relatively safe level. Through the comprehensive evaluation of the ecological security of Laizhou Bay, we can recognize the utilization of marine resources and ecological carrying capacity, guide the rational development and utilization of marine resources, and promote the sustainable development of the marine economy.

Temperature and nutrients drive distinct successions between diatoms and dinoflagellates over the past 40 years: Implications for climate warming and eutrophication.

Diatoms and dinoflagellates are two typical functional groups of phytoplankton, playing important roles in ecosystem processes and biogeochemical cycles. Changes in diatoms and dinoflagellates are thought to be one of the possible mechanisms for the increase in harmful algal blooms (HABs), due to changing hydrological conditions associated with climate change and human activities. However, little is known about their ability to adapt to changing ocean environments, thus making it difficult to know whether and how they are adapting. By analyzing a 44-year monitoring dataset in the central Bohai Sea during 1978-2021, we found that the abundance ratio of diatoms to dinoflagellates showed a decreasing trend seasonally and ecologically, indicating that the phytoplankton community underwent distinct successional processes from diatom dominance to diatom-dinoflagellate co-dominance. These processes exhibited varying responses to temperature, nutrient concentrations and ratios, and their interactions, of which temperature primarily drove the seasonal succession whereas nutrients were responsible for the ecological succession. Specifically, diatoms showed a preference for lower temperatures and higher DIP concentrations, and were able to tolerate lower DIN at lower temperatures. In contrast, dinoflagellates tended to prevail at conditions of warming and high N/P ratios. These different traits of diatoms and dinoflagellates reflected the fact that warming as a result of rising temperature and eutrophication as a consequence of nutrient input would favor dinoflagellates over diatoms. Moreover, the increasing dominance of dinoflagellates indicated that dinoflagellate blooms were likely to become more frequent and intense in the central Bohai Sea.

Environmental controls on the seasonal variations of diatoms and dinoflagellates in the Qingdao coastal region, the Yellow Sea.

Diatoms and dinoflagellates are two typical functional groups of phytoplankton assemblages, which play a crucial role in the structure and functioning of most marine ecosystems. To date, a novel challenge in ecology and biogeochemistry is to address the influences of environmental changes associated with climate change and human activities on the dynamics of diatoms and dinoflagellates. However, the knowledge of the key environmental factors controlling the diatom-dinoflagellate dynamics remains to be improved, particularly in the coastal ecosystems. Therefore, we conducted four cruises along the Qingdao coastline in spring, summer, autumn, and winter 2022 to explore how diatoms and dinoflagellates varied in response to regional environmental changes. The results showed that the phytoplankton communities were dominated by diatoms and dinoflagellates in terms of abundance and species diversity throughout the year in the study region. Yet, there were significant seasonal variability of diatoms and dinoflagellates across the four seasons. For example, diatom species was the most diverse during autumn, and the higher average abundance was observed in the fall and winter. In contrast, the average abundance of dinoflagellates was maximum during the summer and minimum in the autumn season. Moreover, the abundance and species ratios of diatoms/dinoflagellates (dia/dino) also showed significant seasonal variations in the region. The dia/dino abundance ratio was lowest in summer, while the dia/dino species ratio showed an increasing trend from spring to fall and a slight descending trend during winter. Based on the redundancy analysis, we revealed that diatoms and dinoflagellates responded differently to various environmental variables in different seasons, of which temperature and nutrients (especially dissolved inorganic nitrogen, DIN) had highly significant correlations with both the dia/dino abundance and species ratios. Thus, we suggested that temperature and DIN were the key factors controlling the seasonal dynamics of diatoms and dinoflagellates in the Qingdao coastal area.

Effect of increased pCO2 and temperature on the phytoplankton community in the coastal of Yellow Sea.

In order to study the dynamics of marine phytoplankton communities in response to anticipated in temperature and CO2, a shipboard continuous culture experiment (Ecostat) was conducted. The experiment involved simulations under current atmospheric CO2 concentrations (400 ppm) and projected year-2100 CO2 levels (1000 ppm), as well as varying temperature under present (22 °C) versus increased temperature (26 °C) in the Yellow Sea during the summer of 2020. The results showed that both the increased pCO2 and temperature had significant effects on microphytoplankton and picophytoplankton, with the warming effect proving to be more significant. The different responses of various species to acidification and warming and their coupling effect led to the changes in microphytoplankton and picophytoplankton community structure. Elevated temperature and greenhouse treatments promoted the growth of dominant diatoms and Synechococcus, such as Guinardia flaccida and Pseudo-nitzschia delicatissima. This phenomenons widened the ecological niche, and the changes in the growth patterns of dominant species consequently influenced the content of cellular elements. Mantel's analysis further demonstrated that both warming and greenhouse promoted the growth of diatoms and Synechococcus. Projections of marine phytoplankton community trends by the end of the century based on Growth Rate Ratio (GRR), indicated that not only would species with GRR < 1 decrease, but also numerous species with growth rates >1 at elevated pCO2 levels would be ousted from competition. This experiment demonstrates the need to investigate whether extended exposure to increased pCO2 and temperature over more extended time scales would similarly induce shifts in the biological and biogeochemical dynamics of the Yellow Sea.

Silicon fractionations in coastal wetland sediments: Implications for biogeochemical silicon cycling.

Coastal wetland sediment is important reservoir for silicon (Si), and plays an essential role in controlling its biogeochemical cycling. However, little is known about Si fractionations and the associated factors driving their transformations in coastal wetland sediments. In this study, we applied an optimized sequential Si extraction method to separate six sub-fractions of non-crystalline Si (Sinoncry) in sediments from two coastal wetlands, including Si in dissolved silicate (Sidis), Si in the adsorbed silicate (Siad), Si bound to organic matter (Siorg), Si occluded in pedogenic oxides and hydroxides (Siocc), Si in biogenic amorphous silica (Siba), and Si in pedogenic amorphous silica (Sipa). The results showed that the highest proportion of Si in the Sinoncry fraction was Siba (up to 6.6 % of total Si (Sitot)), followed by the Sipa (up to 1.8 % of Sitot). The smallest proportion of Si was found in the Sidis and Siad fractions with the sum of both being <0.1 % of the Sitot. We found a lower Siocc content (188 ± 96.1 mg kg-1) when compared to terrestrial soils. The Sidis was at the center of the inter-transformation among Si fractions, regulating the biogeochemical Si cycling of coastal wetland sediments. Redundancy analysis (RDA) combined with Pearson's correlations further showed that the basic biogenic elements (total organic carbon and total nitrogen), pH, and sediment salinity collectively controlled the Si fractionations in coastal wetland sediments. Our research optimizes sediment Si fractionation procedure and provides insights into the role of sedimentary Si fractions in controlling Si dynamics and knowledge for unraveling the biogeochemical Si cycling in coastal ecosystems.

Unprecedented phytoplankton blooms in autumn/winter in the southern Bohai Sea (China) due to high Yellow River discharge: Implications of extreme rainfall events.

The occurrence of abnormal phytoplankton blooms is one of the significant changes in coastal ecosystems due to climate change. However, the underlying mechanism of such blooms remains poorly understood due to the complexity of the system. In this study, the data from numerous observations was used to elucidate the unprecedented phytoplankton blooms in the autumn and winter of 2021 in Laizhou Bay, a typical aquaculture bay in the southern Bohai Sea of China. The abundance of phytoplankton cells increased by more than tenfold in the southern waters compared to that in the same period from 2019 to 2020. The phytoplankton bloom was first observed in winter in the Bohai Sea, with the cell abundance in the southern bay exceeding 108 cells L-1 in December 2021. The diversity and evenness of phytoplankton communities decreased in the southern area. Cerataulina pelagica was the dominant algae, comprising 69 % of the total phytoplankton in October and 99 % in December. In autumn 2021, the largest flood of the Yellow River in recent decades occurred. This was attributed to extreme rainfall events within the river basin. The input of substantial riverine nutrients played a significant role in promoting phytoplankton blooms. Correlation analysis indicated the important cumulative impact of the Yellow River on phytoplankton blooms rather than a direct short-term effect. Numerical modeling results indicated that exceptionally high Yellow River discharge in autumn could significantly affect the entire bay from autumn to the following spring. This study may contribute to understanding the abnormal phytoplankton blooms in coastal waters and provide valuable insights for environmental management in river basins and coastal waters.

Environmental forcing of phytoplankton carbon-to-diversity ratio and carbon-to-chlorophyll ratio: A case study in Jiaozhou Bay, the Yellow Sea.

The relationships between phytoplankton carbon (C) biomass and diversity (i.e., C-to-H' ratio) and chlorophyll a (i.e., C-to-Chl a ratio) are good indicators of marine ecosystem functioning and stability. Here we conducted four cruises spanning 2 years in Jiaozhou Bay to explore the dynamics of C-to-H' and C-to-Chl a ratios. The results showed that the phytoplankton C biomass and diversity were dominated by diatoms, followed by dinoflagellates. The average C-to-H' ratio ranged from 84.10 to 912.17, with high values occurring in the northern region of the bay. In contrast, the average C-to-Chl a ratio ranged between 15.55 and 89.47, and high values primarily appeared in the northern or northeastern part of the bay. In addition, the redundancy analysis showed that temperature and phosphate (DIP) were significantly correlated with both ratios in most cases, indicating that temperature and DIP may be key factors affecting the dynamics of C-to-H' and C-to-Chl a ratios.

Probiotics and the Potential of Genetic Modification as a Possible Treatment for Food Allergy.

Food allergy is a common condition that affects millions of people worldwide. It is caused by an abnormal immune response to harmless food antigens, which is influenced by genetics and environmental factors. Modulating the gut microbiota and immune system with probiotics or genetically modified probiotics confers health benefits to the host and offers a novel strategy for preventing and treating food allergy. This systematic review aims to summarize the current proof of the role of probiotics in food allergy and propose a promising future research direction of using probiotics as a possible strategy of treatment for food allergy.

The Potential Role of C-Reactive Protein in Metabolic-Dysfunction-Associated Fatty Liver Disease and Aging.

Nonalcoholic fatty liver disease (NAFLD), recently redefined as metabolic-dysfunction-associated fatty liver disease (MASLD), is liver-metabolism-associated steatohepatitis caused by nonalcoholic factors. NAFLD/MASLD is currently the most prevalent liver disease in the world, affecting one-fourth of the global population, and its prevalence increases with age. Current treatments are limited; one important reason hindering drug development is the insufficient understanding of the onset and pathogenesis of NAFLD/MASLD. C-reactive protein (CRP), a marker of inflammation, has been linked to NAFLD and aging in recent studies. As a conserved acute-phase protein, CRP is widely characterized for its host defense functions, but the link between CRP and NAFLD/MASLD remains unclear. Herein, we discuss the currently available evidence for the involvement of CRP in MASLD to identify areas where further research is needed. We hope this review can provide new insights into the development of aging-associated NAFLD biomarkers and suggest that modulation of CRP signaling is a potential therapeutic target.

Contrasting effects of river inflow and seawater intrusion on zooplankton community structure in Jiaozhou bay, the Yellow Sea.

Environmental changes associated with river inflow and seawater intrusion are known to affect zooplankton communities in coastal systems, but how zooplankton respond to these environmental changes remains unclear at present. Here we explored the effects of river inflow and seawater intrusion on zooplankton community structure in Jiaozhou Bay. The results showed that the river inflow and seawater intrusion are key in driving zooplankton dynamics, but with contrasting effects. According to the distinct hydrographic conditions, the sampling area could be geographically divided into the river inflow area with low-salinity and high-nutrient conditions (i.e., EIZ) and the seawater intrusion zone with high-salinity and low-nutrient conditions (i.e., SIZ). There were significant differences in zooplankton communities (e.g., abundance and species composition) between the two regions with seasonal changes. For example, the zooplankton abundance was significantly higher in the SIZ than in the EIZ during spring, whereas an opposite pattern was observed for the summer season. In contrast, the species richness was higher in the EIZ than in the SIZ in spring, while an opposite variation trend was observed during summer. These results together suggested that the river inflow and seawater intrusion had contrasting effects on zooplankton community structure in different seasons. According to the canonical correspondence analysis, we observed that the zooplankton community structure was mainly driven by temperature, chlorophyll a (Chl a), and nutrients in the EIZ, but it was largely affected by salinity in the SIZ. The implication is that changes in temperature, Chl a, and nutrients as a result of river inflow and changes in salinity as a consequence of seawater intrusion are key in driving the dynamics of zooplankton communities in Jiaozhou Bay.

Relationship between cell volume and particulate organic matter for different size phytoplankton.

Phytoplankton biomass is an intuitive indicator to evaluate the stability of marine ecosystems. How to obtain phytoplankton biomass quickly and accurately is the focus of marine research. In this work, we selected 12 phytoplankton species to explore the relationship between cell volume and cellular compounds. Results indicated that the volume of 12 species spanned 6 orders of magnitude from 0.11 µm3 (Synechococcus sp. XM-5) to 1.26 × 105 µm3 (Ditulum brightwellii). Besides, a strong negative linear correlation between cell abundance and volume was observed. For per-cell of phytoplankton, the log-log relationship between cell volume and particulate organic carbon (POC), particulate organic nitrogen (PON), chlorophyll a (Chl-a) results reflected a positive correlation. On the contrary, the density of cellular compounds (POC, PON and Chl-a) presented a negative correlation in cell volume. In addition, for ratio of particulate organic carbon and particulate organic nitrogen (C/N), the average value was 5.46, diatoms had the lowest values, 4.72. Moreover, the ratio of carbon and chlorophyll a (C/Chl-a) varied from 15.09 to 79.75, indicating that the C/Chl-a values affected by the different categories. Meanwhile, diatoms usually have a lower C/Chl-a value for their special structure. Finally, we excluded the Synechococcus group of small individuals and found that the slope of their corresponding carbon-volume relationship was not very different, indicating that the smaller individuals had no effect on the assessment of carbon-volume relationship. The physiological variation, based on the cell volume, provides us with a new perspective to simplify the relationship between carbon and volume, and further simplify the methods and analysis of experiments and model simulations of ecosystem biological rate and flux.

Identifying the key factors affecting the trimethylamine N-oxide content of teleost fishes collected from the marginal seas of China and the epipelagic zone of the northwest Pacific Ocean.

To identify the key factors influencing the trimethylamine N-oxide (TMAO) content of teleost fishes living in shallow seas and the epipelagic zone of the deep ocean, the muscle TMAO content was measured in 152 teleost fishes (21 species) collected from the marginal seas of China and the epipelagic zone of the northwest Pacific Ocean (NWPO) during May-July 2021. The results showed that the TMAO content in all fishes varied from 4.99 to 82.97 mmol kg-1, and it varied notably among different species, e.g., the highest average content (72.71 ± 8.22 mmol kg-1 in Argyrosomus argentatus) was 1 order of magnitude higher than the lowest one (Scomber japonicus), but the ratios of the highest content to the lowest content in each species varied from 1.29 to 3.28, suggesting that the interspecific variations in TMAO content were obviously greater than the intraspecific variations. Moreover, no correlation was observed between the TMAO content of the 152 fishes and the corresponding environmental factors of seawater depth, salinity and temperature, indicating that species played a more important role than environmental factors in driving TMAO accumulation. To exclude the influence of species, intraspecies correlations between TMAO content and environmental factors were analyzed. In the marginal seas of China, only ∼8 % of the TMAO content of teleost fishes (1 species) showed a positive correlation with salinity and depth, but ∼50 % of the TMAO content (5 species) was negatively correlated with temperature. Moreover, the TMAO content of the fish increased by 4.66 ± 1.38 % compared with their corresponding intraspecific average values for every 1 °C of temperature decrease. A similar phenomenon was also found in the TMAO content of pelagic teleost fishes in the NWPO, suggesting that temperature was a key environmental factor affecting the TMAO content of teleost fishes in shallow seas and the epipelagic zone of the deep ocean.

Picocyanobacteria-A non-negligible group for the export of biomineral silica to ocean depth.

Diatoms have long been thought to dominate the marine silicon (Si) cycle, as well as play an important role in the ocean's carbon (C) export, due to density-driven particle sedimentation. Research in the past decade has shed new light on the potential importance of picocyanobacteria to C export, although the sinking mechanism is still unclear. Interestingly, the recent discovery of Si accumulation by picocyanobacteria of the genus Synechococcus has strong implications for the marine Si cycle, which may also have profound influence on the oceanic C export. Understanding the mechanisms of Synechococcus Si accumulation and its ecological effects are therefore critical for addressing wider issues such as Si and C exports by small cells via biological pump. Here, we show that recent advances in process studies indicate that the presence of Si within picocyanobacteria may be a common and universal feature. Subsequently, we generalize four biochemical forms of Si potentially present in picocyanobacterial cells, which are all different from diatomaceous opal-A, and hypothesize that these various structures of Si phases may be several stage products of Si precipitation. At the same time, several aspects of Si dynamics in Synechococcus are also discussed emphatically. In addition, we provide a first estimate of picocyanobacteria Si stock and production for the global ocean, accounting for 12% of the global Si inventory and 45% of the global annual Si production in the surface ocean, respectively. The implication is that picocyanobacteria may exert a significant influence on the marine Si cycle, which is likely to alter our understanding of the long-term control of the oceanic Si cycling by diatoms. Finally, we summarize three possible mechanisms and pathways through which picocyanobacteria-derived Si can be transported to the deep ocean. Altogether, marine picocyanobacteria, despite very small in cell size, are a non-negligible group for the export of biomineral Si to deeper waters and ocean sediments.

Key determinants controlling the seasonal variation of coastal zooplankton communities: A case study along the Yellow Sea.

Zooplankton play key top-down and bottom-up regulatory roles in aquatic food webs, and are also ecologically indicative in marine ecosystems. However, there are relatively limited data on the effects of environmental changes on natural zooplankton communities, especially in coastal ecosystems. In the present study, we systematically evaluated the potential effects of various environmental variables, such as temperature, salinity, and nutrients, on the zooplankton communities along the coastal Yellow Sea during spring, summer, and fall. The results showed that the average abundance of zooplankton decreased in general from spring to autumn, but the biomass exhibited a different seasonal variation trend, with the highest in summer and the lowest in fall. Throughout the three seasons, copepods were the most dominant species within the zooplankton communities, followed by Pelagic larvae and Hydromedusae. However, Noctiluca miliaris accounted for a large proportion of zooplankton abundance during spring. Moreover, the correlation analysis was applied to explore the potential effects of environmental factors on the seasonal variation of zooplankton communities. The results showed that chlorophyll a (Chl a) and salinity were significantly correlated with zooplankton abundance and biomass during spring. The implication is that high phytoplankton biomass (expressed as Chl a) and salinity would benefit the growth of zooplankton in spring. During summer and fall, the effects of dissolved inorganic phosphate (DIP) on the zooplankton abundance and biomass showed a significant positive correlation, indicating that zooplankton were better able to tolerate high DIP during summer and fall. Taken together, Chl a, salinity, and DIP may be the key determinants controlling the seasonal dynamics of zooplankton communities in the coastal Yellow Sea.

Ocean acidification and warming significantly affect coastal eutrophication and organic pollution: A case study in the Bohai Sea.

Most coastal ecosystems are faced with novel challenges associated with human activities and climate change such as ocean acidification, warming, eutrophication, and organic pollution. However, data on the independent or combined effects of ocean acidification and warming on coastal eutrophication and organic pollution at present are relatively limited. Here, we applied the generalized additive models (GAMs) to explore the dynamics of coastal eutrophication and organic pollution in response to future climate change in the Bohai Sea. The GAMs reflected the fact that acidification alone favors eutrophication and organic pollution, while warming alone inhibits these two variables. Differently, the interactions between acidification and warming in the future may further exacerbate the organic pollution but may mitigate the progress of eutrophication. These different responses of eutrophication and organic pollution to acidification and warming may be attributed to algae growth and microbial respiration, as well as some physical processes such as stratification.

Ecological risk assessment of heavy metal pollutants and total petroleum hydrocarbons in sediments of the Bohai Sea, China.

Heavy metals and organic pollutants like total petroleum hydrocarbons (TPHs) in coastal marine sediments are receiving extensive attention, as they may pose a serious threat to the aquatic environment and ecosystem health. To date, however, data on the long-term variations in the levels of sedimentary heavy metals and TPHs as well as their ecological risks are relatively limited. Here, we conducted 12 cruises spanning 3 years in the Bohai Sea and obtained ~1400 sediment samples to explore the long-term variations of heavy metals (i.e., Hg, As, Cu, Zn, Pb, Cd) and TPHs, and to assess their potential ecological risks. The results suggested that the ranges for the levels of Hg, As, Cu, Zn, Pb, Cd, and TPHs in sediments between 2019 and 2021 were <0.01-0.07, 0.23-10.72, 8.07-20.67, 25.52-46.55, 10.94-28.19, 0.14-0.56, and 9.14-18.41 mg kg-1, respectively. Based on the single factor evaluation (Fi) for sediment quality, we found that most of the evaluation factors in the study area met the requirements of sediment quality standard (i.e., Fi < 1), except for the factor of metal Cd in some cases. The implication is that the sediment in the Bohai Sea was fairly clean in terms of heavy metals and TPHs. However, the concentration of metal Cd exceeded the sediment quality standard during May 2019 and 2020 (i.e., Fi > 1), indicating that Cd could be identified as a major pollutant in surface sediments. Also, based on the ecological risk assessment (Ei) of heavy metal pollutants, we found that the metal Cd had reached a level with potential ecological risk in some cases (80 ≤ Ei < 160). As such, we further suggested that the Cd contamination might have a potential risk on the Bohai Sea' ecosystem.

Different responses of phytoplankton and zooplankton communities to current changing coastal environments.

Marine plankton are faced with novel challenges associated with environmental changes such as ocean acidification, warming, and eutrophication. However, data on the effects of simultaneous environmental changes on complex natural communities in coastal ecosystems are relatively limited. Here we made a systematic analysis of biological and environmental parameters in the Bohai Sea over the past three years to suggest that plankton communities responded differently to current changing coastal environments, with the increase of phytoplankton and the decrease of zooplankton. These different changes of phyto- and zooplankton potentially resulted from the fact that both the effect of acidification as a result of pH decline and the effect of warming as a consequence of increasing temperature favored phytoplankton over zooplankton at present. Furthermore, water eutrophication and salinity as well as heavy metals Hg, Zn, and As had more or less diverse consequences for the dynamics of phytoplankton and zooplankton. Differently, with ongoing climate change, we also revealed that both phytoplankton and zooplankton would decrease in the future under the influence of interactions between acidification and warming.

Exploring the dynamics of marine picophytoplankton among the Yellow Sea, Indian Ocean and Pacific Ocean: The importance of temperature and nitrogen.

Marine picophytoplankton (<2 µm) are the most abundant photosynthetic group and also important contributors to global primary production. However, it is still constrained to incorporate picophytoplankton into dynamic ecosystem models, as a result of our limited understanding of their global distribution and abundance. Here, we applied a large dataset consisted of 1817 in situ observations from the Yellow Sea, Indian Ocean, and Pacific Ocean to suggest that picophytoplankton abundance and distribution had a large variability among the three distinct regions. Based on the correlation analysis, aggregated boosted tree analysis, and generalized additive model, we proposed that water temperature and dissolved inorganic nitrogen (N) were key determinants in driving the large-scale variability of marine picophytoplankton. For example, we revealed that high temperature and low N would stimulate the growth of Prochlorococcus. Therefore, these results could provide some insights into the various environmental factors which affect the dynamics of picophytoplankton, as well as the dynamic ecosystem models.

Eutrophication status assessment in the Laizhou Bay, Bohai Sea: Further evidence for the ecosystem degradation.

Laizhou Bay is plagued by many environmental problems (e.g., acidification and eutrophication) that are likely to increase in the coming decades along with social and demographic trends. We thus conducted 12 cruises from July 2019 to December 2021 in the Laizhou Bay to evaluate the potential influences of human activities on its eutrophication status. The results showed that the parameters of dissolved inorganic nitrogen (DIN) and oxygen (DO) exceeded the water quality standard in some cases during the past 3 years, suggesting DIN and DO were the crucial factors controlling water quality in the bay. Meanwhile, DIN was identified as a major pollutant in the region, and played an important role in driving the phytoplankton dynamics. Furthermore, with the increases in human-related nutrient inputs (especially DIN), the bay environment reached the mild eutrophication level and was probably at risk from excessive nutrient loads at present, further evidencing the ecosystem degradation.

Response of the Phytoplankton Sinking Rate to Community Structure and Environmental Factors in the Eastern Indian Ocean.

The phytoplankton sinking rate in the eastern Indian Ocean was measured during spring 2017 based on the SETCOL method. The range of phytoplankton sinking rates was -0.291 to 2.188 md-1, with an average of 0.420 ± 0.646 md-1. The phytoplankton sinking rate in the Equator (EQ) and the eastern boundary of the Indian Ocean near Sumatra (EB) was lower than that in the Bay of Bengal (BOB). The sinking rate above 100 m was low and increased rapidly below 100 m in all the three regions. The phytoplankton community composition had an important impact on the phytoplankton sinking rate in the east Indian Ocean. The strong stratification in BOB resulted in Trichodesmium spp. bloom and a lower phytoplankton diversity and evenness in BOB, while the phytoplankton in the deep layer are senescent cells that sink down from the upper layer and cannot actively regulate the state of the cells, resulting in a higher sinking rate. Depth and temperature have a great impact on the physiological state of phytoplankton. The sinking rate of phytoplankton depend on the dominant groups composing the phytoplankton community. For the eastern Indian Ocean, seawater stratification caused by temperature changes the distribution of nutrients in the upper layer, and phytoplankton are affected by temperature and nutrients, resulting in changes in community structure, and finally showing different subsidence characteristics.