Variations in dissolved oxygen and aquatic biological responses in China's coastal seas

Spatiotemporal distribution patterns and coupling effects of aquatic environmental factors in the dry-wet season over a decade from the Beibu Gulf, South China Sea

Since the 21st century, the Beibu Gulf area has been affected by increasing anthropogenic activities, which makes the coastal aquatic ecosystem extremely concerning. However, the comprehensive exploration and analysis of the long-term scale behavior change characteristics of various water quality environment factors is still limited. Through comprehensively detecting coastal surface water environmental behavior information from 33 locations in the Beibu Gulf from 2005 to 2015, we revealed and quantified mutual response characteristics and patterns of various environmental indicators. The main environmental pollution indicators (e.g., COD, NH4+, NO3-, and DIP) showed a gradual decrease in concentration from the coast to the offshore sea area, and significantly increases during the wet season. The semi-enclosed Maowei Sea exhibited the most prominent performance with significant differences compared to other regions in Beibu Gulf. The average Chlorophyll-a (Chla) content in the coastal area of the Beibu Gulf during the wet season was more than twice that of the dry season, yet the interaction pattern between Chla and environmental factors in the two seasons was opposite to its concentration behavior, accompanied by a closely significant relationship with thermohaline structure and the input of nitrogen and phosphorous nutrients. The multivariate statistical analysis results of total nutrient dynamics suggested that the Beibu Gulf was clearly divided into different regions in both dry and wet season clusters. The present study can provide a comprehensive perspective for the spatial and temporal migration patterns and transformation laws of coastal water environmental factor, which should contribute to improve the prevention countermeasure of nutrient pollution in coastal environment.

Deciphering variations in the surficial bacterial compositions and functional profiles in the intersection between North and South Yellow Sea

The coastal ocean systems play paramount role in the nutrient biogeochemistry because of its interconnected environment. To gain a novel insight into coupling relationships between bacterial community, functioning properties and nutrient metabolism, we conducted analysis on the patterns and driving factors of planktonic bacterial functional community across subsurface water of marine ranching near the Yellow Sea in both summer and winter. Illumina HiSeq Sequencing and a corresponding set of biogeochemical data were used to assess distribution patterns of taxa, adaptive mechanism and metabolic function. Results demonstrated that Proteobacteria, Cyanobacteria, Actinobacteriota and Bacteroidota were dominant phyla both in summer and winter. Taxonomic profiles related to nutrient variation were found to be highly correlated with Dissolved Oxygen (DO) and Chlorophyll fluorescence (FLUO), and distinct diversity differences were also found between summer and winter samples. Functional activity in summer associated with the relative abundance of phototrophy and photoautotrophy were the highest in the subsurface water, while in winter the dominant functional properties were mainly include chemoheterotrophy and aerobic_ chemoheterotrophy. A significant difference related to functional activity between summer and winter, mainly representing ligninolysis and iron_respiration. In general, our study provides a framework for understanding the relative importance of environmental factors, temperature variation and nutrient availability in shaping the metabolic processes of aquatic microorganisms, particularly in ocean mariculture systems.

Environmental DNA metabarcoding reveals the effect of environmental selection on phytoplankton community structure along a subtropical river

The Dongjiang River, a major tributary of the Pearl River system that supplies water to more than 40 million people in Guangdong Province and neighboring regions of China, harbors rich biodiversity, including many endemic and endangered species. However, human activities such as urbanization, agriculture, and industrialization have posed serious threats to its water quality and biodiversity. To assess the status and drivers of phytoplankton diversity, which is a key indicator of aquatic ecosystem health, this study used Environmental DNA (eDNA) metabarcoding combined with machine learning methods to explore spatial variations in the composition and structure of phytoplankton communities along the Dongjiang River, including its estuary. The results showed that phytoplankton diversity exhibited spatial distribution patterns, with higher community structure similarity and lower network complexity in the upstream than in the downstream regions. Environmental selection was the main mechanism shaping phytoplankton community composition, with natural factors driving the dominance of Pyrrophyta, Ochrophyta, and Cryptophyta in the upstream regions and estuaries. In contrast, the downstream regions was influenced by high concentrations of pollutants, resulting in increased abundance of Cryptophyta. The random forest model identified temperature, dissolved oxygen, chlorophyll a, NO2-, and NH4+ as the main factors influencing the primary phytoplankton communities and could be used to predict changes during wet periods. This study provides valuable insights into the factors influencing phytoplankton diversity and community composition in the Dongjiang River, and demonstrates the application value of eDNA metabarcoding technique in large-scale, long-distance river biodiversity monitoring.

The correlation between echinoderms diversity and physicochemical parameters in marine pollution: A case study of the Persian Gulf coastline

This study was conducted with the aim of investigating the correlation between echinoderms diversity and physicochemical parameters in the Persian Gulf coastline in Bushehr province in 4 seasons from March to December 2017. The physicochemical parameters including water temperature, dissolved oxygen (DO), electrical conductivity (EC), salinity, pH and turbidity were measured at each sampling location. The results showed a significant correlation between echinoderms diversity and physicochemical parameters. The correlation coefficient of the Astropecten polyacanthus species with the parameters of temperature, DO, EC, salinity and turbidity was reported as -0.41, 0.64, -0.25, -0.44 and 0.60 respectively. This coefficient for the Ophiothrix fragilis species was reported as -0.68, 0.70, -0.21, -0.36 and -0.55 respectively. The results demonstrated that the most sensitive species were Astropecten polyacanthus and Ophiothrix fragilis respectively. The different species of echinoderms can be used as biological indicators of pollution in evaluating the physicochemical quality of marine environments.

Ecosystem health evaluation of urban rivers based on multitrophic aquatic organisms

The ecosystem health evaluation method of urban rivers is significantly different from natural rivers, because of intensive human interferences and ecological restoration measures. Biotic integrity index (IBI) provides a method to quantify the response of aquatic organisms to environmental stress. Multi-trophic aquatic organisms may exhibit different responses and sensitivities to stress factors, which affects the reliability of the IBIs. This study proposed a hypothesis that the biota with the higher trophic level (whose habitat was not completely destroyed) or that of the biota with the shorter life cycle would be more sensitive in urban rivers. To prove the above hypothesis, the ecosystem health status of urban rivers was evaluated by the IBIs across multitrophic groups, including benthic invertebrates, zooplankton, phytoplankton, periphyton algae and microorganisms. The reliability of the IBIs was assessed by estimating their relationship with water quality index (WQI). The spatial distribution differences of the IBIs were distinguished by spatial autocorrelation analysis. The results showed that the IBI based on benthic invertebrates cannot mask the effects of dredging. Compared with the IBIs from other trophic groups, the correlation coefficients between the IBIs based on zooplankton and microorganisms and WQI were higher. Moreover, the evaluation results of Z (Zooplankton)-IBI and M (Microorganism)-IBI were able to discriminate the least, medium and highly impaired site groups divided by WQI. For the spatial response mode, Z-IBI and M-IBI could identify the high-value river sections under ecosystem restoration projects, and Z-IBI could also identify the low-value river sections under intensive human interferences. Therefore, Z-IBI and M-IBI could be recommended as the priority application in urban rivers. The constructed ecosystem health evaluation framework for urban rivers would play a guiding role in reducing impairments and restoring water ecosystem quality.

Spatiotemporal variation of dissolved oxygen concentration in the northern Gulf of Mexico during the period of 1992-2017

Dissolved oxygen (DO) concentration is an important index of ocean systems. In this study, spatiotemporal variations in DO were analyzed in the northern Gulf of Mexico (NGM) for the period of 1992-2017. Temporal variation involves annual, monthly, and seasonal variations. The spatial scope ranged from the sea surface to 1000 m depth. The results show that: (1) DO decreased from 1992 to 2017 with a brief increase; (2) clear seasonal variation characteristics of DO were presented, and the seasonal average DO was relatively low in summer; (3) the monthly average trends of sea surface DO in different years were relatively consistent; and (4) with increasing water depth from seawater surface to 1000 m, the average DO of 1992 to 2017 tended to decrease and then increase. The results aim at shedding some light on the assessment of DO change in the NGM over the past quarter century.