East China Sea increasingly gains limiting nutrient P from South China Sea

Spring protistan communities in response to warming in the northeastern East China Sea

The northeastern East China Sea is a highly dynamic marine ecosystem influenced by seasonally varying water mass properties. However, despite being among the world's fastest-warming ocean, there has been limited investigation into the impacts of warming on protistan communities. We collected seawater from two stations (E42 and E46) with different natural protist communities and environmental attributes to investigate the acclimation of the two communities to artificially elevated temperatures (ambient T, +2, and +4 °C). Nutrient and Chl-a conditions reflected oceanographic differences, providing insights into protistan community dynamics. Notably, small-sized autotrophic protists prevailed in the phosphate-deficient E42 community, with mid-incubation heterotrophic conversions. Higher temperatures exacerbated the effects of the P deficiency on the E42 community. While the proportions of Bacillariophyta increased only in the nutrient-balanced E46 communities, those of mixotrophic dinoflagellates increased with elevated temperature, regardless of P deficiency, suggesting that mixotrophy likely aids adaptation in changing marine environments. In summary, the findings of this microcosm study illuminate the potential modulation of spring protistan communities in the northeastern East China Sea under anticipated future warming.

Changes in dinoflagellate and diatom blooms in the East China Sea over the last two decades, under different spatial and temporal scale scenarios

Frequent algal blooms in the nearshore area of the East China Sea (ECS) pose a serious threat to both the marine environment and human health. Climate and environmental changes play an important role in the occurrence of diatoms and dinoflagellates blooms. Using the MODIS-Aqua 1-km satellite observations, the outbreaks of dinoflagellate and diatom blooms in the ECS coast in summer during 2003-2022 were mapped. Our results found that although the bloom frequency of dinoflagellate was consistently higher than diatoms, its bloom intensity showed a slightly decline trend in recent decades. The driving factors analysis showed that river runoff and sediments discharge played different effect on the formation of diatom and dinoflagellate blooms. Besides, our results compared the effect of El Niño and La Niña on bloom occurrences. This study was supposed to provide detailed insights into algal blooms, with important implications for relevant meteorological and climate changes in coastal regions.

Effect of ocean warming on pigment and photosynthetic carbon fixation of plankton assemblage in Pingtan Island of Southeast China

Temperature plays an important role in affecting the physiological traits of marine plankton. In this study, we conducted an outdoor incubation experiment to investigate the effects of elevated temperature on Chl a, photosynthetic carbon fixation and the composition of plankton communities in the surface seawater around Pingtan Island, the northwest Taiwan Strait in Autumn 2022. After 3-4 days of incubation, elevated temperature (1-4 °C higher than ambient temperature) led to a decrease in Chl a concentration across all three stations, did not result in significant increases in the particulate organic carbon (POC) and nitrogen (PON) concentrations in seawater with high nitrate concentrations, whereas increased POC and PON concentrations in nitrate-limited seawater. These findings suggest that the effect of temperature on the POC and PON contents of plankton is affected by the availability of nitrate. Diatoms were the dominant phytoplankton group in all three stations. Our results indicate that ocean warming has a potential to increase the POC contents of marine plankton per volume of seawater, which may increase the ability of phytoplankton to absorb atmospheric CO2 and to alleviate global warming.

Vertical dynamics of free-living and particle-associated vibrio communities in the eastern tropical Indian Ocean

Members of the family Vibrionaceae (vibrios) are widely distributed in estuarine, offshore, and marginal seas and perform an important ecological role in the marine organic carbon cycle. Nevertheless, there is little knowledge about whether vibrios play ecological roles in the oligotrophic pelagic area, which occupies a larger water volume. In this study, we investigated the abundance, diversity, and composition of free-living and particle-associated vibrios and their relationships with environmental factors along the water depth in the eastern tropical Indian Ocean (ETIO). The abundance of vibrios in free-living fractions was significantly higher than that of particle-associated fractions on the surface. Still, both were similar at the bottom, indicating that vibrios may shift from free-living lifestyles on the surface to mixed lifestyles at the bottom. Vibrio-specific 16S rRNA gene amplicon sequencing revealed that Paraphotobacterium marinum and Vibrio rotiferianus were dominant species in the water column, and Vibrio parahaemolyticus (a clinically important pathogen) was recorded in 102 samples of 111 seawater samples in 10 sites, which showed significant difference from the marginal seas. The community composition also shifted, corresponding to different depths in the water column. Paraphotobacterium marinum decreased with depth, and V. rotiferianus OTU1528 was mainly distributed in deeper water, which significantly correlated with the alteration of environmental factors (e.g., temperature, salinity, and dissolved oxygen). In addition to temperature and salinity, dissolved oxygen (DO) was an important factor that affected the composition and abundance of Vibrio communities in the ETIO. Our study revealed the vertical dynamics and preferential lifestyles of vibrios in the ETIO, helping to fill a knowledge gap on their ecological distribution in oligotrophic pelagic areas and fully understanding the response of vibrios in a global warming environment.

Nitrate dynamics and source apportionment on the East China Sea shelf revealed by nitrate stable isotopes and a Bayesian mixing model

The excess input of nitrate is one of the primary factors triggering nearshore eutrophication. To estimate the source apportionment of nitrate on the East China Sea (ECS) shelf, the nitrogen and oxygen stable isotopes in nitrate (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) collected in winter and late spring 2016 were analyzed alongside essential physical, chemical and biological parameters. The temporal and spatial distributions and characteristic values of nitrate-bearing water masses were presented. Accordingly, the end-member mixing model and Rayleigh model were applied to systematically analyze biogeochemical processes. The biogeochemical processes of nitrate were weak in winter, except in the southern ECS, where assimilation and nitrification probably occurred. In contrast, the biogeochemical processes were intensive in spring. The stable isotopic fractionations of N and O were unified in the whole area, and the ratio between δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^- was 1.81 ± 0.04, which indicated significant assimilation accompanying nitrification in spring. Furthermore, a Bayesian stable isotope mixing model was used to reveal the source contributions of nitrate on the ECS shelf for the first time, demonstrating that the Changjiang Diluted Water and Kuroshio Subsurface Water were always sustained and provided steady nitrate sources for the whole ECS. The nitrate inputs from the Yellow Sea to the northern ECS increased from approximately 30 % in spring to nearly 70 % in winter, while that from the Taiwan Strait Warm Water to the southern ECS decreased from approximately 40 % in spring to zero in winter. Moreover, although the nitrate contributions from nitrification were significantly weak in the middle and northern ECS during winter, they were important over the entire ECS during spring. This study qualitatively and quantitatively improves the understanding of seasonal nitrate control from various sources, and these findings are important for nutrient management and policy making to mitigate nearshore eutrophication.

Fine-Scale Structuring of Planktonic Vibrio spp. in the Chinese Marginal Seas

Vibrio is ubiquitous in marine environments with high metabolism flexibility and genome plasticity. Studies have investigated the ecological distribution of Vibrio spp. in several narrow zones, but a broad scale pattern of distribution and community assembly is still lacking. Here, we elucidated the distribution of Vibrio spp. in seawater along the Chinese marginal seas with a high spatial range. Comparison of Vibrio abundance between 3- and 0.2-μm-pore-size membranes showed distinction in preferential lifestyle. Vibrio spp. in the Yellow Sea (YS) was low in abundance and adopted a particle-associated lifestyle, whereas that in the East China Sea (ECS) and South China Sea (SCS) was more abundant and was likely in a temporary free-living state as a strategy to cope with nutrient limitation. Vibrio community compositions were also separated by sampling area, with different dominant groups in YS (Vibrio chagasii and Vibrio harveyi), ECS and SCS (Vibrio japonicus and V. chagasii). The community niche breadth was significantly wider in ECS and SCS than that of YS. Among species, V. chagasii and V. harveyi had the largest niche breadths likely reflecting strong competitive positions. Stochastic processes played important roles in shaping the geographical pattern of the vibrionic community. Environmental selection (e.g., temperature, salinity, and dissolved oxygen) had a much greater impact on the community in surface than in bottom water. The large proportions of unexplained variations (78.9%) imply complex mechanisms in their community assembly. Our study provides insights into the spatial distribution patterns and underlying assembly mechanisms of Vibrio at a broad spatial scale. IMPORTANCE Vibrio spp. may exert large impacts on biogeochemical cycling in coastal habitats, and their ecological importance has drawn increasing attention. Here, we investigated the spatial distribution pattern and community assembly of Vibrio populations along the Chinese marginal seas, spanning a wide spatial scale. Our results showed that the abundances of the Vibrio population increased with decreasing latitude and their preferential lifestyle differed among adjacent coastal areas. The compositions of Vibrio spp. were also separated by geographical location, which was mainly attributable to stochastic processes. Overall, this work contributes to the understanding of the ecological distribution patterns and the community assembly mechanisms of marine vibrios at a high spatial range. The large proportion of unexplained variations indicates the existence of complex mechanisms in the assembly of vibrionic community which should be considered comprehensively in future.

Significance of nutrients in oxygen-depleted bottom waters via various origins on the mid-outer shelf of the East China Sea during summer

East China Sea (ECS) is considered one of the largest dissolved oxygen (DO) depleted areas in the world's oceans. To assess the relative importance of water sources and biological processes to modulate low DO water over the ECS shelf, we conducted 7 cruises in the summers between 2004 and 2015. To cover a broad study area, observations were taken by both Chinese and Japanese research vessels in 2013, the consistent DO values were obtained in the intercalibration station from China and Japan. The subsurface/bottom water DO depletion was observed over both the inner and mid-outer shelves. In 2009 and 2013, the low DO (3-4.2 mg L^-1) area covered ca. 4 × 10⁴ km² on the mid-outer shelf, comparable with the reported area of summer hypoxia off the Changjiang estuary. On the basis of a seven endmember mixing model using heavy rare earth elements, temperature and salinity data collected in 2013 and 2015, we determined that on the southern shelf the low DO water mainly originated from Kuroshio Subsurface Water (28-72%). Both the DO level in the dominant source water and organic matter (OM) remineralization modulated the formation and expansion of low DO waters. Oxygen-depleted bottom waters featured with high nutrients were both transported from the water's source regions and produced by OM remineralization on the mid-outer shelf. The estimated regenerated nutrient fluxes derived from OM respiration in the bottom water of the mid-outer shelf were equivalent to 18-37% of the nitrate and nitrite, and 2 to 5-fold the phosphorus, delivered from the Changjiang River in summer. The large quantity of regenerated nutrients from oxygen-depleted bottom waters on the mid-outer shelf could be utilized and support primary production in the adjacent oceans. Our findings provide valuable observation for simulation models of nutrient cycles and budgets in the ECS and adjacent oceans.

Encountering shoaling internal waves on the dispersal pathway of the pearl river plume in summer

Fundamentally, river plume dynamics are controlled by the buoyancy due to river effluent and mixing induced by local forcing such as winds and tides. Rarely the influence of far-field internal waves on the river plume dynamics is documented. Our 5-day fix-point measurements and underway acoustic profiling identified hydrodynamic processes on the dispersal pathway of the Pearl River plume. The river plume dispersal was driven by the SW monsoon winds that induced the intrusion of cold water near the bottom. The river effluent occupied the surface water, creating strong stratification and showing on-offshore variability due to tidal fluctuations. However, intermittent disruptions weakened stratification due to wind mixing and perturbations by nonlinear internal waves (NIWs) from the northern South China Sea (NSCS). During events of NIW encounter, significant drawdowns of the river plume up to 20 m occurred. The EOF deciphers and ranks the contributions of abovementioned processes: (1) the stratification/mixing coupled by wind-driven plume water and NIWs disruptions (81.7%); (2) the variation caused by tidal modulation (6.9%); and (3) the cold water intrusion induced by summer monsoon winds (5.1%). Our findings further improve the understanding of the Pearl River plume dynamics influenced by the NIWs from the NSCS.

Southward spreading of the Changjiang Diluted Water in the La Niña spring of 2008

The La Niña of 2007/2008 was particularly strong, so was the southward flow of the cold, nutrient-rich Changjiang (Yangtze River) Diluted Water (CDW) when the winter monsoon started to blow in the fall. Here we use shipboard data in 2008 in two transects, one in the southwestern East China Sea and one in the southern Taiwan Strait, to show that as late as April in 2008 the CDW was still clearly identifiable when the winter monsoon had weakened. Waters as cold as 16 °C with a salinity lower than 30 still occupied the southwestern East China Sea. Waters of 17 °C and S < 32 could also be found off the coast of China in the central Taiwan Strait. The concentration of NO₃ + NO₂ was higher than 18 μmol L⁻¹ at both places, which was as much as 40 times higher than the northward moving South China Sea (SCS) water to the east. As a result, the Changjiang River plume may be a significant source of nutrients, particularly N, to the oligotrophic, N-poor SCS, especially in the La Niña years. Indeed, colder and more turbid CDW was more intense and went farther south in 2008 compared with the normal springs of 2006, 2007 and 2009.

Spatial Distribution of Phytoplankton Community Composition and Their Correlations with Environmental Drivers in Taiwan Strait of Southeast China

Large-scale dinoflagellate blooms have appeared in recent decades in the Taiwan Strait, Southeast China. To study spatial variability of phytoplankton community composition, physical and chemical environmental drivers in surface seawater of the Taiwan Strait, we conducted cruises in May and July 2019. Cell numbers of dinoflagellates were significantly higher than that of diatoms in most sampling stations during the cruise in May, whereas diatoms were the major contributor to autotrophic biomass in July. Phytoplankton community shifted from a dinoflagellate- and diatom-dominated system in May to diatom dominance in July. The dominant phytoplankton species (genera) were the harmful algal bloom dinoflagellates Prorocentrum donghaiense and Scrippsiella trochoidea and the diatoms Coscinodiscus in May, and Rhizosolenia, Pseudo-nitzschia, and Guinardia in July. Cell densities of dinoflagellates and P. donghaiense reduced exponentially with increasing seawater temperature and salinity and decreasing dissolved inorganic nitrogen (DIN) concentrations. Based on the results of our work and previous studies, it becomes obvious that harmful dinoflagellate blooms are likely to be a major component of the planktonic food web in the Taiwan Strait at a temperature of 17.0–23.0 °C, a salinity of 29.0–33.0 psu, and a DIN concentration higher than 2.0 μmol L–1.

Elevated primary productivity triggered by mixing in the quasi-cul-de-sac Taiwan Strait during the NE monsoon

The Taiwan Strait (TS) connects two of the largest marginal seas in the world, namely the East China Sea (ECS) and the South China Sea (SCS). When the NE monsoon prevails, the fresh, nutrient-rich but P-limited China Coastal Current (CCC) flows southward. Yet, part of the CCC turns eastward after entering the TS and then turns back toward the ECS. In the southern TS, part of the salty, N-limited, northward TS current (TSC) in the eastern part of the strait turns westward and eventually returns to the SCS. That is, the TS acts like a quasi-cul-de-sac during the NE monsoon season. Based on 822 samples from 28 cruises, the highest Chl. a concentration occurs at a salinity around 32 even though the nutrient concentration is not the highest. Mixing the cold-fresh-eutrophic CCC water and the warm-salty-oligotrophic TSC water results in a more suitable condition for biological uptake in both the southern ECS and the northern SCS.