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The amount of fluvial input has important impacts on shelf regions. To understand how the magnitude of fluvial discharge affects plume ecosystems, particularly organic carbon consumption, data over 10 consecutive summers (2003–2012) were examined in the Changjiang River plume of the East China Sea. The area of the Changjiang River plume ranged from approximately 4.90 × 103km2to 94.83 × 103km2and varied in proportion to the freshwater discharge rate. Plankton community respiration was at the medium to high end of the values reported for coastal regions. Total organic carbon consumption over the Changjiang River plume was positively correlated with the Changjiang freshwater discharge. This scaling relationship might be associated with river flows delivering allochthonous organic carbon and dissolved inorganic nutrients into the East China Sea. However, plankton community respiration (per m3basis) decreased as the area of the Changjiang River plume increased; this rate appeared to have been influenced by particulate organic carbon (POC) levels and/or total plankton biomass. Even though POC was dominated by phytoplankton biomass allometrically, bacteria contributed more to plankton community respiration, possibly signifying that these bacteria were more reliant on plankton‐derived organic matter. Even with high phytoplankton biomass and primary productivity, this plume ecosystem was heterotrophic, albeit close to a state of carbon balance. To support high organic carbon consumption, in addition to allochthonous sources from fluvial runoff, a large amount of organic matter might have been autochthonous and derived from plankton in the Changjiang River plume.

Upwelling and internal waves are known to affect the physical and chemical characteristics of marine ecosystems, yet the processes and mechanisms by which internal waves influence phytoplankton biomass and community composition in upwelling regions are still unclear. In this study, a 72‐h time series of observations was conducted in an upwelling system in the northern South China Sea during the summer of 2014. The results showed that the intensity of upwelling was affected by internal waves, which caused nutrient fluctuations in the upper water column. The phytoplankton total chlorophyll aresponded positively to the increase of nutrient concentrations, but only after a time lag of 12–16 h. This overall response was the net result of four different types of responses displayed by nine specific phytoplankton groups. All groups that displayed immediate positive responses continued to respond positively at least 12 h later, whereas all groups that responded negatively showed no time‐lagged responses. Based on the vertical distributions of the nine phytoplankton groups and their known physiological traits, we suggest that these different types of response were the net result of a rapid physical transport effect and a time‐lagged, physiological effect via bottom‐up control.

The importance of biodiversity effects on ecosystem functioning across trophic levels, especially via predatory–prey interactions, is receiving increased recognition. However, this topic has rarely been explored for marine microbes, even though microbial biodiversity contributes significantly to marine ecosystem function and energy flows. Here we examined diversity and biomass of bacteria (prey) and nanoflagellates (predators), as well as their effects on trophic transfer efficiency in the East China Sea. Specifically, we investigated: (i) predator diversity effects on prey biomass and trophic transfer efficiency (using the biomass ratio of predator/prey as a proxy), (ii) prey diversity effects on predator biomass and trophic transfer efficiency, and (iii) the relationship between predator and prey diversity. We found higher prey diversity enhanced both diversity and biomass of predators, as well as trophic transfer efficiency, which may arise from more balanced diet and/or enhanced niche complementarity owing to higher prey diversity. By contrast, no clear effect was detected for predator diversity on prey biomass and transfer efficiency. Notably, we found prey diversity effects on predator–prey interactions; whereas, we found no significant diversity effect on biomass within the same trophic level. Our findings highlight the importance of considering multi-trophic biodiversity effects on ecosystem functioning in natural ecosystems.

The importance of biodiversity effects on ecosystem functioning across trophic levels, especially via predatory–prey interactions, is receiving increased recognition. However, this topic has rarely been explored for marine microbes, even though microbial biodiversity contributes significantly to marine ecosystem function and energy flows. Here we examined diversity and biomass of bacteria (prey) and nanoflagellates (predators), as well as their effects on trophic transfer efficiency in the East China Sea. Specifically, we investigated: (i) predator diversity effects on prey biomass and trophic transfer efficiency (using the biomass ratio of predator/prey as a proxy), (ii) prey diversity effects on predator biomass and trophic transfer efficiency, and (iii) the relationship between predator and prey diversity. We found higher prey diversity enhanced both diversity and biomass of predators, as well as trophic transfer efficiency, which may arise from more balanced diet and/or enhanced niche complementarity owing to higher prey diversity. By contrast, no clear effect was detected for predator diversity on prey biomass and transfer efficiency. Notably, we found prey diversity effects on predator–prey interactions; whereas, we found no significant diversity effect on biomass within the same trophic level. Our findings highlight the importance of considering multi-trophic biodiversity effects on ecosystem functioning in natural ecosystems.

Understanding the potential impacts of internal waves on phytoplankton communities in oligotrophic oceans remains an important research challenge. In this study, we elucidated the impact of internal waves on phytoplankton communities through a comprehensive 154‐hr time‐series of observations in the South China Sea (SCS). We identified distinctive variations in phytoplankton pigment biomass and composition across the upper, middle, and lower layers of the euphotic zone, which we attributed to the perturbations triggered by internal waves. Phytoplankton other than Prochlorococcusin the lower, nutrient‐replete layer likely benefitted from allochthonous nutrients introduced by internal waves, but their growth rates were constrained by light limitation, and their pigment biomass was held in check by microzooplankton grazing. In contrast, in the upper, nutrient‐depleted layer, the relative abundance of Prochlorococcusincreased, likely because of the ammonium regenerated by zooplankton. The middle layer, characterized as the deep chlorophyll maximum layer, exhibited a dynamic equilibrium characterized by nutrient and light co‐limitation. This equilibrium resulted in high nitrate assimilation and growth by phytoplankton. The balancing of those rates by significant grazing losses maintained total chlorophyll aconcentrations at a high level. Based on these findings, we proposed a three‐layer euphotic zone structure characterized by distinct physiological conditions, nutrient‐light dynamics, grazing pressure, and phytoplankton responses to internal waves. This three‐layer paradigm elucidated the intricate interplay between internal waves and phytoplankton communities and provided insights into the mechanisms that govern primary production and carbon cycling in oligotrophic oceanic ecosystems. Internal waves, a common occurrence in stratified oceans, play a crucial role in oligotrophic oceans by delivering nutrients to the euphotic zone. The enhancement of phytoplankton growth and primary productivity by this nutrient supply impacts the phytoplankton community composition. However, the specific mechanisms behind these changes remain uncertain. Our study, conducted at the SouthEast Asia Time‐series Study station, located at 116°E, 18°N in the northern South China Sea, provided valuable insights. Through satellite imagery, 154‐hr time series observations, and barotropic tide data, we firmly established the presence of internal wave activity in this region. Our observations revealed distinct variations in phytoplankton communities across different layers of the euphotic zone, driven by internal wave‐induced perturbations. We introduced a three‐layer structure within the euphotic zone, shedding light on the complex interplay between internal waves and phytoplankton communities. This understanding advances marine primary production estimates and enriches our comprehension of the global carbon cycle. Time‐series observations revealed varying phytoplankton responses to internal waves in euphotic layers of the South China Sea (SCS)Nutrient‐replete and nutrient‐depleted layers exhibited contrasting phytoplankton dynamics under internal wave influenceThree‐layer structure highlighted the complex interplay between internal waves, nutrients, light, and grazing in this oligotrophic system Time‐series observations revealed varying phytoplankton responses to internal waves in euphotic layers of the South China Sea (SCS) Nutrient‐replete and nutrient‐depleted layers exhibited contrasting phytoplankton dynamics under internal wave influence Three‐layer structure highlighted the complex interplay between internal waves, nutrients, light, and grazing in this oligotrophic system

Picophytoplankton are the smallest, most abundant photosynthetic organisms in the ocean. Knowledge of the diel variability of these tiny microbes has important implications for the structure of microbial food webs and key biogeochemical processes. However, insight into the mechanisms that underlie picophytoplanktonic diel dynamics is limited. By combining a field survey with a published dataset, we found that cell numbers and cell sizes/biomasses of picophytoplankton were tightly synchronized to the day‐night cycle, but they were in a quasi‐antiphase relationship to each other. This pattern is a confirmation and extension of previous studies. Mortality rates showed that Prochlorococcusand Synechococcuswere subject to considerable grazing pressure throughout the day and night. The quasi‐antiphase diel cycles in abundance and cell size/biomass are likely determined by the light‐dependent diel behavior of cell growth and division and continuous losses to grazing. This work significantly improves our understanding of autotrophic picoplankton in the oligotrophic ocean. Picophytoplankton are tiny, single‐celled photosynthetic organisms that contribute to almost all primary production in the vast euphotic zones of the oligotrophic ocean. Understanding their roles in that environment is critical but challenging, mainly because of their minuscule size and the complexity of microbial processes and interactions. Time‐series observations based on flow cytometry, a powerful technique that provides information about the numbers and sizes of picophytoplankton cells, have elucidated many ecological and biogeochemical processes associated with picophytoplankton, but some questions remain. A field survey in the northern South China Sea combined with a published dataset revealed that picophytoplankton cell size and biomass tended to decrease (increase) during the night (day) when cell numbers were increasing (decreasing). Such quasi‐antiphase cycles are likely a general feature of near‐steady‐state oligotrophic ecosystems and reflect the cycles of carbon fixation, energy storage, and cell growth during the daytime and cell division and energy depletion during the night. Mortality rates estimated via modified dilution experiments showed that Prochlorococcusand Synechococcuswere subject to considerable grazing pressure throughout the day and night. This work significantly improves our understanding of these microorganisms and may have implications for the carbon cycle in oligotrophic marine ecosystems. Picophytoplanktonic quasi‐antiphase diel cycles in abundance and cell size/biomass are likely a general feature of the oligotrophic oceanGrazing pressure on Prochlorococcusand Synechococcusis as high during the day as during the night Picophytoplanktonic quasi‐antiphase diel cycles in abundance and cell size/biomass are likely a general feature of the oligotrophic ocean Grazing pressure on Prochlorococcusand Synechococcusis as high during the day as during the night

Abstract We examined the spatial distributional relationships between diatom assemblages and water types during the winter in the East China Sea. Principal component analysis was used to identify two water types and two diatom assemblages in the study area. Coastal water types along the mainland China coastline had low temperature and salinity levels, but high nitrate levels. The shelf-mixing water type in the rest of the study area had higher temperatures and salinities and lower levels of nitrate. Diatom assemblage distribution was not spatially consistent with water type. The Kuroshio assemblage had a large standing stock, distributed along the surface of the shelf break. This assemblage is likely the result of Kuroshio surface water coming into contact with nutrient-rich water in the shelf area, triggering proliferation of certain diatom species. A background assemblage with low standing stock level persisted over the entire study area in both coastal water and the shelf-mixing water types. Our results support previous research: the background assemblage is due to poor growth conditions such as the convection of water during winter; there were no significant seasonal variations in the species composition.