The variability of chlorophyll a concentration (Chl a) in the open South China Sea (SCS) was examined using observations from two Bio‐Argo floats. During the period of September 2014 to August 2015, there was a permanent subsurface Chl a maximum (SCM) in the depth range of 48 to 96 m in the central basin of the SCS. In the northern basin, the SCM disappeared in winter, replaced by enhanced surface layer phytoplankton with high Chl a. The values of the SCM were influenced by the vertical displacement of isotherms. Strong wind forcing and surface cooling were the main physical drivers of high surface Chl a in winter. In the north, stronger wind than in the center, lower sea surface temperature (SST) than in the center, and Kuroshio water intrusion were more favorable for the upward transport of nutrient‐rich deep water. A large amount of nitrate could be advected from the Taiwan Strait and shallow continental shelf to the northern basin in winter. A combination of strong wind mixing, surface cooling, Kuroshio water intrusion, and horizontal advection caused the winter surface phytoplankton bloom in the north.

Two Bio‐Argo floats measured the concentration of chlorophyll‐a, the backscattering coefficient, the fluorescence of humic‐like dissolved organic matter, dissolved oxygen, and temperature and salinity in the northern and central basins of the South China Sea for over 2 years. Temporal evolutions of bio‐optical properties were analyzed at surface, subsurface, and in the whole water column, respectively. It was found that (1) The seasonal variability of the surface chlorophyll‐a was highly controlled by photoacclimation, especially in the central basin; (2) backscattering in the upper 150 m was nearly constant, exhibiting no distinct seasonality; (3) with vertical mixing, particles from the deep chlorophyll maxima were entrained into the mixed layer resulting in enhanced surface chlorophyll during the early winter. This phenomenon may mislead a study based on satellite data which is likely to interpret it as blooming rather than a redistribution of phytoplankton within the water column; (4) analysis of a winter bloom and an anticyclonic eddy reveal that physical entrainment and biological photoacclimation modulated the vertical distributions of chlorophyll‐a and particles and potentially also changes of phytoplankton community composition; and (5) fluorescent dissolved organic matter was found to be highly coupled to phytoplankton dynamics in both basins, with a maximum (after removing the contribution of physical convective mixing) located at the depth of chlorophyll‐a subsurface maximum.

The particulate backscattering coefficient (bbp) provides effective proxies for particulate organic carbon (POC) and phytoplankton carbon (Cphy); however, their bio-optical relationships in the oligotrophic ocean are rarely reported. In this work, based on the in-situ synchronous optical and biogeochemical measurements in the oligotrophic South China Sea (SCS) basin, we refined the regional relationships between POC (and Cphy) and bbp and investigated the impacts of phytoplankton community compositions and size classes on the bbp variability. The observations showed that: 1) POC and Cphy exhibited good linear relationships with bbp; 2) the relationship between Cphy and POC could also be fitted in a linear function with a positive POC intercept, and the POC contributed by phytoplankton-covarying non-algal particles was nearly two-fold of Cphy; and 3) the POC-specific bbp (b*bp) was positively correlated with the fraction of the phytoplankton groups haptophytes (Type 8) and diatoms to total Chla, but negatively correlated with the fraction of pico-phytoplankton to Chla (fpico). These findings suggest that in oligotrophic waters, the variability of b*bp was mainly determined by the variability in the relative contribution of large phytoplankton with complex structures.

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Deep ocean microorganisms consume particulate organic matter that is produced in the surface ocean and exported to deeper depths. Such consumption not only enriches inorganic carbon in the deep ocean but also transforms organic carbon into recalcitrant forms, creating an alternative type of carbon sequestration. However, estimates of deep microbial carbon demand substantially exceed the available particulate organic carbon exported from the euphotic zone, resulting in an unbalanced dark ocean carbon budget. Here, we combined field-based microbial activity parameters, integrated multiyear particle export flux data, sinking particle fluxes measured by sediment traps, and optical data from Biogeochemical-Argo floats to quantify the main sources of organic carbon to the dark ocean. Laterally transported particles (including sinking and suspended particles) serve as a major energy source, which directly provide organic carbon and enhance new organic carbon production by dark carbon fixation, reconciling the mismatch in the regional carbon budget. Plain Language Summary Particulate organic matter, produced by phytoplankton in the upper ocean, can sink through the water column and act as a source of organic matter to the deep ocean. These particles are decomposed to carbon dioxide by microorganisms, resulting in dissolved inorganic carbon and organic carbon resistant to decomposition in the deeper ocean. This process controls the biological sequestration of CO 2 by the oceans. However, there is an imbalance between the low amount of organic carbon exported from the photic zone and the high microbial demand for carbon in the dark ocean. We attempted to explain how the deep ocean carbon and energy supply can meet the microbial metabolic demand. Four main organic carbon sources were measured and quantified in the South China Sea: particles that come from the photic zone, particles that move laterally through the ocean, dark carbon fixation, and dissolved organic carbon. We found that laterally transported particles from the surrounding margins provide a direct source of organic carbon and also allow for much new organic carbon production through dark carbon fixation. These particles, which provide a major energy source to dark ocean ecosystems, help resolve the mismatch in the regional carbon budget.

Super typhoon Rammasun (2014) traveled across the South China Sea on July 16–18. Its far-field impacts on phytoplankton dynamics in the upper ocean were documented by a Biogeochemical-Argo (BGC-Argo) float located 200 km to the left of its track. Both surface chlorophyll-a concentration (Chla) and particulate backscattering coefficient (bbp) were observed to increase substantially within two distinct stages. The initial increase occurred during the passage of the typhoon, and the subsequent increase happened 5 days after the typhoon. In contrast, depth-integrated Chla and bbp in the upper 150 m underwent negligible changes throughout the entire period. The key lies in the fact that surface phytoplankton increases in the far-field region resulted from the physically driven vertical redistribution of particles, rather than from biological alternations. The first increase was attributed to the typhoon-induced strong turbulence which deepened the surface mixed layer, and thus entrained subsurface particles to the surface; the second was due to the post-typhoon adiabatic quasi-geostrophic adjustment of the upper ocean that gradually raised the isopycnals (and thus subsurface particles). These results challenge the prevailing wisdom on typhoon impacts, and thus shed new lights on the nature of the upper ocean responses to typhoons from both physical and biological perspectives.

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As China rapidly urbanizes, land resources tend to deplete. This paper aims to identify and propose a resolution of land use conflicts to promote sustainable land use and coordinate the interaction between humans and the environment in urban areas. The methodology of land use conflict assessment within spatial zoning of the Su–Xi–Chang region was evaluated. Taking into consideration the intensity of human activities and the background condition of the natural environment, we divided the study area into a few regions. Furthermore, we developed a methodology by calculating landscape complexity, fragility, and stability based on spatial zoning so as to derive the spatio-temporal characteristics of the land use conflict index (LUCI) in the Su–Xi–Chang region. The results indicate the following: (1) According to the urban spatial attribute index (USAI) statistics, we delineate the core, fringe, and suburban zones of the Su–Xi–Chang region, which accounted for 9.61%, 11.06%, and 79.33% of urban agglomerations respectively; (2) from 1990 to 2018, the fragility index (FI) and LUCI showed S-shaped curves, while the complexity (CI) and the stability indices (SI) exhibited minor fluctuations in the fringe and suburb zones; and (3) intensive and severe conflict is concentrated in core areas due to quite intense human activities and in fringe areas due to an increased interaction between humans and land, while moderate conflict is mainly found in rural and suburban areas that pose as a natural environmental space. The results can serve as a theoretical framework for an effective identification of the LUCI in an urban agglomeration and coordination of the optimal allocation of regional land resources.