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Nutrients limiting phytoplankton growth in the ocean are a critical control on ocean productivity and can underpin predicted responses to climate change. The extensive western subtropical North Pacific is assumed to be under strong nitrogen limitation, but this is not well supported by experimental evidence. Here, we report the results of 14 factorial nitrogen–phosphorus–iron addition experiments through the Philippine Sea, which demonstrate a gradient from nitrogen limitation in the north to nitrogen–iron co-limitation in the south. While nitrogen limited sites responded weakly to nutrient supply, co-limited sites bloomed with up to ~60-fold increases in chlorophyll a biomass that was dominated by initially undetectable diatoms. The transition in limiting nutrients and phytoplankton growth capacity was driven by a gradient in deep water nutrient supply, which was undetectable in surface concentration fields. We hypothesize that this large-scale phytoplankton response gradient is both climate sensitive and potentially important for regulating the distribution of predatory fish.

We examined the evolution of intermittent hypoxia off the Pearl River Estuary during three cruise legs conducted in July 2018: one during severe hypoxic conditions before the passage of a typhoon and two post-typhoon legs showing destruction of the hypoxia and its reinstatement. The lowest ever regional dissolved oxygen (DO) concentration of 3.5 μmol kg−1 (~ 0.1 mg L−1) was observed in bottom waters during Leg 1, with a ~ 660 km2 area experiencing hypoxic conditions (DO < 63 μmol kg−1). Hypoxia was completely destroyed by the typhoon passage but was quickly restored ~ 6 days later, resulting primarily from high biochemical oxygen consumption in bottom waters that averaged −14.6 ± 4.8 μmol O2 kg−1 d−1. The shoreward intrusion of subsurface shelf waters contributed to an additional 8.6 ± 1.7 % of oxygen loss during the reinstatement of hypoxia. Freshwater input-induced stratification, suppressing turbulent mixing induced by wind stress and/or tidal forcing, stabilized the water column and facilitated the hypoxia formation. The rapid reinstatement of summer hypoxia has a comparable timescale with water residence time and that of its initial disturbance from frequent tropical cyclones or high-wind events throughout the season. This has important implications towards better understanding the intermittent nature of hypoxia, and predicting coastal hypoxia in a changing climate.

To distinguish between the distribution and activity of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) in the Pearl River estuary (PRE), we investigated the DNA- and cDNA-based beta-proteobacterial and archaeal amoA genes on three sizefractionated particles of > 3.0 mu m, 0.45-3.0 mu m, and 0.22-0.45 mu m. Results showed that AOB were more abundant in the freshwater with high concentrations of ammonium (NH4+) and low dissolved oxygen, whereas AOA were dominant in the NH4+-depleted seawater and sensitive to temperature. Obvious shifts in ammonia-oxidizing communities were found along the salinity gradient in the PRE. AOB clearly presented a particle-associated nature, as evidenced by higher relative abundance of amoA genes attached to the large particles (> 3.0 mu m) and their transcripts exclusively detected on this fraction. Moreover, higher transcriptional activity (indicated by the cDNA/DNA ratio) of AOB on the large particles, suggesting AOB were actively involved in ammonia oxidation despite their lower abundance in the mid- and lower estuarine regions. In contrast, AOA exhibited higher transcriptional activity on the 0.45-3.0 mu m and 0.22- 0.45 mu m particles, implying the free-living strategy of these microbes. Together, these findings from field observations provide useful information on the ecological strategies of ammonia-oxidizing communities in response to different environmental conditions.

Observations reveal the high spatiotemporal variability of the hypoxia in the western and eastern coastal transition zones (WCTZ and ECTZ) off the Pearl River Estuary (PRE). We utilized data from cruise observations, buoy mooring, and a three-dimensional model based on ROMS to investigate the hypoxia variability response to the typical synoptic variation of the summer monsoon, i.e., from prevailing upwelling-favorable wind (UFW), the ensuing episodic upwelling relaxation (URW), downwelling (DFW), to downwelling relaxation wind (DRW). During the UFW, we found that the northeastward shelf current converges with nutrient-rich river plume, forms strong stratification, and provides hubs for depositing detritus for hypoxia to develop in the CTZ. The ensuing URW and DFW forcing alters the transport to advect the plume and detritus westward, weakens vertical mixing, and enhances hypoxia in the WCTZ. Correspondingly, opposite conditions occur in the ECTZ as the plume veers westward during the URW and DFW. Contrarily, the weakening westward transport during the DRW forcing restores the biophysical conditions to their states when there is an UFW event. We found that varying wind-driven circulation interacts with the plume to jointly regulate the transport of nutrient and detritus, water vertical mixing, and residence time, and, thus, the hypoxia variability in the CTZ. The variability of hypoxia in the CTZ is less controlled by the river discharges once biogeochemical conditions become saturated. Our study illustrates the biophysical control of hypoxia variability that is frequently observed in estuaries and coastal oceans. © 2021. American Geophysical Union. All Rights Reserved.

Coastal downwelling is generally considered to have a limited biological effect compared with coastal upwelling. In this study, downslope transport of nearshore, nutrient-enriched waters during downwelling is found to induce distinct biological productivity in the water column over the northeastern South China Sea (NSCS). By conducting a process-driven study over a widened shelf with intensified downwelling in the NSCS, we investigated the biophysical processes associated with strong nutrient enrichment in the water column of downwelled waters. These processes and underlying mechanisms are largely unreported and remain unclear. Field measurements and a three-dimensional coupled physical-biological model incorporating nitrate (N), phytoplankton (P), zooplankton (Z), and detritus (D) were utilized to investigate distinct cross-shore nutrient transport over the uniquely widened NSCS shelf. We revealed that intensified downwelling circulation, dynamically induced by the widened shelf topography, enhanced chlorophyll a and biological productivity in a strip of well-mixed water over the inner shelf as well as in the downwelled water over the mid-shelf. Strong time lags and spatial differences existed among N, P, and Z because of the physical transport and the ensuing biogeochemical response. The intensified downslope transport of nutrient-rich coastal water formed distinct cross-shore wedge-shaped P, Z, and D structures, while N was rapidly consumed in the water column. This study illustrates the underlying coupled physical-biogeochemical processes associated with the observed biogeochemical response to wind-driven downwelling circulation over the variable shelf, which are commonly found in coastal oceans worldwide. Copyright © 2021 Han, Gan, Dai, Lu, Liang and Zhao.

Globally expanding hypoxia in estuaries and coastal oceans has largely been attributed to the elevated river nutrient inputs, whereas the role of river-delivered terrestrial organic matter (OMterr) in hypoxia formation has been less investigated. This study uses a coupled physical-biogeochemical model and observations to investigate how OMterr directly (via remineralization) and indirectly (via the nutrients released from OMterr remineralization) promotes hypoxia development in the coastal transition zone off the Pearl River Estuary. Results show that direct contribution of OMterr remineralization to total oxygen consumption by terrestrial and marine organic matter negatively correlates with salinity, decreasing from over 60% in the upper estuary to nearly 0% in the far reaches of the river plume, and is higher in the upstream (average 30%) than the downstream region (average 18%). Nevertheless, the nutrients released from OMterr remineralization greatly sustain an indirect contribution to oxygen depletion and hypoxia formation downstream. The increasing relative importance of indirect over the direct effect of OMterr to hypoxia along the plume path is a combined result of the wind-driven eastward shelf current and the OMterr-released nutrients being advected farther downstream than the sinking OMterr. This highlights that without including the indirect effect of OMterr may underestimate the role of OMterr in hypoxia formation in aquatic systems. Examinations of the hypoxia response to varying riverine loads further suggest that reducing the nutrient and OMterr loads is required for hypoxia mitigation in the upstream region while reducing the nutrient load alone is more effective in mitigating hypoxia in the downstream. © 2020 Association for the Sciences of Limnology and Oceanography

Using a validated, three-dimensional, coupled physical-biogeochemical model, this study examines the dynamics of dissolved inorganic carbon (DIC) in the South China Sea (SCS). The model identifies characteristic differences in DIC concentrations between the SCS and adjacent West Philippine Sea (WPS), showing an increase in DIC of up to 140 μmol kg−1 in the upper 2000 m layer from the WPS to the vicinity of the northern SCS. This increasing pattern continues from the northern to the southern SCS, but to a much lower degree (<20 μmol kg−1 increase). Overall, spatial and seasonal variation of DIC in the SCS are evidently modulated by both intrinsic dynamics (e.g., river plumes, vertical mixing, basin-scale upwelling and eddies), and extrinsic exchanges via various straits. The inputs of DIC to the SCS are mainly through the Luzon Strait in the upper layer (depth, h < 600 m) and from the deep WPS at > 1600 m. Exports of DIC occur through other straits and the SCS intermediate water outflow (600 m to 1600 m) via the Luzon Strait. The model shows upward transport of DIC in the upper 400 m and below 1100 m, in contrast to downward DIC transport between 400 m and 1100 m. Seasonally, both DIC concentrations and fluxes in the upper layer have larger dynamic ranges in winter than in summer, attributable to the combined effects of enhanced Kuroshio intrusion, stronger vertical upwelling and greater biogeochemical alterations. A balance of various constitutive DIC terms revealed that horizontal and vertical advection is an order of magnitude greater than the other terms of the DIC budget, which has an opposite sign and thus tends to neutralize each other. The model indicated a decrease in the DIC inventory in the euphotic zone in spring and summer caused by net DIC consumption, and a contrasting increase in fall and winter primarily influenced by DIC vertical transport. In intermediate and deep layers, physical transport processes play a dominant role in the seasonal variation o

Growing human activities in recent decades have collectively resulted in large amounts of nutrients export into coastal oceans. As the most reactive nitrogen species, ammonium (NH4+) plays the critical role in biogeochemical cycles in estuaries and the coastal ocean. In the highly polluted Pearl River Estuary (PRE), NH4+ predominates to be the energy source for nitrification and to be the material source for bacteria and phytoplankton to grow. Both above processes are affected by light, yet in opposite ways. Nevertheless, rare studies paid attention to dual NH4+ transformation processes specifically during dark conditions. By using nitrogen isotope tracer technique, we quantitatively and simultaneously differentiated two distinctive NH4+ consumption pathways, that is, NH4+ oxidation (AO(D)) and assimilation (AA(D)) rates, especially under dark conditions along the PRE during the 2015 and 2017 summer cruises when biological activities were the highest. We found the NH4+ transformations display a bilayer structure with AA(D) > AO(D) in almost all the surface waters and vice versa in all bottom waters, suggesting bacteria and phytoplankton (mainly bacteria) control NH4+ consumption in surface during the night while nitrifiers are the major NH4+ consumer in the bottom waters. Through redundancy analysis, we found that both processes are mainly driven by NH4+ in the PRE during summer. In addition, in the downstream PRE during two cruises, AO(D) at most contributed 27% and 8% of the water column community dissolved oxygen (DO) consumption assuming the Redfield stoichiometry has no direct effect on hypoxia formation in the PRE during summer.

Nitrification plays a central role in the estuarine nitrogen cycle. Previous studies in estuary mainly focused on the niche partition between ammonia-oxidizing archaea (AOA) and bacteria (AOB), while the diversity, activity, biogeography, and ecophysiology of different AOA groups remained unclear. Here, we for the first time report on niche partitioning and differentially distributed active populations among diverse AOA (inferred from amoA gene) in a typical subtropical estuary - Pearl River estuary (PRE). In the water column of the PRE, the AOA communities mainly consisted of water column A (WCA) and SCM1-like (Nitrosopumilus maritimus-like) sublineages. Surprisingly, we observed a strong disagreement in AOA communities at DNA and RNA levels. In DNA samples, WCA generally dominated the AOA community, and the distributional pattern indicated that WCA I and WCA II sublineages preferred oceanic and coastal conditions, respectively. In contrast, diverse SCM1-like sublineages were identified, and outnumbered WCA at RNA level, in which SCM1-like-III was limited to freshwater, while the rest of the sublineages were widely distributed in the estuary. The SCM1-like sublineages strongly correlated with nitrification rate, which indicated their important contribution to ammonia oxidation. Furthermore, intense nitrification contributed significantly to hypoxia conditions (nitrification contributed averaged 12.18% of oxygen consumption) in the estuary. These results revealed different ammonia-oxidizing activities and niche partitioning among different AOA sublineages in estuarine water, which was unexplored in previous DNA and clone library-based studies. The ecological significance and functioning of the diverse AOA should be further explored in the marine ecosystem. © 2020 Author(s).

Globally expanding hypoxia in estuaries and coastal oceans has largely been attributed to the elevated river nutrient inputs, whereas the role of river-delivered terrestrial organic matter (OMterr) in hypoxia formation has been less investigated. This study uses a coupled physical-biogeochemical model and observations to investigate how OMterr directly (via remineralization) and indirectly (via the nutrients released from OMterr remineralization) promotes hypoxia development in the coastal transition zone off the Pearl River Estuary. Results show that direct contribution of OMterr remineralization to total oxygen consumption by terrestrial and marine organic matter negatively correlates with salinity, decreasing from over 60% in the upper estuary to nearly 0% in the far reaches of the river plume, and is higher in the upstream (average 30%) than the downstream region (average 18%). Nevertheless, the nutrients released from OMterr remineralization greatly sustain an indirect contribution to oxygen depletion and hypoxia formation downstream. The increasing relative importance of indirect over the direct effect of OMterr to hypoxia along the plume path is a combined result of the wind-driven eastward shelf current and the OMterr-released nutrients being advected farther downstream than the sinking OMterr. This highlights that without including the indirect effect of OMterr may underestimate the role of OMterr in hypoxia formation in aquatic systems. Examinations of the hypoxia response to varying riverine loads further suggest that reducing the nutrient and OMterr loads is required for hypoxia mitigation in the upstream region while reducing the nutrient load alone is more effective in mitigating hypoxia in the downstream. © 2020 Association for the Sciences of Limnology and Oceanography

Satellite‐derived chlorophyll data, in situ measurements, and time series buoy monitoring revealed the persistent and deteriorating seasonal eutrophication and hypoxia with two distinct centers in the coastal transition zone (CTZ) between the Pearl River Estuary and the adjacent continental shelf off Hong Kong. We used a process‐oriented coupled physical‐biogeochemical numerical model to investigate the intrinsic physical and biogeochemical dynamics under the extrinsic forcing of wind, river discharge, and tides for the hypoxia formation in the CTZ. We found that the convergence induced by cyclonic vortices in the CTZ, which were formed by the buoyancy‐driven currents and wind‐driven shelf currents, created the stable water column with weak mixing and long residence time and accumulated nutrients and organic matter for the eutrophication and hypoxia development. The biophysical responses to spring‐neap cycle further demonstrated the coherent linkages of oceanic currents, vortex formation, and convergence of organic matter within the CTZ and indicated the critical role of mixing in the dissolved oxygen (DO) budgets during the spring‐neap tidal cycle. The study found that biogeochemical substances and processes are necessary conditions for eutrophication and hypoxia formation, and when these necessary conditions are combined with the favorable hydrodynamic conditions, they form a sufficient condition for eutrophication and hypoxia formation in the CTZ.

Global Oceans is one chapter from the State of the Climate in 2019 annual report and is avail-able from https://doi.org/10.1175/BAMS-D-20-0105.1. Compiled by NOAA’s National Centers for Environmental Information, State of the Climate in 2019 is based on contr1ibutions from scien-tists from around the world. It provides a detailed update on global climate indicators, notable weather events, and other data collected by environmental monitoring stations and instru-ments located on land, water, ice, and in space. The full report is available from https://doi.org /10.1175/2020BAMSStateoftheClimate.1.

Estuarine systems are of key importance for the riverine input of silicon (Si) to the ocean, which is a limiting factor of diatom productivity in coastal areas. This study presents a field dataset of surface dissolved Si isotopic compositions (30SiSi(OH)4) obtained in the estuaries of three of the world’s largest rivers, the Amazon (ARE), Yangtze (YRE), and Pearl (PRE), which cover different climate zones. While 30SiSi(OH)4 behaved conservatively in the YRE and PRE supporting a dominant control by water mass mixing, significantly increased 30SiSi(OH)4 signatures due to diatom utilization of Si(OH)4 were observed in the ARE and reflected a Si isotopic enrichment factor 30 of −1.0±0.4‰ (Rayleigh model) or −1.6±0.4‰ (steady state model). In addition, seasonal variability of Si isotope behavior in the YRE was observed by comparison to previous work and most likely resulted from changes in water residence time, temperature, and light level. Based on the 30 value obtained for the ARE, we estimate that the global average 30SiSi(OH)4 entering the ocean is 0.2-0.3‰ higher than that of the rivers due to Si retention in estuaries. This systematic modification of riverine Si isotopic compositions during estuarine mixing, as well as the seasonality of Si isotope dynamics in single estuaries, needs to be taken into account for better constraining the role of large river estuaries in the oceanic Si cycle.

One interesting question in the study of marine microbial ecology is how ocean currents affect the geographic distributions of microorganisms. The Zhe-Min Coastal Current (ZMCC) is strong in winter and could influence the plankton communities in Hong Kong coastal waters by bringing species from far north. In this study, we investigated the spatial distribution of microbial eukaryotic communities (MEC) in the ZMCC and evaluated the relative contribution of environmental and spatial factors on the variations of MEC. The results showed that diverse taxa of diatoms (Bacillariophyceae) and Dinophyceae (Dinoflagellata) dominated the sequences in all samples. MEC compositions in the Hong Kong coastal waters were closer to the communities in the ZMCC than other adjacent areas. Abundant and rare taxa exhibited a similar geographic pattern of distribution. Environmental factors alone explained 14.1% and 27.6% of the variations in the entire and abundant communities, respectively, which were higher than pure spatial factors (8.3% and 19.5%, respectively). The lower importance of spatial factors (dispersal limitation) might be explained by the passive dispersal favored by current transportation and was further supported by the high immigration rate computed in the neutral community model (NCM). High fitness to the NCM (R-2 = 0.74) indicated the predominant role of neutral process in shaping the MEC. The large proportion of unexplained variations in rare taxa suggested the importance of other potential factors, especially biotic interactions. Our study demonstrates the influences of coastal current on local and regional structures of MEC.

The South China Sea (SCS) is one of the largest marginal seas in the world, yet the processes that drive its nutrient distribution are not well understood. Using the three-dimensional China Sea Multiscale Ocean Modeling System (CMOMS) embedded with a nitrogen-phosphorus-phytoplankton-zooplankton-detritus ecosystem model, we investigated nutrient transport and dynamics in the SCS. The model was well validated with field and remotely sensed observations. We found that the unique layered distribution of nutrients that increases from north to south in the upper layer and increases in the opposite direction in the middle and deep layers. We found that the lateral and vertical fluxes associated with three-dimensional ocean circulation and biogeochemical processes shape the distribution. The lateral fluxes are provided extrinsically by the sandwich-like transport through Luzon Strait (LS) and the exchanges through other straits, and intrinsically by the interior vertical motion due to flow-slope topography interaction. The nutrient deficiency in the upper layer due to dilution by inflow from the Western Pacific Ocean is mainly compensated by the upward nutrient-rich deep water. The nutrient transport is jointly regulated by volume transport and the uneven spatiotemporal distribution of nutrients. We demonstrated that labile organic matter plays an important role in total nutrient budgets. In euphotic layer, biological production is the major sink, but remineralization contributions to PO4 are more important than to NO3 due to the higher recycling rate of PO4. In the shelf water, coastal upwelling serves as major source for both NO3 and PO4, vertical diffusion is another important source of PO4 due to the strong downward increasing gradient. © 2020 Elsevier Ltd