Journal of Geophysical Research-Oceans最新文献

A record-breaking marine heatwave (MHW) occurred in the South China Sea (SCS) during the summer of 2020, causing substantial ecological impacts, while its influence on phytoplankton dynamics remains unknown. Here we employed a regionally optimized physical-biogeochemical model to examine the phytoplankton response to this event. An analysis of the mixed-layer heat budget in the MHW region revealed that the MHW was driven by enhanced shortwave radiation and reduced horizontal advection of cold water, associated with weakened upwelling off the coast of Vietnam. For phytoplankton, average surface chlorophyll-a (Chla) concentrations during the MHW experienced a significant decline in June and July, and a slight reduction in September compared to normal conditions, which can be attributed to reduced horizontal advection of nutrients associated with weakened upwelling. Furthermore, we found that the reduction in depth-integrated Chla in the euphotic zone was much smaller during the MHW due to the increase in subsurface Chla, which was also attributable to the changed lateral transport. This study highlights the importance of non-local effects of MHWs on phytoplankton distributions in the SCS.

Periodic outbreaks of crown-of-thorns starfish have led to severe coral reef degradation, with one hypothesized cause being eutrophication. Phosphorus (P) is one of the driving factors for eutrophication, but researches on the spatiotemporal distributions of P in coral reefs are limited, impeding our understanding of the P cycling in coral reefs and its correlation with starfish outbreaks. This study undertook an analysis of various P species, migration and transformation in seawater, sediments, and crown-of-thorns starfish within the coral reefs of the Xisha Islands, northern South China Sea. The results show that in seawater, P predominantly existed in the dissolved phase, with organic P constituting the largest fraction (>67%). Conversely, in sediments, P primarily existed as inorganic form (43%−95%), with calcium-bound P comprising the majority (26%−46%). Notably, exchangeable P accounted for a relatively smaller fraction (9%−20%) but played a significant role as a source of P released from sediments into seawater (0.01−0.17 mmol m⁻² hr⁻¹). During starfish outbreaks, starfish (1,000 individuals per hectare) assimilated P from seawater via phytoplankton or corals, resulting in substantial accumulation of both organic and inorganic P in their tissues (69−315 μmol g⁻¹). Meanwhile, starfish excretion released P back into the seawater, which contributes to the migration and transformation of various P forms. Historical data also show a close relationship between P content and starfish density. These findings highlight the interactions between starfish and P cycling within coral reefs, and provide valuable insights into conservation and restoration on coral reefs, especially those severely affected by starfish outbreaks.

Marine heatwaves (MHWs), extreme warm ocean temperature events, greatly impact marine ecosystems. While most MHW studies in the South China Sea (SCS) focus on the sea surface, subsurface characteristics remain less explored. This study uses high-resolution (1/12°) ocean reanalysis data sets to assess MHW properties with depth in the SCS from 1993 to 2022. We find that MHW intensities are typically stronger at subsurface levels (30–150 m) than at the surface, with significant spatial variations. The vertical structures of MHW maximum and cumulative intensities peak between 70 and 100 m with 2.7°C and 63°C days, respectively; MHW annual days and duration increase gradually from 10 to 2,000 m with duration peaks at 2,000 m (∼60 days), four times longer than that at the surface. The SCS experienced two major periods of extensive El Niño-related subsurface MHWs from 1997 to 2002 and 2008 to 2014, with MHWs reaching depths of 150 m and covering 70% of the upper 40 m. Strong El Niño events strengthen the western Pacific subtropical high, reducing summer monsoon winds, weakening coastal upwelling east of Vietnam, and increasing water temperatures. Regions of elevated eddy kinetic energy in the SCS suggest that significant mesoscale eddy activities are situated east of Vietnam. In addition, notable eddy heat transport was observed in the above region and west of the Luzon Strait in the upper 1,000 m. This may help to explain the high maximum and cumulative intensities of MHWs exceeding 100 m in the northern SCS.

Ferromanganese nodules are important marine storehouses for critical metals and windows for changing oceans. Although advanced in situ analytical techniques have been applied to visualize the elemental distribution in the nodule cross-sections, their spatial distribution remains largely uncertain. This study addresses this gap by employing micro X-ray fluorescence mapping of parallel nodule cross-sections to delineate the spatial distributions of critical metals (Mn, Fe, Co, Ti, Ni, and Cu) in three Co-rich ferromanganese nodules from the northwestern Pacific Ocean. The 10-layer Os isotopic compositions of one nodule closely align with the well-documented marine Os isotope evolution of seawater, providing a chronological framework and a maximum age of ∼36 Ma for these nodules. Three concentric chemostratigraphic layers, labeled L1, L2, and L3, were identified from the inside out, based on microscopic structures and the distributions of critical metals. The early growth stage was marked by Mn-rich, Si-rich, and high Mn/Fe ratios, suggesting a diagenetic-driven process attributed to high paleoproductivity conditions because of the low latitude of the study area at that time. The subsequent growth stages are all hydrogenetic in origin to be rich in Fe, Co, and Ti with low Mn/Fe ratios. The apparent detritus present during the second growth stage of the nodules may correspond to the stronger bottom currents in the early Miocene. The final mineralization stage indicates a more stable environment with diminished bottom current activity, leading to the formation of a dense, laminar hydrogenetic layer.

The Arctic Ocean is characterized by substantial seasonal and inter-annual variability, of which the sources and impacts are not yet fully understood. Here, we analyze how much of the variability found in in situ observations of biogeochemical and ecological variables collected at the Long-Term Ecological Research Observatory HAUSGARTEN can be explained by differences in the physical conditions in the water masses passing through the Fram Strait (FS). Employing a size-based plankton ecosystem model with nine distinct size classes of protist phyto- and zooplankton, we simulate standing stocks and fluxes within the nutrient, phytoplankton, zooplankton, and detritus pools in water parcels that follow trajectories tracing the opposing East-Greenland and West-Spitsbergen currents through the FS. Our model results agree with in situ observations of biogeochemical tracers, plankton size measurements, climatological data, and remote sensing observations. They show distinct temporal developments in plankton size composition, growth, and export in trajectory ensembles, highlighting how variable physical conditions affect the communities' specific growth histories. Our study indicates that 10%–72% of the variability in upper water column tracer concentrations observed in the FS can be attributed to differences in water parcel trajectories. The maxima of net primary production and vertical export along the trajectories occurred in some (spatial and temporal) distance upstream of the sites of in situ sampling. This study shows that Lagrangian modeling helps clarify complex biogeochemical-ecological relationships in highly dynamic systems such as the FS, which is urgently needed to understand the role of climate change in the Arctic carbon cycle.

The size distribution of submicron particles is essential for understanding their biogeochemical and optical roles, but it has seldom been measured. This study utilizes ViewSizer 3000, an instrument that tracks Brownian motions of particles, to measure the particle size distributions (PSD) from 250 to 1,050 nm in the North Pacific Ocean (NP) and the North Atlantic Ocean (NA) at depths from 5 to 500 m. The concentration of particles varies over one order of magnitude at any given size bin, with greater variations up to two orders of magnitude at sizes >600 nm. In both locations, concentrations decrease with depth. Bacterioplankton are a dominant component, accounting for 65%–90% of the submicron particles in the surface waters (<100 m) and approximately 30%–40% at depths >150 m at both sites. In the NP, the volume mean diameter increased approximately 5% from the morning to noon at the surface, probably resulting from the diurnal growth of bacterioplankton. In the NA, the concentration and mean size increased by >60% and ∼10% respectively after one storm that introduced a different particle population into the study area.