A pivotal role for ocean eddies in the distribution of microbial communities across the Antarctic Circumpolar Current.
ABSTRACT: Mesoscale variability and associated eddy fluxes play crucial roles in ocean circulation dynamics and the ecology of the upper ocean. In doing so, these features are biologically important, providing a mechanism for the mixing and exchange of nutrients and biota within the ocean. Transient mesoscale eddies in the Southern Ocean are known to relocate zooplankton communities across the Antarctic Circumpolar Current (ACC) and are important foraging grounds for marine top predators. In this study we investigated the role of cyclonic and anti-cyclonic eddies formed at the South-West Indian Ridge on the spatial variability and diversity of microbial communities. We focused on two contrasting adjacent eddies within the Antarctic Polar Frontal Zone to determine how these features may influence the microbial communities within this region. The water masses and microbiota of the two eddies, representative of a cyclonic cold core from the Antarctic zone and an anti-cyclonic warm-core from the Subantarctic zone, were compared. The data reveal that the two eddies entrain distinct microbial communities from their points of origin that are maintained for up to ten months. Our findings highlight the ecological impact that changes, brought by the translocation of eddies across the ACC, have on microbial diversity.
Project description:Coherent oceanic mesoscale eddies with unique dynamical structures have great impacts on ocean transports and global climate. Eddy kinetic energy (EKE), derived from time-dependent circulation, is commonly used to study mesoscale eddies. However, there are three deficiencies of EKE when focusing on the analysis of coherent mesoscale eddies. Here, we propose a comprehensive concept-Lagrangian EKE (LEKE) as an additional metric which is a combination of gridded EKE calculated in Eulerian framework and tracked coherent mesoscale eddies in Lagrangian framework. Evidences suggest that LEKE can make up these deficiencies as an effective supplement. In this study, regional application over Northwestern Pacific Ocean is taken as an example. It clearly demonstrates that LEKE reveals more accurate and detailed characteristics of both cyclonic and anticyclonic eddies than EKE when coherent mesoscale eddies are the specific focus, such as the variation rates of kinetic energy during the eddy propagation, spatial-temporal differences of kinetic energy between cyclonic and anticyclonic eddies. Overall, using LEKE to analyze coherent mesoscale eddies gives the rise to understand the spatial-temporal contrasts between eddies with different polarities, and provides a new perspective to recognize the crucial role played by coherent mesoscale eddies in the ocean.
Project description:Mesoscale eddies are ubiquitous features of ocean circulation that modulate the supply of nutrients to the upper sunlit ocean, influencing the rates of carbon fixation and export. The popular eddy-pumping paradigm implies that nutrient fluxes are enhanced in cyclonic eddies because of upwelling inside the eddy, leading to higher phytoplankton production. We show that this view does not hold for a substantial portion of eddies within oceanic subtropical gyres, the largest ecosystems in the ocean. Using space-based measurements and a global biogeochemical model, we demonstrate that during winter when subtropical eddies are most productive, there is increased chlorophyll in anticyclones compared with cyclones in all subtropical gyres (by 3.6 to 16.7% for the five basins). The model suggests that this is a consequence of the modulation of winter mixing by eddies. These results establish a new paradigm for anticyclonic eddies in subtropical gyres and could have important implications for the biological carbon pump and the global carbon cycle.
Project description:Intraseasonal oscillation of deep currents in the Kuroshio Extension region is examined using observations from a collection of current meter moorings. The moored observations reveal variability with characteristic time scales of 23-38 days for velocity time series and of 38-99 days for temperature time series. The time series of normalized temperature (NT) in the deep ocean change correspondingly with sea level anomaly (SLA). The maximum correlation coefficient between NT in the deep ocean and SLA is also up to 0.7. Positive correlation is observed between deep currents and surface geostrophic current. Furthermore, the influence of mesoscale eddies on deep currents is examined by analyzing the data collected when cyclonic and anticyclonic eddies crossed the current meter mooring. Whether anticyclonic or cyclonic eddy intensified the deep currents from 2000 m to 4000 m in the same direction and increased the amplitude. These results provide observational evidence of intraseasonal oscillation in the deep ocean and the effect of mesoscale eddies on deep currents in the Kuroshio Extension region.
Project description:Oceanic mesoscale eddies with horizontal scales of 50-300?km are the most energetic form of flows in the ocean. They are the oceanic analogues of atmospheric storms and are effective transporters of heat, nutrients, dissolved carbon, and other biochemical materials in the ocean. Although oceanic eddies have been ubiquitously observed in the world oceans since 1960s, our understanding of their three-dimensional (3D) structure, generation, and dissipation remains fragmentary due to lack of systematic full water-depth measurements. To bridge this knowledge gap, we designed and conducted a multi-months field campaign, called the South China Sea Mesoscale Eddy Experiment (S-MEE), in the northern South China Sea in 2013/2014. The S-MEE for the first time captured full-depth 3D structures of an anticyclonic and cyclonic eddy pair, which are characterized by a distinct vertical tilt of their axes. By observing the eddy evolution at an upstream versus downstream location and conducting an eddy energy budget analysis, the authors further proposed that generation of submesoscale motions most likely constitutes the dominant dissipation mechanism for the observed eddies.
Project description:Mesoscale eddies are critical components of the ocean's "internal weather" system. Mixing and stirring by eddies exerts significant control on biogeochemical fluxes in the open ocean, and eddies may trap distinctive plankton communities that remain coherent for months and can be transported hundreds to thousands of kilometers. Debate regarding how and why predators use fronts and eddies, for example as a migratory cue, enhanced forage opportunities, or preferred thermal habitat, has been ongoing since the 1950s. The influence of eddies on the behavior of large pelagic fishes, however, remains largely unexplored. Here, we reconstruct movements of a pelagic predator, the blue shark (Prionace glauca), in the Gulf Stream region using electronic tags, earth-observing satellites, and data-assimilating ocean forecasting models. Based on >2,000 tracking days and nearly 500,000 high-resolution time series measurements collected by 15 instrumented individuals, we show that blue sharks seek out the interiors of anticyclonic eddies where they dive deep while foraging. Our observations counter the existing paradigm that anticyclonic eddies are unproductive ocean "deserts" and suggest anomalously warm temperatures in these features connect surface-oriented predators to the most abundant fish community on the planet in the mesopelagic. These results also shed light on the ecosystem services provided by mesopelagic prey. Careful consideration will be needed before biomass extraction from the ocean twilight zone to avoid interrupting a key link between planktonic production and top predators. Moreover, robust associations between targeted fish species and oceanographic features increase the prospects for effective dynamic ocean management.
Project description:Microbial population size, production, diversity, and community structure are greatly influenced by the surrounding physicochemical conditions, such as large-scale biogeographic provinces and water masses. An oceanic mesoscale dipole consists of a cyclonic eddy and an anticyclonic eddy. Dipoles occur frequently in the ocean and usually last from a few days to several months; they have significant impacts on local and global oceanic biological, ecological, and geochemical processes. To better understand how dipoles shape microbial communities, we examined depth-resolved distributions of microbial communities across a dipole in the South China Sea. Our data demonstrated that the dipole had a substantial influence on microbial distributions, community structure, and functional groups both vertically and horizontally. Large alpha and beta diversity differences were observed between anticyclonic and cyclonic eddies in surface and subsurface layers, consistent with distribution changes of major bacterial groups in the dipole. The dipole created uplift, downward transport, enrichment, depletion, and horizontal transport effects. We also found that the edge of the dipole might induce strong subduction, indicated by the presence of Prochlorococcus and Synechococcus in deep waters. Our findings suggest that dipoles, with their unique characteristics, might act as a driver for microbial community dynamics.IMPORTANCE Oceanic dipoles, which consist of a cyclonic eddy and an anticyclonic eddy together, are among the most contrasted phenomena in the ocean. Dipoles generate strong vertical mixing and horizontal advection, inducing biological responses. This study provides vertical profiles of microbial abundance, diversity, and community structure in a mesoscale dipole. We identify the links between the physical oceanography and microbial oceanography and demonstrate that the dipole, with its unique features, could act as a driver for microbial community dynamics, which may have large impacts on both the local and global marine biogeochemical cycles.
Project description:Vertical heat transport by ocean mesoscale eddies plays an important role in maintaining western boundary current extension fronts. Oceanic fronts associated with strong western boundary current extensions vent a vast amount of heat into the atmosphere, anchoring mid-latitude storm tracks and facilitating ocean carbon sequestration. However, it remains unclear how the surface heat reservoir is replenished by ocean processes to sustain the atmospheric heat uptake. Using high-resolution climate simulations, we find that the vertical heat transport by ocean mesoscale eddies acts as an important heat supplier to the surface ocean in frontal regions. This vertical eddy heat transport is not accounted for by the prevailing inviscid and adiabatic ocean dynamical theories such as baroclinic instability and frontogenesis but is tightly related to the atmospheric forcing. Strong surface cooling associated with intense winds in winter promotes turbulent mixing in the mixed layer, destructing the vertical shear of mesoscale eddies. The restoring of vertical shear induces an ageostrophic secondary circulation transporting heat from the subsurface to surface ocean.
Project description:Mesoscale eddies, which contribute to long-distance water mass transport and biogeochemical budget in the upper ocean, have recently been taken into assessment of the deep-sea hydrodynamic variability. However, how such eddies influence sediment movement in the deepwater environment has not been explored. Here for the first time we observed deep-sea sediment transport processes driven by mesoscale eddies in the northern South China Sea via a full-water column mooring system located at 2100 m water depth. Two southwestward propagating, deep-reaching anticyclonic eddies passed by the study site during January to March 2012 and November 2012 to January 2013, respectively. Our multiple moored instruments recorded simultaneous or lagging enhancement of suspended sediment concentration with full-water column velocity and temperature anomalies. We interpret these suspended sediments to have been trapped and transported from the southwest of Taiwan by the mesoscale eddies. The net near-bottom southwestward sediment transport by the two events is estimated up to one million tons. Our study highlights the significance of surface-generated mesoscale eddies on the deepwater sedimentary dynamic process.
Project description:In this study, the effects of oceanic mesoscale eddies on the looping path of the Kuroshio intrusion (KI) were symmetrically investigated by composite analysis using merged satellite data. We found that the mesoscale eddies propagating from the east have a significant impact on the looping path over a time scale of 30-60 days. Cyclonic eddies (CEs) enhance the looping path, but anticyclonic eddies decrease it. We also found that strong eddies do not have strong effects on the looping path. For instance, strong CEs induce the strong surface intrusion of the Kuroshio, but the looping currents are weak due to the presence of the strong Luzon Cold Eddy in the South China Sea, which tends to prevent loop formation. The complicated relationship between eddies and the path of the KI results in a nonsignificant correlation coefficient between the KI and eddy activities in the western Pacific.
Project description:Like most benthic marine organisms, coral reef fishes produce larvae that traverse open ocean waters before settling and metamorphosing into juveniles. Where larvae are transported and how they survive is a central question in marine and fisheries ecology. While there is increasing success in modelling potential larval trajectories, our knowledge of the physical and biological processes contributing to larval survivorship during dispersal remains relatively poor. Mesoscale eddies (MEs) are ubiquitous throughout the world's oceans and their propagation is often accompanied by upwelling and increased productivity. Enhanced production suggests that eddies may serve as important habitat for the larval stages of marine organisms, yet there is a lack of empirical data on the growth rates of larvae associated with these eddies. During three cruises in the Straits of Florida, we sampled larval fishes inside and outside five cyclonic MEs. Otolith microstructure analysis revealed that four of five species of reef fish examined had consistently faster growth inside these eddies. Because increased larval growth often leads to higher survivorship, larvae that encounter MEs during transit are more likely to contribute to reef populations. Successful dispersal in oligotrophic waters may rely on larval encounter with such oceanographic features.