Abstract
Ocean stratification and the vertical extent of the mixed layer influence the rate at which the ocean and atmosphere exchange properties. This process has direct impacts for anthropogenic ...heat and carbon uptake in the Southern Ocean. Submesoscale instabilities that evolve over space (1–10 km) and time (from hours to days) scales directly influence mixed layer variability and are ubiquitous in the Southern Ocean. Mixed layer eddies contribute to mixed layer restratification, while down-front winds, enhanced by strong synoptic storms, can erode stratification by a cross-frontal Ekman buoyancy flux. This study investigates the role of these submesoscale processes on the subseasonal and interannual variability of the mixed layer stratification using four years of high-resolution glider data in the Southern Ocean. An increase of stratification from winter to summer occurs due to a seasonal warming of the mixed layer. However, we observe transient decreases in stratification lasting from days to weeks, which can arrest the seasonal restratification by up to two months after surface heat flux becomes positive. This leads to interannual differences in the timing of seasonal restratification by up to 36 days. Parameterizing the Ekman buoyancy flux in a one-dimensional mixed layer model reduces the magnitude of stratification compared to when the model is run using heat and freshwater fluxes alone. Importantly, the reduced stratification occurs during the spring restratification period, thereby holding important implications for mixed layer dynamics in climate models as well as physical–biological coupling in the Southern Ocean.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Polar oceans are poorly monitored despite the important role they play in regulating Earth’s climate system. Marine mammals equipped with biologging devices are now being used to fill the data gaps ...in these logistically difficult to sample regions. Since 2002, instrumented animals have been generating exceptionally large data sets of oceanographic CTD casts (>500,000 profiles), which are now freely available to the scientific community through the MEOP data portal (http://meop.net). MEOP (Marine Mammals Exploring the Oceans Pole to Pole) is a consortium of international researchers dedicated to sharing animal-derived data and knowledge about the polar oceans. Collectively, MEOP demonstrates the power and cost-effectiveness of using marine mammals as data-collection platforms that can dramatically improve the ocean observing system for biological and physical oceanographers. Here, we review the MEOP program and database to bring it to the attention of the international community.
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.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Community composition is one of the main factors influencing the ecological functioning of any given ecosystem, with a more diverse community providing a larger set of services. Benthic community ...composition can vary at spatial and temporal scales, with the dynamics of primary production and benthic food availability being key determinants of community structure of a given system. Studies have indicated shifts in benthic community composition at sub-Antarctic Islands over a period of 30 years, linked to variability in food availability driven by climate change. Here, we aim to evaluate possible short-term variability in benthic community composition at the sub-Antarctic archipelago Prince Edward Islands (PEIs), sampling stations across three hydrographically dynamic regions over two consecutive years, 2016–2017. The results indicated significantly higher species richness and abundance in 2017 than in 2016, with some taxa also showing variation among regions around the PEIs. Such effects can be linked to different substratum type or hydrographic regime. This study further contributes to understanding variability and changes in benthic communities in the near future, an essential information to develop efficient management strategies for this vulnerable marine system.
Hydrographic transects of the Antarctic Circumpolar Current (ACC) south of Africa are projected into baroclinic stream function space parameterized by pressure and dynamic height. This produces a ...two‐dimensional gravest empirical mode (GEM) that captures more than 97% of the total density and temperature variance in the ACC domain. Weekly maps of absolute dynamic topography data, derived from satellite altimetry, are combined with the GEM to obtain a 16 year time series of temperature and salinity fields. The time series of thermohaline fields are compared with independent in situ observations. The residuals decrease sharply below the thermocline and through the entire water column the mean root‐mean‐square (RMS) error is 0.15°C, 0.02, and 0.02 kg m−3 for temperature, salinity, and density, respectively. The positions of ACC fronts are followed in time using satellite altimetry data. These locations correspond to both the observed and GEM‐based positions. The available temperature and salinity information allow one to calculate the baroclinic zonal velocity field between the surface and 2500 dbar. This is compared with velocity measurements from repeat hydrographic transects at the GoodHope line. The net accumulated transports of the ACC, derived from these different methods are within 1–3 Sv of each other. Similarly, GEM‐produced cross‐sectional velocities at 300 dbar compare closely to the observed data, with the RMS difference not exceeding 0.03 m s−1. The continuous time series of thermohaline fields, described here, are further exploited to understand the dynamic nature of the ACC fronts in the region, and which is given by Swart and Speich (2010).
The Southern Ocean is undergoing rapid environmental change, which has impacted its ecosystems and food webs. There is need for ecosystem models that incorporate all levels of the biotic system and ...consider physical context. Using an end-to-end ecosystem model of the Prince Edward Islands (PEIs), we investigated the importance of the input of oceanic nutrients, oceanic plankton, and run-off of terrestrial ammonium to nearshore production. We compared ecosystem state as group production rates and as the relative scale of pelagic versus demersal food web pathways under alternate ocean current regimes, assumptions of macrozooplankton advection into shelf waters, and assumptions of plankton retention within the nearshore region. The major outcomes are: (1) oceanic plankton, more than oceanic nutrients or terrestrial nutrients, is the major driver of production for all groups within the nearshore ecosystem. Island run-off of ammonium is a minor driver of production but is most important among groups with higher reliance upon detrital food chains (benthic invertebrates, demersal fishes, Gentoo penguins); (2) groups most sensitive to changes in ocean current regime and assumptions of macrozooplankton advection into shelf waters are planktivores (southern rockhopper penguins, Macaroni penguins) and piscivores whose diets rely heavily upon planktivorous fishes; (3) zooplankton populations cannot support estimated levels of predation pressure within the nearshore ecosystem if they behave as purely passive drifters. Our findings suggest changes to physical processes, such as a postulated southward shift in the position of the sub-Antarctic front leading to intensification of currents approaching the islands, has already had and will continue to have significant impacts on the PEIs ecosystem.
The eastern side of the South Atlantic Meridional overturning circulation Basin-wide Array (SAMBA) along 34.5° S is used to assess the nonlinear, mesoscale dynamics of the Cape Basin. This array ...presently consists of current meter moorings and bottom mounted Current and Pressure recording Inverted Echo Sounders (CPIES) deployed across the continental slope. These data, available from September 2014 to December 2015, combined with satellite altimetry allow us to investigate the characteristics and the impact of mesoscale dynamics on local water mass distribution and cross-validate the different data sets. We demonstrate that the moorings are affected by the complex dynamics of the Cape Basin involving Agulhas rings, cyclonic eddies and anticyclonic eddies from the Agulhas Bank and the South Benguela upwelling front and filaments. Our analyses show that exchange of water masses happens through the advection of water by mesoscale eddies but also via wide water mass intrusions engendered by the existence of intense dipoles. These complex dynamics induce strong intra-seasonal upper-ocean velocity variations and water mass exchanges between the shelf and the open ocean but also across the subantarctic and subtropical waters. This work presents the first independent observations comparison between full-depth moorings and CPIES data sets within the eastern South Atlantic region that gives some evidence of eastern boundary buoyancy anomalies associated with migrating eddies. It also highlights the need to continuously sample the full water depth as inter-basin exchanges occur intermittently and affect the whole water column.
Rafting invertebrate species represent an important link for the dispersal and colonisation of islands in the Southern Ocean. They are sensitive to changes in hydrodynamics through effects on ...transport and food availability, particularly in the case of filter-feeding species. This gives them the potential to act as sentinels of environmental change. The rafting kelp-associated
Gaimardia trapesina
is one such species, widely distributed in the Southern Ocean. In 2015 and 2016, the size structure, diet, and distribution of
G. trapesina
were examined around the Sub-Antarctic Prince Edward Islands (PEI) to establish benchmark data and explore their potential use for monitoring long-term change.
Gaimardia trapesina
and its potential food sources were collected and analysed for size structure, attachment strength, and diet, using stable isotope and fatty acid analyses. The populations examined were dominated by relatively small individuals. The strength of attachment to kelp blades was low and the byssal threads showed high elasticity. The PEI lie in the path of the west–east flowing Antarctic Circumpolar Current and the highest abundances of
G. trapesina
were found on the downstream sides of both islands, while the species was absent from the western, upstream coasts. Both diet analyses and SIAR mixing models indicated that
G. trapesina
feeds on suspended particulate matter, while kelp particles contribute minimally to the diet. Considering the wide distribution of
G. trapesina
, its importance as food for birds and fish, and its sensitivity to environmental conditions, there is good potential to use this species as an indicator of environmental change in the Southern Ocean.
The 2013 Intergovernmental Panel on Climate Change report, using CMIP5 and EMIC model outputs suggests that the Atlantic Meridional Overturning Circulation (MOC) is very likely to weaken by 11–34% ...over the next century, with consequences for global rainfall and temperature patterns. However, these coupled, global climate models cannot resolve important oceanic features such as the Agulhas Current and its leakage around South Africa, which a number of studies have suggested may act to balance MOC weakening in the future. To properly understand oceanic changes and feedbacks on anthropogenic climate change we need to substantially improve global ocean observations, particularly within boundary current regions such as the Agulhas Current, which represent the fastest warming regions across the world’s oceans. The South African science community, in collaboration with governing bodies and international partners, has recently established one of the world’s most comprehensive observational networks of a western boundary current system, measuring the Greater Agulhas Current System and its inter-ocean exchanges south of Africa. This observational network, through its design for long-term monitoring, collaborative coordination of resources and skills sharing, represents a model for the international community. We highlight progress of the new Agulhas System Climate Array, as well as the South African Meridional Overturning Circulation programme, which includes the Crossroads and GoodHope hydrographic transects, and the South Atlantic MOC Basin-wide Array. We also highlight some of the ongoing challenges that the programmes still face.Significance:• Large mooring arrays have been successfully deployed to monitor the Greater Agulhas Current system.• Capacity development is ongoing, although established, in marine science around South Africa.• Challenges exist with regard to retention of skilled staff, resources and funding.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Understanding the impact of oceanographic features on marine microbial ecosystems remains a major ecological endeavour. Here we assess microbial diversity, community structure and functional capacity ...along the Agulhas Current system and the Subtropical Front in the South Indian Ocean (SIO). Samples collected from the epipelagic, oxygen minimum and bathypelagic zones were analysed by 16S rRNA gene amplicon and metagenomic sequencing. In contrast to previous studies, we found high taxonomic richness in surface and deep water samples, but generally low richness for OMZ communities. Beta-diversity analysis revealed significant dissimilarity between the three water depths. Most microbial communities were dominated by marine Gammaproteobacteria, with strikingly low levels of picocyanobacteria. Community composition was strongly influenced by specific environmental factors including depth, salinity, and the availability of both oxygen and light. Carbon, nitrogen and sulfur cycling capacity in the SIO was linked to several autotrophic and copiotrophic Alphaproteobacteria and Gammaproteobacteria. Taken together, our data suggest that the environmental conditions in the Agulhas Current system, particularly depth-related parameters, substantially influence microbial community structure. In addition, the capacity for biogeochemical cycling of nitrogen and sulfur is linked primarily to the dominant Gammaproteobacteria taxa, whereas ecologically rare taxa drive carbon cycling.
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