Cross Shelf Research Articles

Sedimentary molybdenum and uranium cycling under seasonally contrasting redox conditions on the Namibian Shelf

Sedimentary molybdenum (Mo) and uranium (U) enrichments have been widely used as a proxy for redox conditions in oxygen-depleted marine paleo-environments. However, in a dynamic upwelling system the seasonal fluctuations from oxic to completely anoxic-sulfidic bottom waters and lateral sediment transport can modify the primary Mo and U signal of the sediment, which in turn may impact paleo-redox interpretations. In this study we present pore water and solid phase data collected at two cross shelf transects during the ‘more oxygenated’ austral winter and ‘anoxic’ austral summer to study the influence of spatially and seasonally contrasting redox conditions on the formation of authigenic Mo and U enrichments in organic carbon (TOC) rich mud belt sediments on the Namibian shelf. A mass balance was established for each element based on diffusive fluxes and element mass accumulation rates to evaluate the respective mechanisms of trace metal delivery, accumulation and recycling.Mo is delivered to the sediment in its dissolved form via diffusion across the sediment–water interface, especially during austral summer when bottom waters are anoxic and surface sediments are highly sulfidic. In the center of the inner shelf mud belt, the benthic Mo fluxes of up to 37 nmol cm−2 yr−1 into sulfidic surface sediments are the highest ever reported for reducing sulfidic systems and agree with the rate of Mo accumulation in the solid phase. Concurrently, high sedimentation rates and low terrigenous input limit solid phase Mo accumulation on the Namibian shelf. In ancient marine sediments, this mode of Mo cycling can be identified by low Mo/TOC ratios of ∼2 similar to those found in sediments deposited below the perennial oxygen minimum zone on the Peruvian shelf and to those found in deposits of the Cretaceous Oceanic Anoxic Event 2. Diffusive U fluxes into the sediment are generally too low to account for the sedimentary enrichment leading to the conclusion that U is delivered mainly in particulate form. In areas with anoxic bottom water, shallow dissolved U maxima directly below the sediment water interface and rather low sedimentary U content indicate that particulate U is recycled and largely released back into the bottom water. At sites where bottom water oxygen concentrations vary from anoxic to completely oxic on seasonal timescales, the depth at which Mo and U are removed from pore waters moves vertically within the sediment column thus defining a layer between the sediment surface and ∼20 cm depth, in which Mo and U accumulate in the solid phase. Our results emphasize the importance of short-term redox fluctuations in the bottom waters and underlying sediments, as well as lateral sediment transport for the authigenic enrichment of redox-sensitive trace metals in reducing shelf sediments. The relative enrichment patterns identified might be useful for the reconstruction of open marine anoxia in the geological past.

Modelling the biogeochemical footprint of rivers in the Hauraki Gulf, New Zealand

Building accurate physical-biogeochemical models of processes driving climate and eutrophication-related stressors in coastal waters is an essential step in managing the impacts of these stressors. Here we develop a coupled physical-biogeochemical model to investigate present day processes for a key marine ecosystem in Aotearoa, New Zealand: The Hauraki Gulf/Firth of Thames system. Simulation results compared well with an accompanying long-term (decadal) observational dataset, indicating that the model captured most of the physical and biological dynamics of the Hauraki Gulf/Firth of Thames system. This model was used to investigate the riverine and cross shelf exchanges of nutrients in the region and showed that only a small number of large rivers within the Firth of Thames dominated the freshwater inputs, with phytoplankton concentrations driven by nutrient inputs from these rivers. However, while riverine inputs dominated the biological response in the Firth of Thames, cross-shelf fluxes dominated the biological response in the outer Hauraki Gulf region. Nutrients from both sources were balanced by a sediment denitrification flux. Analyses were conducted to examine agreement of observations with subsampled and climatological model outputs. These revealed that modelling effort needs to focus on the representation of sediment fluxes and parameterizations during the autumn, and the observational effort needs to focus on increased temporal data collection during summer to better understand biases in seasonal climatologies derived from model and observations. These results are valuable for demonstrating effects of land-derived and oceanic drivers of the biogeochemical dynamics of the Hauraki Gulf/Firth of Thames system.

Distribution and off–shelf transport of dissolved manganese in the East China Sea

To gain a better understanding of the geochemical behavior of dissolved manganese (Mn) in the marginal seas with respect to distribution and exchange fluxes, more than 200 water samples were collected in the East China Sea (ECS) in May, August, and October of 2013. The concentration of dissolved Mn in the ECS ranged from 1.1 to 81.5 nM, with a gradual decrease with distance from the shore. Seasonal distribution of dissolved Mn varies significantly in the Changjiang estuary, mainly regulated by freshwater input from the Changjiang (Yangtze River) and redox variations. The ECS continental shelf is an important source of Mn for adjacent waters, and the export of Mn–rich coastal waters had an important effect on its re-distribution and internal cycling. The dynamic variation fluxes of water and dissolved Mn across the 100– and 200–m isobaths in the ECS were calculated with an aid of the Finite−Volume Coastal Ocean Model (FVCOM). The ECS continental shelf exported (5.69 ± 1.14) × 108 mol/yr of Mn into the East/Japan Sea from the Tsushima Strait. The Kuroshio surface waters receive an additional (1.02 ± 3.12) × 108 mol/yr of Mn from the ECS continental shelf through a cross–shelf exchange process, which could potentially affect dissolved Mn in the Northwest Pacific. Our data suggest that off-shelf transport from the ECS continental shelf is essential for understanding the biogeochemical cycles of trace metals in the Northwest Pacific Ocean and the East/Japan Sea.

Inter-annual variability patterns of reef cryptobiota in the central Red Sea across a shelf gradient

The combination of molecular tools, standard surveying techniques, and long-term monitoring programs are relevant to understanding environmental and ecological changes in coral reef communities. Here we studied temporal variability in cryptobenthic coral reef communities across the continental shelf in the central Red Sea spanning 6 years (three sampling periods: 2013–2019) and including the 2015 mass bleaching event. We used a combination of molecular tools (barcoding and metabarcoding) to assess communities on Autonomous Reef Monitoring Structures (ARMS) as a standardized sampling approach. Community composition associated with ARMS for both methodologies (barcoding and metabarcoding) was statistically different across reefs (shelf position) and time periods. The partition of beta diversity showed a higher turnover and lower nestedness between pre-bleaching and post-bleaching samples than between the two post-bleaching periods, revealing a community shift from the bleaching event. However, a slight return to the pre-bleaching community composition was observed in 2019 suggesting a recovery trajectory. Given the predictions of decreasing time between bleaching events, it is concerning that cryptobenthic communities may not fully recover and communities with new characteristics will emerge. We observed a high turnover among reefs for all time periods, implying a homogenization of the cryptobiome did not occur across the cross shelf following the 2015 bleaching event. It is possible that dispersal limitations and the distinct environmental and benthic structures present across the shelf maintained the heterogeneity in communities among reefs. This study has to the best of our knowledge presented for the first time a temporal aspect into the analysis of ARMS cryptobenthic coral reef communities and encompasses a bleaching event. We show that these structures can detect cryptic changes associated with reef degradation and provides support for these being used as long-term monitoring tools.

Fate of internal solitary wave and enhanced mixing in Manado Bay, North Sulawesi, Indonesia

The Sulawesi Sea is one of the most active regions where internal solitary waves (ISWs) are frequently observed through satellite images. These waves that result from the nonlinear evolution of internal tides are generated over the Sibutu Sills and propagate over a long distance, up to 700 km before shoaling in coastal areas. To date there is yet no specific in-situ observation or numerical studies dedicated to characterize their dynamics. We present results from the combination of two simulations and in-situ observations which allow to describe the full life cycle from generation to breaking of ISWs in the Sulawesi Sea. A first set of 2D non-hydrostatic numerical simulations reproduces the generation of ISW from the non-linear steepening of the internal tides. The tidal energy flux emanated from the generation site is of about 15 kW m−1 with progressive decrease in the along flux direction resulting the growth of ISWs. The ISW energy flux increases up to large fraction of 32% of the total energy flux at 700 km from the generation point while the M2 internal tide contributes to 64% of the total energy flux. A second simulation at higher resolution focusing on the shallow coastal area of the Manado Bay reproduces the breaking of the ISWs at the shelf slope and was compared with in-situ observations. About 3.5% of the energy contained in an ISWs packet is dissipated over a 3 km wide cross shelf region of the Manado Bay in the simulation. Both simulations and observations revealed the presence of energetic boluses identified as a sequence of high-frequency waves of elevations in the stratified bottom layer. A high level of turbulence was diagnosed in the observations and simulations from the Thorpe Method. The mean eddy kinetic energy dissipation rate and vertical diffusivity in the observation reaches as 10−6 W kg−1 and 10−3 m2 s−1, respectively; while the dissipation rate is one order of magnitude higher and the vertical diffusivity is one order lower in the model. This discrepancy likely reflects the sparse sampling in the observations as well as differences in the stratification and the absence of surface forcing in the model.

Large scale patches of Calanus finmarchicus and associated hydrographic conditions off the Lofoten archipelago

Large swarms of individuals at different spatiotemporal scales characterise the distributions of many animal species. In the ocean several mesozooplankton taxa aggregate in large patches or swarms driven by active behavioural responses to hydrographic structures, although intrinsic biotic characteristics of species´ life cycles not related with the environment can also affect spatial distributions. To understand the mechanisms of aggregation and disentangle environmental and pure spatial effects contributing to patch formation, we extensively sampled a large Calanus finmarchicus patch off the Lofoten islands, northern Norway, in spring 2017 by means of Laser Optical Plankton Counter (LOPC) cross shelf transects and biophysical samplings at fixed stations. We observed a clear association between the buoyant layer of the fresh, cold Norwegian Coastal Current (NCC) and high surface copepod abundances. Off shelf lateral displacements of the NCC along its path may retain copepods from the offshore basins. At deeper layers, copepod abundances increased markedly at regions with low sea level anomalies and reduced vertical flows. Our results also suggest that copepods performed short range diel vertical migration within the patch. Potential future changes in the seasonality and structure of the NCC and their impact on swarm occurrence and formation are discussed.

Benthic foraminifera as indicators of recent mixed turbidite-contourite sediment transport system in the Eastern Mediterranean upper continental slope, offshore Israel

Nile derived siliciclastic sediments are the main source of sedimentation along the Levant continental margins of the Mediterranean Sea. These sediments are transported along the southeastern Mediterranean coast via well-documented longshore currents, mainly operating along the shelf. However, the cross shelf component of sediment transport, responsible for conveying sediments toward the upper slope, is less known. To better understand the cross-shelf vs. the longshore component of sediment transport we studied a ~6 m long piston core (DOR280) sampled on the upper continental slope (280 m water depth) and analyzed benthic foraminiferal assemblages and their shell taphonomy alongside the particle size distribution and mineralogy of bulk sediments, to document both the source and the transport mechanism of those upper continental-slope sediments. The radiocarbon dating at the core-base is ~650 Cal. Yrs BP, indicating an exceptionally high average sedimentation rate of ~800 cm/kyr.DOR280 consists of two alternating distinct sedimentary facies: (1) laminated (L) intervals up to ~40 cm thick each and showing a high ratio of allochthonous vs. autochthonous (allo/auto) benthic foraminiferal species and a high percentage of broken shells, indicating contribution of transported sediments originating from mid-shelf habitats; (2) non-laminated (NL) intervals up to ~200 cm thick each and showing a low allo/auto ratio and low percentages of broken shells, indicating mostly in-situ hemipelagic deposition. The L intervals are interpreted here as turbidites. The sedimentation rates calculated only for the NL intervals are still exceptionally high, thus excluding hemipelagic sedimentation as the sole mechanism for the NL deposition. Therefore, a contour bottom current transported component is suggested here as a source of the NL sedimentation.We conclude that a mixed contourite-turbidite system actively prevails along the upper continental slope of the Levant coast, offshore Israel. The mid-shelf to upper slope sediment transport is nearly continuous and presents occasional turbidite events of average decadal reoccurrence time.

EOF Analysis for Wind-driven Along and Cross Shelf Exchange around a Sandbank over the Daesan Inner Shelf (Yellow Sea, Korea)

Ryu, H.-R.; Kim, S.-S.; Kim, D.-H.; Jang, W.-S., and Lee, S.-O., 2021. EOF analysis for wind-driven along and cross shelf exchange around a sandbank over the Daesan inner shelf (Yellow Sea, Korea). In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 141–145. Coconut Creek (Florida), ISSN 0749-0208. It is essential to understand the shelf circulation dynamics qualitatively and, if possible quantitively, since the inner shelf is an important zone located between the surf zone and the continental shelf in terms of the heat exchange, the transport of terrigenous sediment, and so on. In this study, from the compound results of current velocity profiles measured by the moored ADCP and wind observations obtained by KMA (Korea Meteorological Administration) over the Daesan inner shelf located in Yellow Sea, Korea, we found that the inner shelf circulation was mainly driven by a combination of wind stress, pressure gradient, Coriolis force and submarine topography. The Empirical Orthogonal Function (EOF) analysis was used to comprehend the shelf circulation dynamics induced by the wind stress and the wavelet coherence was used to analyze the time-frequency distribution of the coherence between currents, which was reconstructed by EOF method and shelf-aligned winds. The thickness of the surface and bottom frictional layers mainly depended on wind forcing and seabed topography. Also, it was found that the subtidal currents were strongly dominated along-shelf due to the irregular seabed topography such as a sandbank. The vertical structure of the along-shelf currents was mostly uniform regardless of water depth change. In mode 2 of EOF analysis, it was shown that the cross-shelf currents were polarized into two layers. Mostly wind in the ocean may cause down-welling or up-welling along the direction, however, in the study area, variation of wind induced current and the Ekman layer are not clearly distinguishable. Additionally, it is resulted that the cross-shelf exchange caused by the wind was not significant because of complicated seabed topography including Jangan sandbank and tidal force even though the Ekman depth was changed by the wind forcing in the Daesan inner shelf. From a result of wavelet coherence analysis, the correlation between wind and currents was also insignificant according to shelf-aligned U, V directions and water depths.

Generalized linear models to assess environmental drivers of paralytic shellfish toxin blooms (Southeast Tasmania, Australia)

Paralytic shellfish toxin (PST) events are often difficult to predict because synergistic processes, involving multiple temporal and spatial scales, are responsible for their development. In this study, we consider a suite of factors, from local to regional processes, driving paralytic shellfish toxin (PST) events associated with Alexandrium catenella on the east coast of Tasmania (Australia). PST concentrations in shellfish were compared against environmental variables, including daily wind speed and direction, satellite chlorophyll-a, sea surface temperature and river discharge. Relationships were tested using generalized linear models (GLMs). GLM coefficients confirm that high PST events were significantly favored by cooler temperatures and higher chlorophyll-a concentrations. PST events were seasonal, occurring between August and November and showed a significant relationship with sustained coastal upwelling and wind transition events after one to three months of predominantly upwelling winds. Satellite imagery shows evidence of cross shelf transport moderating PST events. Overall, the results suggest that a combination of factors (currents, upwelling conditions and changes in sea surface temperature) work together to initiate and moderate PST events. The use of GLMs provides a useful tool for understanding synergistic relationships. These techniques are of value for developing reliable predictive models to forecast and manage the effects of harmful and costly PST events.

Cross shelf gradients of scleractinian corals in the Spermonde Islands, South Sulawesi, Indonesia

Abstract. Yusuf S, Beger M, Tassakka ACMAR, Brauwer MD, Pricella A, Rahmi, Umar W, Limmon GV, Moore AM, Jompa J. 2021. Cross shelf gradients of scleractinian corals in the Spermonde Islands, South Sulawesi, Indonesia. Biodiversitas 22: 1415-1423. Coral reef ecosystems around the world have suffered extensive degradation, including the reefs of the Wallacea region within the Coral Triangle global biodiversity hotspot. Anthropogenic and natural threats can reduce the level of coral reef biodiversity differentially across environmental or impact gradients. The purpose of this study was to evaluate the changes in hard coral (Scleractinia) diversity and community structure across an inshore-offshore zonation gradient in the Spermonde Islands, South Sulawesi, Indonesia. Data on coral colony species and abundance as well as live coral cover were collected from 10 m2 belt transects at a depth of 6 to 8 m. A total of 72 transects were placed around the 12 island stations in three zones: the inner mid-shelf zone, outer mid-shelf zone and outer shelf zone. Data were analyzed to determine the species richness, and three ecological indices for the hard coral communities were calculated: the Shannon Diversity Index (H'), Similarity Index (E), and Dominance Index (C). A total of 310 hard coral species belonging to 62 genera were recorded. The coral communities were dominated by the genera Fungia, Montipora and Porites, and coral cover was in the 'moderate' category. The number of species was directly proportional to the number of colonies within each zone. Live coral cover was higher in the inner mid-shelf zone and outer shelf zone than the outer mid-shelf zone; conversely, the species richness and coral colony abundance were higher in the outer mid-shelf zone. However, the differences were not statistically significant. The indices H’, C, and E did not differ significantly between the zones. However, Tambakulu Island in Zone 4 had the lowest values of E and H’ and the highest value of C. Findings suggest that most-hard coral communities in the cross-shelf zones of the Spermonde Islands are stable communities characterized by relatively high diversity and low dominance indices.

High resolution geochemical analysis of massive Porites spp. corals from the Wet Tropics, Great Barrier Reef: rare earth elements, yttrium and barium as indicators of terrigenous input

Abstract Catchment modification and increased sediment loads in coastal regions are impacting many of the world's coral reefs, yet relevant water quality data are limited by the availability of instrumental records. Massive Porites spp. corals offer the potential to reconstruct water quality by use of the geochemical proxies that are incorporated into their skeleton. Here we present ∼ monthly resolved rare earth element (REE), yttrium (Y) and barium/calcium (Ba/Ca) concentrations obtained from four Porites corals collected across a known water quality gradient from the Great Barrier Reef, Australia. Sub-annual REE time series patterns were comparable among corals despite having significantly different concentrations. Whilst Ba/Ca did not reflect the known water quality gradient of the region, Y/holmium ratios reflected broad cross shelf water quality gradients and REE concentrations reflected local scale variance in water quality. This study demonstrates that REE analysis of Porites corals offers great potential for assessing changes in water quality in reef environments.

Seasonal variations in the surficial sediment grain size in the East China Sea continental shelf and their implications for terrigenous sediment transport

This study discusses the seasonal variations in surficial sediment grain size and their implications for sediment transport driven by the hydrodynamics in the East China Sea, based on grain size analysis of surficial sediments and salinity data collected from the East China Sea continental shelf during two cruises in 2011. The results show that the grain size distribution on the East China Sea continental shelf presents apparent spatial and seasonal variations. The mud area exhibits more variation than the sand area, which indicates that the sediment supply is the main factor controlling the surficial sediment grain size distribution on the East China Sea continental shelf. The grain size near the Yangtze River Estuary is relatively coarse in summer, because of the inherited characteristics from the previous winter, whereas it is much finer in autumn, due to the input of fine-grained materials during the flood period of the Yangtze River. A high fine-grained sediment concentration was found along the Zhejiang-Fujian coast in autumn when the Zhejiang-Fujian Coastal Current forms. The sediments in the northeast of the study region (the mud patch southwest of Cheju Island) are much coarser, and the fine-grained area is narrower in autumn than in summer because of seasonal variations in the Yellow Sea Coastal Current and the frequency of storms. Simultaneously, the tongue-shaped fine-grained cross shelf front located along the Zhejiang-Fujian coast occurs farther to the south in autumn than in summer. However, the surficial grain size distribution patterns on the middle and outer shelf are similar in the two seasons.

Cross shelf transport of terrigenous organic matter in surface sediments from outer shelf to Okinawa Trough in East China Sea

Identification of sources and distribution of terrigenous organic matter (TOM) are significant to understand the carbon transport in East China Sea (ECS). However, sources of TOM buried in the ECS are diverse and complex, which are likely derived from the Changjiang River, Yellow River and/or small rivers from China mainland and Taiwan. In order to evaluate the fate of TOM offshore in the ECS, the study sampled 80 surface sediments from outer shelf-slope-Okinawa Trough (OT) region in the ECS. Both terrigenous and marine organic biomarkers were detected. The concentration (ng/g dry weight sediment) of terrigenous biomarkers including ƩC27 + C29 + C31 n-alkanes ranged from 12.2 to 990.9; while the content of marine biomarkers varied from 1.3 to 3891.2 for C37 alkenones, 2.6 to 922.6 for brassicasterol and 7.1 to 1535.8 for dinosterol. Bulk organic carbon and TOM distributions were controlled by the hydrodynamic conditions. The terrigenous and marine biomarker ratio (TMBR) (ƩC27 + C29 + C31 n-alkanes)/(ƩC27 + C29 + C31 n-alkanes + (brassicasterol + dinosterol +alkenones)) increased first and decreased from the outer shelf to OT spatially, showing the distribution pattern of relative proportion of TOM. Hotspots of significant deposition of TOM were observed in the ECS by δ13C and TMBR proxies that the mechanism was unclear yet. Binary model revealed higher percentage of TOM based on TMBR than δ13C for both slope (32.1% vs. 20.0%) and trough (23.1% vs. 15.2%) region. An underestimation of C4 vegetation contribution likely biased the bulk carbon model. Carbon preference index suggested a zonation distribution of TOM that outer shelf had different sources from the slope and OT attributed to cross shelf transport of terrigenous organic matter.

Elemental composition of suspended particles across the southeastern continental shelf off the coast of North Florida and South Georgia: Provenance, transport, fate and implications to mid-outer shelf water column processes.

This paper reports on the chemistry of suspended particulates collected in waters of the southeastern US continental shelf off the coasts of North Florida and South Georgia. Approximately 350 suspended sediment samples were analyzed for particulate organic carbon and nitrogen (POC and PON), particulate aluminum and calcium and a suite of trace metals which included particulate alkaline earths (magnesium, strontium, barium), particulate rare earth metals (lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium and dysprosium), particulate transition metals (cobalt, copper, iron, manganese, nickel. scandium, titanium and vanadium) and others: particulate cadmium, zinc, thorium and uranium. The samples were collected in the fall of 1987 during a multidiscipline expedition (FLEX) focused on cross shelf exchange of water and materials during northward wind stress and fall Gulf Stream intrusions. Although samples were analyzed shortly after collection, archived data have not been previously presented. Results are interpreted with regard to how they relate to particle provenance, transport and fate.Results indicate that significant influence of terrestrial sources of suspended particles is limited to the inner shelf, however the concentrations of some metals, such as rare earths, appear to reflect this provenance across the entire shelf. Mid-outer shelf particles are dominated by re-suspended biogenic carbonates and biogenic organic carbon rich particles formed in response to Gulf Stream up welled nutrients. Particulate trace element concentrations and primary production in mid-outer shelf waters are interpreted as a response to not only nutrient, but also trace metal enrichment of up welled Gulf Stream water intruded onto the shelf. Based on particle chemistry, micro nutrients such as Fe in upwelled waters may be limiting in sustaining mid-outer shelf production. Other trace metals such as Sc appear to be enriched due to adsorption on organic rich particles. And the formation SrSO4 particles, originating from Acantharian spicules, appear to create an environment for the precipitation of barite in outer shelf surface waters.

Carbon outwelling across the shelf following a massive mangrove dieback in Australia: Insights from radium isotopes

Abstract Mangrove soil carbon stocks are known to decrease following forest loss due to respiration and enhanced soil CO2 emissions. However, changes in carbon outwelling to the coastal ocean due to mangrove forest disturbance have not been considered. In December 2015, an extremely large mangrove dieback event (∼7000 hectares, spanning 1000 km of coastline) occurred in the Gulf of Carpentaria, Australia. To assess the effect this dieback event had on carbon outwelling, we used radium isotopes and dissolved carbon measurements (dissolved organic carbon, DOC, dissolved inorganic carbon, DIC, and total alkalinity, TAlk) to estimate cross-shelf carbon transport from living and dead mangrove areas and to calculate the carbon losses from living and dead forest soils via SGD. Radium distributions imply cross shelf eddy diffusivity of 107.5 ± 26.9 and 104.6 ± 23.9 m−2 s−1 from dead and living areas and radium water ages reveal that mangrove carbon reaches 10 km offshore within 7 days. Outwelling rates from living and dead areas were explained by soil carbon losses via SGD. This study suggests a decrease in carbon outwelling to the ocean from dead forest areas compares to living areas by 0–12% for DOC, 50–52% for DIC and by 37–51% for TAlk ∼8 months after the dieback event occurred. Changes to oceanic carbon outwelling rates following mangrove loss are likely driven by a gradual depletion of carbon stocks from the sediment profile.

Wind Conditions on the Great Barrier Reef Influenced the Recruitment of Snapper (Lutjanus carponotatus)

Most coral reef fishes have a pelagic larval stage before recruiting to reefs. The survival of larvae and their subsequent recruitment can drive the dynamics of reef populations. Here we show that the recruitment of the snapper Lutjanus carponotatus to One Tree Island in the Capricorn Bunker Group, in the southern Great Barrier Reef, was highly variable over 23 years. We predicted that the currents in the Capricorn Bunker Group, including their wind driven components and the Capricorn Eddy (a nearby transient oceanic eddy), would affect patterns of recruitment. A biophysical model was used to investigate this prediction. L. carponotatus were collected from One Tree Island and the dates when they were in the plankton as larvae were determined from their otoliths. The winds present during the pelagic phases of the fish were examined; they were found to have survived either longshore (SSE) winds that induced little cross shelf movement in the larval plume or cross shelf (ENE) winds that induced little longshore movement. The unidirectional transportation of the larval plume in these conditions was favourable for recruitment as it kept the plume concentrated in the Capricorn Bunker Group. These winds were more prevalent in the periods of peak L. carponotatus production that preceded high recruitment. Dispersal under average winds (6.2 m s-1 from the prevailing ESE) and strong winds (velocity 1.5 times average), with and without the Capricorn Eddy, was also modelled. Each of these combinations were less favourable for recruitment than the longshore and cross shelf winds larval L. carponotatus survived before reaching OTI. The larval plume was comparatively less concentrated in the Capricorn Bunker Group under average winds. Strong winds transported the larval plume far longshore, to the NW, away from the Capricorn Bunker Group, while the Capricorn Eddy transported larvae seaward into oceanic waters. Larval swimming could counteract these dispersive forces; however, significant dispersion had occurred before larvae developed strong swimming and orientation abilities. This study provides a physical proxy for the recruitment of snapper. Further, we have demonstrated that great insights into recruitment variability can be gained through determining the specific conditions experienced by survivors.

Past ice stream and ice sheet changes on the continental shelf off the Sabrina Coast, East Antarctica

Our understanding of the response of the Antarctic ice sheet to climate and ocean changes requires the improvement of current ice-atmosphere-ocean models and the accurate determination of boundary conditions such as ice thickness and extent at key time intervals, so that satellite gravity observations and isostatic models can be adjusted. However, large portions of the Antarctic margin remain understudied or lack suitable data. One key area where data are lacking, is the Sabrina Coast portion of the East Antarctic Ice Sheet (EAIS) margin where the Totten Glacier, which has the largest ice discharge in East Antarctica, is accelerating, thinning and loosing mass at high rates. In this work, we present the results of the first geological and geophysical marine survey to the continental shelf offshore of the Dalton Ice Tongue and Moscow University ice shelf, east of the Totten Glacier. The data presented include multibeam swath bathymetry and multichannel seismic, focusing on the sea floor morphology and sedimentary section above a regional angular unconformity separating pre- and post-Miocene glacial strata. Sea floor scouring and iceberg keel marks on the outer shelf, associated with gullies on the upper slope indicate that ice expanded in the past and grounded ~5 km from the shelf edge at ~450–500 mbsl, extending ~155 km north of the current Moscow University Ice Shelf. A nearly 1000 m deep area in the inner-middle shelf, oriented NW with paleo-ice flow direction indicated by mega scale glacial lineations (MSGL) and drumlins, is interpreted as a cross shelf glacial trough. A series of geomorphic associations on the north-eastern side of the glacial trough includes glacial lobes, grounding zone wedges (GZW), glacial lineations and transverse ridges, which indicates slower ice, grounding line stabilization and collapse. These geomorphic associations are organized in 4 four sets representing different past ice-flow configurations reflecting changes in ice flow direction, grounding line position, location of fast and slow ice areas, and retreat pattern. Some of the geomorphic features identified are compatible with the presence of an organized subglacial drainage, and others are with rapid grounding line collapse. A well-preserved series of GZWs occurring at different water depths implies they were formed during different glacial stages or cycles. The inferred diminishing ice thickness for consecutives GZWs indicates that the margin of the Antarctic ice sheet evolved to a less extensive coverage of the continental shelf through successive glacial stages or cycles.The identification of different ice flow configurations, evidence of subglacial water and past ice margin collapse indicates a dynamic ice sheet margin with varying glacial conditions and retreat modes. We observe that some of the described morphological associations are similar to those found in the Amundsen sea sector of the West Antarctic Ice Sheet (WAIS) where they are associated with ice sheet and ice stream collapse. Although further studies are needed to assess the precise timing and rates of the glacial processes involved, we conclude that there is enough evidence to support the hypothesis that the EAIS margin can behave as dynamically as the WAIS margin, especially during glacial retreat and icesheet margin collapse.

Wind-Driven Circulation in a Shelf Valley. Part II: Dynamics of the Along-Valley Velocity and Transport

AbstractThe dynamics controlling the along-valley (cross shelf) flow in idealized shallow shelf valleys with small to moderate Burger number are investigated, and analytical scales of the along-valley flows are derived. This paper follows Part I, which shows that along-shelf winds in the opposite direction to coastal-trapped wave propagation (upwelling regime) force a strong up-valley flow caused by the formation of a lee wave. In contrast, along-shelf winds in the other direction (downwelling regime) do not generate a lee wave and consequently force a relatively weak net down-valley flow. The valley flows in both regimes are cyclostrophic with O(1) Rossby number. A major difference between the two regimes is the along-shelf length scales of the along-valley flows Lx. In the upwelling regime Lx depends on the valley width Wc and the wavelength λlw of the coastal-trapped lee wave arrested by the along-shelf flow Us. In the downwelling regime Lx depends on the inertial length scale |Us|/f and Wc. The along-valley velocity scale in the upwelling regime, given byis based on potential vorticity (PV) conservation and lee-wave dynamics (Hs and Hc are the shelf and valley depth scales, respectively, and f is the Coriolis parameter). The velocity scale in the downwelling regime, given by is based on PV conservation. The velocity scales are validated by the numerical sensitivity simulations and can be useful for observational studies of along-valley transports. The work provides a framework for investigating cross-shelf transport induced by irregular shelf bathymetry and calls for future studies of this type under realistic environmental conditions and over a broader parameter space.

Seasonal and spatial variability in the optical characteristics of DOM in a temperate shelf sea

The Celtic Sea is a productive temperate sea located on the Northwest European Shelf. It is an important pathway for the delivery of land-derived material to the North Atlantic Ocean, including dissolved organic matter (DOM). The aim of this study was to determine the seasonal and spatial variability in the magnitude, source and composition of DOM at three sites representing on shelf, central shelf and shelf edge regions in the Celtic Sea, using observations collected during the UK Shelf Sea Biogeochemistry (SSB) research programme (November 2014 – August 2015). The concentration of dissolved organic carbon (DOC) alongside DOM absorbance and fluorescence indices were measured and fluorescence Excitation and Emission Matrices (EEMs) combined with Parallel Factor Analysis (PARAFAC) were used to assess DOM composition and lability. The PARAFAC model identified four unique fluorescent components for autumn (November 2014), winter (March 2015), spring (April 2015) and summer (July 2015) consisting of two humic-like components attributed to terrestrial (C1) and marine sources (C2), and two protein components identified as tyrosine-like (C3) and tryptophan-like (C4) attributed to in situ production. DOC varied seasonally and there were strong cross shelf trends. The protein components (C3 and C4) exhibited large seasonal and within season variability particularly during productive periods. In contrast, there were persistent cross shelf gradients in the CDOM absorption coefficient at 305 nm (a305), the UV specific absorbance at 280 nm (SUVA280), the humification index (HIX), and the humic-like fluorescent components (C1 and C2), which were higher in the on shelf region and decreased towards the shelf edge. The humic-like components and the slope ratio (SR) were significantly correlated with salinity throughout all seasons, indicating a strong influence of terrestrially-derived organic matter in the Celtic Sea, with potentially up to 35% of DOC in the central shelf during winter originating from terrestrial inputs. Results from this study illustrate the importance of monitoring DOM quantitatively and qualitatively for a better understanding of the supply, production, cycling and export of this dynamic organic carbon pool in shelf seas.

Organic matter in sediments of a tropical and upwelling influenced region of the Brazilian continental margin (Campos Basin, Rio de Janeiro)

Lipid biomarkers [fatty acids (FAs), sterols and alcohols] and carbon stable isotopes (δ13C values) in surface sediments from 9 cross shelf transects (25–3000 m water depth) from the Campos Basin, SE Brazilian continental margin were analyzed. The aim was to investigate the link between the prevailing regional specific oceanographic conditions (upwelling events, intrusion of cold and nutrient-rich water, low river input) and the nature and distribution of organic matter (OM) in the basin. A general predominance of OM from autochthonous processes, but with a relevant spatial gradient in the quality and quantity of the sedimentary OM, was observed. On the shelf (<150 m), concentrations of lipids were usually low, except in areas influenced by upwelling, but the presence of labile compounds suggested the occurrence of fresh OM in the sediment. The export of continental OM was observed only in shelf sediments near the Paraíba do Sul River. The upper and middle slope (400–1300 m) exhibited the highest concentrations of total organic carbon (TOC) and lipids, but lipid biomarkers suggested the presence of OM with a high degree of bacterial degradation. This may result from the export of material from shallow areas, possibly due to the action of eddies and meandering of the Brazil Current and bottom currents in the region. On the lower slope (1900–3000 m), only the more recalcitrant compounds were above detection limit. The presence of labile lipids in high amount in the shelf and slope suggests the presence of OM with a high potential for supplying the food requirements of heterotrophic organisms in the sediment, which may in turn have a major influence on the ecology of benthic communities.