Showing papers by "Scottish Association for Marine Science published in 2013"

Threats and knowledge gaps for ecosystem services provided by kelp forests: a northeast Atlantic perspective.

Abstract: Kelp forests along temperate and polar coastlines represent some of most diverse and productive habitats on the Earth. Here, we synthesize information from >60 years of research on the structure and functioning of kelp forest habitats in European waters, with particular emphasis on the coasts of UK and Ireland, which represents an important biogeographic transition zone that is subjected to multiple threats and stressors. We collated existing data on kelp distribution and abundance and reanalyzed these data to describe the structure of kelp forests along a spatial gradient spanning more than 10° of latitude. We then examined ecological goods and services provided by kelp forests, including elevated secondary production, nutrient cycling, energy capture and flow, coastal defense, direct applications, and biodiversity repositories, before discussing current and future threats posed to kelp forests and identifying key knowledge gaps. Recent evidence unequivocally demonstrates that the structure of kelp forests in the NE Atlantic is changing in response to climate- and non-climate-related stressors, which will have major implications for the structure and functioning of coastal ecosystems. However, kelp-dominated habitats along much of the NE Atlantic coastline have been chronically understudied over recent decades in comparison with other regions such as Australasia and North America. The paucity of field-based research currently impedes our ability to conserve and manage these important ecosystems. Targeted observational and experimental research conducted over large spatial and temporal scales is urgently needed to address these knowledge gaps.

Genome structure and metabolic features in the red seaweed Chondrus crispus shed light on evolution of the Archaeplastida.

Abstract: Red seaweeds are key components of coastal ecosystems and are economically important as food and as a source of gelling agents, but their genes and genomes have received little attention. Here we report the sequencing of the 105-Mbp genome of the florideophyte Chondrus crispus (Irish moss) and the annotation of the 9,606 genes. The genome features an unusual structure characterized by gene-dense regions surrounded by repeat-rich regions dominated by transposable elements. Despite its fairly large size, this genome shows features typical of compact genomes, e.g., on average only 0.3 introns per gene, short introns, low median distance between genes, small gene families, and no indication of large-scale genome duplication. The genome also gives insights into the metabolism of marine red algae and adaptations to the marine environment, including genes related to halogen metabolism, oxylipins, and multicellularity (microRNA processing and transcription factors). Particularly interesting are features related to carbohydrate metabolism, which include a minimalistic gene set for starch biosynthesis, the presence of cellulose synthases acquired before the primary endosymbiosis showing the polyphyly of cellulose synthesis in Archaeplastida, and cellulases absent in terrestrial plants as well as the occurrence of a mannosylglycerate synthase potentially originating from a marine bacterium. To explain the observations on genome structure and gene content, we propose an evolutionary scenario involving an ancestral red alga that was driven by early ecological forces to lose genes, introns, and intergenetic DNA; this loss was followed by an expansion of genome size as a consequence of activity of transposable elements.

Cold-water coral reefs.

Abstract: Scleractinian hard corals in deep, cold waters have been known since the eighteenth century. However, advances in deep-ocean exploration are now revealing the true scale and distribution of cold-water coral reefs. This prompted a resurgence of interest with dramatic discoveries of deep-water reef and coral carbonate mound provinces rapidly followed by improved understanding of coral reproduction, feeding, and molecular genetics. Hundreds of tropical coral species build shallow reefs, but less than 10 cold-water species form deep reef frameworks. Of these, the best characterized is Lophelia pertusa , which dominates in the Northeast Atlantic. Assemblages of octocorals and hydrocorals are found in other parts of the world's oceans, such as the North Pacific. Cold-water coral skeletons provide well-preserved, high-resolution paleoclimatic archives and recent advances have been made in interpreting geochemical proxies for seawater temperature and ocean ventilation history. The reefs form structurally complex habitats supporting many other species. This complexity makes them vulnerable to mechanical damage from deep-water bottom trawling and modeled scenarios suggest that cold-water coral reefs may be threatened by ocean acidification. Integrated basin-scale studies are needed to understand linkages between reef provinces as a critical step toward developing meaningful systems of protected areas for their conservation.

High rates of microbial carbon turnover in sediments in the deepest oceanic trench on Earth

Abstract: Microbes regulate the decomposition of organic matter in marine sediments. Measurements at the deepest oceanic site on Earth reveal high rates of microbial activity, potentially fuelled by the deposition of organic matter. Microbes control the decomposition of organic matter inmarine sediments. Decomposition, in turn, contributes to oceanic nutrient regeneration and influences the preservation of organic carbon1. Generally, rates of benthic decomposition decline with increasing water depth, although given the vast extent of the abyss, deep-sea sediments are quantitatively important for the global carbon cycle2,3. However, the deepest regions of the ocean have remained virtually unexplored4. Here, we present observations of microbial activity in sediments at Challenger Deep in the Mariana Trench in the central west Pacific, which at almost 11,000 m depth represents the deepest oceanic site on Earth. We used an autonomous micro-profiling system to assess benthic oxygen consumption rates. We show that although the presence of macrofauna is restricted at Challenger Deep, rates of biological consumption of oxygen are high, exceeding rates at a nearby 6,000-m-deep site by a factor of two. Consistently, analyses of sediments collected from the two sites reveal higher concentrations of microbial cells at Challenger Deep. Furthermore, analyses of sediment 210Pb profiles reveal relatively high sediment deposition in the trench. We conclude that the elevated deposition of organic matter at Challenger Deep maintains intensified microbial activity at the extreme pressures that characterize this environment.

Does Presence of a Mid-Ocean Ridge Enhance Biomass and Biodiversity?

Abstract: In contrast to generally sparse biological communities in open-ocean settings, seamounts and ridges are perceived as areas of elevated productivity and biodiversity capable of supporting commercial fisheries. We investigated the origin of this apparent biological enhancement over a segment of the North Mid-Atlantic Ridge (MAR) using sonar, corers, trawls, traps, and a remotely operated vehicle to survey habitat, biomass, and biodiversity. Satellite remote sensing provided information on flow patterns, thermal fronts, and primary production, while sediment traps measured export flux during 2007–2010. The MAR, 3,704,404 km2 in area, accounts for 44.7% lower bathyal habitat (800–3500 m depth) in the North Atlantic and is dominated by fine soft sediment substrate (95% of area) on a series of flat terraces with intervening slopes either side of the ridge axis contributing to habitat heterogeneity. The MAR fauna comprises mainly species known from continental margins with no evidence of greater biodiversity. Primary production and export flux over the MAR were not enhanced compared with a nearby reference station over the Porcupine Abyssal Plain. Biomasses of benthic macrofauna and megafauna were similar to global averages at the same depths totalling an estimated 258.9 kt C over the entire lower bathyal north MAR. A hypothetical flat plain at 3500 m depth in place of the MAR would contain 85.6 kt C, implying an increase of 173.3 kt C attributable to the presence of the Ridge. This is approximately equal to 167 kt C of estimated pelagic biomass displaced by the volume of the MAR. There is no enhancement of biological productivity over the MAR; oceanic bathypelagic species are replaced by benthic fauna otherwise unable to survive in the mid ocean. We propose that globally sea floor elevation has no effect on deep sea biomass; pelagic plus benthic biomass is constant within a given surface productivity regime.

Framework for understanding marine ecosystem health

Abstract: Although the terms 'health' and 'healthy' are often applied to marine ecosystems and communicate information about holistic condition (e.g. as required by the Ecosystem Approach), their meaning is unclear. Ecosystems have been understood in various ways, from non-interacting populations of species to complex integrated systems. Health has been seen as a metaphor, an indicator that aggregates over system components, or a non-localized emergent system property. After a review, we define good ecosystem health as: 'the condition of a system that is self-maintaining, vigorous, resilient to externally imposed pressures, and able to sustain services to humans. It contains healthy organisms and populations, and adequate functional diversity and functional response diversity. All expected trophic levels are present and well interconnected, and there is good spatial connectivity amongst subsystems.' We equate this condition with good ecological or environmental status, e.g. as referred to by recent EU Directives. Resilience is central to health, but difficult to measure directly. Ecosystems under anthropogenic pressure are at risk of losing resilience, and thus of suffering regime shifts and loss of services. For monitoring whole ecosystems, we propose an approach based on 'trajectories in ecosystem state space', illustrated with time-series from the northwestern North Sea. Change is visualized as Euclidian distance from an arbitrary reference state. Variability about a trend in distance is used as a proxy for inverse resilience. We identify the need for institutional support for long time-series to underpin this approach, and for research to establish state space co-ordinates for systems in good health. Changes in the northern North Sea, 1958-2008, plotted in a state space defined by the breeding success of kittiwakes, abundance of copepods Calanus spp., and simulated annual primary production.© Inter-Research 2013. www.int-res.com.

Beyond climate change attribution in conservation and ecological research

Abstract: There is increasing pressure from policymakers for ecologists to generate more detailed 'attribution' analyses aimed at quantitatively estimating relative contributions of different driving forces, including anthropogenic climate change (ACC), to observed biological changes. Here, we argue that this approach is not productive for ecological studies. Global meta-analyses of diverse species, regions and ecosystems have already given us 'very high confidence' [sensu Intergovernmental Panel on Climate Change (IPCC)] that ACC has impacted wild species in a general sense. Further, for well-studied species or systems, synthesis of experiments and models with long-term observations has given us similarly high confidence that they have been impacted by regional climate change (regardless of its cause). However, the role of greenhouse gases in driving these impacts has not been estimated quantitatively. Should this be an ecological research priority? We argue that development of quantitative ecological models for this purpose faces several impediments, particularly the existence of strong, non-additive interactions among different external factors. However, even with current understanding of impacts of global warming, there are myriad climate change adaptation options already developed in the literature that could be, and in fact are being, implemented now.

Hadal disturbance in the Japan Trench induced by the 2011 Tohoku–Oki Earthquake

Abstract: In situ video observations and sediment core samplings were performed at two hadal sites in the Japan Trench on July, 2011, four months after the Tohoku–Oki earthquake. Video recordings documented dense nepheloid layers extending ~30–50 m above the sea bed. At the trench axis, benthic macrofauna was absent and dead organisms along with turbid downslope current were observed. The top 31 cm of sediment in the trench axis revealed three recent depositions events characterized by elevated 137Cs levels and alternating sediment densities. At 4.9 km seaward from the trench axis, little deposition was observed but the surface sediment contained 134Cs from the Fukushima Dai–ichi nuclear disaster. We argue that diatom blooms observed by remote sensing facilitated rapid deposition of 134Cs to hadal environment and the aftershocks induced successive sediment disturbances and maintained dense nepheloid layers in the trench even four months after the mainshock.

Heterotrophic Compensation: A Possible Mechanism for Resilience of Coral Reefs to Global Warming or a Sign of Prolonged Stress?

Abstract: Thermally induced bleaching has caused a global decline in corals and the frequency of such bleaching events will increase. Thermal bleaching severely disrupts the trophic behaviour of the coral holobiont, reducing the photosynthetically derived energy available to the coral host. In the short term this reduction in energy transfer from endosymbiotic algae results in an energy deficit for the coral host. If the bleaching event is short-lived then the coral may survive this energy deficit by depleting its lipid reserves, or by increasing heterotrophic energy acquisition. We show for the first time that the coral animal is capable of increasing the amount of heterotrophic carbon incorporated into its tissues for almost a year following bleaching. This prolonged heterotrophic compensation could be a sign of resilience or prolonged stress. If the heterotrophic compensation is in fact an acclimatization response, then this physiological response could act as a buffer from future bleaching by providing sufficient heterotrophic energy to compensate for photoautotrophic energy losses during bleaching, and potentially minimizing the effect of subsequent elevated temperature stresses. However, if the elevated incorporation of zooplankton is a sign that the effects of bleaching continue to be stressful on the holobiont, even after 11 months of recovery, then this physiological response would indicate that complete coral recovery requires more than 11 months to achieve. If coral bleaching becomes an annual global phenomenon by mid-century, then present temporal refugia will not be sufficient to allow coral colonies to recover between bleaching events and coral reefs will become increasingly less resilient to future climate change. If, however, increasing their sequestration of zooplankton-derived nutrition into their tissues over prolonged periods of time is a compensating mechanism, the impacts of annual bleaching may be reduced. Thus, some coral species may be better equipped to face repeated bleaching stress than previously thought.

Megacities and large urban agglomerations in the coastal zone: interactions between atmosphere, land, and marine ecosystems.

Abstract: Megacities are not only important drivers for socio-economic development but also sources of environmental challenges. Many megacities and large urban agglomerations are located in the coastal zone where land, atmosphere, and ocean meet, posing multiple environmental challenges which we consider here. The atmospheric flow around megacities is complicated by urban heat island effects and topographic flows and sea breezes and influences air pollution and human health. The outflow of polluted air over the ocean perturbs biogeochemical processes. Contaminant inputs can damage downstream coastal zone ecosystem function and resources including fisheries, induce harmful algal blooms and feedback to the atmosphere via marine emissions. The scale of influence of megacities in the coastal zone is hundreds to thousands of kilometers in the atmosphere and tens to hundreds of kilometers in the ocean. We list research needs to further our understanding of coastal megacities with the ultimate aim to improve their environmental management.

Combining in-situ measurements and altimetry to estimate volume, heat and salt transport variability through the Faroe Shetland Channel

Abstract: . From 1994 to 2011, instruments measuring ocean currents (Acoustic Doppler Current Profilers; ADCPs) have been moored on a section crossing the Faroe–Shetland Channel. Together with CTD (Conductivity Temperature Depth) measurements from regular research vessel occupations, they describe the flow field and water mass structure in the channel. Here, we use these data to calculate the average volume transport and properties of the flow of warm water through the channel from the Atlantic towards the Arctic, termed the Atlantic inflow. We find the average volume transport of this flow to be 2.7 ± 0.5 Sv (1 Sv = 106 m3 s–1) between the shelf edge on the Faroe side and the 150 m isobath on the Shetland side. The average heat transport (relative to 0 °C) was estimated to be 107 ± 21 TW (1 TW = 1012 W) and the average salt import to be 98 ± 20 × 106 kg s−1. Transport values for individual months, based on the ADCP data, include a large level of variability, but can be used to calibrate sea level height data from satellite altimetry. In this way, a time series of volume transport has been generated back to the beginning of satellite altimetry in December 1992. The Atlantic inflow has a seasonal variation in volume transport that peaks around the turn of the year and has an amplitude of 0.7 Sv. The Atlantic inflow has become warmer and more saline since 1994, but no equivalent trend in volume transport was observed.

Integrating multiple data sources to assess the distribution and abundance of bottlenose dolphins Tursiops truncatus in Scottish waters

Abstract: The distribution, movements and abundance of highly mobile marine species such as bottlenose dolphins Tursiops truncatus are best studied at large spatial scales, but previous research effort has generally been focused on relatively small areas, occupied by populations with high site fidelity. We aimed to characterize the distribution, movements and abundance of bottlenose dolphins around the coasts of Scotland, exploring how data from multiple sources could be integrated to build a broader-scale picture of their ecology. We reviewed existing historical data, integrated data from ongoing studies and developed new collaborative studies to describe distribution patterns. We adopted a Bayesian multi-site mark-recapture model to estimate abundance of bottlenose dolphins throughout Scottish coastal waters and quantified movements of individuals between study areas. The majority of sightings of bottlenose dolphins around the Scottish coastline are concentrated on the east and west coasts, but records are rare before the 1990s. Dedicated photo-identification studies in 2006 and 2007 were used to estimate the size of two resident populations: one on the east coast from the Moray Firth to Fife, population estimate 195 [95% highest posterior density intervals (HPDI): 162–253] and the second in the Hebrides, population estimate 45 (95% HPDI: 33–66). Interaction parameters demonstrated that the dolphins off the east coast of Scotland are highly mobile, whereas those off the west coast form two discrete communities. We provide the first comprehensive assessment of the abundance of bottlenose dolphins in the inshore waters of Scotland. The combination of dedicated photo-identification studies and opportunistic sightings suggest that a relatively small number of bottlenose dolphins (200–300 individuals) occur regularly in Scottish coastal waters. On both east and west coasts, re-sightings of identifiable individuals indicate that the animals have been using these coastal areas since studies began.

Physiological and biogeochemical traits of bleaching and recovery in the mounding species of coral Porites lobata: implications for resilience in mounding corals.

Abstract: Mounding corals survive bleaching events in greater numbers than branching corals. However, no study to date has determined the underlying physiological and biogeochemical trait(s) that are responsible for mounding coral holobiont resilience to bleaching. Furthermore, the potential of dissolved organic carbon (DOC) as a source of fixed carbon to bleached corals has never been determined. Here, Porites lobata corals were experimentally bleached for 23 days and then allowed to recover for 0, 1, 5, and 11 months. At each recovery interval a suite of analyses were performed to assess their recovery (photosynthesis, respiration, chlorophyll a, energy reserves, tissue biomass, calcification, δ13C of the skeletal, δ13C, and δ15N of the animal host and endosymbiont fractions). Furthermore, at 0 months of recovery, the assimilation of photosynthetically acquired and zooplankton-feeding acquired carbon into the animal host, endosymbiont, skeleton, and coral-mediated DOC were measured via 13C-pulse-chase labeling. During the first month of recovery, energy reserves and tissue biomass in bleached corals were maintained despite reductions in chlorophyll a, photosynthesis, and the assimilation of photosynthetically fixed carbon. At the same time, P. lobata corals catabolized carbon acquired from zooplankton and seemed to take up DOC as a source of fixed carbon. All variables that were negatively affected by bleaching recovered within 5 to 11 months. Thus, bleaching resilience in the mounding coral P. lobata is driven by its ability to actively catabolize zooplankton-acquired carbon and seemingly utilize DOC as a significant fixed carbon source, facilitating the maintenance of energy reserves and tissue biomass. With the frequency and intensity of bleaching events expected to increase over the next century, coral diversity on future reefs may favor not only mounding morphologies but species like P. lobata, which have the ability to utilize heterotrophic sources of fixed carbon that minimize the impact of bleaching and promote fast recovery.

Polycyclovorans algicola gen. nov., sp. nov., an Aromatic-Hydrocarbon-Degrading Marine Bacterium Found Associated with Laboratory Cultures of Marine Phytoplankton

Abstract: A strictly aerobic, halotolerant, rod-shaped bacterium, designated strain TG408, was isolated from a laboratory culture of the marine diatom Skeletonema costatum (CCAP 1077/1C) by enrichment with polycyclic aromatic hydrocarbons (PAHs) as the sole carbon source. 16S rRNA gene sequence analysis placed this organism within the order Xanthomonadales of the Gammaproteobacteria . Closest relatives included representatives of the Hydrocarboniphaga-Nevskia-Sinobacter clade ( Sinobacteraceae . The strain exhibited a narrow nutritional spectrum, preferring to utilize aliphatic and aromatic hydrocarbon compounds and small organic acids. Notably, it displayed versatility for degrading two- and three-ring PAHs. Moreover, catechol 2,3-dioxygenase activity was detected in lysates, indicating that this strain utilizes the meta -cleavage pathway for aromatic compound degradation. Cells produced surface blebs and contained a single polar flagellum. The predominant isoprenoid quinone of strain TG408 was Q-8, and the dominant fatty acids were C 16:0 , C 16:1 ω7 c and C 18:1 ω7 c . DNA G+C content for the isolate was 64.3 ± 0.34 mol%. On the basis of distinct phenotypic and genotypic characteristics, strain TG408 represents a novel genus and species in the class Gammaproteobacteria , for which the name Polycyclovorans algicola gen. nov., sp. nov. is proposed. Quantitative PCR primers targeting the 16S rRNA gene of this strain were developed and used to show that this organism is found associated with other species of marine phytoplankton. Phytoplankton may be a natural biotope in the ocean where new species of hydrocarbon-degrading bacteria await discovery and which contribute significantly to natural remediation processes.

A Novel and Low-Cost Sea Ice Mass Balance Buoy

Abstract: The understanding of sea ice mass balance processes requires continuous monitoring of the seasonal evolution of the ice thickness. While autonomous ice mass balance (IMB) buoys deployed over the past two decades have contributed to scientists' understanding of ice growth and decay processes, deployment has been limited, in part, by the cost of such systems. Routine, basinwide monitoring of the ice cover is realistically achievable through a network of reliable and affordable autonomous instrumentation. This paper describes the development of a novel autonomous platform and sensor that replaces the traditional thermistor strings for monitoring temperature profiles in the ice and snow using a chain of inexpensive digital temperature chip sensors linked by a single-wire data bus. By incorporating a heating element into each sensor, the instrument is capable of resolving material interfaces (e.g., air–snow and ice–ocean boundaries) even under isothermal conditions. The instrument is small, low cost, a...

Marine renewable energy development: assessing the Benthic Footprint at multiple scales

Abstract: Increasing concerns over global climate change and sustainable fuel procurement are driving the search for new ways to derive energy from the seas. Globally, the offshore wind energy sector has progressed rapidly, and wave and tidal-current energy converters are now approaching deployment at commercial scales. To date, most studies of the ecological effects of marine renewable energy development have concentrated on birds and marine mammals. Here, we focus on the consequences for benthic flora and fauna, and for benthic habitats across a variety of scales. We use a “Benthic Footprint” concept to discuss the potential for species-specific environmental responses, and to consider the poorly understood cumulative effects of wind, wave, and tidal-current energy operations on marine ecosystems. Collaborations between ecologists, industry specialists, and government bodies, as well as better designs for devices, arrays, and developments consisting of multiple arrays, can contribute to the goal of reducing the B...

Ecological network indicators of ecosystem status and change in the Baltic Sea.

Abstract: Several marine ecosystems under anthropogenic pressure have experienced shifts from one ecological state to another In the central Baltic Sea, the regime shift of the 1980s has been associated with food-web reorganization and redirection of energy flow pathways These long-term dynamics from 1974 to 2006 have been simulated here using a food-web model forced by climate and fishing Ecological network analysis was performed to calculate indices of ecosystem change The model replicated the regime shift The analyses of indicators suggested that the system’s resilience was higher prior to 1988 and lower thereafter The ecosystem topology also changed from a web-like structure to a linearized food-web

Decadal Freshening of the Antarctic Bottom Water Exported from the Weddell Sea

Abstract: Recent decadal changes in Southern Hemisphere climate have driven strong responses from the cryosphere. Concurrently, there has been a marked freshening of the shelf and bottom waters across a wide sector of the Southern Ocean, hypothesised to be caused by accelerated glacial melt in response to a greater flux of warm waters from the Antarctic Circumpolar Current onto the shelves of West Antarctica. However, the circumpolar pattern of changes has been incomplete: no decadal freshening in the deep layers of the Atlantic sector had been observed. In this study, we document a significant freshening of the Antarctic Bottom Water exported from the Weddell Sea, which is the source for the abyssal layer of the Atlantic overturning circulation, and we trace its possible origin to atmospheric-forced changes in the ice shelves and sea ice on the eastern flank of the Antarctic Peninsula that include an anthropogenic component. These findings suggest that the expansive and relatively cool Weddell gyre does not insulate the bottom water formation regions in the Atlantic sector from the ongoing changes in climatic forcing over the Antarctic region.

Tidal downwelling and implications for the carbon biogeochemistry of cold‐water corals in relation to future ocean acidification and warming

Abstract: Cold-water coral (CWC) reefs are recognized as ecologically and biologically significant areas that generate habitats and diversity. The interaction between hydrodynamics and CWCs has been well studied at the Mingulay Reef Complex, a relatively shallow area of reefs found on the continental shelf off Scotland, UK. Within ‘Mingulay Area 01’ a rapid tidal downwelling of surface waters, brought about as an internal wave, is known to supply warmer, phytoplankton-rich waters to corals growing on the northern flank of an east-west trending seabed ridge. This study shows that this tidal downwelling also causes short-term perturbations in the inorganic carbon (CT) and nutrient dynamics through the water column and immediately above the reef. Over a 14 h period, corresponding to one semi-diurnal tidal cycle, seawater pH overlying the reef varied by ca. 0.1 pH unit, while pCO2 shifted by >60 ?atm, a shift equivalent to a ca. 25 year jump into the future, with respect to atmospheric pCO2. During the summer stratified period, these downwelling events result in the reef being washed over with surface water that has higher pH, is warmer, nutrient depleted, but rich in phytoplankton-derived particles compared to the deeper waters in which the corals sit. Empirical observations, together with outputs from the European Regional Shelf Sea Ecosystem Model, demonstrate that the variability that the CWC reefs experience changes through the seasons and into the future. Hence, as ocean acidification and warming increase into the future, the downwelling event specific to this site could provide short-term amelioration of corrosive conditions at certain times of the year; however, it could additionally result in enhanced detrimental impacts of warming on CWCs. Natural variability in the CT and nutrient conditions, as well as local hydrodynamic regimes, must be accounted for in any future predictions concerning the responses of marine ecosystems to climate change.

Effects of transient bottom water currents and oxygen concentrations on benthic exchange rates as assessed by eddy correlation measurements

Abstract: [1] Eddy correlation (EC) measurements in the benthic boundary layer (BBL) allow estimating benthic O2 uptake from a point distant to the sediment surface. This noninvasive approach has clear advantages as it does not disturb natural hydrodynamic conditions, integrates the flux over a large foot-print area and allows many repetitive flux measurements. A drawback is, however, that the measured flux in the bottom water is not necessarily equal to the flux across the sediment-water interface. A fundamental assumption of the EC technique is that mean current velocities and mean O2 concentrations in the bottom water are in steady state, which is seldom the case in highly dynamic environments like coastal waters. Therefore, it is of great importance to estimate the error introduced by nonsteady state conditions. We investigated two cases of transient conditions. First, the case of transient O2 concentrations was examined using the theory of shear flow dispersion. A theoretical relationship between the change of O2 concentrations and the induced vertical O2 flux is introduced and applied to field measurements showing that changes of 5–10 μM O2 h−1 result in transient EC-fluxes of 6–12 mmol O2 m−2 d−1, which is comparable to the O2 uptake of shelf sediments. Second, the case of transient velocities was examined with a 2D k-e turbulence model demonstrating that the vertical flux can be biased by 30–100% for several hours during changing current velocities from 2 to 10 cm s−1. Results are compared to field measurements and possible ways to analyze and correct EC-flux estimates are discussed.

Differentiating the wild or farmed origin of Mediterranean fish: a review of tools for sea bream and sea bass

Abstract: This study was financed by the European Union 7th Research Framework Project ‘Prevent Escape’ (no 226885; wwwpreventescapeeu)

Atlantic Meridional Overturning Circulation slowdown cooled the subtropical ocean

Abstract: [1] Observations show that the upper 2 km of the subtropical North Atlantic Ocean cooled throughout 2010 and remained cold until at least December 2011. We show that these cold anomalies are partly driven by anomalous air-sea exchange during the cold winters of 2009/2010 and 2010/2011 and, more surprisingly, by extreme interannual variability in the ocean's northward heat transport at 26.5°N. This cooling driven by the ocean's meridional heat transport affects deeper layers isolated from the atmosphere on annual timescales and water that is entrained into the winter mixed layer thus lowering winter sea surface temperatures. Here we connect, for the first time, variability in the northward heat transport carried by the Atlantic Meridional Overturning Circulation to widespread sustained cooling of the subtropical North Atlantic, challenging the prevailing view that the ocean plays a passive role in the coupled ocean-atmosphere system on monthly-to-seasonal timescales.

Rapid cross-density ocean mixing at mid-depths in the Drake Passage measured by tracer release

Abstract: Diapycnal mixing (across density surfaces) is an important process in the global ocean overturning circulation1, 2, 3. Mixing in the interior of most of the ocean, however, is thought to have a magnitude just one-tenth of that required to close the global circulation by the downward mixing of less dense waters4. Some of this deficit is made up by intense near-bottom mixing occurring in restricted ‘hot-spots’ associated with rough ocean-floor topography5, 6, but it is not clear whether the waters at mid-depth, 1,000 to 3,000 metres, are returned to the surface by cross-density mixing or by along-density flows7. Here we show that diapycnal mixing of mid-depth (~1,500 metres) waters undergoes a sustained 20-fold increase as the Antarctic Circumpolar Current flows through the Drake Passage, between the southern tip of South America and Antarctica. Our results are based on an open-ocean tracer release of trifluoromethyl sulphur pentafluoride. We ascribe the increased mixing to turbulence generated by the deep-reaching Antarctic Circumpolar Current as it flows over rough bottom topography in the Drake Passage. Scaled to the entire circumpolar current, the mixing we observe is compatible with there being a southern component to the global overturning in which about 20 sverdrups (1 Sv = 106 m3 s−1) upwell in the Southern Ocean, with cross-density mixing contributing a significant fraction (20 to 30 per cent) of this total, and the remainder upwelling along constant-density surfaces. The great majority of the diapycnal flux is the result of interaction with restricted regions of rough ocean-floor topography.