Upper-water Column Research Articles

Variation in subsurface water temperature and its link to the Kuroshio Current in the Okinawa Trough during the last 38.5 kyr

An 820.4-cm long piston core (YK12-15 PC-01) was obtained from site OT-1B (28°07.15′N, 127°11.97′E, 1050 m water depth), located in the middle of the Okinawa Trough in the southern East China Sea (ECS). For core YK12-15 PC-01, oxygen and carbon isotopes (δ18O and δ13C) of planktic foraminifera Globigerinoides ruber sensu stricto (250–355 μm size fraction) and Globorotalia inflata (>250 μm size fraction) together with total organic carbon (TOC), nitrogen content (TN), and δ13C of organic matter were analyzed for the last 38.5 thousand years (kyr). The δ18O records of G. ruber (δ18Oruber) and G. inflata (δ18Oinflata) were used to estimate the temperature/salinity of the surface and subsurface waters, respectively. In addition, we estimated the vertical temperature gradient between the surface and the subsurface based on the difference in δ18O between G. ruber and G. inflata (Δδ18Oruber-inflata). The δ18Oinflata and Δδ18Oruber-inflata indicate millennial-scale variations in association with the oscillations in Antarctic temperatures during the Marine Isotope Stage (MIS) 3. These results suggest that variations in subsurface temperature and vertical temperature gradient in the upper water column in the ECS were closely related to the Atlantic bipolar seesaw during MIS 3. The subsurface temperature in the ECS was higher at the beginning of Greenland stadials when the Antarctic temperatures showed the lowest values. The warming of the subsurface is likely attributed to the stronger Kuroshio transport. High peaks in the TOC and TN records coincide with the low peaks in δ18Oinflata (high temperature), suggesting higher primary productivity, which is in good agreement with the interpretation of the intensified Kuroshio. However, the subsurface warming was not recognized during Herinch events, indicating different temperature responses in the upper water column from other stadials.

A novel upward-looking hydroacoustic method for improving pelagic fish surveys

For ethical reasons and animal welfare, it is becoming increasingly more important to carry out ecological surveys with a non-invasive approach. Information about fish distribution and abundance in the upper water column is often fundamental. However, this information is extremely hard to obtain using classical hydroacoustic methods. We developed a rigid frame system for pushing upward looking transducers of the scientific echo sounder (38 and 120 kHz) in front of the research vessel. The efficiency of the new approach for monitoring juvenile fish at night was investigated by comparing the results with a quantitative fry trawl in the Římov Reservoir in the Czech Republic. The experimental setup enabled comparisons for the 0–3 m and 3–6 m depth layers, which are utilized by almost all juvenile fish in summer. No statistically significant differences in the estimated abundance of juveniles were found between the two sampling methods. The comparison of abundance estimates gathered by the two frequencies were also not significantly different. The predicted mean lengths from acoustic sampling and the trawl catches differed by less than 10 mm in all comparisons. Results suggest that mobile hydroacoustic upward-looking systems can fill the methodological gap in non-invasive surveying of surface fishes.

Vertical thermal gradient history in the eastern equatorial Pacific during the early to middle Miocene: Implications for the equatorial thermocline development

AbstractKnowledge of the equatorial thermocline is essential for understanding climate changes in the tropical Pacific. Multispecies planktic foraminiferal analyses provide a way to examine temperature distributions and thus the structure of the thermocline. Although the secular thermocline development has been documented back to the late Miocene, the early to middle Miocene interval has rarely been examined. In addition, relationships with the dynamic Antarctic ice sheets remain unclear. Here we investigate the vertical thermal gradient in the upper water column at Integrated Ocean Drilling Program Site U1337 in the eastern equatorial Pacific (EEP) throughout the early to middle Miocene (23.1 to 11.7 Ma). The gradient increased over the Miocene Climatic Optimum, whereas it decreased during the East Antarctic Ice Sheet Expansion (EAIE). Comparison of the EEP record with its western equatorial Pacific (WEP) counterpart suggests that sea surface temperature was more stable in the WEP than in the EEP. We further estimated equatorial thermocline from two diagonal gradients between the EEP and the WEP: thermocline shoaled from 16.7 to 15.7 Ma and tilt weakened between 16.5 and 13.8 Ma. The onset of the “Monterey Excursion” and the reduced Antarctic ice sheet volume would have affected thermocline depth and tilt, respectively. Thermocline depth was likely much deeper compared to Pliocene‐to‐modern conditions. Furthermore, a 4‐point‐based distribution of isotherms (4DI index) was used as a metric of the evenness or unevenness of the isotherm distributions. The 4DI index considerably reduced at around the EAIE and other Mi‐events, reflecting the evenly distributed isotherms under a more glaciated Antarctica.

Crustacean zooplankton release copious amounts of dissolved organic matter as taurine in the ocean.

Taurine (Tau), an amino acid‐like compound, is present in almost all marine metazoans including crustacean zooplankton. It plays an important physiological role in these organisms and is released into the ambient water throughout their life cycle. However, limited information is available on the release rates by marine organisms, the concentrations and turnover of Tau in the ocean. We determined dissolved free Tau concentrations throughout the water column and its release by abundant crustacean mesozooplankton at two open ocean sites (Gulf of Alaska and North Atlantic). At both locations, the concentrations of dissolved free Tau were in the low nM range (up to 15.7 nM) in epipelagic waters, declining sharply in the mesopelagic to about 0.2 nM and remaining fairly stable throughout the bathypelagic waters. Pacific amphipod–copepod assemblages exhibited lower dissolved free Tau release rates per unit biomass (0.8 ± 0.4 μmol g−1 C‐biomass h−1) than Atlantic copepods (ranging between 1.3 ± 0.4 μmol g−1 C‐biomass h−1 and 9.5 ± 2.1 μmol g−1 C‐biomass h−1), in agreement with the well‐documented inverse relationship between biomass‐normalized excretion rates and body size. Our results indicate that crustacean zooplankton might contribute significantly to the dissolved organic matter flux in marine ecosystems via dissolved free Tau release. Based on the release rates and assuming steady state dissolved free Tau concentrations, turnover times of dissolved free Tau range from 0.05 d to 2.3 d in the upper water column and are therefore similar to those of dissolved free amino acids. This rapid turnover indicates that dissolved free Tau is efficiently consumed in oceanic waters, most likely by heterotrophic bacteria.

Coastal sources, sinks and strong organic complexation of dissolved cobalt within the US North Atlantic GEOTRACES transect GA03

Abstract. Cobalt is the scarcest of metallic micronutrients and displays a complex biogeochemical cycle. This study examines the distribution, chemical speciation, and biogeochemistry of dissolved cobalt during the US North Atlantic GEOTRACES transect expeditions (GA03/3_e), which took place in the fall of 2010 and 2011. Two major subsurface sources of cobalt to the North Atlantic were identified. The more prominent of the two was a large plume of cobalt emanating from the African coast off the eastern tropical North Atlantic coincident with the oxygen minimum zone (OMZ) likely due to reductive dissolution, biouptake and remineralization, and aeolian dust deposition. The occurrence of this plume in an OMZ with oxygen above suboxic levels implies a high threshold for persistence of dissolved cobalt plumes. The other major subsurface source came from Upper Labrador Seawater, which may carry high cobalt concentrations due to the interaction of this water mass with resuspended sediment at the western margin or from transport further upstream. Minor sources of cobalt came from dust, coastal surface waters and hydrothermal systems along the Mid-Atlantic Ridge. The full depth section of cobalt chemical speciation revealed near-complete complexation in surface waters, even within regions of high dust deposition. However, labile cobalt observed below the euphotic zone demonstrated that strong cobalt-binding ligands were not present in excess of the total cobalt concentration there, implying that mesopelagic labile cobalt was sourced from the remineralization of sinking organic matter. In the upper water column, correlations were observed between total cobalt and phosphate, and between labile cobalt and phosphate, demonstrating a strong biological influence on cobalt cycling. Along the western margin off the North American coast, this correlation with phosphate was no longer observed and instead a relationship between cobalt and salinity was observed, reflecting the importance of coastal input processes on cobalt distributions. In deep waters, both total and labile cobalt concentrations were lower than in intermediate depth waters, demonstrating that scavenging may remove labile cobalt from the water column. Total and labile cobalt distributions were also compared to a previously published South Atlantic GEOTRACES-compliant zonal transect (CoFeMUG, GAc01) to discern regional biogeochemical differences. Together, these Atlantic sectional studies highlight the dynamic ecological stoichiometry of total and labile cobalt. As increasing anthropogenic use and subsequent release of cobalt poses the potential to overpower natural cobalt signals in the oceans, it is more important than ever to establish a baseline understanding of cobalt distributions in the ocean.

Fine-scale spatial and interannual cadmium isotope variability in the subarctic northeast Pacific

We present dissolved cadmium (Cd) concentrations, [Cd], and stable isotope compositions, ε112/110Cd, in high-resolution depth profiles from five stations along the Line P transect in the subarctic northeast Pacific Ocean. In addition to profiles collected in 2012, subsurface isopycnal samples and surface samples were collected in 2013 and 2014 respectively, providing both temporal and spatial coverage. Surface waters are characterized by Cd depletion relative to phosphate (PO43−) compared to deepwater Cd:PO43−, and high inferred remineralization ratios in the nutricline (0.45nmolμmol−1) are observed, consistent with Cd enrichment relative to phosphorus (P) in surface-derived biogenic particles. The correlation between Cd and PO43− weakens at depths where oxygen is highly depleted as shown by local minima in dissolved [Cd] and the tracer Cd⁎. The decoupling, which is driven by a deficit of Cd relative to PO43−, appears consistent with the recent hypothesis of dissolved Cd removal in oxygen-depleted regions by insoluble metal sulfide formation.Dissolved ε112/110Cd indicates a biologically driven fractionation in surface waters with more positive (heavy) values in the upper water column and lower (light) values in deeper waters. The highest ε112/110Cd observed in our sample set (5.19±0.23) is comparable to observations from the Southern Ocean but is significantly lighter than maximum reported surface values from the subtropical North Pacific of ε112/110Cd≥15. A global compilation of low [Cd] surface water shows similar differences in maximum ε112/110Cd. A surface water intercalibration should be prioritized to help determine if these differences at low [Cd] reflect true physical or biological variability or are due to analytical artefacts. Surface samples from the 2012 sampling campaign fit a closed-system Rayleigh fractionation model; however, surface waters sampled in 2014 had much lower [Cd] with relatively constant ε112/110Cd that cannot be explained by a closed-system Rayleigh model. These results correspond with a warm water surface anomaly found along Line P in 2014 and demonstrate that there is interannual variability in the biogeochemical cycling of Cd and its isotopes in the subarctic North Pacific. In contrast to other ocean basins where vertical variability in ε112/110Cd is observed at depth, deep and intermediate waters in the North Pacific have a near-uniform ε112/110Cd value (mean of 1.14±0.37, n=43, 2SD) representative of nearly all samples at or below 1000 m depth. Imprinted upon this nearly homogeneous intermediate and deep North Pacific ε112/110Cd signature are fine-scale spatial trends, with heavier values observed toward the coastal end of Line P than the oceanic end at intermediate depths, and with slightly heavier values in subtropical North Pacific deep water compared to the subarctic North Pacific. The nearly constant Cd isotopic composition of North Pacific deep waters is consistent with the inflow of Circumpolar Deep Water at depth in the Pacific basin, along with deep remineralization, and supports the potential of ε112/110Cd as a tracer of global deepwater circulation.

Distribution of reactive aluminum under the influence of mesoscale eddies in the western South China Sea

To understand the distribution of aluminum (Al) under the influence of mesocale eddies in the western South China Sea (SCS), sea level anomaly, geostrophic current, environmental parameters and reactive Al were investigated in the western SCS in August 2013. The highest reactive Al concentration ((180±64) nmol/L) was observed in the surface waters, indicating a substantial atmospheric input. Vertically, the reactive Al decreased from the surface high concentration to the subsurface minima at the depth of chlorophyll a (Chl a) maxima and then increased again with depth at most of the stations. The average concentration of reactive Al in the upper 100 m water column was significantly lower in the cyclonic eddy ((137±6) nmol/L) as compared with that in the noneddy waters ((180±21) nmol/L). By contrast, the average concentrations of Chl a and silicate in the upper 100 m water column were higher in the cyclonic eddy and lower in the anticyclonic eddy. There was a significant negative correlation between the average concentrations of reactive Al and Chl a in the upper 100 m water column. The vertical distribution of reactive Al and the negative correlation between reactive Al and Chl a both suggest that the reactive Al in the upper water column was significantly influenced by biological removal processes. Our results indicate that mesoscale eddies could regulate the distribution of reactive Al by influencing the primary production and phytoplankton community structure in the western SCS.

Did the Deepwater Horizon oil spill affect growth of Red Snapper in the Gulf of Mexico?

The explosion of the Deepwater Horizon (DWH) oil platform in 2010 released more than 200 million gallons of crude oil into the northern Gulf of Mexico (GoM). Elevated levels of carcinogenic polycyclic aromatic hydrocarbons (PAHs) were abundant in the upper water column throughout the event. Previous research suggests that PAHs may have negatively affected fishes in the northern GoM. Our objective was to test whether crude oil contamination from the DWH oil spill was correlated with changes in growth rates in adult Red Snapper, Lutjanus campechanus. We fit von Bertalanffy growth curves and back-calculated length-at-age using data collected from 2011–2013 during long-line surveys in the northern GoM and on the West Florida Shelf. No significant variation in von Bertalanffy growth parameters existed among the catch years; a combined-years model gave L∞, k, and t0 values of 82.91, 0.20, and 0.43, respectively. No significant difference existed between pre- (back-calculated) and post-DWH growth curves. However, annual widths of the fourth, fifth, and sixth increments (the dominant cohorts in the population) declined significantly post-DWH (2010–2012) by 13%, 15%, and 22%, respectively, and were significantly smaller than the mean width of each respective increment in pre-spill years (2006–2009). While the DWH event was related temporally to growth declines in the dominant adult age groups, other environmental factors (winds, temperature, and river discharge) may also affect growth. Accordingly, meridional (V) and zonal (U) winds, sea level height anomalies (a proxy for water temperature variation), and Mississippi River discharge were compared to increment widths but none of the factors were strongly correlated with variation in age-specific growth increments (maximum Pearson’s r=0.47). Therefore, we are unable to reject the hypothesis that the DWH resulted in growth rate declines as opposed to climatic variation.

Increase in acidifying water in the western Arctic Ocean

Ocean acidification has expanded in the western Arctic Ocean. Observations from the 1990s to 2010 show that aragonite saturation levels have decreased, with low saturation water deepening to 250 m and increasing in area more rapidly than seen in other oceans. The uptake of anthropogenic CO2 by the ocean decreases seawater pH and carbonate mineral aragonite saturation state (Ωarag), a process known as Ocean Acidification (OA). This can be detrimental to marine organisms and ecosystems1,2. The Arctic Ocean is particularly sensitive to climate change3 and aragonite is expected to become undersaturated (Ωarag < 1) there sooner than in other oceans4. However, the extent and expansion rate of OA in this region are still unknown. Here we show that, between the 1990s and 2010, low Ωarag waters have expanded northwards at least 5°, to 85° N, and deepened 100 m, to 250 m depth. Data from trans-western Arctic Ocean cruises show that Ωarag < 1 water has increased in the upper 250 m from 5% to 31% of the total area north of 70° N. Tracer data and model simulations suggest that increased Pacific Winter Water transport, driven by an anomalous circulation pattern and sea-ice retreat, is primarily responsible for the expansion, although local carbon recycling and anthropogenic CO2 uptake have also contributed. These results indicate more rapid acidification is occurring in the Arctic Ocean than the Pacific and Atlantic oceans5,6,7,8, with the western Arctic Ocean the first open-ocean region with large-scale expansion of ‘acidified’ water directly observed in the upper water column.

Incidental capture of leatherback sea turtles in fixed fishing gear off Atlantic Canada

Abstract Incidental capture in commercial fishing gear is a threat to many populations of marine megafauna, including sea turtles. While research has largely focused on pelagic longline impacts on sea turtles, fixed‐gear fisheries are a significant, historically understudied source of injury and mortality. The present study assesses the interaction of endangered leatherback sea turtles (Dermochelys coriacea) with fixed‐gear fisheries in high‐latitude seasonal foraging habitat where sub‐adult and adult turtles aggregate. Records of leatherback‐fishery interactions (n = 205) were compiled from databases of publicly‐reported sea turtle sightings in Atlantic Canada (1998–2014) to identify the spatio‐temporal distribution of these events; to identify corresponding fisheries and gear types; and to describe the mechanics and outcomes of entanglements in fixed gear. Most reports came from coastal Nova Scotia (n = 136) and Newfoundland (n = 40), with reporting rates peaking in the mid‐to‐late 2000s. The majority of entanglements were reported during the summer months of July and August when leatherbacks are seasonally resident and several fisheries are active in continental shelf waters. Entanglements were most commonly reported in pot gear (e.g. snow crab, lobster, whelk) and trap nets (e.g. mackerel), reflecting extensive use of polypropylene lines distributed in the upper water column where leatherback foraging activity is concentrated. Given reporting biases and uncertainty regarding post‐release survivorship, entanglement mortalities should be considered a gross underestimate of true mortality rates. This study highlights both the importance of looking beyond pelagic longlines to evaluate leatherback interactions with fixed‐gear fisheries in high‐use continental shelf foraging habitat, and of involving the fishing industry in developing mitigation measures to reduce entanglement rates and associated turtle mortality.

Decline in global oceanic oxygen content during the past five decades.

Ocean models predict a decline in the dissolved oxygen inventory of the global ocean of one to seven per cent by the year 2100, caused by a combination of a warming-induced decline in oxygen solubility and reduced ventilation of the deep ocean. It is thought that such a decline in the oceanic oxygen content could affect ocean nutrient cycles and the marine habitat, with potentially detrimental consequences for fisheries and coastal economies. Regional observational data indicate a continuous decrease in oceanic dissolved oxygen concentrations in most regions of the global ocean, with an increase reported in a few limited areas, varying by study. Prior work attempting to resolve variations in dissolved oxygen concentrations at the global scale reported a global oxygen loss of 550 ± 130 teramoles (1012 mol) per decade between 100 and 1,000 metres depth based on a comparison of data from the 1970s and 1990s. Here we provide a quantitative assessment of the entire ocean oxygen inventory by analysing dissolved oxygen and supporting data for the complete oceanic water column over the past 50 years. We find that the global oceanic oxygen content of 227.4 ± 1.1 petamoles (1015 mol) has decreased by more than two per cent (4.8 ± 2.1 petamoles) since 1960, with large variations in oxygen loss in different ocean basins and at different depths. We suggest that changes in the upper water column are mostly due to a warming-induced decrease in solubility and biological consumption. Changes in the deeper ocean may have their origin in basin-scale multi-decadal variability, oceanic overturning slow-down and a potential increase in biological consumption.

Distribution and sea-to-air fluxes of volatile halocarbons in the Bohai Sea and North Yellow Sea during spring.

Concentrations of volatile halocarbons (VHCs), such as CHBr2Cl, CHBr3, C2HCl3, and C2Cl4, in the Bohai Sea (BS) and North Yellow Sea (NYS) were measured during the spring of 2014. The VHC concentrations varied widely and decreased with distance from the coast in the investigated area, with low values observed in the open sea. Depth profiles of the VHCs were characterized by the highest concentration generally found in the upper water column. The distributions of the VHCs in the BS and NYS were clearly influenced by the combined effects of biological production, anthropogenic activities, and riverine input. The sea-to-air fluxes of CHBr2Cl, CHBr3, C2HCl3, and C2Cl4 in the study area were estimated to be 47.17, 56.63, 162.56, and 104.37nmolm-2d-1, respectively, indicating that the investigated area may be a source of atmospheric CHBr2Cl, CHBr3, C2HCl3, and C2Cl4 in spring.

Distribution of transparent exopolymer particles (TEP) across an organic carbon gradient in the western North Atlantic Ocean

In this study, the abundance of transparent exopolymer particles (TEP) is examined across a surface water gradient of organic carbon from a phytoplankton bloom region in the western North Atlantic to oligotrophic waters in the Sargasso Sea, including a coastal region sampled near Cape Cod. TEP are macrogels that reach up to millimeters in size and abiotically assemble from dissolved acidic polysaccharides secreted by phytoplankton. Due to their great stickiness, TEP self aggregate and also form aggregates with non-TEP particulate organic carbon (POC). Aggregation enhances ballasting, thereby mutually facilitating export and subsequent sedimentation of both TEP and POC, increasing the efficiency of the biological pump. Here, four distinct regions with varying chlorophyll a concentrations, temperature, and salinity were sampled in the upper column, at the surface, and from the sea surface microlayer (SML). While TEP in seawater shows no correlation to chlorophyll a, nutrients, or total organic carbon (TOC) concentration, a strong correlation exists between TEP and TOC in the SML; and concentrations of both variables are inversely proportional to surface productivity as indicated by chlorophyll a concentrations. As open ocean regions show greater enrichments of TEP and TOC in the SML compared to the coastal region, we suggest that the role of the SML in organic carbon cycling is dependent on regional biogeochemistry and productivity. We hypothesize that a lower abundance of particles in oligotrophic regions compared to bloom regions limits TEP export by sinking, thus increasing the residence time of TEP in the upper water column and the SML.

First Limnological Characterization of Crater Lake Billy Mitchell (Bougainville Island, Papua New Guinea)1.

In this study we present a first limnological characterization of Lake Billy Mitchell [1,013 m above sea level (a.s.l.), 88.3 m depth, 3 km2 surface area] in central Bougainville Island, Papua New Guinea. Physicochemical depth profiles indicated mixis of the entire water body with oxygen saturation reaching 55% in the deepest layers. A shallow thermocline was eroded at night, indicating atelomixis. and Na+, Ca2+, Mg2+ were the dominant anions and cations, respectively, leading to a conductivity of around 1,230 μS cm-1. The pH was close to neutral throughout the water column, and no accumulation of CO2 was observed at greater depths. With a total phosphorus concentration of around 25 μg liter-1 the lake can be considered as meso-to eutrophic. The phytoplankton community consisted of 18 taxa. The dinophyte Peridiniopsis cf. penardii and the filamentous green alga Planctonema lauterbornii dominated in the uppermost layer and reached a total biovolume around 16 mm3 liter-1. Six macrophyte taxa were found (three Spermatophyta/three Bryophyta), with the water chestnut Eleocharis dulcis covering the shoreline and Ceratophyllum demersum spreading to at least 3 m depth. Seven ciliate species were detected (<5 individuals ml-1) with bacterivorous scuticociliates and the prostomatid Coleps hirtus hirtus dominating the assemblage. The micrometazoan plankton community comprised the rotifer Anuraeopsis fissa, the copepod Mesocyclops cf. affinis, and a cladoceran species with-in the Ceriodaphnia cornuta group all concentrating in the upper water column. The only fish species found in the lake was the eel Anguilla megastoma, whereas in the effluent river this species occurred together with Anguilla marmorata.

Phosphorus dynamics in biogeochemically distinct regions of the southeast subtropical Pacific Ocean

The southeast subtropical Pacific Ocean was sampled along a zonal transect between the coasts of Chile and Easter Island. This remote area of the world’s ocean presents strong gradients in physical (e.g., temperature, density and light), chemical (e.g., salinity and nutrient concentrations) and microbiological (e.g., cell abundances, biomass and specific growth rates) properties. The goal of this study was to describe the phosphorus (P) dynamics in three main ecosystems along this transect: the upwelling regime off the northern Chilean coast, the oligotrophic area associated with the southeast subtropical Pacific gyre and the transitional area in between these two biomes. We found that inorganic phosphate (Pi) concentrations were high and turnover times were long (>210nmoll−1 and >31d, respectively) in the upper water column, along the entire transect. Pi uptake rates in the gyre were low (euphotic layer integrated rates were 0.26mmolm−2d−1 in the gyre and 1.28mmolm−2d−1 in the upwelling region), yet not only driven by decreases in particle mass or cell abundance (particulate P- and cell- normalized Pi uptake rates in the euphotic layer were ∼1–4 times and ∼3–15 times lower in the gyre than in the upwelling, respectively). However these Pi uptake rates were at or near the maximum Pi uptake velocity (i.e., uptake rates in Pi amended samples were not significantly different from those at ambient concentration: 1.5 and 23.7nmoll−1d−1 at 50% PAR in the gyre and upwelling, respectively). Despite the apparent Pi replete conditions, selected dissolved organic P (DOP) compounds were readily hydrolyzed. Nucleotides were the most bioavailable of the DOP substrates tested. Microbes actively assimilated adenosine-5′-triphosphate (ATP) leading to Pi and adenosine incorporation as well as Pi release to the environment. The southeast subtropical Pacific Ocean is a Pi-sufficient environment, yet DOP hydrolytic processes are maintained and contribute to P-cycling across the wide range of environmental conditions present in this ecosystem.

A distinctive biomarker assemblage in an Infracambrian oil and source rock from western India: Molecular signatures of eukaryotic sterols and prokaryotic carotenoids

Molecular fossils record aspects of Neoproterozoic-Early Cambrian palaeobiology in the Bikaner-Nagaur Basin, western India. Biomarker hydrocarbons from crude oils and sedimentary rock extracts belonging to the Marwar Supergroup, India, were analyzed using gas chromatography–mass spectrometry, including molecular specific metastable reaction monitoring (MRM) transitions. The distinctive biomarker assemblage indicates significant contribution from both eukaryotes and bacteria. The steranes record contributions from algae as well as demosponges. Dominance of C29 sterane isomers signifies that green algae were important primary producers while detection of traces of dinosteranes suggests input from dinoflagellates or their ancestors. The high C30 sterane ratio (24-isopropylcholestane/24-n-propylcholestane; ∼5) indicates substantial proliferation of Demospongiae which are widely considered as basal metazoa. The community of algae and other taxa, including sponges which require oxygen, thrived in a thin oxygenated layer in the upper water column. However, the presence of an underlying euxinic water column is demonstrated by the presence of gammacerane, extremely low values of pristane/phytane (around 0.4) and carotenoids derived from both green (Chlorobi) and purple (Chromatiaceae) sulfur bacteria. Chlorobactane, isorenieratane and β-isorenieratane indicate both green- and brown-pigmented Chlorobi, whereas the detection of okenane and renierapurpurane are diagnostic for the Chromatiaceae. Paleorenieratane, a carotenoid pigment from an apparently ‘extinct’ Chlorobi taxon is present in high abundance particularly in the sedimentary rock extract. Excellent preservation of the biomarkers and an extremely mild thermal history is indicated by sterane and triterpane molecular ratios with preferences for biological stereoisomers. The entire biomarker assemblage is indicative of a restricted, salinity-stratified depositional environment possibly including the presence of extensive, anoxygenic microbial mats. Some of the observed characteristics mirror those of other coeval successions in Oman, Eastern Siberia and Australia but the particular combination of features appears to be unique to this setting.

Sinkers or floaters? Contribution from salp pellets to the export flux during a large bloom event in the Southern Ocean

Salp fecal pellets are rich in organic matter and have been shown to sink at very high velocities. In recent years, salp abundances have been increasing in the Southern Ocean where they seem to be replacing krill as the dominant grazers on phytoplankton. As salps can form large swarms with high pellet production rates, it has been suggested that they will become increasingly important for the vertical export of particulate organic matter in the Southern Ocean. However, detailed studies combining both investigations of pellet production rates, turnover, and export are still needed in order to determine whether salp pellets are important for export (‘sinkers’) or recycling (‘floaters’) of organic matter. Our results suggest that pellets are produced at high rates in the upper few hundred meters of the water column. Although we observed high sinking velocities and low microbial degradation rates of the produced salp pellets, only about one third of the produced pellets were captured in sediment traps placed at 100m and about ~13% of the produced pellets were exported to sediment traps placed at 300m. The high retention of these fast-settling pellets seems to be caused by break-up and loosening of the pellets, possibly by zooplankton and salps themselves. We measured 3-fold lower size-specific sinking velocities in loosened and fragmented compared to freshly produced intact pellets-. This enhanced the residence times (>1day) of both small and large pellets in the upper water column. We postulate that the fragile nature of salp pellets make them more important for recycling of organic matter in the upper mesopelagic layer rather than as a conduit for export of particulate organic matter to the seafloor.

Water-column cooling and sea surface salinity increase in the upwelling region off central-south Chile driven by a poleward displacement of the South Pacific High

Here we present results of direct observations of seawater temperature and salinity over the continental shelf off central-south Chile that shows an unprecedented cooling of the entire water column and an increase in upper layer salinity during 2002 to 2013. We provide evidence that this phenomenon is related to the intensification but mostly to a recent southward displacement of the South Pacific High over the same period, from 2007 on. This in turn has accelerated alongshore, equatorward, subtropical coastal upwelling favorable winds, particularly during winter, injecting colder water from below into the upper water column. Consequently, the environmental conditions on the shelf off central-south Chile shifted from a warmer (fresher) to a cooler (saltier) phase; water column temperature dropped from 11.7°C (2003–2006) to 11.3°C (2007–2012) and upper layer salinity rose by 0.25; water column stratification gradually decreased. The biological impacts of such abrupt cooling are apparently already happening in this coastal ecosystem, as recent evidence shows substantial changes in the plankton community and negative trends in zooplankton biomass over the same period.

The metabolic response of thecosome pteropods from the North Atlantic and North Pacific oceans to high CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and low O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;

Abstract. As anthropogenic activities directly and indirectly increase carbon dioxide (CO2) and decrease oxygen (O2) concentrations in the ocean system, it becomes important to understand how different populations of marine animals will respond. Water that is naturally low in pH, with a high concentration of carbon dioxide (hypercapnia) and a low concentration of oxygen, occurs at shallow depths (200–500 m) in the North Pacific Ocean, whereas similar conditions are absent throughout the upper water column in the North Atlantic. This contrasting hydrography provides a natural experiment to explore whether differences in environment cause populations of cosmopolitan pelagic calcifiers, specifically the aragonitic-shelled pteropods, to have a different physiological response when exposed to hypercapnia and low O2. Using closed-chamber end-point respiration experiments, eight species of pteropods from the two ocean basins were exposed to high CO2 ( ∼ 800 µatm) while six species were also exposed to moderately low O2 (48 % saturated, or ∼ 130 µmol kg−1) and a combined treatment of low O2/high CO2. None of the species tested showed a change in metabolic rate in response to high CO2 alone. Of those species tested for an effect of O2, only Limacina retroversa from the Atlantic showed a response to the combined treatment, resulting in a reduction in metabolic rate. Our results suggest that pteropods have mechanisms for coping with short-term CO2 exposure and that there can be interactive effects between stressors on the physiology of these open ocean organisms that correlate with natural exposure to low O2 and high CO2. These are considerations that should be taken into account in projections of organismal sensitivity to future ocean conditions.

Effects of coral extracts on survivorship, swimming behavior, and settlement of Pocillopora damicornis larvae

Coral larvae released into the water column encounter different dissolved chemical cues secreted by organisms on the substratum that are engaged in spatial competition. These cues initiate vital biological processes in the larvae. In this study, the effects of crude extracts from the scleractinian corals Pocillopora damicornis and Porites cylindrica, and the soft coral Sarcophyton glaucum, on the survivorship, swimming behavior, and settlement of Pocillopora damicornis larvae were evaluated. High concentrations of heterospecific coral extracts caused a significant decline in both larval survivorship and settlement whereas conspecific extracts did not result in any larval mortality. Exposure to heterospecific extracts resulted in repulsion of larvae as indicated by a transient change in larval morphology and sustained swimming in the upper water column. In contrast, the conspecific extract enhanced larval exploration of both the water column and substratum and was able to induce larval settlement. These results suggest that chemical cues emanating from conspecifics and other coral species are capable of altering the biology of coral larvae, thus potentially influencing settlement behavior and species distribution on the reef.