Upper-water Column Research Articles

Quantification of ammonia oxidation rates and ammonia‐oxidizing archaea and bacteria at high resolution in the Gulf of California and eastern tropical North Pacific Ocean

Ammonia‐oxidizing microorganisms compete with phytoplankton for reduced nitrogen in the euphotic zone and provide oxidized nitrogen to other microbes present in the sea. We report 15NH4+ oxidation rate measurements made at 5–20‐m resolution using an in situ array and quantification of ammonia‐oxidizing archaea (AOA) and ammonia‐oxidizing bacteria (AOB) in corresponding samples from the upper water column and oxygen minimum zone (OMZ) of the Gulf of California (GOC) and eastern tropical North Pacific Ocean (ETNP). 15NH4+ oxidation rates varied substantially with depth and between stations: they were greatest at the base of the euphotic zone, and maximum rates were up to 28‐fold greater than rates measured within 5–10 m. Pyrosequencing and quantitative polymerase chain reactions (QPCR) indicated that AOA were present throughout the water column at all latitudes and always outnumbered AOB. AOB constituted only 39 of 432,240 16S ribosomal ribonucleic acid gene sequences produced via pyrosequencing but were more abundant at greater depths and higher latitudes. 15NH4+ oxidation rates were correlated with AOA abundance at some stations and were detectable in 96% of samples, including depths where oxygen concentrations were < 5 µmol kg−1 and depths within the euphotic zone, where up to 42% of ammonia oxidation occurred. Ammonia is rapidly oxidized within discrete depth intervals in the GOC and ETNP; while pyrosequencing and QPCR demonstrate that AOB are confined to deeper portions of the water column, AOA appear to be active within the euphotic zone, where they may quickly respond to nitrogen inputs.

Size distribution of particles and zooplankton across the shelf-basin system in southeast Beaufort Sea: combined results from an Underwater Vision Profiler and vertical net tows

Abstract. The size distribution and mean spatial trends of large particles (>100 μm, in equivalent spherical diameter, ESD) and mesozooplankton were investigated across the Mackenzie Shelf (southeast Beaufort Sea, Arctic Ocean) in July–August 2009. Our main objective was to combine results from an Underwater Vision Profiler 5 (UVP5) and traditional net tows (200 μm mesh size) to characterize the structural diversity and functioning of the Arctic shelf-basin ecosystem and to assess the large-scale correspondence between the two methodological approaches. The core dataset comprised 154 UVP5 profiles and 29 net tows conducted in the shelf (<100 m isobath), slope (100–1000 m) and basin (>1000 m) regions of the study area. The mean abundance of total particles and zooplankton in the upper water column (<75 m depth) declined exponentially with increasing distance from shore. Vertical and latitudinal patterns in total particle concentration followed those of chlorophyll a (chl a) concentration, with maximum values between 30 and 70 m depth. Based on the size-spectra derived from the UVP5 dataset, living organisms (0.1–10 mm ESD) accounted for an increasingly large proportion of total particle abundance (from 0.1 % to >50 %) when progressing offshore and as the ESD of particles was increasing. Both the UVP5 and net tows determined that copepods dominated the zooplankton community (~78–94 % by numbers) and that appendicularians were generally the second most abundant group (~1–11 %). The vertical distribution patterns of copepods and appendicularians indicated a close association between primary production and the main grazers. Manual taxonomic counts and ZooScan image analyses shed further light on the size-structure and composition of the copepod community – which was dominated at ~95 % by a guild of 10 typical taxa. The size distributions of copepods, as evaluated with the 3 methods (manual counts, ZooScan and UVP5), showed consistent patterns co-varying in the same order of magnitude over the upper size range (>1 mm ESD). Copepods <1 mm were not well quantified by the UVP5, which estimated that only ~13–25 % of the assemblage was composed of copepods <1 mm ESD compared with ~77–89 % from the net tow estimates. However, the biovolume of copepods was overwhelmingly dominated (~93–97 %) by copepods >1 mm ESD. Our results illustrate that the combination of traditional sampling methods and automated imaging techniques is a powerful approach that enabled us to conclude on the prevalence of a relatively high productivity regime and dominant herbivorous food web over the shelf when compared with the low-productive recycling system detected offshore.

Bottom sediment as a source of organic contaminants in Lake Mead, Nevada, USA

Treated wastewater effluent from Las Vegas, Nevada and surrounding communities’ flow through Las Vegas Wash (LVW) into the Lake Mead National Recreational Area at Las Vegas Bay (LVB). Lake sediment is a likely sink for many hydrophobic synthetic organic compounds (SOCs); however, partitioning between the sediment and the overlying water could result in the sediment acting as a secondary contaminant source. Locating the chemical plumes may be important to understanding possible chemical stressors to aquatic organisms. Passive sampling devices (SPMDs and POCIS) were suspended in LVB at depths of 3.0, 4.7, and 6.7 (lake bottom) meters in June of 2008 to determine the vertical distribution of SOCs in the water column. A custom sediment probe was used to also bury the samplers in the sediment at depths of 0–10, 10–20, and 20–30cm. The greatest number of detections in samplers buried in the sediment was at the 0–10cm depth. Concentrations of many hydrophobic SOCs were twice as high at the sediment–water interface than in the mid and upper water column. Many SOCs related to wastewater effluents, including fragrances, insect repellants, sun block agents, and phosphate flame retardants, were found at highest concentrations in the middle and upper water column. There was evidence to suggest that the water infiltrated into the sediment had a different chemical composition than the rest of the water column and could be a potential risk exposure to bottom-dwelling aquatic organisms.

Arctic Bay Formation, Borden Basin, Nunavut (Canada): Basin evolution, black shale, and dissolved metal systematics in the Mesoproterozoic ocean

The Arctic Bay Formation (Nunavut, Canada) represents a late Mesoproterozoic muddy terrigenous ramp and contains >200m of black shale. The formation was studied in order to decipher the tectonostratigraphic and geochemical evolution of the basin, address the origin of metal enrichment, and determine whether this frontier basin has the potential to host sedimentary-exhalative or polymetallic black shale deposits. Samples were analysed in the laboratory for major and trace elements, total organic carbon (TOC), 4-step loss-on-ignition (LOI), and Pb isotopes. Non-calcareous black shale exhibits neither Ce nor Y anomalies, reflecting euxinia in the lower water column, whereas slightly dolomitic black shale has both Ce and Y anomalies, reflecting the dolomite's probable origin as a precipitate in the upper water column. The stratigraphic distribution of the rare earth elements (REEs) indicates an evolving sediment provenance, and Pb isotopic data indicate that the source of clay in the black shale was dominated by weathered, juvenile, mantle-derived material. Base metals and redox-sensitive metals, expressed as enrichment ratios relative to conservative lithophile elements, are elevated and exhibit coherent covariations in the black shale. Enrichment in the redox-sensitive elements, such as Mo and U, correlates with dolomite content of the shale, rather than with organic C or Fepy. From a deep-time ocean evolution perspective, this important observation suggests that enrichment in these metals cannot necessarily be attributed to metal incorporation at an interface between sediment and euxinic water. Instead, in Arctic Bay Formation black shale, the metals were either scavenged onto dolomite as it precipitated in the water column, or secondarily re-distributed within the sediment according to its dolomite content. The base metals that are concentrated in the black shale (e.g., Zn) were probably sourced from diffuse hydrothermal venting, and although there is no evidence at the studied location for a nearby point source of metals (vent), persistent bottom-water euxinia would have ensured the effective scavenging of any dissolved metals supplied, and so the basin has at the very minimum a hypothetical potential for SEDEX and polymetallic mineralisation. Whole-rock U–Th–Pb isotope analysis of black shale yielded a date of 1092±59Ma, which is considered to be the Arctic Bay Formation's depositional age.

Observations of the Surf-Zone Turbulent Dissipation Rate

Abstract The contributions of surface (breaking wave) boundary layer (SBL) and bottom (velocity shear) boundary layer (BBL) processes to surf-zone turbulence is studied here. The turbulent dissipation rate ε, estimated on a 160-m-long cross-shore instrumented array, was an order of magnitude larger within the surf zone relative to seaward of the surf zone. The observed ε covaried across the array with changing incident wave height, tide level, and alongshore current. The cross-shore-integrated depth times ε was correlated with, but was only 1% of, the incident wave energy flux, indicating that surf-zone water-column turbulence is driven directly (turbulence injected by wave breaking) or indirectly (by forcing alongshore currents) by waves and that the bulk of ε occurs in the upper water column. This small fraction is consistent with laboratory studies. The surf-zone-scaled (or Froude-scaled) ε is similar to previous field observations, albeit somewhat smaller than laboratory observations. A breaking-wave ε scaling is applicable in the midwater column at certain locations, indicating a vertical diffusion of turbulence and ε balance. However, observations at different cross-shore locations do not collapse, which is consistent with a cross-shore lag between wave energy gradients and the surface turbulence flux. With strong alongshore currents, a BBL-scaled ε indicates that shear production is a significant turbulence source within the surf zone, particularly in the lower water column. Similarly for large currents at one location, the dissipation to shear production ratio approaches one. Both dissipation scalings depend upon wave energy flux gradients. The ratio of BBL to SBL ε has complex dependencies but is larger for a deeper part of the surf zone and more obliquely incident waves.

Physical control on methanotrophic potential in waters of the Santa Monica Basin, Southern California

Methane concentrations and turnover rates were measured throughout the water column at nine stations in the Santa Monica Basin (SMB), at one station in the San Pedro Basin (SPB), and at one station in the Santa Catalina Basin (SCtB) in July 2007 and September 2009. Methane concentrations were elevated throughout the water column, with subsurface (4–15 nmol L−1), midwater (7.5–100 nmol L−1), and bottom‐water (5–242 nmol L−1) maxima. The SMB water column was divided into four depth‐dependent regimes, based on temperature‐salinity relationships and patterns in methane concentration and fractional turnover rate. We propose that distinct physical controls on methane dynamics distinguish these regimes. In the upper water column, methanotrophic activity appears to be controlled by borderland‐scale, seasonal fluctuations in circulation. The midwaters represent a transition regime between northern‐sourced shallow waters and southern‐sourced deep waters, wherein methanotrophic activity decreases with depth to its minimum at 500 m, coinciding with the midwater methane maxima. Below 500 m, methanotrophic activity increases to the sill depth of 737 m, coincident with increasingly restricted circulation of water. Bottom waters are restricted from through‐basin circulation. These waters were oxygen depleted, but methane input sustained an active psychrotolerant methanotrophic community and had some of the fastest aerobic methane oxidation rates yet reported in the marine environment.

STUDY OF TRANSMISSION LOSS VARIATION AFFECTED BY INTERNAL TIDES IN THE SEA AREA NORTHEAST OF TAIWAN

Internal tides can cause density and sound speed fluctuations in the ocean, and the thickness of the mixing layer can fluctuate in short periods of time. This phenomenon affects underwater sound propagation and induces transmission loss variations. In the sea area northeast of Taiwan, regular internal tide activity is observed. The topography and sediment there are also complex, which makes the effect on transmission loss by internal tides even more complicated. This paper studies how transmission loss variation is affected by internal tides in the sea area northeast of Taiwan. The hydrographic fields with influence from the internal tides are provided by an experiment survey on the 4th of September, 2008 and a 3-D tide model. The transmission loss comparison between different thicknesses of the mixing layer in the internal tide fields is shown. Furthermore, the correlation coefficient is calculated and shows that layer depth and transmission loss have a medium to high negative correlation in the upper water column during the summer spring-tide period, and a low to medium negative correlation during the summer neap-tide period.

Decreased production of surface-active organic substances as a consequence of the oligotrophication in the northern Adriatic Sea

The dynamics of surface-active substances (SAS) in the northern Adriatic Sea were studied based on monthly sampling from 1998 to 2010. The data from two stations on the eastern and the western part of the northern Adriatic basin, differing significantly in terms of their trophic status, are presented and discussed. As a consequence of oligotrophication the concentration of SAS, primarily produced by phytoplankton, decreased lately (period 2005–2010) in the upper water column. A higher decrease was observed at a site further away from the influence of freshwater, implying that oligotrophic regions are much more sensible to the environmental changes. Besides the change in SAS concentration, the changes in its distribution between the dissolved and the particulate fraction and their acidity characteristics along the water column have been also observed in the recent period. We suggest that production of SAS shifted to deeper layers. This probably due to the phytoplankton depth increasing in response to the changed vertical distribution of nutrients. This implies increased importance of regenerated nutrients near the bottom for the phytoplankton blooming lately. In the period 2005–2010 pico- and nano-phytoplankton might significantly contribute to new SAS production near the bottom during summer. The observed changes of SAS may result in the changes of biogeochemical cycling of macro- and microconstituents of sea.

Nitrogen fixation in the East China Sea and southern Yellow Sea during summer 2006

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 447:77-86 (2012) - DOI: https://doi.org/10.3354/meps09509 Nitrogen fixation in the East China Sea and southern Yellow Sea during summer 2006 Run Zhang1, Min Chen1,2,*, Jianping Cao1, Qiang Ma1, Junhong Yang1, Yusheng Qiu1,2 1Department of Oceanography, and 2State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, 361005, PR China *Email: mchen@xmu.edu.cn ABSTRACT: Nitrogen fixation in the subtropical East China Sea (ECS) and the southern Yellow Sea (YS) were measured using 15N2 tracer assay during June and July 2006. Depth-integrated nitrogen fixation (2 to 221 µmol N m−2 d−1) was highest in the oceanic main path of the Kuroshio Current in the northeastern ECS, and in the mesohaline (surface salinity 30 to 34) stations. Very little N2 fixation was encountered in the low-salinity (surface salinity <30) area, and we suggest that N2 fixation was hindered by the nutrient conditions (lack of ‘excess’ phosphate relative to nitrate, xsPO4 = [PO4] − [NO3]/16) near the Changjiang (Yangtze River) mouth. In the mesohaline waters, N2 fixation was positively correlated with the vertical density (σt) gradient in the upper water column (30 m), indicating that N2 fixation can also be controlled by physical regime, and enhanced water column stratification may promote N2 fixation during summer. N2 fixation met 0.01 to 4.6% of nitrogen demand by primary production, suggesting that N2 fixation was not a major contributor to primary production in the study area. Estimated new N flux by N2 fixation to the ECS continental shelf (13 Gg N) during summer is at the same order of magnitude as atmospheric deposition, but much lower than either the Kuroshio subsurface water upwelling or the Changjiang riverine input. KEY WORDS: Nitrogen fixation · East China Sea · Southern Yellow Sea · 15N2 assay Full text in pdf format PreviousNextCite this article as: Zhang R, Chen M, Cao J, Ma Q, Yang J, Qiu Y (2012) Nitrogen fixation in the East China Sea and southern Yellow Sea during summer 2006. Mar Ecol Prog Ser 447:77-86. https://doi.org/10.3354/meps09509 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 447. Online publication date: February 13, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Utility of dissolved barium in distinguishing North American from Eurasian runoff in the Arctic Ocean

Dissolved barium has been shown to have the potential to distinguish Eurasian from North American (NA) river runoff. As part of the ARK-XXII/2 Polarstern expedition in summer 2007, Ba was analyzed in the Barents, Kara, Laptev seas, and the Eurasian Basins as well as the Makarov Basin up to the Alpha and Mendeleyev Ridges. By combining salinity, δ18O and initial phosphate corrected for mineralization with oxygen (PO4*) or N/P ratios we identified the water mass fractions of meteoric water, sea ice meltwater, and marine waters of Atlantic as well as Pacific origin in the upper water column. In all basins inside the lower halocline layer and the Arctic intermediate waters we find Ba concentrations close to those of the Fram Strait branch of the lower halocline (41–45nM), reflecting the composition of the incoming Atlantic water. A layer of upper halocline water (UHW) with higher Ba concentrations (45–55nM) is identified in the Makarov Basin. Atop of the UHW, the Surface Mixed Layer (SML), including the summer and winter mixed layers, has high concentrations of Ba (58–67nM). In the SML of the investigated area of the central Arctic the meteoric fraction can be identified by assuming a conservative behavior of Ba to be primarily of Eurasian river origin. However, in productive coastal regions biological removal compromises the use of Ba to distinguish between Eurasian and NA rivers. As a consequence, the NA river water fraction is underestimated in productive surface waters or waters that have passed a productive region, whereas this fraction is overestimated in subsurface waters containing remineralised Ba, particularly when these waters have passed productive shelf regions. Especially in the Laptev Sea and small regions in the Barents Sea, Ba concentrations are low in surface waters. In the Laptev Sea exceptionally high Ba concentrations in shelf bottom waters indicate that Ba is removed from surface waters to deep waters by biological activity enhanced by increasing ice-free conditions as well as by scavenging by organic matter of terrestrial origin. We interpret high Ba concentrations in the UHW of the Makarov Basin to result from enrichment by remineralisation in bottom waters on the shelf of the Chukchi Sea and therefore the calculated NA runoff is an artefact. We conclude that no NA runoff can be demonstrated unequivocally anywhere during our expedition with the set of tracers considered here. Small contributions of NA runoff may have been masked by Ba depletion and could only be resolved by supportive tracers on the uptake history. We thus suggest that Ba has to be used with care as it can put limits but not yield quantitative water mass distributions. Only if the extra Ba inputs exceed the cumulative biological uptake the signal can be unequivocally attributed to NA runoff.

Multifrequency species classification of acoustic-trawl survey data using semi-supervised learning with class discovery

Acoustic surveys often use multifrequency backscatter to estimate fish and plankton abundance. Direct samples are used to validate species classification of acoustic backscatter, but samples may be sparse or unavailable. A generalized Gaussian mixture model was developed to classify multifrequency acoustic backscatter when not all species classes are known. The classification, based on semi-supervised learning with class discovery, was applied to data collected in the eastern Bering Sea during summers 2004, 2007, and 2008. Walleye pollock, euphausiids, and two other major classes occurring in the upper water column were identified.

Fluorometric detection of total dissolved zinc in the southern Indian Ocean

Zinc acts as a micronutrient in the ocean, capable of influencing and potentially controlling phytoplankton productivity and community structure. Thus, it is important to quantify the distribution of dissolved Zn in the oceans, in addition to understanding the biogeochemical behavior of this important element. Meeting this objective has been elusive since dissolved Zn concentrations in the upper water column can be extremely low, and it is difficult to eliminate contamination during sample collection and analysis. Our approach to this problem was to utilize a Flow Injection Analysis (FIA) method initially described by Nowicki et al. (1994), and collecting uncontaminated seawater using a trace-metal clean rosette system (Measures et al., 2008).Samples for total dissolved Zn analysis were during the 2009 CLIVAR I5 cruise across the southern Indian Ocean (from Cape Town, South Africa to Fremantle, Australia). Dissolved Zn concentrations have not been previously reported for this region. Extremely low dissolved Zn concentrations (0.02nM) were observed in surface waters of the central Indian Ocean gyre, documenting the extreme biological depletion of Zn typical of the open ocean. Concentrations of Zn and Si both increased with depth. The highest concentrations measured for dissolved Zn (>3.5nM) were collected at 1300m off western Australian. Total dissolved Zn concentrations were observed to be oceanographically consistent, and well correlated with dissolved silicate across the transect. The linear regression of total dissolved Zn vs. Si for all of the data yielded a slope of 0.059±0.003 (nM Zn/μM Si), which is consistent with the values reported for the north Pacific and thus support the previously reported nutrient-type Zn–silicate relationship. The zonal section of the dissolved Zn/Si ratios also exhibit broad maxima and minima, consistent with variable sources for Zn and different recycling rates for Zn vs. Si.

The organic complexation of iron in the marine environment: a review

Iron (Fe) is an essential micronutrient for marine organisms, and it is now well established that low Fe availability controls phytoplankton productivity, community structure, and ecosystem functioning in vast regions of the global ocean. The biogeochemical cycle of Fe involves complex interactions between lithogenic inputs (atmospheric, continental, or hydrothermal), dissolution, precipitation, scavenging, biological uptake, remineralization, and sedimentation processes. Each of these aspects of Fe biogeochemical cycling is likely influenced by organic Fe-binding ligands, which complex more than 99% of dissolved Fe. In this review we consider recent advances in our knowledge of Fe complexation in the marine environment and their implications for the biogeochemistry of Fe in the ocean. We also highlight the importance of constraining the dissolved Fe concentration value used in interpreting voltammetric titration data for the determination of Fe speciation. Within the published Fe speciation data, there appear to be important temporal and spatial variations in Fe-binding ligand concentrations and their conditional stability constants in the marine environment. Excess ligand concentrations, particularly in the truly soluble size fraction, seem to be consistently higher in the upper water column, and especially in Fe-limited, but productive, waters. Evidence is accumulating for an association of Fe with both small, well-defined ligands, such as siderophores, as well as with larger, macromolecular complexes like humic substances, exopolymeric substances, and transparent exopolymers. The diverse size spectrum and chemical nature of Fe ligand complexes corresponds to a change in kinetic inertness which will have a consequent impact on biological availability. However, much work is still to be done in coupling voltammetry, mass spectrometry techniques, and process studies to better characterize the nature and cycling of Fe-binding ligands in the marine environment.

Diets and Trophic Linkages of Epipelagic Fish Predators in Coastal Southeast Alaska during a Period of Warm and Cold Climate Years, 1997–2011

AbstractThis study identifies important trophic links for epipelagic marine fish predators in Southeast Alaska to improve understanding of marine ecosystem dynamics in response to climate change. Fish predators can be viewed as autonomous samplers whose diets should integrate the available prey taxa commensurate with environmental conditions. We examined fish predators from annual (1997–2011) surveys conducted in May to September by the Southeast Coastal Monitoring (SECM) project of Auke Bay Laboratories in the marine waters of Southeast Alaska. This project has emphasized long‐term monitoring of strait and coastal marine habitats used by juvenile Pacific salmon Oncorhynchus spp. and associated epipelagic fishes to understand how environmental variation affects the sustainability of salmon resources. From 1,295 surface trawl hauls, trophic links were identified for 2,473 fish representing 19 predator species, principally adult and immature salmon, immature walleye pollock Theragra chalcogramma, and spiny dogfish Squalus acanthias. The most common fish prey consumed were fish larvae, juvenile salmon, Pacific herring Clupea pallasii, capelin Mallotus villosus, walleye pollock, lanternfishes (Myctophidae), and Pacific sand lance Ammodytes hexapterus, whereas the most common invertebrate prey consumed were euphausiids, decapod larvae, pteropods, and amphipods. This study describes the degree of piscivory, incidence of juvenile salmon prey, and frequency and weight composition of prey in the diets of epipelagic fish predators, but it did not clearly detect an effect of warm‐versus‐cold climate years on the diets of key planktivorous or piscivorous predators over the 15‐year time series. Identifying the persistence of trophic links in epipelagic waters over time is important because climate‐related changes in the upper water column have the potential to impact Southeast Alaska marine ecosystem dynamics and the productivity of important regional fisheries by altering key prey resources and trophic interactions.Received December 2, 2011; accepted May 9, 2012

Particle-reactive radionuclides (&amp;lt;sup&amp;gt;234&amp;lt;/sup&amp;gt;Th, &amp;lt;sup&amp;gt;210&amp;lt;/sup&amp;gt;Pb, &amp;lt;sup&amp;gt;210&amp;lt;/sup&amp;gt;) as tracers for the estimation of export production in the South China Sea

Abstract. The time-series station, SEATS (18° N, 116° E) in the South China Sea was visited six times during October 2006–December 2008 to carry out seawater sampling and floating trap deployments for the determination of distributions and fluxes of POC, PIC, PN, 234Th, 210Pb, and 210Po in the upper 200 m of the water column. Radionuclide deficiencies resulted in removal fluxes from the euphotic layer of 1.1×103–1.8×103 dpm m−2d−1 and 7.1–40.2 dpm m−2d−1 for 234Th and 210Po, respectively. Due to atmospheric input, an excess of 210Pb relative to 226Ra is commonly observed in the upper water column. Sinking fluxes of total mass, POC, PIC, PN, 234Th, 210Pb, and 210Po measured at the euphotic depth were low in summer-fall and high in winter-spring, reflecting the seasonal variability of biological pumping. Excluding the suspiciously low primary productivity data point in July 2007, a relatively high e-ratio of 0.28–0.69 was estimated by the ratio of the POC flux at the euphotic depth and the integrated primary productivity. The ratios of 234Th, 210Pb, and 210Po to organic carbon, inorganic carbon, and nitrogen in the sinking particles were combined with the disequilibria of 234Th–238U, 210Pb–226Ra, and 210Po–210Pb to estimate export fluxes of POC, PIC, and PN from the euphotic layer. Compared with measured fluxes by the sediment trap and estimated fluxes by other approaches, it is concluded that the export production in the South China Sea, ranging from 1.8 to 21.3 mmol-C m−2d−1, can be reasonably estimated using 234Th, 210Pb, and 210Po as carbon proxies.

Disruption of a cyclonic eddy circulation by wind stress in Prince William Sound, Alaska

Oceanographic observations made during the Sound Predictions 2009 field experiment in Prince William Sound, Alaska, have documented rapid changes in the upper water column (0–40m) circulation. An assortment of drifting buoys, sampling four different depths, and HF radar surface current mapping, revealed three modes of circulation: anticyclonic, open cyclonic, and closed cyclonic. Each mode was observed at least once within an 18-day window, and the transition between them took as little as a day. Time-resolved hydrographic measurements show that the mass field was variable, but generally arranged such that the surface geostrophic flow should be in a closed-core cyclonic eddy configuration. Observations show that the mass field was likely influenced by relatively low salinity water flowing into Prince William Sound from the shelf, and from local freshwater input. We quantitatively examine why a closed-core circulation was not always observed by focusing on the transition between the closed and open cyclonic flow patterns. The western region of the central sound is a key area for this transition. Here, the high-frequency radar revealed that the closed circulation was established when the net flow shifted direction from northward to southward. A detailed comparison of the meridional geostrophic and wind-driven flows, using measured winds and hydrographic data from CTD profiles and two autonomous vehicles, shows that the geostrophic flow was mostly southward while the wind-driven flow was mostly northward. A net southward flow can be caused by a decrease in the northward wind-driven flow or an increase in the southward geostrophic flow.

Microbial methane production in oxygenated water column of an oligotrophic lake

The prevailing paradigm in aquatic science is that microbial methanogenesis happens primarily in anoxic environments. Here, we used multiple complementary approaches to show that microbial methane production could and did occur in the well-oxygenated water column of an oligotrophic lake (Lake Stechlin, Germany). Oversaturation of methane was repeatedly recorded in the well-oxygenated upper 10 m of the water column, and the methane maxima coincided with oxygen oversaturation at 6 m. Laboratory incubations of unamended epilimnetic lake water and inoculations of photoautotrophs with a lake-enrichment culture both led to methane production even in the presence of oxygen, and the production was not affected by the addition of inorganic phosphate or methylated compounds. Methane production was also detected by in-lake incubations of lake water, and the highest production rate was 1.8-2.4 nM⋅h(-1) at 6 m, which could explain 33-44% of the observed ambient methane accumulation in the same month. Temporal and spatial uncoupling between methanogenesis and methanotrophy was supported by field and laboratory measurements, which also helped explain the oversaturation of methane in the upper water column. Potentially methanogenic Archaea were detected in situ in the oxygenated, methane-rich epilimnion, and their attachment to photoautotrophs might allow for anaerobic growth and direct transfer of substrates for methane production. Specific PCR on mRNA of the methyl coenzyme M reductase A gene revealed active methanogenesis. Microbial methane production in oxygenated water represents a hitherto overlooked source of methane and can be important for carbon cycling in the aquatic environments and water to air methane flux.

Dissolution of Calcite in the Twilight Zone: Bacterial Control of Dissolution of Sinking Planktonic Carbonates Is Unlikely

We investigated the ability of bacterial communities to colonize and dissolve two biogenic carbonates (Foraminifera and oyster shells). Bacterial carbonate dissolution in the upper water column is postulated to be driven by metabolic activity of bacteria directly colonising carbonate surfaces and the subsequent development of acidic microenvironments. We employed a combination of microsensor measurements, scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM) and image analysis and molecular documentation of colonising bacteria to monitor microbial processes and document changes in shell surface topography. Bacterial communities rapidly colonised shell surfaces, forming dense biofilms with extracellular polymeric substance (EPS) deposits. Despite this, we found no evidence of bacterially mediated carbonate dissolution. Dissolution was not indicated by Ca2+ microprofiles, nor was changes in shell surface structure related to the presence of colonizing bacteria. Given the short time (days) settling carbonate material is actually in the twilight zone (500–1000 m), it is highly unlikely that microbial metabolic activity on directly colonised shells plays a significant role in dissolving settling carbonates in the shallow ocean.

Diagnostic modeling of dimethylsulfide production in coastal water west of the Antarctic Peninsula

The rate of gross biological dimethylsulfide (DMS) production at two coastal sites west of the Antarctic Peninsula, off Anvers Island, near Palmer Station, was estimated using a diagnostic approach that combined field measurements from 1 January 2006 through 1 March 2006 and a one-dimensional physical model of ocean mixing. The average DMS production rate in the upper water column (0–60m) was estimated to be 3.1±0.6nMd−1 at station B (closer to shore) and 2.7±0.6nMd−1 at station E (further from shore). The estimated DMS replacement time was on the order of 1d at both stations. DMS production was greater in the mixed layer than it was below the mixed layer. The average DMS production normalized to chlorophyll was 0.5±0.1(nMd−1)/(mgm−3) at station B and 0.7±0.2(nMd−1)/(mgm−3) at station E. When the diagnosed production rates were normalized to the observed concentrations of total dimethylsulfoniopropionate (DMSPt, the biogenic precursor of DMS), we found a remarkable similarity between our estimates at stations B and E (0.06±0.02 and 0.04±0.01(nMDMSd−1)/(nMDMSP), respectively) and the results obtained in a previous study from a contrasting biogeochemical environment in the North Atlantic subtropical gyre (0.047±0.006 and 0.087±0.014(nMDMSd−1)/(nMDMSP) in a cyclonic and anticyclonic eddy, respectively). We propose that gross biological DMS production normalized to DMSPt might be relatively independent of the biogeochemical environment, and place our average estimate at 0.06±0.01(nMDMSd−1)/(nMDMSPt). The significance of this finding is that it can provide a means to use DMSPt measurements to extrapolate gross biological DMS production, which is extremely difficult to measure experimentally under realistic in situ conditions.

Upper ocean climate of the Eastern Mediterranean Sea during the Holocene Insolation Maximum – a model study

Abstract. Nine thousand years ago (9 ka BP), the Northern Hemisphere experienced enhanced seasonality caused by an orbital configuration close to the minimum of the precession index. To assess the impact of this "Holocene Insolation Maximum" (HIM) on the Mediterranean Sea, we use a regional ocean general circulation model forced by atmospheric input derived from global simulations. A stronger seasonal cycle is simulated by the model, which shows a relatively homogeneous winter cooling and a summer warming with well-defined spatial patterns, in particular, a subsurface warming in the Cretan and western Levantine areas. The comparison between the SST simulated for the HIM and a reconstruction from planktonic foraminifera transfer functions shows a poor agreement, especially for summer, when the vertical temperature gradient is strong. As a novel approach, we propose a reinterpretation of the reconstruction, to consider the conditions throughout the upper water column rather than at a single depth. We claim that such a depth-integrated approach is more adequate for surface temperature comparison purposes in a situation where the upper ocean structure in the past was different from the present-day. In this case, the depth-integrated interpretation of the proxy data strongly improves the agreement between modelled and reconstructed temperature signal with the subsurface summer warming being recorded by both model and proxies, with a small shift to the south in the model results. The mechanisms responsible for the peculiar subsurface pattern are found to be a combination of enhanced downwelling and wind mixing due to strengthened Etesian winds, and enhanced thermal forcing due to the stronger summer insolation in the Northern Hemisphere. Together, these processes induce a stronger heat transfer from the surface to the subsurface during late summer in the western Levantine; this leads to an enhanced heat piracy in this region, a process never identified before, but potentially characteristic of time slices with enhanced insolation.