Deep Water Column Research Articles

The speciation of water in sea water and in gelatinous marine animals

Abstract Raman spectra provide important information on the speciation of water in sea water, and its relationship to temperature, pressure, and salinity. These spectra reveal two primary water bands: a bending mode doublet peak at 1640 cm −1 , and a dominant water stretching mode band with Raman shifts in the 2900–3800 cm −1 region. Early laboratory work revealed the presence of an ‘isosbestic point’ within the water spectra, which is consistent with a two component structure. Ocean scientists working with aircraft based pulsed time gated laser systems used the temperature dependence of the two component water structure observed within the water stretching band to determine upper ocean temperature profiles. We find that our results are consistent with the Walrafen water pentamer as the primary structure in rapid exchange with H 2 O monomers, and we have deconvoluted the stretching band into 5 peaks. From laboratory pressure cell data very clear trends in water structure with temperature, pressure, and to a lesser degree salinity can be observed, and a plot of the natural log of the water peak amplitudes versus the inverse of the absolute temperature permits estimation of the strength of the hydrogen bond. While there is no definitive way to distinguish between a fully “intact” and a fully “broken” hydrogen bond and the hydrogen bonded water structures are evanescent, our observations are consistent with ΔH = 2.48 kcal/mol. From our compiled expeditionary data (primarily from Monterey Bay, California) from ~ 280 m to 2740 m depth we find strong compensation of the opposing temperature and pressure effects such that the observed water peaks show a remarkable consistency throughout the deep water column. In situ Raman observations of the gelatinous mass (mesoglea) of many deep-sea animals show that liquid water there is absent, and essentially all salts are excluded. Within the mesoglea the water bending modes are eliminated and the O H stretching modes are composed of only 3 bands.

Sequestered oil pollution mapping, and tracking active oil breakouts in sensitive rivers, bays, and estuaries

ABSTRACT Oil on the water surface represents just the American Petroleum Institute API > 10 gravity component of any crude oil spill or well blowout, and is identified and tracked by conventional remote sensing means. However, the API ≤ 10 components of the hydrocarbons are not readily accessible by these means. UV sensors on underwater vehicles can sample just a few cubic centimeters at a time and are subject to fouling. Side-scan sonar, under certain conditions, can “see” gas bubbles on the near outer shell of a subsurface plume if they exist early on during a blowout, but cannot assess the entire volume. Oil sequestered in bed sediments in oceans or rivers is not visible to UV sensors, nor is it visible to divers. It is apparent that this sensing gap problem needed to be addressed. A new technology developed by the US Geological Survey, working closely with Williamson & Associates of Seattle, holds promise for rapid mapping and characterization of below-surface hydrocarbons. Crude oil drifting in the deep ocean water column, oil blanketing the seafloor, and oil sequestered in seafloor and river bottom sediments can now be quickly mapped in 3D. If drifting in the seawater column, dispersed oil can be tracked as its distribution evolves over time. This technology also is potentially useful for mapping combined storm-water overflow (CSO, or sewage) deposits, as well as Superfund sites in Puget Sound and other sensitive rivers, bays, and estuaries close to cities that pose serious hazards to both humans and wildlife. The technology is based on a surface-sensitive electro-physical property known as induced polarization (IP). This surface-sensitivity means that highly dispersed IP-reactive materials have more surface area exposed to surrounding water, and are thus more responsive than undispersed materials of the same mass. IP technology has been used successfully for many decades to map disseminated porphyry sulfide deposits on land, but has only been applied commercially at sea since 2007. Recent laboratory and Puget Sound experiments have verified that the IP response of oil dispersed in water and sequestered in sediments is unusually strong: at least 20 times greater than a strong “hit” in an IP survey for sulfide minerals on land. The marine IP system has been towed behind a small boat in as little as 1–2 meters water depth, while one version has been tested (using a towed sled) to 3,500 meters depth. Depending on the cable-streamer design, the depth of detection of chargeable materials in sediments can be greater than 20 meters. It can be used to monitor active drill platforms for leaks. Finally, IP is also strongly reactive to buried pipelines; in the Gulf of Mexico there are over 43,000 miles of poorly-located, often hidden, corroding pipe. We can now map it precisely.

Molybdenum isotope fractionation and speciation in a euxinic lake—Testing ways to discern isotope fractionation processes in a sulfidic setting

The molybdenum (Mo) isotope composition in euxinic shales has been used as a proxy for the global distribution of anoxic conditions in ancient oceans, and since more recently also as a proxy for sulfide concentrations in depositional environments. However, there is currently no way to distinguish isotope fractionation at low bottom water sulfide concentrations in ‘local’ basins from ‘global’ secular isotope variations associated with changing seawater composition. This uncertainty is challenging the use of Mo isotopes for paleoceanographic reconstructions.To explore this further, we present new data from sediments deposited over the past ~9800years in one of the best studied euxinic localities in the world: Lake Cadagno in Switzerland. The sample set allows us to test ways to discern isotope fractionation processes at play in a highly restricted euxinic basin.Most of our drill core samples (n=18) show high δ98Mo values similar to previously studied shallow sediments, indicative of quantitative Mo removal from the water column (Dahl et al. 2010a). However, a few samples (n=3) deposited between about 1200 and 3400years ago carry low δ98Mo values and have been isotopically fractionated in the lake. Sedimentological and geochemical characterizations show that these δ98Mo-fractionated sediments formed during times of frequent injection of O2- and sediment-rich river water into the deep sulfidic water column. A positive correlation between δ98Mo and sedimentary Mo contents suggests that isotope fractionation occurred during times of non-quantitative Mo removal, although Mn-oxide cycling at the chemocline might also contribute a subordinate proportion of (98Mo-depleted) molybdenum into the sulfidic zone. Sedimentary Mo/U enrichments relative to oxic lake water further supports the hypothesis that a particulate Mo shuttle was most efficient during times of quantitative Mo removal. Therefore, periods with inefficient Mo capture are ascribed to incomplete conversion of molybdate to particle reactive Mo species when bottom water H2S levels were low or less stable than today.Using XAFS spectroscopy, we found that the two distinct Mo compounds predominating in the sediments (MoIV-S and MoVI-OS) are not diagnostic for isotope fractionation that has occurred in Lake Cadagno. Instead, we infer that δ98Mo-fractionated products (forming via a low-sulfide Mo pathway) can be subsequently altered with little or no isotopic imprint during remobilization and re-precipitation (e.g., at higher sulfide levels in the sediments) as well as during post-depositional oxidation.Future work could investigate local δ98Mo-fractionation processes expressed in other euxinic settings and explore other sedimentary metrics to constrain the steps involved in the euxinic burial pathway(s). One tantalizing prospect of this is to distinguish between local bottomwater sulfide levels and variations in the fraction of global seafloor anoxia from the Mo isotope composition in ancient euxinic mudrocks.

Changes in Marine Prokaryote Composition with Season and Depth Over an Arctic Polar Year

As the global climate changes, the higher latitudes are seen to be warming significantly faster. It is likely that the Arctic biome will experience considerable shifts in ice melt season length, leading to changes in photoirradiance and in the freshwater inputs to the marine environment. The exchange of nutrients between Arctic surface and deep waters and their cycling throughout the water column is driven by seasonal change. The impacts, however, of the current global climate transition period on the biodiversity of the Arctic Ocean and its activity are not yet known. To determine seasonal variation in the microbial communities in the deep water column, samples were collected from a profile (1-1000m depth) in the waters around the Svalbard archipelago throughout an annual cycle encompassing both the polar night and day. High-throughput sequencing of 16S rRNA gene amplicons was used to monitor prokaryote diversity. In epipelagic surface waters (200m), seasonality subsequently had much less effect on community composition. In summer, phytoplankton-associated Gammaproteobacteria and Flavobacteriia dominated surface waters, whilst in low light conditions (surface waters in winter months and deeper waters all year round), the Thaumarchaeota and Chloroflexi-type SAR202 predominated. Alpha-diversity generally increased in epipelagic waters as seasonal light availability decreased; OTU richness also consistently increased down through the water column, with the deepest darkest waters containing the greatest diversity. Beta-diversity analyses confirmed that seasonality and depth also primarily drove community composition. The relative abundance of the eleven predominant taxa showed significant changes in surface waters in summer months and varied with season depending on the phytoplankton bloom stage; corresponding populations in deeper waters however, remained relatively unchanged. Given the significance of the annual phytoplankton bloom pattern on prokaryote diversity in Arctic waters, any changes to bloom dynamics resulting from accelerated global warming will likely have major impacts on surface marine microbial communities, those impacts inevitably trickling down into deeper waters.

Depositional environment of oil shale within the second member of Permian Lucaogou Formation in the Santanghu Basin, Northwest China

The non-marine Santanghu Basin in northwest China hosts one of the richest and thickest Permian lacustrine source rock intervals in the world. Conventional oil in Jurassic sandstone reservoirs and tight oil in the tuffaceous Permian Tiaohu Formation were sourced from mudrocks in the underlying second member of Lucaogou Formation, which also have tight oil potential proved by recent commercial discoveries. Based on inorganic and organic geochemistry, organic petrography, and stable isotope geochemistry, the depositional environment and the tight oil potential of the second member of Permian Lucaogou Formation were investigated. The data imply a gradual evolution of the depositional environment from a stratified, saline to a freshwater lake. The succession can be subdivided into a lower, middle, and an upper unit, each characterized by decreasing water salinity. High bioproductivity has been caused by the bloom of algae and photosynthetic cyanobacteria. Water column stratification accelerated the activity of methanothrophs, as indicated by low δ13C values of hopanes. Biomarker composition provides evidence for decreasing contributions of cyanobacteria to the biomass and increasing abundance of Prasinophytae compared to other algae with decreasing salinity. Enhanced terrigenous organic matter input appeared during periods of high freshwater inflow. Low total organic carbon (TOC) contents in the lower unit are most likely caused by rapid sedimentation rate. High bioproductivity and excellent preservation conditions resulted in high TOC contents in the middle unit. Terrigenous organic matter input increased together with fresh water inflow in the upper unit, resulting in high TOC values during periods of possibly low sedimentation rates in a deep water column. The samples of the second member exhibit a good to very good potential to generate conventional oil. High TOC and extractable organic matter yields, together with the thickness of the fine-grained sediments, show that the middle and upper units of the second member of Lucaogou Formation hold significant tight oil potential. Mineralogical composition of this dolomite-rich shale with respect to abundance of brittle minerals (including quartz and carbonates) and rare clay testifies a good tight oil potential.

Elevated Concentrations of 4-Bromobiphenyl and 1,3,5-Tribromobenzene Found in Deep Water of Lake Geneva Based on GC×GC-ENCI-TOFMS and GC×GC-μECD.

We quantified the concentrations of two little-studied brominated pollutants, 1,3,5-tribromobenzene (TBB) and 4-bromobiphenyl (4BBP), in the deep water column and sediments of Lake Geneva. We found aqueous concentrations of 625 ± 68 pg L–1 for TBB and 668 ± 86 pg L–1 for 4BBP over a depth range of 70–191.5 m (near-bottom depth), based on duplicate measurements taken at five depths during three separate 1 month sampling periods at our sampling site near Vidy Bay. These levels of TBB and 4BBP were 1 or 2 orders of magnitude higher than the quantified aqueous concentrations of the components of the pentabrominated biphenyl ether technical mixture, which is a flame retardant product that had a high production volume in Europe before 2001. We observed statistically significant vertical concentration trends for both TBB and 2,2′,4,4′,6-pentabromobiphenyl ether in the deep water column, which indicates that transport and/or degradation processes affect these compounds. These measurements were enabled by application of a comprehensive two-dimensional gas chromatograph coupled to an electron capture negative chemical ionization time-of-flight mass spectrometer (GC×GC-ENCI-TOFMS) and to a micro-electron capture detector (GC×GC-μECD). GC×GC-ENCI-TOFMS and GC×GC-μECD were found to be >10× more sensitive toward brominated pollutants than conventional GC×GC-EI-TOFMS (with an electron impact (EI) ionization source), the latter of which had insufficient sensitivity to detect these emerging brominated pollutants in the analyzed samples. GC×GC also enabled the estimation of several environmentally relevant partitioning properties of TBB and 4BBP, further confirming previous evidence that these pollutants are bioaccumulative and have long-range transport potential.

Lead concentrations and isotopic compositions in the Western Philippine Sea

Dissolved lead (Pb) concentrations and isotope ratios in seawater of the Western Philippine Sea (WPS) were determined to investigate sources of Pb in the region. Surface seawater concentrations at seven sampling stations ranged from 36.2 to 68.1pmolkg−1. Isotopic composition of surface water, with 206Pb/207Pb ranging from 1.162 to 1.170, 208Pb/207Pb ranging from 2.445 to 2.451, and 206Pb/204Pb ranging from 18.14 to 18.27, reflects Asian anthropogenic aerosols input to the WPS. Shallow water Pb concentrations within the Kuroshio Current domain are about 15pmol kg−1 lower than at other sites and, together with a distinct isotopic signature (206Pb/207Pb=1.167–1.170, 208Pb/207Pb=2.447–2.451 and 206Pb/204Pb=18.22–18.27), reflect water originating from the Equatorial Pacific that is relatively less impacted by contamination from anthropogenic inputs. An isotopically distinct sub-surface Pb maximum at about 100 to 250m, representing water originating from the Western North Pacific where anthropogenic Pb deposition is high, was seen at all seven sites. Lead concentrations in deep water in the stations further from shore are typically lower than in the surface layer and ranged from 11.2 to 51.6pmolkg−1. Lead isotopic signatures in deep water at these sites (206Pb/207Pb=1.162–1.184; 208Pb/207Pb=2.448–2.471; 206Pb/204Pb=18.13–18.51) are offset from pre-anthropogenic values and suggest that anthropogenic sources have penetrated the deep water column. Elevated concentrations and isotopic signatures observed in the deep water at stations closer to shore, where sediment transport is prevalent, indicate that sedimentary input is a major source of dissolved Pb at these sites. Differences in Pb concentrations and isotopic signatures between samples collected from the Pacific Deep Water (PDW) water-mass during this study and those collected a decade ago suggest that Pb inputs even in deep water change on decadal scales.

Seasonal Changes and Spatial Variation in Water Quality of a Large Young Tropical Reservoir and Its Downstream River

This study examined the water quality of the large young tropical Bakun hydroelectric reservoir in Sarawak, Malaysia, and the influence of the outflow on the downstream river during wet and dry seasons. Water quality was determined at five stations in the reservoir at three different depths and one downstream station. The results show that seasons impacted the water quality of the Bakun Reservoir, particularly in the deeper water column. Significantly lower turbidity, SRP, and TP were found during the wet season. At 3–6 m, the oxygen content fell below 5 mg/L and hypoxia was also recorded. Low NO2--N, NO3--N, and SRP and high BOD5, OKN, and TP were observed in the reservoir indicating organic pollution. Active logging activities and the dam construction upstream resulted in water quality deterioration. The outflow decreased the temperature, DO, and pH and increased the turbidity and TSS downstream. Elevated organic matter and nutrients downstream are attributable to domestic discharge along the river. This study shows that the downstream river was affected by the discharge through the turbines, the spillway operations, and domestic waste. Therefore, all these factors should be taken into consideration in the downstream river management for the health of the aquatic organisms.

Screening of potential zooplankton control technologies for wastewater treatment High Rate Algal Ponds

Zooplankton taxa including cladocerans and rotifers are one of the greatest challenges for effective management of High Rate Algal Ponds (HRAPs) for wastewater treatment as they can establish and rapidly consume beneficial microalgae. Harmful zooplankton need to be controlled using cost effective treatments, and here we tested under controlled laboratory conditions the efficacy of chemical CO2 asphyxiation, biological control of rotifers using competitor species expected to graze only moderately on colonial microalgae, and mechanical disruption using hydrodynamic shear stress. CO2 asphyxiation caused acute mortality of all zooplankton species (t<10min). Increasing the cladoceran Moina tenuicornis to densities >2500individuals/L was associated with a decrease in rotifer populations (~23% of the population in the control). The ostracod Heterocypris incongruens at densities >1000 individuals/L were also associated with a decrease in rotifer densities that were ~27% of the population in the control. Hydrodynamic shear stress killed 100% of M. tenuicornis and ~80% of the rotifer Brachionus calyciflorus after a single pass. Furthermore, M. tenuicornis was concentrated in the upper 50mm of a 300mm deep water column using vertical migration induced by CO2 concentrations of between 25 and 180mg/L. All of these treatments have potential for use to control zooplankton blooms in WW HRAPs.

Depth Preference of Settling Red Drum (Sciaenops ocellatus) Larvae in Relation to Benthic Habitat Color and Water-Column Depth

A variety of sensory cues could be operating to aid fishes in finding benthic settlement habitat. Visual cues are thought to be most important over short distances when light is adequate. Preference for substrate color (black, green, and yellow) at two water-column heights (85 and 33.3 cm) was tested on red drum, Sciaenops ocellatus, at pre-settlement, settlement, and post-settlement sizes. For settlement- and post-settlement-size classes, no effect of substrate color or water-column height was detected on mean distance of fish from the bottom or strength of depth preference. Pre-settlement larvae in the deep water column exhibited greater strength of depth preference than those in the shallow water column (9.6 ± 7.1 vs. 5.6 ± 4.1 cm 5 min−1, respectively). Pre-settlement larvae were closer to the bottom in the shallow water column than the deep water column (19 ± 9 vs. 44 ± 28 cm). Over green and yellow substrates, they were closer to the bottom in the shallow water column compared to the deep water column (19 ± 9 vs. 53 ± 28 cm over green and 15 ± 10 vs. 41 ± 26 cm over yellow). However, no difference was detected in mean depth over the black substrate (21 ± 8 and 39 ± 28 cm for shallow and deep, respectively). These results suggest pre-settlement-size larvae respond to benthic cues prior to settlement by avoiding light colors of estuarine substrates and not descending toward the substrate in the presence of dark colors associated with the offshore environment.

Ventilation time and anthropogenic CO2 in the South China Sea based on CFC-11 measurements

The South China Sea (SCS) is the largest semi-enclosed marginal sea in Southeast Asia, and is bounded by the Asian continent, Philippine Archipelago, and Great Sunda Islands. Due to the wide shelves on its northwestern and southern ends as well as the presence of numerous islets, atolls and reefs, the average depth of the SCS is only 1350m. In this study we used measurements of the transient tracer CFC-11 from the SCS to calculate ventilation time-scales and the concentration of anthropogenic CO2 (Cant) based on the transit time distribution. The CFC-11 concentrations decreased consistently with depth and the deep and bottom water in the SCS had a CFC-11 value close to the detection limit (0.01pmolkg−1 or 0.5ppt). The ventilation times (mean ages) for the deep and bottom water column were ~500–600 years, and based on the mean age profiles the southern part of the intermediate SCS water was older than the northern part. The ventilation time distribution was in agreement with the existence of mean annual cyclonic circulation in the SCS. The mean column inventory of Cant in the northern SCS was 28.9molCm−2 (error range (ER): 22.8–35.6molCm−2), while in the southern SCS it was 28.4molm−2 (ER: 21.9–35.2molCm−2). The total Cant inventory was estimated to be 1 Pg C (ER: 0.8–1.3 Pg C, referenced to the year 2011), suggesting that the SCS stores less Cant then the adjacent seas. The vertical diffusivity was estimated to be 2–4.6×10−4m2s−1 in the SCS based on the “transient steady state”. The upwelling was estimated as 13–34myr−1. The high diffusivity was probably due to the strong internal tide, while the strong upwelling was due to the persistent counterclockwise (cyclonic) circulation.

An impaired metabolic response to hydrostatic pressure explains Alcanivorax borkumensis recorded distribution in the deep marine water column.

Alcanivorax borkumensis is an ubiquitous model organism for hydrocarbonoclastic bacteria, which dominates polluted surface waters. Its negligible presence in oil-contaminated deep waters (as observed during the Deepwater Horizon accident) raises the hypothesis that it may lack adaptive mechanisms to hydrostatic pressure (HP). The type strain SK2 was tested under 0.1, 5 and 10 MPa (corresponding to surface water, 500 and 1000 m depth, respectively). While 5 MPa essentially inactivated SK2, further increase to 10 MPa triggered some resistance mechanism, as indicated by higher total and intact cell numbers. Under 10 MPa, SK2 upregulated the synthetic pathway of the osmolyte ectoine, whose concentration increased from 0.45 to 4.71 fmoles cell−1. Central biosynthetic pathways such as cell replication, glyoxylate and Krebs cycles, amino acids metabolism and fatty acids biosynthesis, but not β-oxidation, were upregulated or unaffected at 10 MPa, although total cell number was remarkably lower with respect to 0.1 MPa. Concomitantly, expression of more than 50% of SK2 genes was downregulated, including genes related to ATP generation, respiration and protein translation. Thus, A. borkumensis lacks proper adaptation to HP but activates resistance mechanisms. These consist in poorly efficient biosynthetic rather than energy-yielding degradation-related pathways, and suggest that HP does represent a major driver for its distribution at deep-sea.

First steps toward standardizing dynamic light regimes for the quantitative study of light-controlled growth in shade-limiting water columns

The diurnal solar cycle is the single most common influential variable for algae growing in a natural environment. In spite of its importance, relatively few laboratory studies have been directed towards photophysiological responses to dynamic irradiance regimes. Further, most of these studies have utilized optically thin cultures. However, phycological technologies utilize optically dense cultures grown in depths measured by centimeters. In this case, algae are harvested under self-shading conditions (all usable incident irradiance being absorbed), and the response of the entire water column to the dynamic irradiance regime is of interest. Thus, the area integrated under some modeled curve for a dynamic irradiance regime becomes the independent variable for the photobiological response being measured of a water column in laboratory studies, and this is important. But what curve should be used to model a dynamic irradiance regime? We suggest that a Gaussian (normal) distribution be used to model this dynamic irradiance because of its well-known statistical attributes. In this study, Thalassiosira pseudonana (marine diatom) was grown under shade-limited growth conditions in a 20-cm deep water column with a dynamic irradiance regime using our SolarStat™. A Gaussian distribution modeled this regime having an 11-h day length (standard deviation ±1.85 h) with an irradiance maximum of 1900 μmol photons m−2 s−1. The daily productivity of batch culture in the linear phase of the light-controlled growth dynamic is compared with those of semi-continuous cultures maintained at 1.0, 0.5, and 0.25 doublings day−1.

Response of calcareous nannoplankton to the Late Cretaceous Oceanic Anoxic Event 2 at Oued Bahloul (central Tunisia)

Oued Bahloul is a reference section for the Cenomanian–Turonian that was located on the North African palaeomargin, where Oceanic Anoxic Event 2 (OAE2) is represented by black limestones and grey marls. Biostratigraphic study allowed the identification of calcareous nannofossil UC3 to UC6 zones. Quantitative analyses of calcareous nannofossil assemblages indicate that high-fertility surface-water conditions predominated throughout most of the OAE2 interval. The most eutrophic conditions probably developed during deposition of the lowermost part of the Whiteinella archaeocretacea Zone (UC3 to UC4 zones), where nannofossil-derived fertility indices reach high values and the small Zeugrhabdotus gr. predominates over Biscutum spp. The temperature index based on calcareous nannofossil assemblages indicates a warming at the beginning of the OAE2, followed by progressive cooling (the same trend indicated by δ18Obulk values). The significant proportion of holococcoliths suggests high-stress conditions in surface waters during most of the OAE2 interval. The enrichment in redox-sensitive elements, together with the abundance of low-oxygen tolerant benthic foraminifera, indicates dysoxic conditions both in the deep-water column and sea-bottom at the beginning of the OAE2 (W. archaeocretacea Zone). Geochemical detrital proxies indicate an enhanced rate of terrigenous supply during the OAE2 interval resulting from increased riverine influx related to a more humid climate. Enhanced fluvial run-off increased nutrient input to marine surface waters contributing to eutrophication (recorded by calcareous nannoplankton and planktic foraminifera). Plankton consumption of surface water nutrients increased organic matter production, whose decay contributed to the decrease in dissolved oxygen, generating increasingly dysoxic/anoxic conditions and favoring preservation of organic matter. Although low-latitude nannofossil assemblages such as those of Oued Bahloul reflect an increase in nutrient availability during the OAE2 interval, assemblages from mid-latitude areas record a decreased nutrient content.

Reservoirs are hotspots of nitrogen cycling in peatland catchments

AbstractThis study presents input–output budgets of total dissolved nitrogen (TDN), dissolved organic N (DON) and dissolved inorganic N (DIN) for a reservoir in a peatland catchment in the south Pennines (UK). This site receives high levels of atmospheric inorganic N deposition, in the range of 26 kg N ha−1 yr−1. The results show that the reservoir retains ~21 to 31% of the annual TDN input (8806 ± 741 kg N). Approximately 39 to 55% of DON (3782 ± 653 kg N) and 6 to 13% of DIN (5024 ± 349 kg N) were retained/processed. A long water retention time (104 days), average annual pH of 6.5, high concentrations of DIN in the reservoir water and a deep water column suggest that denitrification is potentially a key mechanism of N retention/removal. The results also demonstrate that DON is potentially photodegraded and utilized within the reservoir, particularly during the summer season when 58 to 80% of DON input (682 ± 241 kg N) was retained, and a net export of DIN (~34 kg N) was observed. The findings therefore suggest that DON may play a more crucial role in the biogeochemistry of peat‐dominated acid sensitive upland freshwater systems than previously thought. Reservoirs, impoundments and large lakes in peatland catchments may be important sites in mediating downstream N transport and speciation. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Modeling the thickness of perennial ice covers on stratified lakes of the Taylor Valley, Antarctica

ABSTRACTA 1-D ice cover model was developed to predict and constrain drivers of long-term ice thickness trends in chemically stratified lakes of Taylor Valley, Antarctica. The model is driven by surface radiative heat fluxes and heat fluxes from the underlying water column. The model successfully reproduced 16 a (between 1996 and 2012) of ice thickness changes for the west lobe of Lake Bonney (average ice thickness = 3.53 m) and Lake Fryxell (average ice thickness = 4.22 m). Long-term ice thickness trends require coupling with the thermal structure of the water column. The heat stored within the temperature maximum of lakes exceeding a liquid water column depth of 20 m can either impede or facilitate ice thickness change depending on the predominant climatic trend (cooling or warming). As such, shallow (&lt;20 m deep water columns) perennially ice-covered lakes without deep temperature maxima are more sensitive indicators of climate change. The long-term ice thickness trends are a result of surface energy flux and heat flux from the deep temperature maximum in the water column, the latter of which results from absorbed solar radiation.

Hawaiian imprint on dissolved Nd and Ra isotopes and rare earth elements in the central North Pacific: Local survey and seasonal variability

Dissolved neodymium isotopes (143Nd/144Nd, expressed as εNd) and rare earth elements (REEs) have the potential to trace the provenance of lithogenic material as well as water masses. The central North Pacific is poorly investigated with respect to its Nd isotope signature and REE cycling, and little is known about the contributions of volcanic islands, such as Hawaii, relative to dust input from Asian deserts to the surface water REE budgets. Here we present dissolved Nd isotope and REE data along with long-lived radium isotope activities from Hawaii Ocean Time-Series Station ALOHA and coastal waters from Oahu, sampled for a GEOTRACES process study in February 2011. The data are supplemented with seasonal samples from ALOHA. Our results show a clear influence of the Hawaiian Islands on the coastal ocean and surface waters at ALOHA during February, expressed by higher surface water Ra activities, radiogenic surface εNd (εNd=+1.4 to −1.0), and elevated Eu anomalies (Eu/Eu∗⩾1.3). Seasonal cycles of Asian dust deposition most likely contribute to the seasonal εNd variability of surface waters at ALOHA, as suggested by more negative εNd and the lack of Eu anomalies in summer. Neodymium isotopes in the intermediate and deep water column at ALOHA trace typical North Pacific water masses, such as North Pacific Intermediate Water and North Pacific Deep Water. We suggest that a radiogenic εNd excursion in 1000–2000m water depth, observed in various North Pacific profiles, is controlled by advection of a modified Upper Circumpolar Deep Water or North Equatorial Pacific Intermediate Water. We further present an updated average εNd signature of −3.5±0.5 for North Pacific Deep Water and show that REE patterns of deep waters at ALOHA are dominantly controlled by vertical processes.

Excess of 236U in the northwest Mediterranean Sea

In this work, we present first 236U results in the northwestern Mediterranean. 236U is studied in a seawater column sampled at DYFAMED (Dynamics of Atmospheric Fluxes in the Mediterranean Sea) station (Ligurian Sea, 43°25′N, 07°52′E). The obtained 236U/238U atom ratios in the dissolved phase, ranging from about 2×10−9 at 100m depth to about 1.5×10−9 at 2350m depth, indicate that anthropogenic 236U dominates the whole seawater column. The corresponding deep-water column inventory (12.6ng/m2 or 32.1×1012 atoms/m2) exceeds by a factor of 2.5 the expected one for global fallout at similar latitudes (5ng/m2 or 13×1012 atoms/m2), evidencing the influence of local or regional 236U sources in the western Mediterranean basin. On the other hand, the input of 236U associated to Saharan dust outbreaks is evaluated. An additional 236U annual deposition of about 0.2pg/m2 based on the study of atmospheric particles collected in Monaco during different Saharan dust intrusions is estimated. The obtained results in the corresponding suspended solids collected at DYFAMED station indicate that about 64% of that 236U stays in solution in seawater. Overall, this source accounts for about 0.1% of the 236U inventory excess observed at DYFAMED station. The influence of the so-called Chernobyl fallout and the radioactive effluents produced by the different nuclear installations allocated to the Mediterranean basin, might explain the inventory gap, however, further studies are necessary to come to a conclusion about its origin.

On the Mediterranean Water Composition

AbstractThe Mediterranean Outflow Water (MOW) spills from the Mediterranean Sea (east North Atlantic basin) west off the Strait of Gibraltar. As MOW outflows, it entrains eastern North Atlantic Central Waters (ENACW) and Intermediate Waters to form the neutrally buoyant Mediterranean Water (MW) that can be traced over the entire North Atlantic basin. Its high salinity content influences the thermohaline properties of the intermediate–deep water column in the North Atlantic and its dynamics. Here, the composition of MW in its source region (the Gulf of Cádiz, west off Strait of Gibraltar) is investigated on the basis of an optimum multiparameter analysis. The results obtained indicate that mixing of MOW (34.1% ± 0.3%) occurs mainly with overlying ENACW (57.1% ± 0.8%) in a process broadly known as central water entrainment. A diluted form (80% of dilution) of the Antarctic Intermediate Water (AAIW) reaches the region and also takes part in MW formation (8.3% ± 0.5%). Finally, the underlying Labrador Sea Water (LSW) also contributes (0.4% ± 0.1%) to the characteristics of MW. From these results and considering 0.74 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) as the mean outflow of MOW, the MW exportation rate was inferred (2.2 Sv), which, decomposing MW, means that the MOW outflow is accompanied by 1.24 Sv of entrained ENACW, 0.18 Sv of AAIW, and &lt;0.01 Sv of LSW.

Predator-guided sampling reveals biotic structure in the bathypelagic.

We targeted a habitat used differentially by deep-diving, air-breathing predators to empirically sample their prey's distributions off southern California. Fine-scale measurements of the spatial variability of potential prey animals from the surface to 1,200 m were obtained using conventional fisheries echosounders aboard a surface ship and uniquely integrated into a deep-diving autonomous vehicle. Significant spatial variability in the size, composition, total biomass, and spatial organization of biota was evident over all spatial scales examined and was consistent with the general distribution patterns of foraging Cuvier's beaked whales (Ziphius cavirostris) observed in separate studies. Striking differences found in prey characteristics between regions at depth, however, did not reflect differences observed in surface layers. These differences in deep pelagic structure horizontally and relative to surface structure, absent clear physical differences, change our long-held views of this habitat as uniform. The revelation that animals deep in the water column are so spatially heterogeneous at scales from 10 m to 50 km critically affects our understanding of the processes driving predator-prey interactions, energy transfer, biogeochemical cycling, and other ecological processes in the deep sea, and the connections between the productive surface mixed layer and the deep-water column.