Deep Slope Waters Research Articles

Trophic ecology of barrelfish (Hyperoglyphe perciformis) in oceanic waters of southeast Florida

Deep-water demersal fishes represent an understudied but ecologically important group of organisms. Select species of demersal fishes rely on pelagic prey items, representing a direct transport of surface carbon to greater depths. Barrelfish,Hyperoglyphe perciformis (Mitchell, 1818), which inhabit deep slope waters, are a species that has been suggested to fill this role, as congeners consume primarily pelagic gelatinous zooplankton. However, there is a dearth of information on the trophic ecology of barrelfish. Stomach content and stable isotope analyses were conducted on 29 barrelfish caught by recreational fishers from November 2014 to November 2015 off Miami, Florida, to improve our understanding of the feeding of this species.Pyrosoma atlanticum Peron, 1804, a pelagic, vertically migrating tunicate, represented 89% of the barrelfish diet by weight. Mesopelagic fish and shrimp contributed much smaller proportions. Standard ellipse areas corrected for sample size (SEAc ) showed a substantially smaller isotopic niche width for barrelfish (n = 26, 0.606 ‰2) than dolphinfish, Coryphaena hippurus Linnaeus, 1758 (n = 36, 2.16 ‰2), king mackerel,Scomberomorus cavalla (Cuvier, 1829) (n = 17, 3.04 ‰2), or wahoo,Acanthocybium solandri (Cuvier, 1832) (n = 12, 1.97 ‰2). Coupled with dependence on a singular prey item, the low SEAc of barrelfish suggests they occupy a limited trophic niche space. Overlap of barrelfish SEAc with dolphinfish (99.5% overlap) and king mackerel (100% overlap) indicate that the carbon sources and the number of trophic steps for barrelfish are similar to king mackerel and dolphinfish, and are linked to surface waters. This trophic linkage suggests that barrelfish may play a role in carbon export and further study of their behavior and daily consumption rates is warranted for quantifying this role.

Regional patterns in ammonia-oxidizing communities throughout Chukchi Sea waters from the Bering Strait to the Beaufort Sea

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 79:273-286 (2017) - DOI: https://doi.org/10.3354/ame01834 Regional patterns in ammonia-oxidizing communities throughout Chukchi Sea waters from the Bering Strait to the Beaufort Sea Julian Damashek1,2,**, Kade P. Pettie1,3,**, Zachary W. Brown1,4, Matthew M. Mills1, Kevin R. Arrigo1, Christopher A. Francis1,* 1Department of Earth System Science, Stanford University, Stanford, CA 94305, USA 2Present address: Department of Marine Sciences, University of Georgia, Athens, GA 30602, USA 3Present address: Department of Biology, Stanford University, Stanford, CA 94305, USA 4Present address: Inian Islands Institute, Gustavus, AK 99826, USA *Corresponding author: caf@stanford.edu**These authors contributed equally to this study ABSTRACT: The shallow Chukchi Sea is a highly productive region of the Arctic Ocean, fed by Pacific water transported north through the Bering Strait. Nitrification in Chukchi Sea bottom waters oxidizes significant amounts of regenerated ammonium, which increases the pool of nitrate available for denitrification and changes the nutrient balance of this water prior to export to the Canada Basin. However, little is known about the ammonia-oxidizing microbial communities in Chukchi Sea waters. We used quantitative polymerase chain reaction assays to determine the abundance and transcriptional activity of both ammonia-oxidizing Archaea (AOA) and ammonia-oxidizing Bacteria (AOB) along coastal Chukchi Sea bottom waters, as well as waters of Atlantic-origin over the Beaufort slope. While AOA significantly outnumbered AOB in deeper Beaufort slope waters, AOB were more abundant at most coastal stations, potentially due to a greater concentration of regenerated ammonium trapped in coastal bottom waters. Quantification of 2 marine AOA ecotypes, Water Column A (WCA) and Water Column B (WCB), showed a surprisingly high abundance of WCB, generally considered a deep-water ecotype, in shallow coastal waters as well as deep slope waters, likely due to transport via shelfbreak upwelling. Transcript abundances also suggested WCA and WCB in coastal waters were transcriptionally active at comparable levels. The relatively high abundance of AOB and WCB suggests the Chukchi Sea has unique ammonia-oxidizing communities compared to most shallow coastal oceans, highlighting the importance of regional biogeochemical processes (ammonium regeneration) and physical processes (upwelling) in structuring coastal microbial communities. KEY WORDS: Thaumarchaeota · Ammonia oxidizers · Arctic · Ammonium · Ecotypes · Nitrification Full text in pdf format Supplementary material PreviousCite this article as: Damashek J, Pettie KP, Brown ZW, Mills MM, Arrigo KR, Francis CA (2017) Regional patterns in ammonia-oxidizing communities throughout Chukchi Sea waters from the Bering Strait to the Beaufort Sea. Aquat Microb Ecol 79:273-286. https://doi.org/10.3354/ame01834 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 79, No. 3. Online publication date: July 05, 2017 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2017 Inter-Research.

Observed wintertime tidal and subtidal currents over the continental shelf in the northern South China Sea

AbstractSynthesis analyses were performed to examine characteristics of tidal and subtidal currents at eight mooring sites deployed over the northern South China Sea (NSCS) continental shelf in the 2006–2007 and 2009–2010 winters. Rotary spectra and harmonic analysis results showed that observed tidal currents in the NSCS were dominated by baroclinic diurnal tides with phases varying both vertically and horizontally. This feature was supported by the CC‐FVCOM results, which demonstrated that the diurnal tidal flow over this shelf was characterized by baroclinic Kelvin waves with vertical phase differences varying in different flow zones. The northeasterly wind‐induced southwestward flow prevailed over the NSCS shelf during winter, with episodic appearances of mesoscale eddies and a bottom‐intensified buoyancy‐driven slope water intrusion. The moored current records captured a warm‐core anticyclonic eddy, which originated from the southwestern coast of Taiwan and propagated southwestward along the slope consistent with a combination of β‐plane and topographic Rossby waves. The eddy was surface‐intensified with a swirl speed of >50 cm/s and a vertical scale of ∼400 m. In absence of eddies and onshore deep slope water intrusion, the observed southwestward flow was highly coherent with the northeasterly wind stress. Observations did not support the existence of the permanent wintertime South China Sea Warm Current (SCSWC). The definition of SCSWC, which was based mainly on thermal wind calculations with assumed level of no motion at the bottom, needs to be interpreted with caution since the observed circulation over the NSCS shelf in winter included both barotropic and baroclinic components.

Nutrients and water masses in the Gulf of Maine–Georges Bank region: Variability and importance to blooms of the toxic dinoflagellate Alexandrium fundyense

We report here the results of ten oceanographic survey cruises carried out in the Gulf of Maine–Georges Bank region of the Northwest Atlantic during the late spring to summer period in 2007, 2008 and 2010, for which we examine and characterize relationships among dissolved inorganic nutrient fields, water mass dynamics and cell densities of the toxic dinoflagellate Alexandrium fundyense. Nutrients are supplied to continental shelf waters of the Gulf of Maine–Georges Bank region by inflows of deep offshore water masses; once in the Gulf they are transported with the residual circulation and mix with surface waters, both in the Gulf and on the Bank. Those fluxes of offshore water masses and their nutrient loads are the major source of nutrients for phytoplankton production in the region, including annual blooms of A. fundyense in the Gulf and on Georges Bank. This much is already known. We suggest here that the locations and magnitude of A. fundyense blooms are controlled in part by variable nutrient fluxes to the interior Gulf of Maine from offshore, and, those interior Gulf of Maine waters are, in turn, the main nutrient source to Georges Bank, which are brought onto the Bank by tidal pumping on the Northern Flank. We present evidence that nitrate is the initial form of nitrogenous nutrient for A. fundyense blooms, but it is quickly depleted to limiting concentrations of less than 0.5µM, at which time continued growth and maintenance of the population is likely fueled by recycled ammonium. We also show that phosphate may be the limiting nutrient over much of Georges Bank in summer, allowing recycled ammonium concentrations to increase. Our temperature–salinity analyses reveal spatial and temporal (seasonal and interannual) variability in the relative proportions of two deep source waters that enter the Gulf of Maine at depth through the Northeast Channel: Warm Slope Water (WSW) and Labrador Slope Water (LSW). Those two source waters are known to vary in their nutrient loads, with nitrate concentrations about 50% higher in WSW than LSW, for example, and as such the proportions of these two water masses to one another are important determinants of the overall nutrient loads in the interior Gulf. In addition to these deep slope water fluxes, we show evidence here of episodic fluxes of relatively fresh and low-nutrient shelf waters from the Nova Scotian Shelf, which enter the Gulf in pulses at depths between the surface and approximately 150m, displacing deep slope waters, and consequently they significantly dilute the Gulf's interior waters, reducing nutrient concentrations and, in turn, affect the magnitude of A. fundyense blooms.

The effects of continental margins and water mass circulation on the distribution of dissolved aluminum and manganese in Drake Passage

A total of 232 samples were analyzed for concentrations of dissolved aluminum ([Al]) and manganese ([Mn]) in Drake Passage. Both [Al] and [Mn] were extremely low (∼0.3 and 0.1 nM, respectively) in the surface layer of the middle Drake Passage, most likely due to limited input and biological uptake/scavenging. Elevated [Al] (>14 nM) and [Mn] (>2 nM) over the South American continental shelf are related to land run‐off, whereas elevated concentrations (>1 nM and >2 nM, respectively) near the Antarctic Peninsula are most likely related to sediment re‐suspension. Re‐suspension of sedimentary particles and pore waters influences the distribution of [Al] and [Mn] over the continental slopes on both sides of Drake Passage. The influence of the continental margins and accumulated dust input potentially explains the higher [Al] observed eastward in the Atlantic section of the Southern Ocean. In the northern Drake Passage, elevated [Al] (∼0.8 nM) and [Mn] (∼0.3 nM) near the seafloor are most likely the result of bottom sediment re‐suspension by the relatively strong currents. In the deep southern Drake Passage sediment re‐suspension associated with the inflow of Weddell Sea Deep Water appears to cause elevated [Al] (>1 nM) and [Mn] (∼0.4 nM). In the deep northern Drake Passage, North Atlantic Deep Water brings in elevated [Al] and Southeast Pacific Deep Slope Water brings in the signature of Pacific hydrothermal vents. Elevated [Mn] andδ3He were correlated in this water layer and are most likely originating from the volcanically active ridges in the Pacific Ocean.

Short-term variations of deep water masses in Drake Passage revealed by a multiparametric analysis of the ANT-XXIII/3 bottle data

A multiparametric analysis is applied on the ANT-XXIII/3 bottle data to determine the spreading and mixing of water masses across Drake Passage during January–February 2006, focusing our interest on the flow of the deep water masses. Mixing proportions are quantified along both the southward and northward journeys between the tip of South America and the Antarctic Peninsula. The method is sensitive enough to detect a marked variability in the flow pathways of the Circumpolar Deep Water, the Southeast Pacific Deep Water (SPDW) and the Weddell Sea Deep Water at the intersection of the Shackleton Fracture Zone (SFZ) and the West Scotia Ridge (WSR) between the southward and northward journeys. Indeed the presence of a sharp meander of the southern branch of the Polar Front (PF S) together with the displacement of the northern branch of the Southern Antarctic Circumpolar Current Front (SACCF N) above the complex bathymetric structure of the WSR induced a strong perturbation in the deep flow field during the southward journey. The height of the SFZ crests clearly sets the lower limit of the SPDW in the Ona Basin. An enhanced δ 3He signature is confined to the north of the Polar Front between 55.5°S and 56.5°S in the 1500–2200 m depth range. It clearly indicates the signature of the core of the Southeast Pacific Deep Slope Water. The entry pathway in the ACC of the SPDSW during ANT-XXIII/3 was farther to the south off the continental slope of South America than it was during the WOCE A21 expedition in 1990, probably due to the sharp changes in the slope orientation at the extreme tip of the South America.

A changing nutrient regime in the Gulf of Maine

Recent oceanographic observations and a retrospective analysis of nutrients and hydrography over the past five decades have revealed that the principal source of nutrients to the Gulf of Maine, the deep, nutrient-rich continental slope waters that enter at depth through the Northeast Channel, may have become less important to the Gulf's nutrient load. Since the 1970s, the deeper waters in the interior Gulf of Maine (>100 m) have become fresher and cooler, with lower nitrate (NO3) but higher silicate (Si(OH)4) concentrations. Prior to this decade, nitrate concentrations in the Gulf normally exceeded silicate by 4–5 μM, but now silicate and nitrate are nearly equal. These changes only partially correspond with that expected from deep slope water fluxes correlated with the North Atlantic Oscillation, and are opposite to patterns in freshwater discharges from the major rivers in the region. We suggest that accelerated melting in the Arctic and concomitant freshening of the Labrador Sea in recent decades have likely increased the equatorward baroclinic transport of the inner limb of the Labrador Current that flows over the broad continental shelf from the Grand Banks of Newfoundland to the Gulf of Maine. That current system now brings a greater fraction of colder and fresher deep shelf waters into the Gulf than warmer and saltier offshore slope waters which were previously thought to dominate the flux of nutrients. Those deep shelf waters reflect nitrate losses from sediment denitrification and silicate accumulations from rivers and in situ regeneration, which together are altering the nutrient regime and potentially the structure of the planktonic ecosystem.

Occurrence and distribution of cetaceans in São Tomé and Príncipe tropical archipelago and their relation to environmental variables

São Tomé and Príncipe is an archipelago located in the Gulf of Guinea. This archipelago seems to be an important area for cetaceans, probably due to large concentrations of prey, as well as the existence of several small bays and shallow water that constitute preferred rest areas. In comparison to other areas of the world, little is known about cetacean communities in this archipelago. Boat-based surveys to record sightings of cetaceans were conducted between 2002 and 2005, which represents the first approach to study the occurrence of whales and dolphins in this tropical region. Sightings of humpback whales(Megaptera novaeangliae), bottlenose dolphins(Tursiops truncatus), pantropical spotted dolphins(Stenella attenuata), killer whales(Orcinus orca), sperm whales(Physeter macrocephalus)and pilot whales(Globicephala melas)were recorded. Cetaceans were found throughout a range of sea surface temperature between 24 and 29°C and a wide range of depths. Significant differences were found regarding the occurrence of some species. Humpback whales showed a preference for shallower waters between 20 and 100 m. Bottlenose dolphins occurred most commonly along the continental shelf (20 to 100 m) and pantropical spotted dolphins occurred in deep slope waters (>1000 m).

Influence of local and external processes on the annual nitrogen cycle and primary productivity on Georges Bank: A 3-D biological–physical modeling study

Georges Bank is one of the world's most highly productive marine areas, but the mechanisms of nutrient supply to support such high productivity remain poorly understood. Intrusions of nutrient-poor Labrador Slope Water (LSW) into the Gulf of Maine (NAO-dependent) potentially can reduce nutrient delivery to the bank, but this mechanism has not been quantitatively examined. In this paper, we present the first whole-year continuous model simulation results using a biological–physical model developed for the Gulf of Maine/Georges Bank region. This high-resolution three-dimensional coupled model consists of the Finite Volume Coastal Ocean Model (FVCOM) and a Nitrogen–Phytoplankton–Zooplankton–Detritus (NPZD) model, and was used to examine the influences of local and external processes on nitrogen and phytoplankton dynamics on Georges Bank. The model captured the general pattern of spatial-temporal distributions of nitrogen and phytoplankton and provided a diagnostic analysis of different processes that control nitrogen fluxes on Georges Bank. Specifically, numerical experiments were conducted to examine seasonal variation in nitrogen transport into the central bank (new nitrogen supply) versus nitrogen regenerated internally in this region. Compared with previous observation-based studies, the model provided a quantitative estimate of nitrogen flux by integrating the transport over a longer time period and a complete spatial domain. The results suggest that, during summer months, internal nitrogen regeneration is the major nitrogen source for primary production on the central bank, while nitrogen supply through physical transport (e.g. tidal pumping) contributes about 1/5 of the total nitrogen demand, with an estimated on-bank nitrogen transport at least 50% less than previous estimates. By comparing the model runs using different nitrogen concentrations in deep Slope Water, the potential influence of NAO-dependent intrusions of LSW was examined. The results suggest that the change of nitrogen concentration in the deep Slope Water may not have a significant impact on nitrogen and phytoplankton dynamics on the well-mixed central bank, largely due to limited nutrient exchange across the tidal mixing front and enhanced near-frontal nutrient uptake. However, relatively more significant impact was observed in the model simulations if both well-mixed and seasonally-stratified areas (inside 100 m isobath of the bank) were considered in flux calculations.

Bayesian analysis of allozyme markers indicates a single genetic population of kingklip Genypterus capensis off South Africa

Kingklip Genypterus capensis inhabit deep continental shelf and slope waters off southern Africa and are now largely harvested as a bycatch in trawl and longline fisheries for Cape hakes Merluccius spp. Regional differences in growth, vertebral count and otolith morphology indicate that kingklip may consist of three stocks. In this study, allozyme markers are used to test whether the two southern stocks, the putative Cape and South Coast stocks, are genetically distinguishable. Standard electrophoretic methods detected Mendelian polymorphisms at six (23%) of 26 enzyme-encoding loci in five samples extending from about 29°S on the West Coast to about 25°E on the South Coast. No deviations from Hardy-Weinberg proportions were observed in the samples or in the pooled sample. Gene diversities (heterozygosity) ranged from HS = 0.039 to 0.042 among samples, and averaged 0.041. Genetic differentiation among localities was remarkably small, FST = 0.003, and did not differ significantly from zero. A Bayesian analysis of population structure, based on detecting departures from Hardy-Weinberg proportions and gametic phase equilibria between loci, indicated with a high likelihood that the five samples were drawn from a single genetic population. In combination, these results are consistent with the occurrence of a single population off South Africa. However, the most conservative management strategy is to set harvest quotas individually on the Cape and South Coast stocks.

An additional deep-water mass in Drake Passage as revealed by 3He data

We present 3He data from a repeat section across Drake Passage, from three sections off the South American continent in the Pacific, at 28°S, 35°S, and 43°S, and from three sections in the Atlantic, eastward of the Malvinas, close to 35°W, and near the Greenwich Meridian. In Drake Passage, a distinct high- 3He signal is observed that is centered just above the boundary of the Lower and the Upper Circumpolar Deep Water (LCDW, UCDW), and is concentrated towards the northern continental slope. 3He concentrations in the Antarctic Circumpolar Current (ACC) upstream of Drake Passage (World Ocean Circulation Experiment section P19 at 88°W) are markedly lower than those found in Drake Passage, and a regional source of primordial helium in the path of the ACC that might cause the high- 3He feature can be ruled out. We explain the feature by addition of high- 3He waters present at the 43°S Pacific section. This supports a previous, similar interpretation of a low-salinity anomaly in Drake Passage (Naveira Garabato et al., Deep-Sea Research I 49 (2002) 681), that is strongly related to the high- 3He feature. Employing multiparameter water mass analysis (including 3He as a parameter), we find that deep waters as met at the 43°S Pacific section, flowing south along the South American continental slope, contribute substantially to the ACC waters in Drake Passage (fractions exceed 50% locally). Lesser, but laterally more extended contributions are found east of the Malvinas, and still smaller ones are present at 35°W and at the Greenwich Meridian. Using velocity measurements from one of the two Drake Passage sections, we estimate the volume transport of these waters to be 7.0±1.2 Sv, but the average transport may be somewhat lower as the other realization had a less pronounced signal. The enhanced 3He signature in Drake Passage is essentially confined north of the Polar Front. Further downstream the signature crosses this front, to the extent that at 35°W the contributions south and north of it are of similar magnitude. At the same time, the 3He levels north of the front are reduced due to a substantial admixture of low- 3He North Atlantic Deep Water, such that 3He becomes highest south of the front. The flow of Southeast Pacific deep slope waters entering the ACC constitutes the predominant exit pathway of the primordial helium released in the deep Pacific, and represents a considerable fraction of the deep water return flow from the Pacific into the ACC. Therefore and also because the density range of the added deep slope waters is intermediate between those of UCDW and LCDW, they must be considered a distinct water mass.

Temporal and regional variability in sources and cycling of DOC and POC in the northwest Atlantic continental shelf and slope

The D 14 C and d 13 C distributions in total dissolved and suspended particulate organic carbon (DOC and POC, respectively) and dissolved inorganic carbon (DIC) were measured throughout the Middle Atlantic Bight (MAB) continental shelf and slope in April–May 1994, March 1996, and July–August 1996. The highest D 14 C values for both DOC (up to � 29%) and POC (up to 78%) were observed in relatively low-salinity shelf waters. The D 14 C values for DOC from the shelf and shallow (B5 m depth) slope generally increased progressively with proximity to the coast, and along-shelf from northeast to southwest. A significant system-wide increase in D 14 C-DOC values occurred between spring and summer 1996, indicating a major flux of young DOC to the MAB over this timeframe. The lowest D 14 CDOC values (as low as � 476%) were found in deep (\300 m depth) slope waters, and they were lower than values measured previously at similar depths in the open North Atlantic. A significant inverse relationship was found between D 14 C-DOC and d 13 C-DOC for shelf and shallow slope waters, which we speculate may be due to variable contributions of young, 14 C-enriched organic matter of terrestrial and/or riverine origin. The suspended POC of shelf and shallow slope waters was isotopically distinct from DOC, and exhibited little of the characteristic areal variability of DOC concentrations and D 14 C across the MAB. In deep slope waters, suspended POC had the greatest ages ever observed for water column POC in any marine system. These 14 C-depleted POC values correlated positively and significantly with d 13 C-POC values, which ranged from � 31% to � 23% in deep slope waters. A multiple-source dual isotopic mass balance model was applied to the observed D 14 C and d 13 C distributions to evaluate the potential sources of DOC and POC to shelf and slope waters of the MAB. With the exception of the highly 14 C- and 13 C-depleted POC from the deep slope, all other DOC and POC values could be constrained by the isotopic signatures of various potential source materials measured for the MAB region. These include varying contributions from potential sources of open ocean, terrestrial and riverine, and bottom nepheloid layer colloidal organic matter, as well as from recent shelf and slope primary production. r 2002 Elsevier Science Ltd. All rights reserved.

Decomposition of dissolved organic matter from the continental margin

Decomposition of dissolved organic carbon, nitrogen and phosphorus (DOC, DON, DOP) was measured for surface and bottom waters of the middle Atlantic bight (MAB) and deep slope water adjacent to the MAB on two occasions in March and August 1996. We used standard bottle incubation techniques to measure the decrease in dissolved organic matter (DOM) concentrations over a 180-day interval. Generally DOM concentrations in the MAB were elevated (125mM DOC, 10.2mM DON and 0.30mM DOP) relative to the surface ocean and deep slope water (46.7mM DOC, 2.76mM DON, 0.03mM DOP). On average the C:N:P ratio of shelf DOM (431:36:1) was substantially higher than the Redfield ratio, but not nearly as high for that of deep slope water (2700:215:1). Decomposition time course data were fit to a three-pool (very labile, labile, and recalcitrant pools) multi-G model using a Marquardt fitting routine. The threepool model was superior to a simple exponential decay model assuming a single pool of DOM. We observed no significant changes in concentration of DOM in deep-water samples, attesting to the old age of this material, its recalcitrant nature, and the cleanliness of our technique for measuring decomposition. There were major differences in the relative amount of very labile, labile and recalcitrant fractions of shelf-water DOC, DON and DOP as a result of preferential remineralization of P over N and N over C. Averaged over stations, the decomposable portion of the bulk DOC, DON and DOP pools increased from 30% to 40% to 81% for C, N and P. There was a wide range in decay coefficients for the very labile and labile DOM pools: average decay coefficient for the very labile pool was 0.219 d � 1 , and 0.018 d � 1 for the labile pool. Average half-lives calculated from the decay coefficients were 4, 12 and 8 days for the very labile DOC, DON and DOP pools, and 54, 113 and 90 days for the labile DOC, DON and DOP pools. On the basis of pool turnover times relative to shelf-water residence time (B100 days) we conclude that autochthonous algal production is the source of the very labile DOM pools. Its rate of production is sufficient to sustain estimated rates of bacteria C demand in continental margins. Our results for the MAB indicate that while substantial amounts of DOM are remineralized in the same time frame as shelf-water residence time, there is substantial DOM remaining that is depleted in N and P relative to C. Strong concentration gradients in DOM occur between shelf and ocean waters and between surface and deeper waters. Coupled with appropriate vertical and horizontal advective and eddy diffusive transports, DOM export from the MAB and other shelf systems may be a significant component of ocean C dynamics. r 2002 Elsevier Science Ltd. All rights reserved.

Sources and cycling of dissolved and particulate organic radiocarbon in the Northwest Atlantic Continental Margin

Continental shelves and slopes are productive and dynamic ocean margin systems that also regulate the fluxes of terrestrial, riverine, and estuarine materials between the continents and oceans. In order to evaluate the ages, potential sources, and transformations of organic matter in an ocean margin system, we measured the radiocarbon (Δ14C) and δ13C distributions of total dissolved organic carbon (DOC), suspended particulate organic carbon (POC), and dissolved inorganic carbon (DIC) in waters of the Middle Atlantic Bight (MAB) continental shelf and slope in April‐May 1994. The Δ14C of DOC was greatest (as high as −39‰) in surface waters of the shelf, decreasing rapidly offshore and with depth, even in relatively shallow (25‐50 m depth) shelf waters. The lowest Δ14C‐DOC values were observed in deep slope waters, where they were significantly lower than values measured previously for the deep Sargasso Sea. There was a strong inverse relationship between Δ14C‐DOC and δ13C‐DOC in all shelf and surface slope waters of the MAB, which is likely attributable to varying contributions of young, 14C‐enriched organic matter of terrestrial and/or riverine origin. The more highly 14C‐depleted DOC in deep slope waters (as low as −442‰) generally had a correspondingly lower δ13C (as low as −22.3‰) component. However, this must originate from relic terrestrial material either in the MAB itself or be discharged to the MAB from rivers and estuaries. The isotopic signatures of POC were clearly differentiable from DOC and indicate that this pool also contained a broad range of both old and young material of terrestrial (δ13C as low as − 24.9‰) and marine (δ13C as high as −19.9‰) origin throughout the MAB shelf and slope. The highest Δ14C‐POC values (up to 78‰) were observed in shallow shelf waters of the southern MAB. Conversely, the lowest Δ14C‐POC values (as low as − 394‰) were found in MAB deep slope waters and were also significantly more depleted in 14C than POC from the central north Atlantic (Sargasso Sea). A multiple‐source isotopic mass balance model employing both 14C and 13C was used to evaluate the relative contributions of both young and old terrigenous versus marine organic matter to DOC and POC in the MAB. The results indicate that shelf and slope DOC is comprised of an old “marine” fraction (represented by offshore Sargasso Sea material) and either a young “terrestrial/riverine/ estuarine” (TRE) component (in shelf and shallow slope waters) or a relic TRE component (in deep and some shallow slope waters). In contrast, suspended POC from the MAB appears to originate predominantly from a mixture of recent MAB primary production and an old, TRE component, similar to that observed in one of the major subestuaries of the Chesapeake Bay. These results suggest that both young and old sources of terrestrial and riverine organic matter can comprise a significant fraction of the DOC and POC in ocean margins. Preliminary calculations indicate that the export of this compositionally unique DOC and suspended POC may be significant terms in the organic carbon budgets of the MAB and other margin systems.

Notes on the distribution and length composition of Raja linteaR. fyllae, R. hyperborea andBathyraja spinicauda (Pisces: Rajidae) in the deep northeastern North Sea and on the slope of the eastern Norwegian Sea

The distribution and length composition of four skate species inhabiting deep shelf and upper slope waters of the Northeastern Atlantic are compared. Raja lintea and Raja fyllae primarily inhabited the Norwegian Deep of the northeastern North Sea, but also occurred in the upper slope waters of the eastern Norwegian Sea, and they had species-specific but partially overlapping distributions. Their areas of distribution were associated with relatively warm Atlantic water masses. In the deeper slope areas with colder Norwegian Sea Deep Water, Raja hyperborea was the only abundant skate, whilst Bathyraja spinicauda seemed associated with the frontal zone between the warm and cold water masses on the upper slope. The entire size range of Raja fyllae was found, but mainly large Raja lintea. The catches of Raja hyperborea consisted of mostly large individuals, whereas the Bathyraja spinicauda were small, immature specimens. 2001 International Council for the Exploration of the Sea

Descriptions of two new species of Bodianus (Perciformes: Labridae) from Australasian waters

Two new species of the labrid genus Bodianus are described from deep shelf and upper slope waters of eastern Australia, New Zealand, and the submarine ridges south‐east and south‐west of New Caledonia. Bodianus flavipinnis sp. nov., living in shelf waters of eastern Australia and New Zealand, is most similar to B. unimaculatus and other species of the B. oxycephalus complex, differing from them most noticeably in lacking prominent broken red stripes in both adult stages. Bodianus flavifrons sp. nov., found on slopes of submarine ridges, is closely related to B. frenchii of southern Australian waters. It differs from that species in having distinctive broad yellow bands on the head. The new species and their close relatives constitute the subgenus Bodianus (Verreo), with a subtropical to warm temperate distribution. A key to the species of the subgenus is provided.

Sources and cycling of nitrogen in the Gulf of Maine

An analysis of water mass flows and nitrogen fluxes in the Gulf of Maine region shows that deep Slope Water that enters the Gulf through the Northeast Channel, and Scotian Shelf Water that enters at the surface, dominate the flux of nitrogen into the Gulf. A box model is developed that examines internal vertical nitrogen fluxes, and reveals that the flux of nitrogen into surface waters is sufficient to explain only about 59 gC m −2 yr −1 of new primary production, which is 20% of the total estimated Gulf of Maine primary production of 290 gC m −2 yr −1. This means that the Gulf-wide f ratio (of “new” NO 3-based production to the total production based on both new NO 3 and recycled NH 4) is 0.20, which is more typical of oligotrophic oceans than a productive continental shelf sea like the Gulf of Maine. The expected f ratio is nearer to 0.4, which would require an additional flux of new NO 3 into the Gulf equal to about 40% of the total flux already accounted for by all sources: Slope Water, Scotian Shelf Water, rivers and atmospheric deposition. This additional supply of “new” nitrogen is argued to be the result of water column nitrification. The box model also shows, surprisingly, that nutrients delivered to surface waters of the Gulf by Scotian Shelf Water are roughly equal to that of Slope Water. It is concluded that better estimates are needed of water flows into and out of the Gulf, along with more measurements of their nutrient loads, and that measurements should be made of water column nitrification rates. An overall conclusion is that the energetics of vertical mixing processes that deliver nutrients to the productive surface waters set the upper limit to biological production in the Gulf of Maine, and that construction of carbon and nitrogen budgets that consider only fluxes into and out of the Gulf, and not internal recycling, will be in error.

Sources and transport of land-derived particulate and dissolved organic matter in the Gulf of Mexico (Texas shelf/slope): The use of ligninphenols and loliolides as biomarkers

The fate and transport of terrestrial organic matter across the continental margin in the Gulf of Mexico was studied in 1992 and 1993 using chemical biomarkers. Lignin-phenols were utilized as biomarkers for terrestrial inputs and indicated that much of the terrestrial organic matter inputs were deposited on the shelf/break and slope. The lignin-phenol concentrations (normalized to carbon) in POC, HMW DOC, and sediments in slope waters were considerably higher than at other open ocean sites studied previously. The dominant mechanism for transport of terrestrially-derived POC and HMW DOC across the shelf and slope was hypothesized to be advection of riverine and estuarine discharges through benthic nepheloid layers. Based on loliolide concentrations in the water column, we believe that lateral transport of these materials at the shelf/break (through extensions of benthic nepheloid layers) may have been an important mechanism for the injection of terrestrially-derived organic matter into deep slope waters.

Stoichiometry of dissolved organic matter dynamics on the continental shelf of the northeastern U.S.A.

Dissolved organic carbon (DOC), nitrogen (DON), phosphorus (DOP), inorganic nitrogen and inorganic phosphorus were measured from the Georges Bank region during spring 1993 and summer 1994. Concentrations of DOC in surface waters ranged from 65–92 μM C and decreased with depth to about 50 μM in shelf slope water at a depth of 1500 m. DON and DOP concentrations showed a similar pattern with depth, being higher in surface water and lower in deep water (∼5 to 3 μM N and 0.17 to 0.02 μM P). There was an inverse relation between inorganic nutrient concentrations and DON and DOP. While DOC concentrations were elevated in regions of highest primary production, there was little evidence of corresponding elevations for DON and DOP. Slopes of regression lines relating total dissolved N and P to inorganic N and P indicated that 19 and 15% of remineralized N and P that accumulates in deeper water is derived from the export and decomposition of dissolved organic material (DOM). While overall inorganic N and P dynamics followed Redfield, A. C. (1958) ( American Scientist, 46, 205–221) stoichiometry (C:N:P, 106:16:1), the C:N:P stoichiometry of dissolved organic matter deviated substantially. Near-surface stoichiometry of DOM was 400–800:1 for DOC:DOP, 11–15:1 for DOC:DON and 24–55:1 for DON:DOP. Relationships between DOC, DON and DOP indicated that N and P were preferentially remineralized relative to carbon resulting in C:N: P ratios for deeper water DOM that deviated even further from Redfield. Decomposition studies revealed that DOC decomposition rate was related to initial DOC concentration. The higher the DOC concentration the more quickly it was mineralized. DOC first order decay coefficients ranged from undetectable for deep slope water with an initial DOC concentration of 50 μM to 0.0024 day −1 for water collected from the Georges Bank shoals with an initial DOC concentration of 92 μM. DON and DOP decomposition occurred concomitantly with DOC remineralization. Loss rates ranged from undetectable to 0.00064 day −1 for DON and from undetectable to 0.0023 day −1 for DOP. During DOM decomposition we noted increases in the C:N, C:P and N:P ratios, which indicated preferential remineralization of N and P relative to C. Vertical concentration gradients and slow DOM turnover times relative to water residence times in the Georges Bank continental shelf region indicate the potential for export of DOM to deeper water. DOM export to depth and preferential remineralization of N and P relative to C may be important mechanisms for pumping C to depth in the ocean.