Phytoplankton dynamics within Gulf Stream intrusions on the southeastern United States continental shelf during summer 1981

During July and August 1981 subsurface intrusion of upwelled nutrient-rich Gulf Stream water was the dominant process affecting temporal and spatial changes in phytoplankton biomass and productivity of the southeastern United States continental shelf between 29 and 32°N latitude. Intruded waters in the study area covered as much as 10¹ km including virtually all of the middle and outer shelf and approximately 50% of the inner shelf area.Within 2 weeks following a large intrusion event in late July, middle shelf primary production and Chl a reached 3 to 4 gC m⁻ d⁻¹ and 75 mg m⁻, respectively. At the peak of the bloom 80% of the water column primary production occurred below the surface mixed-layer, and new primary production (i.e., NO₃-supported) exceeded 90% of the total. Chl a-normalized photosynthetic rates were very high as evidenced by high mean assimilation number (15.5 mg C mg Chl a⁻¹ h⁻¹), high mean α (14 mg C mg Chl a⁻¹ Ein⁻¹ m), and no photoinhibition. As a result of the high photosynthetic rates, mean light-utilization index (Ψ) was 2 to 3 times higher than reported for temperature sub-arctic and arctic waters.The results imply a seasonal (June to August) middle shelf production of 150 g C m⁻¹, about 15% higher than previous estimates of annual production on the middle shelf. Intrusions of the scale we observed in 1981 may not occur every summer. However, when such events do occur, they are by far the most important processes controlling summer phytoplankton dynamics of the middle and outer shelf and of the inner shelf in the southern half of the study area.     

Impact of eddies on ocean diapycnal mixing in Gulf Stream region

Based on high-resolution,Array for Real-time Geostrophic Oceanography(Argo)profiles and Sea Level Anomaly(SLA)data,this study statistically analyzes and compares turbulent diapycnal mixing profiles inside and outside mesoscale eddies in the Gulf Stream region.The result indicates that average diapycnal diffusivity at 300–540 m depths in anticyclonic eddies reaches4.0×10-5 m2 s-1.This is significantly higher than the 1.6×10-5 m2 s-1 outside eddies and 0.8×10-5 m2 s-1 in cyclonic eddies.Probabilities of diapycnal diffusivity greater than 10-4 m2 s-1 within anticyclonic and cyclonic eddies and outside eddies are29%,5%and 12%,respectively.However,magnitudes of average diapycnal diffusivity at 540–900 m depths in these three cases are of the same order,10-5 m2 s-1.Twenty-four of a total 38 anticyclonic eddies had enhanced mixing in the ocean interior,and 22 were observed during or shortly after strong winds.The coincidence between enhanced mixing and strong wind stress indicates that more wind-induced,near-inertial wave energy propagates downward in anticyclonic eddies.The deeper part of 12 profiles(below 540 m)in anticyclonic eddies had vertical overturns with Thorpe scale exceeding 5 m,among which three profiles had overturns reaching 20 m.Enhanced mixing may have occurred in deep layers of some profiles,although it was not evident in average conditions.     

Seasonality of the submesoscale dynamics in the Gulf Stream region

Frontogenesis and frontal instabilities in the mixed layer are known to be important processes in the formation of submesoscale features. We study the seasonality of such processes in the Gulf Stream (GS) region. To approach this problem, a realistic simulation with the Hybrid Coordinate Ocean Model is integrated for 18 months at two horizontal resolutions: a high-resolution (1/48°) simulation able to resolve part of the submesoscale regime and the full range of mesoscale dynamics, and a coarser resolution (1/12°) case, in which submesoscales are not resolved. Results provide an insight into submesoscale dynamics in the complex GS region. A clear seasonal cycle is observed, with submesoscale features mostly present during winter. The submesoscale field is quantitatively characterized in terms of deviation from geostrophy and 2D dynamics. The limiting and controlling factor in the occurrence of submesoscales appears to be the depth of the mixed layer, which controls the reservoir of available potential energy available at the mesoscale fronts that are present most of the year. Atmospheric forcings are the main energy source behind submesoscale formation, but mostly indirectly through mixed layer deepening. The mixed layer instability scaling suggested in the (Fox-Kemper et al., J Phys Oceanogr 38:1145–1165, 2008) parametrization appears to hold, indicating that the parametrization is appropriate even in this complex and mesoscale dominated area.     

Numerical simulation of a mesolow over a Gulf Stream filament

A three-dimensional mesoscale numerical model is used to investigate mesoscale circulation over a Gulf Stream filament. Two numerical experiments are performed with different initial uniform ambient wind speeds (U=0.1 m s^–1, 3.5 m s^–1 and 7 m s^–1) for a typical winter day. It is found that for both low and moderate winds, a closed mesoscale circulation forms over the Gulf Stream filament. When the Gulf Stream filament was removed, the model did not predict a mesoscale circulation. The modeled circulation over the filament is in agreement with the observations, suggesting that the atmospheric circulations over the filaments may be an important mechanism in the U.S. East Coast cyclogenesis.     

Distribution and photoreactivity of chromophoric dissolved organic matter in northern Gulf of Mexico shelf waters

The distribution and photoreactivity of chromophoric dissolved organic matter (CDOM) in the northern Gulf of Mexico along the Louisiana coastal shelf were examined during three cruises in summer 2007, fall 2007, and summer 2008. The influence of the Mississippi River plume was clearly evident as CDOM levels (defined as a₃₀₅) and dissolved organic carbon (DOC) concentrations were well-correlated with salinity during all cruises. Elevated CDOM and CDOM:DOC ratios of surface samples collected offshore of Atchafalaya Bay and the Breton-Chandeleur Sound complex indicated emanations of organic-rich waters from coastal wetlands are also an important source to nearshore shelf waters. Generally, CDOM and DOC levels were highest in surface waters and decreased with depth, but during summer 2007 and summer 2008, CDOM levels in near-bottom samples were occasionally higher than at mid-depths without concomitant increases in DOC. CDOM photobleaching was measured during 24 irradiations using a SunTest XLS+ solar simulator with photobleaching rate coefficients (k₃₀₅) ranging from 0.011 to 0.32 h⁻¹. For fall 2007 and summer 2008, higher k₃₀₅ values were generally observed in samples with higher initial CDOM levels. However, samples collected during summer 2007 did not exhibit a similar pattern nor were there differences in photobleaching rates between surface and bottom samples. Spectral slope coefficients (S_275-295 or S_350-400) and DOC levels were largely unchanged after 24 h irradiations. Modeled CDOM photobleaching for northern Gulf of Mexico mid-shelf waters predicts that during the summer when solar irradiance is high and the water column becomes stratified, nearly 90% of the CDOM in the upper 1 m may be lost to photobleaching, with losses up to 20% possible even at 10 m depth.     

Vertical mixing of gravel over a long flood series

Vertical sediment exchange is a fundamental component of bedload transport in gravel‐bed channels. This paper describes the characteristic depth of exchange achieved over a long flood series. Analysis is based on 11 recoveries of magnetically tagged gravels deployed in Carnation Creek, Canada, completed between 1990 and 2008. Vertical grain exchange mixes gravels throughout the streambed relatively rapidly. Within one to eight floods the mean burial depth approaches two times the surface layer thickness, quantified by the 90th percentile of the size distribution. Finer gravels are mixed more rapidly into the bed than coarser gravels. Both active and passive grain exchanges throughout most of the bed produce the overall vertical distribution of marked grains. Gravel exchanges exhibit fairly consistent patterns once tracers are well mixed by large floods. Results highlight the role of flood sequence in determining exchange depths, support the notion of an upper limit to exchange, and underscore the importance of passive grain exchange. Copyright © 2011 John Wiley & Sons, Ltd.     

Development and validation of biocriteria using benthic macroinvertebrates for North Carolina estuarine waters

Biological criteria were developed to assess water quality in mesohaline and polyhaline estuarine waters in North Carolina. The criteria used a combination of three metrics: a new 'Hilsenhoff-type' biotic index incorporating the sensitivity to pollution of each taxon and its relative abundance, total number of taxa, and number of amphipod and caridean shrimp taxa. Agreement with an independent data set was 90%, as was within site repeatability of this biocriteria over a range of water qualities and salinities. The versatility of the biotic index is discussed relative to other sampling methods and in other parts of the world.     

Interannual variability of Gulf Stream warm-core rings in response to the North Atlantic Oscillation

Analysis of a quality-controlled database of Gulf Stream warm-core rings (WCRs) between 75° and 50°W during 1978–1999 demonstrates a significant correlation between WCR occurrences and variations in large-scale atmospheric forcing related to the state of the North Atlantic Oscillation (NAO). The mechanisms for linking the NAO with the rate of WCR occurrences are two-fold: (1) the influence of the NAO on Gulf Stream (GS) position, which could affect the interaction of the Gulf Stream with the New England Seamounts chain and thus allow for a higher/lower number of WCR occurrences; (2) the NAO-induced eddy kinetic energy (EKE) variability in the Gulf Stream region (GSR), which is indicative of the baroclinic instability processes necessary for WCR formation. Variability in GS movement is studied by analyzing annual mean positions of the Gulf Stream North Wall obtained from satellite-derived sea surface temperature (SST) frontal charts. Response of GSR EKE to fluctuations in the state of the NAO is examined with a numerical simulation of the North Atlantic basin from 1980–1999. The North Atlantic basin is simulated using a 1/6°-resolution eddy-resolving Regional Ocean Modeling System (ROMS) model that spins up with Southampton Oceanography Center (SOC) ocean-atmosphere atlas-derived atmospheric forcing fields. Model-derived EKE estimates are observed to be in good agreement with TOPEX/Poseidon altimeter-based EKE estimates as well as with results from other modeling studies for the North Atlantic basin. We suggest that lateral movement of the GS may not be the primary mechanism causing variation in the rate of WCR occurrences, because GS position is observed to respond at a lag of one year, whereas annual rates of WCR occurrences respond at 0-year lag to the NAO. Based on results from numerical simulations of the North Atlantic basin, adjustment to NAO-induced wind forcing is seen to impact the GSR EKE intensity and possibly the related baroclinic instability structure of the GS at 0-year lag. These results suggest that NAO-induced interannual variability in GSR EKE is the most likely mechanism affecting WCR occurrences. Numerical simulations show that high (low) phases in the state of the NAO exhibit higher (lower) EKE in the GSR, providing a greater (lesser) source of baroclinic instability to the GS front, possibly resulting in higher (lower) occurrences of WCRs.     

Coupling of early diagenetic processes and sedimentary dynamics in tropical shelf environments: the Gulf of Papua deltaic complex

Tropical mobile mud belts represent a major class of biogeochemical and diagenetic systems characterized by extensive and frequent physical reworking of fine-grained, organic-rich deposits underlying oxygenated waters. Large regions of the Gulf of Papua, Papua New Guinea deltaic complex are dominated by such conditions. A reworked mud belt lies within the inner shelf between 10 and 20 m depth on a sedimentary clinoform derived from coalescing deltas. Deposits across the topset are typically suboxic, nonsulfidic over the upper 0.5–1 m, and have low to moderate maximum pore water concentrations of dissolved Fe(II) and Mn(II) (100–200, but up to 800 μM). Sediments are reactive, with surficial ΣCO₂ production 0.1–0.3 mM d⁻¹ and benthic O₂ fluxes 23±15 mmol m⁻² d⁻¹ (upper 20 cm). The highest rates occur within inner topset deposits (10–20 m) and near the high accumulation rollover region of the topset–foreset beds (40–50 m). Lower rates are found inshore along intertidal channels—mangrove fringe and within scoured or exposed consolidated deposits of the middle topset region. Remineralization rate patterns are independent of relative dominance by terrestrial or marine carbon in sediments. Dissolved O₂ usually penetrates 2–5 mm into surface sediments when macrofaunal burrows are absent. More than 75% of the highly reactive sedimentary Fe(III) pool (350–400 μmol g⁻¹) is typically diagenetically reduced in the upper 0.5 m. Pore water can be measureably depleted at depths >0.5 m, but dissolved H₂S generally remains below detection over the upper 1–2 m. As in other deltaic topset regions, concentration gradients often indicate that compared to many marine deposits of similar sediment accumulation rates, relatively refractory C_org is supplied to the SO₄ reducing zone. Sedimentary C/S ratios are 4–6 within the suboxic topset regions but decrease to <3 in offshore foreset beds where sulfidic diagenesis dominates. Only 15–20% of the diagenetically reduced Fe(II) is pyritic and a maximum of 10–25% is carbonate, implying that most Fe(II) is associated with authigenic or lithogenic silicates or oxides. The dominance of suboxic, nonsulfidic diagenetic processes reflect coupling between delivery of oxide-rich terrestrial debris, remobilization and reoxidation of deposits, and repetitive entrainment/remineralization of both labile and refractory organics. Distinct sedimentary indicators of reactive, suboxic mobile mud belts within tropical climatic zones are: abundant total highly reactive Fe (ΣFeR )>300 μmol g⁻¹; most reactive Fe is diagenetically reduced (ΣFe(II)/ΣFeR0.7–0.8); the proportion of diagenetically reduced Fe present as pyrite is low (Py–Fe(II)<0.2); C/S 4–8; and C_org/particle surface area <0.4 (mg C m⁻²). These depositional environments must be most common in tropical climates during high sea stand.     

Anatomy of a shoreface sand ridge revisited using foraminifera: False Cape Shoals, Virginia/North Carolina inner shelf

Certain details regarding the origin and evolution of shelf sand ridges remain elusive. Knowledge of their internal stratigraphy and microfossil distribution is necessary to define the origin and to determine the processes that modify sand ridges. Fourteen vibracores from False Cape Shoal A, a well-developed shoreface-attached sand ridge on the Virginia/North Carolina inner continental shelf, were examined to document the internal stratigraphy and benthic foraminiferal assemblages, as well as to reconstruct the depositional environments recorded in down-core sediments. Seven sedimentary and foraminiferal facies correspond to the following stratigraphic units: fossiliferous silt, barren sand, clay to sandy clay, laminated and bioturbated sand, poorly sorted massive sand, fine clean sand, and poorly sorted clay to gravel. The units represent a Pleistocene estuary and shoreface, a Holocene estuary, ebb tidal delta, modern shelf, modern shoreface, and swale fill, respectively. The succession of depositional environments reflects a Pleistocene sea-level highstand and subsequent regression followed by the Holocene transgression in which barrier island/spit systems formed along the Virginia/North Carolina inner shelf 5.2 ka and migrated landward and an ebb tidal delta that was deposited, reworked, and covered by shelf sand.     

The density of North Atlantic and North Pacific deep waters

The densities of seventeen samples of seawater from GEOSECS stations 27 (North Atlantic) and 217 (North Pacific) have been measured with a vibrating flow densimeter at 25°C. The densities of the deep samples were found to be 5 ± 1.5and16 ± 3.6ppm greater, for the North Atlantic and North Pacific, respectively, than predicted by the equation of state of Millero, Gonzalez and Ward (1976) derived for seawaters of constant relative composition. The results are in good agreement with the density anomalies predicted by Brewer and Bradshaw (1975) on the basis of the observed increase of dissolved silica, alkalinity and total carbon dioxide in oceanic deep waters. The application of these corrections results in an agreement with the Millero, Gonzalez and Ward (1976) equation of state to ±4 ppm.     

Vertical dynamics of summer phytoplankton in a stratified estuary (Gulf of Finland,Baltic Sea)

We present the results of multiparametric observations designed to follow the phytoplankton dynamics and interrelated physical, chemical and biological processes in the Gulf of Finland (Baltic Sea). Data were acquired by an autonomous moored water column profiler, an acoustic Doppler current profiler, a flow-through system installed aboard a ferry and by profiling and discrete water sampling aboard research vessels in July and August 2009. The main aim of the study was to investigate the processes responsible for the formation and maintenance of sub-surface maxima of phytoplankton biomass. We suggest that the environmental conditions caused by the prevailing atmospheric and oceanographic forcing (wind; vertical stratification; basin-wide, mesoscale and sub-mesoscale processes) are preferred by certain species/taxonomic groups and explain the migration patterns of phytoplankton. Nocturnal downward migration of phytoplankton with a swimming speed up to 1.6 m h⁻¹ occurred when the community was dominated by the dinoflagellate Heterocapsa triquetra. The observed splitting of the population into two vertically separated biomass maxima suggests that the H. triquetra cells, which reached the sub-surface layers with high nutrient concentrations, experienced bi-diurnal or asynchronous (when swimming upwards) vertical migration. The most intense sub-surface biomass maxima, on some occasions with the biomass much higher than that in the surface layer, were detected in connection to the sub-mesoscale intrusions below the depth of the strongest vertical density gradient.     

The seasonal occurrence of Neocalanus cristatus,Neocalanus plumchrus,and Eucalanus bungii over the shelf of the northern Gulf of Alaska

Field collections of zooplankton from the northern Gulf of Alaska demonstrate that the large oceanic copepods Neocalanus cristatus, N. plumchrus, and Eucalanus bungii are seasonal members of shelf and coastal zooplankton communities. They account for more than 25% of the net zooplankton biomass (0.333 mm) during the spring and summer months.The presence, absence, and subsequent reappearance of these copepods over the shelf is apparently related to their seasonal shifts in vertical distributions and to a persistent pattern of onshore Ekman transport. During periods when these copepods occur in the surface water of the bordering deep ocean (late autumn to summer) they are swept shoreward over the shelf. During times when they occur below the Ekman layer, they are generally absent from the shelf.The ecological significance of this seasonally introduced biomass is discussed in terms of what is understood about these species from studies elsewhere in the North Pacific Ocean.     

Generation mechanisms for mesoscale eddies in the Gulf of Lions: radar observation and modeling

Coastal mesoscale eddies were evidenced during a high-frequency radar campaign in the Gulf of Lions (GoL), northwestern Mediterranean Sea, from June 2005 to January 2007. These anticyclonic eddies are characterized by repeated and intermittent occurrences as well as variable lifetime. This paper aims at studying the link between these new surface observations with similar structures suggested at depth by traditional acoustic Doppler current profiler measurements and investigates the eddy generation and driving mechanisms by means of an academic numerical study. The influence of the wind forcing on the GoL circulation and the eddy generation is analyzed, using a number of idealized configurations in order to investigate the interaction with river discharge, buoyancy, and bathymetric effects. The wind forcing is shown to be crucial for two different generation mechanisms: A strong northerly offshore wind (Mistral) generates a vortex column due to the bathymetric constraint of a geostrophic barotropic current, which can surface after the wind relaxes; a southerly onshore wind generates a freshwater bulge from the Rhône river discharge, which detaches from the coast and forms a well-defined surface anticyclonic eddy based on buoyancy gradients. These structures are expected to have important consequences in terms of dispersion or retention of biogeochemical material at local scales.     

Regionality and seasonality of submesoscale and mesoscale turbulence in the North Pacific Ocean

The kinetic energy (KE) seasonality has been revealed by satellite altimeters in many oceanic regions. Question about the mechanisms that trigger this seasonality is still challenging. We address this question through the comparison of two numerical simulations. The first one, with a 1/10° horizontal grid spacing, 54 vertical levels, represents dynamics of physical scales larger than 50 km. The second one, with a 1/30° grid spacing, 100 vertical levels, takes into account the dynamics of physical scales down to 16 km. Comparison clearly emphasizes in the whole North Pacific Ocean, not only a significant KE increase by a factor up to three, but also the emergence of seasonal variability when the scale range 16–50 km (called submesoscales in this study) is taken into account. But the mechanisms explaining these KE changes display strong regional contrasts. In high KE regions, such the Kuroshio Extension and the western and eastern subtropics, frontal mixed-layer instabilities appear to be the main mechanism for the emergence of submesoscales in winter. Subsequent inverse kinetic energy cascade leads to the KE seasonality of larger scales. In other regions, in particular in subarctic regions, results suggest that the KE seasonality is principally produced by larger-scale instabilities with typical scales of 100 km and not so much by smaller-scale mixed-layer instabilities. Using arguments from geostrophic turbulence, the submesoscale impact in these regions is assumed to strengthen mesoscale eddies that become more coherent and not quickly dissipated, leading to a KE increase.     

Modelling the tidal mixing fronts and seasonal stratification of the Northwest European Continental shelf

We investigate mixing processes under stratified conditions on the Northwest European Continental shelf using a numerical model (POLCOMS). Our results indicate that convection induced by vertical shearing of horizontal density gradients (‘shear-induced convection’) is a regularly occurring feature in the bottom and surface boundary layers in this open shelf-sea situation. Two types of turbulence models are investigated to study their capability for reproducing the observed location of tidal mixing fronts, and the physical processes occurring in seasonally stratified waters. The first model is a one-equation variant of the Mellor–Yamada model, whereas the second model combines a more recent second-momentum closure with a two-equation model. It is found that generally mean frontal positions (as estimated from ICES data) are predicted more accurately by the two-equation model. The one-equation model reproduces the mean frontal locations to 18.1 km (<3 grid spacings) and the two-equation model to 17.1 km; although in the Celtic Sea the accuracy is ∼33 and ∼12 km, respectively. Comparison with historical tide gauges, current metres, CTD stations, and thermistor chain data from the North Sea Project all show an improvement in accuracy when the two-equation model is used. This is particularly apparent in the model's ability to reproduce the spring–neap variability during stratification. We find that in the presence of shear-induced convection the routinely applied clipping of the turbulent length-scale, previously thought to be a minor ingredient in a turbulence model, has a dramatic effect on the results: if the length-scale clipping is not applied, substantial over-mixing is observed to occur. The causes and possible remedies of this effect are investigated. Overall our results demonstrate a sensitivity to the details of the turbulence model that is significantly greater than previously thought.     

Eurasian Arctic shelf hydrography: Exchange and residence time of southern Laptev Sea waters

The hydrography of the Laptev Sea is significantly influenced by river water and sea-ice processes, which are highly variable over the annual cycle. Despite of an estuarine structure the inner and outer shelf regions are decoupled at times as documented by the stability of a warm intermediate layer formed during summer below the Lena River plume. We demonstrate that a remnant of this warm layer is preserved below the fast ice until the end of winter, while only slightly farther to the north, offshore of the landfast ice in the polynya region, the pycnocline is eroded and no signature of this layer is found. The warm intermediate layer (WIL) formed during summer can be used as tracer for Laptev Sea shelf waters throughout the winter. Thereby, residence times of southern Laptev Sea waters can be estimated to be at least from summer to the end of winter/spring of the following year.