Bacterial pathogens in coastal sediments may pose a health risk to users of beaches. Although recent work shows that beach sands harbor both indicator bacteria and potential pathogens, it is not known how deep within beach sands the organisms may persist nor if they may be exposed during natural physical processes. In this study, sand cores of approximately 1 m depth were collected at three sites across the beach face in Kitty Hawk, North Carolina before, during and after large waves from an offshore hurricane. The presence of DNA from the fecal indicator bacterium Enterococci was detected in subsamples at different depths within the cores by PCR amplification. Erosion and accretion of beach sand at the three sites also was determined for each sampling day. The results indicate that ocean beach sands with persisting enterococci signals could be exposed and redistributed when wind, waves, and currents cause beach erosion or accretion.
Field observations of flow and sediment transport in a tributary channel through intertidal mudflats indicate that suspended sediment was closely linked to advection and dispersion of a tidal salinity front. During calm weather when tidal forcing was dominant, high concentrations of suspended sediment advected up the mudflat channel in the narrow region between salty water from San Francisco Bay and much fresher runoff from the small local watershed. Salinity and suspended sediment dispersed at similar rates through each tidal inundation, such that during receding ebbs the sediment pulse had spread spatially and maximum concentrations had decreased. Net sediment transport was moderately onshore during the calm weather, as asymmetries in stratification due to tidal straining of the salinity front enhanced deposition, particularly during weaker neap tidal forcing. Sediment transport by tidal forcing was periodically altered by winter storms. During storms, strong winds from the south generated wind waves and temporarily increased suspended sediment concentrations. Increased discharge down the tributary channels due to precipitation had more lasting impact on sediment transport, supplying both buoyancy and fine sediment to the system. Net sediment transport depended on the balance between calm weather tidal forcing and perturbations by episodic storms. Net transport in the tributary channel was generally off-shore during storms and during calm weather spring tides, and on-shore during calm weather neap tides.
Here we present observations of the hydrography of the Patagonian Shelf, shelf break and offshore waters, with reference to the environmental conditions present during the period of peak coccolithophore abundance. Analysis of a hydrographic dataset collected in December 2008 (austral spring/summer), as part of the Coccolithophores of the Patagonian Shelf (COPAS) research cruise, identified 5 distinct surface water masses in the region between 37°S and 55°S. These water masses, identified through salinity gradients, displayed varying mixed layer depths, macronutrient inventories and chlorophyll-a fluorescence. Subantarctic Shelf Water (SSW), located to the north of the Falkland Islands and extending north along the shelf break, was also host to a large coccolithophore bloom. The similarities between the distribution of calcite, as seen in remote sensing data, and SSW indicate that the coccolithophore bloom encountered conditions conducive to bloom development within this water mass. Analysis of chemical and environmental data also collected during the COPAS cruise revealed that many of the commonly cited conditions for coccolithophore bloom development were present within SSW (e.g. low N:P ratio, high N:Si ratio, shallow mixed layer depth). In the other water masses present on the Patagonian Shelf greater variability in these same parameters may explain the more diffuse concentration of calcite, and the smaller size of possible coccolithophore blooms. The distribution of SSW is strongly influenced by the latitudinal variation in shelf break frontal width, which varies from 20 to 200 km, and consequently strong hydrographic controls underlie the position of the coccolithophore bloom during austral summer.
Intertidal marine systems are highly dynamic systems which are characterized by periodic fluctuations in environmental parameters. Microbial processes play critical roles in the remineralization of nutrients and primary production in intertidal systems. Many of the geochemical and biological processes which are mediated by microorganisms occur within microenvironments which can be measured over micrometer spatial scales. These processes are localized by cells within a matrix of extracellular polymeric secretions (EPS), collectively called a “microbial biofilm”. Recent examinations of intertidal systems by a range of investigators using new approaches show an abundance of biofilm communities. The purpose of this overview is to examine recent information concerning the roles of microbial biofilms in intertidal systems. The microbial biofilm is a common adaptation of natural bacteria and other microorganisms. In the fluctuating environments of intertidal systems, biofilms form protective microenvironments and may structure a range of microbial processes. The EPS matrix of biofilm forms sticky coatings on individual sediment particles and detrital surfaces, which act as a stabilizing anchor to buffer cells and their extracellular processes during the frequent physical stresses (e.g., changes in salinity and temperature, UV irradiation, dessication). EPS is an operational definition designed to encompass a range of large microbially-secreted molecules having widely varying physical and chemical properties, and a range of biological roles. Examinations of EPS using Raman and Fourier-transform infared spectroscopy, and atomic-force microscopy suggest that some EPS gels possess physical and chemical properties which may hasten the development of sharp geochemical gradients, and contribute a protective effect to cells. Biofilm polymers act as a sorptive sponge which binds and concentrates organic molecules and ions close to cells. Concurrently, the EPS appear to localize extracellular enzyme activities of bacteria, and hence contribute to the efficient biomineralization of organics. At larger spatial scales, the copious secretion of specific types of EPS by diatoms on the surfaces of intertidal mudflats may stabilize sediments against resuspension. Biofilms exert important roles in environmental- and public health processes occurring within intertidal systems. The sorptive properties of EPS effectively chelate toxic metals and other contaminants, which then act as an efficient trophic-transfer vehicle for the entry of contaminants into food webs. In the water column, biofilm microenvironments in suspended flocs may form a stabilizing refugia that enhances the survival and propagation of pathogenic (i.e., disease-causing) bacteria entering coastal waters from terrestrial and freshwater sources. The EPS matrix affords microbial cells a tremendous potential for resiliency during periods of stress, and may enhance the overall physiological activities of bacteria. It is emphasized here that the influences of small-scale microbial biofilms must be addressed in understanding larger-scale processes within intertidal systems.
A secondary depression crossing southern Wales and England on the 13 December 1981 resulted in the highest water levels experienced in the Bristol Channel this century and severe flooding along the north Somerset coast. Both a numerical model-based prediction scheme and theLennon (1963,Quarterly Journal of the Royal Meteorological Society,89, 381–394) criteria for west coast storm surges failed to provide adequate warning of the expected levels. A numerical reconstruction of the event shows that the failure of the surge model forecast was due to an incorrect prediction by the atmospheric model used to provide the meteorological input to the sea model. The fallibility of the Lennon criteria suggests their reappraisal, particularly in the light of a subsequent failure in November 1984. Some of the difficulties in the identification of storm surge residuals within the Bristol Channel are shown to be associated with tidal measurement and prediction problems which are, as yet, unresolved.
Data on the temporal and spatial variations of 99Tc and 137Cs in UK coastal waters are provided from surveys carried out over the period 1993–1998; prior to and post the discharge of elevated quantities of 99Tc from the nuclear fuel reprocessing plant at Sellafield (Cumbria, UK). 99Tc seawater concentrations in the Irish Sea increased by more than an order of magnitude within the study period concomitant with the increase in discharges. 137Cs discharges were relatively low (∼0.3% of their peak values in the 1970s). Simplistically, the 137Cs distribution reflected varying hydrographic conditions superimposed upon the residual concentrations of a system approaching steady state. Differences between the 137Cs/99Tc ratio in Sellafield discharges and seawater indicate that 137Cs remobilisation, from sediments contaminated by large discharges in the 1970s, is presently the predominant (∼90%) source term to the water column. The 137Cs/99Tc ratio in the Irish Sea decreased significantly within the period of these surveys (e.g. reduced from ∼14 in 1993 to 0.4 in 1996, within the vicinity of Sellafield). However, it is difficult to extrapolate this information to assess the contribution from Sellafield in distant waters because the low levels of 137Cs are continuously perturbed by additional inputs during transit (e.g. remobilisation from sediments outside the Irish Sea). Examination of 99Tc data for the North Sea indicates that the leading edge of the first EARP pulse, entering via the Scottish coastal current, may have migrated to the limit of the current flowing south along the British coastal margin within 9 months. The estimated transfer factor (TF) for Sellafield discharges in this current is 52 Bq m−3/PBq annum−1 with transit times from Sellafield to the Pentland Firth and Lowestoft of ∼9 and ∼24 months, respectively. These transit times, derived from the 99Tc data, are significantly shorter than previous estimates based on 137Cs and 90Sr data from the 1970s and 1980s.
Vertical profiles of and have been collected at micro-, meso- and macrotidal mudflats in order to calculate present accumulation rates. Sediment cores were taken at the mudflat at Kongsmark in the microtidal Lister Dyb tidal area (Denmark), at the BOA bridge in the mesotidal Dollard estuary (Netherlands), and at the Skeffling mudflat in the macrotidal Humber estuary (UK). Except for the Kongsmark site and site A in the Humber estuary, no accurate calculation of accumulation rates was possible for the investigated sites. The accumulation rate obtained for the Kongsmark site is confirmed by other independent data and shows that the accumulation is not supply limited at this site at present. For site A in the Humber estuary a modified CIC method was applied to the -profile and the result agrees with results from the nearby Welwick Marsh. The salt marsh at Skeffling is advancing out onto the fringing mudflat with accumulation at the innermost part of the mudflat but with erosion taking place further offshore. The reasons for the generally poor dating results seems to be either erosion (two Humber sites) or postdepositional reworking of the deposited material (bioturbation and continued resuspension) and the data set suggests that the mixing depth may increase with tidal range and thereby hydrodynamic forcing.
The deposition record of 137Cs was traced in the SE Black Sea sediments adjacent to the Coruh river mouth in comparison with the earlier studied chronology of 137Cs deposition in front of the Danube delta (NW Black Sea). In both cases, the 137Cs profiles showed two subsurface peaks attributable to maximum fallout of ‘bomb’ and Chernobyl radionuclides. The Coruh profile revealed a larger contribution of ‘bomb’ 137Cs in comparison with the Chernobyl input, suggesting different coverage of NW and SE Black Sea regions with the Chernobyl fallout. The 137Cs-derived dating showed that maximum deposition of particulate bound 137Cs in sediments adjacent to the Coruh river mouth was delayed for ∼14 yr relative to date of Chernobyl accident, reflecting a buffer effect of the watershed soils. This transit time is 3 times longer than in the Danube catchment area, indicating a difference in retention processes in these mountainous (Coruh) and lowland (Danube) river basins. The 137Cs profile in Coruh sediments showed penetration of 137Cs to much greater depth than would be expected from 137Cs fallout chronology, suggesting the sediment mixing rate of 1.3 cm2 yr−1. This value was used to evaluate deposition chronology of 137Cs, applying the model developed for pulse fallout case. Comparing the measured and modelled data has allowed differentiation of the flood-induced discharge of the 137Cs-containing suspended matter and the slower transit of eroded soil particles from the contaminated catchment areas. The obtained results may be used for the prediction of period when the pollutants, deposited over the river basins, can reach the Black Sea.
Dating techniques based on the natural and anthropogenic radionuclides 210Pb and 137Cs were applied to the study of the sedimentation regime in Cienfuegos Bay, Cuba. Core samples were collected from different locations in the bay and analysed to evalute the accumulation rate. Results evidenced significant changes in the sedimentation rate during the last 40 years: the recent sediment accumulation rates (0.47–0.50 g cm−2 yr−1) in the Northen basin are almost double those estimated before 1965 (0.30 g cm−2 yr−1). The 210Pb profiles show significant deviations from a simple exponential decline and abrupt variations between 1966 and 1970. These irregularities match closely periods of changes in land use (intense de-forestation and regimentation of the Arimao and Caonao rivers in the late 1960s and early 1970s) and exceptional natural events (Hurricane “Camille” in 1969 and the intense rainfall of 1988) which occurred in Cienfuegos. The 137Cs and chlorite minerals profiles validate the results obtained from 210Pb dating and confirm the effect of exceptional events and changes in the natural hydrological regime of the bay during the past 40 years.
The net storage of the radionuclides 238Pu (2.8 TBq), 239, 240Pu (75 TBq), 137Cs (730 TBq), 241Am (40 TBq) and 210Pbxs (2700 TBq) in North Sea sediments has been estimated from their concentrations in 40 boxcores. From comparison of 238Pu/239, 240Pu ratios between those found in the sediments and those in atmospheric fallout and discharges from the nuclear fuel reprocessing plants at Sellafield (UK) and la Hague (France), it is concluded that the latter is the main supplier of plutonium to the Southern Bight, while the former is dominant in the remainder of the North Sea. About 7% of all Sellafield plutonium discharge is stored in North Sea sediments. The German Bight receives and stores plutonium and fine particulate matter mainly from the Elbe estuary. Of all 137Cs transported through the North Sea, about 2% is stored in the sediment. 241Am is scavenged faster than plutonium and is probably transported from Sellafield to the North Sea as 241Pu.
A model for radionuclide transport in the marine environment is described and applied, in the Irish Sea, to the movement of caesium (137Cs) and plutonium (239/240Pu) discharged from Sellafield. The model resolves processes operating on tidal time scales, whilst being simple enough to run over decadal time periods. Modelled processes include transport in the water column, exchange of contaminants between dissolved and particulate phases, wave–current sediment resuspension, sediment transport and mixing of material within the seabed. Transport in the water column is by a combination of tide, wind and density-driven flows. Sediment transport is based on the erosion, advection and deposition of three sediment classes representing sand, flocculated mud/silt and fine background components. Transfer of radionuclides between the dissolved and sediment phases is implemented using rate equations. A layered seabed is incorporated with transfers between layers representing biological and physical mixing processes. We demonstrate that the model is able to reproduce observed concentrations of 137Cs and 239/240Pu in the water column, the build-up of 239/240Pu in the seabed over a 30-year time span, including recent changes in seabed inventory distributions. The model is used to identify and quantify the mechanisms responsible for this recent redistribution of 239/240Pu in the region. The results suggest that whilst sediment transport plays a role in redistributing contaminated sediment in the eastern Irish Sea, desorption, followed by transport in the dissolved phase and reabsorption onto particulate material, is the primary mechanism for redistribution of 239/240Pu.
The Camamu Bay (CMB) is located on the narrowest shelf along the South American coastline and close to the formation of two major Western Boundary Currents (WBC), the Brazil/North Brazil Current (BC/NBC). These WBC flow close to the shelf break/slope region and are expected to interact with the shelf currents due to the narrowness of the shelf. The shelf circulation is investigated in terms of current variability based on an original data set covering the 2002–2003 austral summer and the 2003 austral autumn. The Results show that the currents at the shelf are mainly wind driven, experiencing a complete reversal between seasons due to a similar change in the wind field. Currents at the inner-shelf have a polarized nature, with the alongshore velocity mostly driven by forcings at the sub-inertial frequency band and the cross-shore velocity mainly supra-inertially forced, with the tidal currents playing an important role at this direction. The contribution of the forcing mechanisms at the mid-shelf changes between seasons. During the summer, forcings in the two frequency bands are important to drive the currents with a similar contribution of the tidal currents. On the other hand, during the autumn season, the alongshore velocity is mostly driven by sub-inertial forcings and tidally driven currents still remain important in both directions. Moreover, during the autumn when the stratification is weaker, the response of the shelf currents to the wind forcing presents a barotropic signature. The meso-scale processes related to the WBC flowing at the shelf/slope region also affect the circulation within the shelf, which contribute to cause significant current reversals during the autumn season. Currents at the shelf-estuary connection are clearly supra-inertially forced with the tidal currents playing a key role in the generation of the along-channel velocities. The sub-inertial forcings at this location act mainly to drive the weak ebb currents which were highly correlated with both local and remote wind forcing during the summer season.
We evaluated changes in siliceous export production and the source of organic matter preserved in sediment core MD07-3109H recovered from the Gulf of Ancud, Chiloé Inner Sea (42°S, 72°W, water column depth: 328 m), southern Chile. We analyzed the abundance of siliceous microfossils (diatoms, silicoflagellates, sponge spicules, Chrysophyte cysts, phytoliths), geochemical proxies (weight percent silicon %SiOPAL, organic carbon, total nitrogen, C/N molar), and sediment stable isotopes (δ13Corg, δ15N). Chronology based on 210Pb and 14C provided an accumulated age of 144 years at the base of the core.Sediments of core MD07-3109H are predominantly marine in origin, averaging δ13Corg=–20.75‰±0.82, δ15N=8.7±0.35‰, and C/N=8.76±0.36. Marine diatoms compose 94% of the total assemblage of siliceous microfossils. Our record of productivity based on the mass accumulation rates of organic carbon, total nitrogen, SiOPAL, and total diatoms showed high values between 1863 and 1869 AD followed by a declining trend until 1921 AD, a transition period from 1921 to 1959 AD with fluctuating values, and a clear decreasing pattern from 1960 AD to the present. This marked reduction in productivity was associated with decreased precipitation and Puelo River streamflow (41°S), as well as a warmer and more stratified water column, especially since the 1980s.
We have evaluated the feasibility of using the terminal restriction fragment length polymorphism (T-RFLP) pattern of polymerase chain reaction (PCR) amplified 16S rRNA sequences to track the changes of the free-living bacterial community for the Pearl River Estuary surface waters. The suitability of specific PCR primers, PCR bias induced by thermal cycles, and field-sampling volumes were critically evaluated in laboratory tests. We established a workable protocol and obtained TRF patterns that reflected the changes in the bacterial population. The temporal dynamics over a 24 h period were examined at one anchored station, as well as the spatial distribution pattern of the bacterial community at several stations, covering the transects along the river discharge direction and across the river plume. The TRF pattern revealed 9 dominant bacterial groups. Changes in their relative abundance reflecting the changes in the bacterial community composition were documented. Many culturable species were isolated from each field sample and a portion of the 16S rRNA gene for each species was sequenced. The species was identified based on sequence data comparison. In this region, the dominant species belong to the γ-subdivision of proteobacteria and the Bacillus/Clostridium group of Firmicutes. We also detected the wide spread distribution of Acinetobacter spp.; many of these species are known nosocomial pathogen for humans.
Part of the Northeast Atlantic mackerel population migrates towards the southern spawning area (Cantábrian Sea) at the end of winter. In this seasonal handline fishery targeting mackerel, the most important in the study area that targets this species, the timing of the peak of catches has shifted forward (later) in recent years. This paper presents results pointing to the likelihood that this shift is due to a change in the timing of the spawning migration to the southern area of the Northeast Atlantic mackerel population. Three types of fleet have been identified within this fishery, and in all of them there is a forward shift in time in effort exerted. Moreover, a new model has been defined for the standardization of catch per unit effort (CPUE). The fishing season appears to have shifted forward by 29 days between 2000 and 2006. Nevertheless, changes have been detected neither in the exploitation pattern nor in the duration of the fishing season during the period studied. A shift on this scale has important consequences for the management of the resource, the fleets that exploit it and the resource assessment survey designs that will have to be adapted to this new scenario.
Kraternaya Bight (Yankich Island, the Kuril Islands) is a submerged crater of the active Ushishir Volcano characterized by terrestrial and underwater gasohydrothermal activity. The gasohydrothermal activity of the volcano produces a great influence on the physico-chemical characteristics of water column, as well as planktonic and bottom communities of marine organisms of the bight. The bivalve mollusks Macoma golikovi and Macoma calcarea are dominant macrobenthos species of Kraternaya Bight, which inhabit all the zones of underwater volcanic activity. Macoma golikovi occurs at a depth of 1–45 m. This species forms the densest aggregations (with an average biomass of 500–800 g m−2 and a population density of 500–700 ind. m−2) at a depth of 3–15 m, on steep underwater crater slopes (40–45°), on slightly silted gravel. The majority of gasohydrothermal vents are located at that depth. Macoma calcarea occurs at a depth of 5–55 m. In contrast to Macoma golikovi, this species is dominant at a depth of 25–40 m, on gentle slopes of the crater (to 20°), on silt and silty sand. That depth is less subjected to the influence of volcanic products. Its average biomass is 450–750 g m−2 with a population density of 200–450 ind. m−2 there. It is assumed that the differences in the distribution are due the fact that Macoma golikovi is mainly a suspension-feeder in Kraternaya Bight, and Macoma calcarea is a deposit-feeder. The studies conducted between 1987 and 1997 on the quantitative distribution of Macoma golikovi and Macoma calcarea in four areas of the bight, which had different intensity, temperature and chemical composition of gasohydrothermal vents and volcanic water seeping showed that both species formed settlements with lesser density in the area of the gasohydrothermal vents containing hydrogen sulfide. Beyond the area of volcanic activity, these species were most abundant. In 1993 and 1995, the mortality of the populations of the bivalve mollusks was observed in the areas of hydrogen sulfide gasohydrothermal vents and volcanic water seeping. In other areas of the bight, their abundance decreased sharply suggesting a relationship to the gasohydrothermal activity of the volcano in those years. Circumstantial evidence for this suggestion was that in 1993, the maximal concentrations of Fe, Zn, Cd, and Mn were observed in the tissues of both species. Macoma golikovi accumulated 5–10 times more Zn than Macoma calcarea. The concentration of Zn in the tissues of Macoma golikovi reached 2000 μg g−1.
A hand-operated, rotating-scoop sediment sampler (HORS) has been developed for coarse-grained, shallow-marine sediment. Design of the 10 kg HORS was inspired by the 60 kg, winch-operated Shipek sampler which also has a 180° rotating scoop. At one-sixth of the Shipek mass, the HORS typically collects about one-sixth as much sediment. The HORS is recommended only if a winch is unavailable for a Shipek grab sampler.Grab samplers without a 180° rotating scoop rely on one or two pairs of jaws which close toward the center along arcs of 90°. These center-closing devices commonly fail to sample coarse sediment because a large grain, e.g. a shell fragment, impedes closure and allows the sample to escape during ascent. Center-closing jaws which meet rather than overlap eventually fail to meet completely because of wear along the jaw edge or misalignment. All grab samplers, including rotating-scoop types, may induce some loss of the finest fraction and so are best reserved for reconnaissance work. The sampler described herein has facilitated reconnaissance of the modern equivalent of oolitic ironstone in the southeastern Caribbean.
A recently extended and spatially rich English Channel sea level dataset has been used to evaluate changes in extreme still water levels throughout the 20th century. Sea level records from 18 tide gauges have been rigorously checked for errors and split into mean sea level, tidal and non-tidal components. These components and the interaction between surge and tide have been analysed separately for significant trends before determining changes in extreme sea level. Mean sea level is rising at 0.8–2.3 mm/year, depending on location. There is a small increase (0.1–0.3 mm/year) in the annual mean high water of astronomical tidal origin, relative to mean sea level, and an increase (0.2–0.6 mm/year) in annual mean tidal range. There is considerable intra- and inter-decadal variability in surge intensity with the strongest intensity in the late 1950s. Storm surges show a statistically significant weak negative correlation to the winter North Atlantic Oscillation index throughout the Channel and a stronger significant positive correlation at the boundary with the southern North Sea. Tide–surge interactions increase eastward along the English Channel, but no significant long-term changes in the distribution of tide–surge interaction are evident. In conclusion, extreme sea levels increased at all of the 18 sites, but at rates not statistically different from that observed in mean sea level.
This paper presents an overview of the significant research on morphodynamics and sediment dynamics on intertidal mudflats in China (1961–1994), particularly in the past 15 years (1980–1994). Development of intertidal mudflats has long been regarded as the response of the intertidal profile to tides, waves and storms. It has been found that there were long-term and short-term cyclic developments of intertidal mudflats in China. Three sedimentological zones have generally been identified from land to sea within the intertidal zone: high mudflat, middle mudflat and low mudflat. In addition, the sediments in the middle mudflat are relatively coarser than those in the high mudflat and low mudflat. Storms have great impacts on the intertidal morphology, sediment textures and sedimentary structures. Based on field investigations of intertidal sedimentary processes, many researchers have found that “settling and scour lags” were only applicable to intertidal cohesive sediment transport during periods of weak waves, but not during storms. In fact, flood fronts, waves, storm surges and longshore drift play important roles in suspended sediment transport on open intertidal mudflats in China. Despite of these extensive studies in the past several decades, there is still a need for an improved understanding of fundamental physical and biological processes governing erosion and deposition of cohesive fine sediment within the intertidal zone in China.
Monthly time series of nutrient pools and transport in the Baltic Entrance were computed for the period 1974–1999 from a combination of hydrodynamic model results and observational data obtained in national monitoring programmes. Significant seasonal variations in both nutrient pools and transport were found. For example, inflowing dense nutrient-rich Skagerrak water causes net transport of dissolved inorganic phosphorus (DIP) and inorganic nitrogen (DIN) towards the Baltic during summer where surface concentrations are low. Surface-water concentrations of DIN and DIP increase during winter and the net transport may reverse. The computations indicated that the pools of dissolved inorganic phosphorus and total phosphorus (TP) were larger in the 1980s than in previous and following decades. Variations in pools of dissolved inorganic nitrogen (DIN) and total nitrogen (TN) were more irregular throughout the period. Computed nutrient transports likewise exhibited long-term variations. Net transport of DIN and DIP was southward, from the Skagerrak to the Kattegat and further into the Belt Sea, during the 1970s. In the same period, the transport of DIN and DIP from the Baltic Sea to the Baltic entrance was below average. In both the Kattegat and the Belt Sea transports reversed during the 1980s. In the 1990s the southward transport of DIP was restored, but the transport of DIN was not restored to the same extent. Between the Kattegat and the Skagerrak the transport diminished while the Belt Sea continued to export DIN to the Kattegat. Northward transports of TN and TP persisted throughout the period. A notable maximum TP transport was found to have taken place during the 1980s.
The subinertial circulation within 2 to 12 km of the Long Island coast is described from a set of observations of the velocity and density fields during August 1978. The flow was characteristically to the southwest, increasing offshore and decreasing with depth. This southwest flow was driven by an externally imposed shoreward rising sea level which was modified at the coast by local wind-driven divergences and convergences. Upwelling and downwelling circulations maintained isopycnal tilts which in turn geostrophically sheared the flow. Nearly complete compensation was observed during steady periods, reducing the near-bottom flow to within ∼80% of its barotropic magnitude. This decreased the effectiveness of bottom stress on the subinertial flow. Even with fairly uniform alongshore bathymetry, the flow was substantially not two-dimensional, because of bathymetric divergences, bottom slope interaction, and bottom stress variations. These effects, in addition to the greater responsiveness of the shallower water columns, tend to define a nearshore regime partially uncoupled from a midshelf regime. The boundary in this case was often observed between 25- and 30-m contours (4 to 8 km offshore).
Currents, coastal winds, coastal sea level, and coastal ocean temperatures were observed at a number of northern Baja California Pacific sites between October 1978 and October 1979. Coastal winds were weakly southward on average and fluctuated north-south throughout the year. Fluctuations were uncorrelated over alongshore separations of 200 km. Coastal winds differed considerably from the large-scale offshore wind estimate (Bakun's Index) both in strength and in variability. At 30°50′N the mean currents were weakly equatorward at 25 m depth and weakly poleward at 42 and 60 m in a water depth of 75 m. The seasonal mean flow was equatorward from October to April and poleward from April to October at the shallower level but poleward all year near bottom. The fluctuations on a time scale of days to weeks were about an order of magnitude stronger than the seasonal variation, were oriented predominantly alongshore, and were quasi-barotropic in nature. Empirical orthogonal analyses showed that almost 90% of the variability could be represented by a single empirical mode. The alongshore fluctuations were significantly correlated with coastal sea level from October to July but during the rest of the year correlation was insignificant, possibly as a result of offshore eddies approaching the shelf.
Based on two years (July 1978–June 1980) of current meter array measurements and bimonthly STD/XBT transects over the continental margin off central California, the mean California Undercurrent has a jet-like core in excess of 15 cm s−1 which is generally confined to the upper 300 m of the water column and to within 30 km of the coast. A comparison of measured current velocities with relative geostrophic velocities suggests, on average, a poleward flow of 10 cm s−1 at the reference level, 450 db. The variability is predominantly annual with a maximum poleward flow in May–June. Equatorward flow was suprisingly weak and infrequent in the transect studied. The alongshore flow has characteristic fluctuations of 15 cm s−1 on time scales of a few days to weeks. The strongest and most coherent element of variation is the “spring transition”, which had different manifestations in the two years observed, perhaps due to differences in the combination of local and remote forcing. The spring transition is characterized by an abrupt decrease in subsurface temperature which is well-correlated with a coastal sea surface temperature (SST) decrease, an upwelling index increase, and a transient reduction or reversal of the poleward flow. The subsurface current and temperature fluctuations are associated with coastal SST and wind fluctuations, and with satellite infra-red imagery patterns and changes.
Temperatures recorded over the Georgia shelf and at near-bottom locations on the slope are compared with records of thermocline elevation from inverted echo sounders on the slope and with shore wind observations, for the period January to April 1978.Both upwelling and downwelling events are observed. Upwelling events are associated with upwelling-favorable wind and/or Gulf stream frontal eddies. Strong upwellings are accompanied by risings of the thermocline over the slope, but some weaker eddies occur without noticeable thermocline disturbance. Downwelling events are associated with onshore meander crests of the Gulf Stream, and sometimes with downwelling-favorable wind. The influence of downwelling does not extend seaward of the upper slope.A long-term (1 month) variation of the thermal structure is observed at the shelf break and under the Gulf Stream. It is related to a filament of cold water on the slope, varying in path and size. This cold filament appears to be Antarctic Intermediate Water which is variably advected through the Straits of Florida. Theory suggest that the variability may be caused by changes in the thermocline depth and tilt across the Gulf Stream in response to transport fluctuations.
Previous work concerning Gulf Stream warm-core rings (WCRs) and their associated shelf water entrainments have been based upon single surveys or time series from individual WCRs. To date, estimates of annual shelf water volume entrained into the Slope Sea by WCRs and its interannual variability have not been made. Using a long time series of satellite-derived sea surface temperature (SST) observations of Slope Sea WCRs, we have completed an analysis of 22 years of WCR data (1978–1999) between 75°W and 50°W to understand the interannual variability of WCRs and their role in entraining shelf water. Satellite-derived SST data digitized at Bedford Institute of Oceanography are analyzed using an ellipse-fitting feature model to determine key WCR characteristics including WCR center position, radius and orientation. Key characteristics are then used to compute WCR swirl velocity by finite-differencing WCR orientations (θ) obtained from the feature model time series. Global mean WCR-edge swirl velocity calculated from all observations is 105.72±10.7 km day−1 (122.36±12.4 cm s−1), and global mean WCR radius is 64.8±6.2 km. Primary and derived WCR data are incorporated into a two-dimensional ring entrainment model (RM) using the quasi-geostrophic approximation of the potential vorticity equation. The RM defines ambient water as entrained by a WCR only if the gradient of relative vorticity term (horizontal shear) dominates the potential vorticity. Proximity of a WCR to the position of the shelf-slope front (SSF) is then used to determine whether the ambient water is entrained from the outer continental shelf. WCR-induced shelf entrainment derived from the RM displays considerable spatial variability, with maximum entrainment occurring offshore of Georges Bank, advecting a mean total annual shelf water volume of 7500 km3 year−1 from the region. Estimates of shelf water fluxes display significant interannual variability, which may be in part due to the observed covariance between WCR occurrences and the state of the North Atlantic Oscillation (NAO). Increased (decreased) occurrences of WCRs are evidenced during positive (negative) phases of the NAO. The total mean annual shelf-wide WCR-induced shelf water transport is estimated to be 23,700 km3 year−1 (0.75 Sv), accounting for nearly 25% of the total transport in the Slope Sea region neighboring the outer continental shelf.
A continuous 11-year time-series of aerial and shipboard survey data in the Great South Channel region reveals a remarkably consistent pattern of right whale distribution, with several interesting variations. Right whales occur in the area during the spring, with a distinct peak in May. Arrival and departure dates appear to vary by a month or more between years. The distribution is concentrated in the deeper basin north of the V-shaped 100-m isobath and a thermal front which approximately parallels the isobath. Within a single year, this concentration tends to be within a relatively small area and may remain stable for days or weeks. The center of distribution changes between years, and in 1984, 1987 and 1989 was on the eastern side of the basin rather than the “typical” western side. At any one time, nearly all of the right whales may aggregate into one to several clusters of <10 km2 each. A gradual southward shift in the center of distribution can be observed as the season progresses. Single-day abundance estimates, uncorrected for animals missed while submerged, ranged up to 179 animals. There was a statistically significant long-term increase in right whale sighting rates in the region, after correcting for estimated increases in survey efficiency, of 3.8% per year. This suggests that the population might be recovering, albeit slowly, from extreme depletion. It is likely that a significant proportion of the western North Atlantic right whale population utilizes the Great South Channel region as a feeding area each spring, aggregating to exploit exceptionally dense copepod patches.
The distribution of chlorophyll on a transect of the English Channel was measured during 1980 and 1981. In both years, high concentrations of chlorophyll a were measured in midchannel in July and August and this was due to a bloom of Gyrodinium aureolum. At a near-shore station close to Plymouth, regular measurements of water transparency and primary production were made during 1981. Values of diffuse attenuation coefficient increased in the spring with increasing chlorophyll concentration; this was followed by a period of low attenuation coefficients when chlorophyll maxima developed on the thermocline. The attenuation coefficient was greatly increased in late summer as the result of a bloom of G. aureolum. The high cell density resulted in self-limitation and specific rates of photosynthetic carbon fixation were low during the bloom. The total water-column light utilization index (Ψ) is calculated to be 0.48 g C g Chl a−1 E−1 m−2 and the possible use of this index to calculate production from depth-integrated chlorophyll a concentrations is discussed.
When a relatively warm year (1981) in the southeastern Bering Sea is compared with a cooler year (1980), the upper layer of both the middle shelf and outer shelf warmed at a faster rate in the warmer year, but the spring bloom of phytoplankton took place at approximately the same time both years. The middle front near the 100 m isobath separated the two major communities of zooplankton both years. Offshore of the front, large calanoid copepods such as Neocalanus plumchrus, Neocalanus cristatus, Eucalanus bungii, and Metridia pacifica dominated, while inshore of that front Pseudocalanus spp., Acartia spp., and Calanus marshallae dominated. Over the outer shelf (Sta. 5) at the end of April, N. plumchrus, N. cristatus, E. bungii, and Pseudocalanus spp. were significantly more abundant in 1980 than in 1981, while over the middle shelf (Sta. 12) all stages of Pseudocalanus spp. and C. marshallae were more abundant in 1981 than in 1980. Gradients in abundance of N. plumchrus and N. cristatus across the outer shelf reversed after periods of wind favourable to subsurface onshore flow suggesting that the shelf population of large calanoids is derived from overwintering slope populations during spring and summer storms. N. plumchrus and C. marshallae, which were thought to reproduce once per year based on 1980 data, produced two cohorts at some stations in 1981, and both E. bungii and C. marshallae reproduced earlier in 1981 than in 1980. Survivorship of N. plumchrus and N. cristatus over the outer shelf (Sta. 5), where only one cohort was produced both years, was higher in 1981 than in 1980, suggesting that predation occurred later in 1981 than in 1980. The abundance of the chaetognath Sagitta elegans over the middle shelf was higher in 1980 than in 1981 suggesting that both increased temperature and reduced predation may account for the larger numbers of small copepods found over the middle shelf in the warmer year.
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.
Thirteen years of weekly sea surface temperature (SST) fields derived from satellite data (January 1982–December 1994) are used to investigate spatial and temporal variabilities of SST in the western region of the South Atlantic Ocean. The analysis is focused on the large-scale variations over the continental shelf and slope region with depths less than 1000 m, between 42 and 22°S.Within the 13-year span, a total of 13 cold and seven warm SST anomalies are observed during and right after ENSO onset. Northward-propagating cold and warm anomalies have average speeds of the order of 18 and 14 cm/s, respectively. Southward-advected cold and warm anomalies, on the other hand, have slower speeds. From a principal component analysis, the first two modes for the inner along-shelf transect, which account for more than 65% of the total non-seasonal variance, suggest a tendency for cold SST anomalies (SSTAs) to be related to ENSO events. Possible connections between SSTAs and the Southern Oscillation Index are also explored. Consideration of the SSTA analyses seem to confirm the hypothesis that the anomalous cold (warm) water extensions to the north on the shelf occur generally in every warm (cold) ENSO+1 year in the present climatology.
Extensive sampling in the Lagoon and Gulf of Venice during the 1980s provided material to establish ‘base’ conditions for subsequent studies of distributions of benthic foraminiferal biotopes and sediments.In 1983, the marine area around the Lido Inlet was characterised by two predominant sedimentary provinces: (a) Sandy Marine Province, along the shore, of very fine sand (3.25–3.50Φ) and (b) Residual Sand Marine Province, along the beach area south of the inlet, of fine sand (2.00–2.75Φ). Similar Residual Sand Lagoon (2.00–2.75Φ) and Sandy Lagoon Provinces (3.50–3.75Φ) predominated near the inlet within the Lagoon. Within these different sediment provinces, the foraminiferal assemblages were essentially uniform in the marine and lagoon area around the Lido Inlet.In 2006, sampling was repeated at 23 of the 1983 sites using the same methodology, and identical laboratory processing. Whilst the foraminiferal assemblages of the two sampling periods are very similar, the sediment grain size data differ with the 2006 samples having an overall decrease in the predominant sand sizes. Thus the primary mode in the 1983 samples ranges from 2.00 to 4.50Φ, whereas for the 2006 samples the primary mode is always between 3.50 and 3.75Φ.The high similarity in the foraminiferal assemblages of the two sampling periods strongly suggests that the physical and chemical characteristics of the water masses, within the sampled area, have not greatly changed during the intervening 23 years.The high level of grain sorting and the presence of a very restricted grain size in the 2006 samples (between 3 and 4Φ) suggest, for the bottom sediment at the Lido Inlet, an origin due to human intervention, which has been particularly extensive since 2003.
Knowledge of long-term movements of water-masses in the English Channel has been substantially improved using hydrodynamic modelling coupled with radio-tracers studies; nevertheless, the precision of results so obtained is still largely dependent on measurement precision. New tools are now available to make more accurate determinations of radio-tracer distribution: (1) Repositioning of station locations at the same tide reference-time, giving a homogeneous spatial data set, coupled with the possibility of interpolating and quantifying the amounts of dissolved radioactivity flowing through the English Channel; (2) the first measurements of tritium () in seawater on a large scale in the English Channel demonstrate that this fully conservative radionuclide is a clearly identifiable marker of industrial releases; (3) recent campaigns carried out during the FLUXMANCHE II CCE (1994) programme show the general distribution of dissolved radionuclides , , , , and in the English Channel and the Irish Sea; and (4) the re-utilisation of data from previous campaigns (1983, 1986, 1988) provides indications, at any given location in the English Channel, about the average dilution and distribution of releases derived from the La Hague reprocessing plant. Excesses and losses of radionuclides are now quantified with respect to known source terms; estimates of losses are provided for non-conservative radionuclides, while an excess of was observed in the English Channel during the period 1983–1994. This excess, which has the same order of magnitude as the quantities released from La Hague plant in the English Channel, could be explained by about 1% of the Sellafield reprocessing plant releases entering the Channel. These results confirm and give a more detailed picture of the previously known distribution of water masses in the English Channel. They lead to clear information about transit times and dilution at this scale, and provide directly comparable data for the validation of hydrodynamic models.
Nearshore surface assemblages of phytoplankton and microzooplankton (mostly tintinnids) from water pumped continuously through a 53 μm net were collected along a 70 km longshore transect off southern California and a shorter offshore transect. The low biomass of the larger phytoplankton and the presence of many warm-water species was consistent with the prevailing El Nin˜o conditions. This was especially marked in the northern part of the longshore transect. To the south, assemblages were more “coastal” and there was a bloom of Stenosemella sp., the presence or absence of which was largely responsible for the pattern of the major microplankton assemblages, as established by dendrograms resulting from the use of the Bray-Curtis index for measuring similarity between samples. Patch length of assemblages was usually 10 km or less. The stormy conditions at the time of the cruise resulted in unusually high runoff from the coastal lagoons and were possibly related to the microplankton structure along the longshore transect.
A multidisciplinary SEEP—I study of particle formation, transformation, transport, sedimentation, and storage was conducted across the shelf-break south of Martha's Vineyard and Long Island during July 1983–October 1984. Moored arrays of current meters, thermistors, transmissometers, fluorometers, and sediment traps provided time-series data on the distributions of currents, temperature, light transmission, fluorescence, organic carbon, carbonate, and particulate 210Pb within shelf and slope waters of the Mid-Atlantic Bight. Shipboard and aircraft measurements focused on the birth and death processes of plankton during the spring bloom, when seasonal fluxes of carbon, carbonate, and210Pb to slope waters and sediments were maximal.Simulation models of nitrogen, carbon, and oxygen transfers during the 1979 and 1984 spring blooms suggested that 50–60% of the daily primary production was consumed by the rest of the shelf food web. Surviving shelf particles enter a near-bottom physical regime on the continental slope which favors their downslope transport and deposition. On an annual basis, 210Pb budgets of the slope sediments suggest that only 20–60% of organic debris from the adjacent shelf ecosystem may exit the shelf-break in the SEEP—I region, with the remainder entrained downstream towards Cape Hatteras.After spring export, continued oxidation of organic matter within slope sediments of the SEEP—I region allows carbon accumulation rates here of <1% of the annual primary production on the shelf. The SEEP—I study area represents a region of minimal export of particles from the Mid-Atlantic Bight, however. For example, 250 km farther south on the 1000-m isobath, the annual fluxes of organic carbon and 210Pb at 25 m above bottom in 1985–1986 were two- to three-fold greater than those at equivalent depths in the SEEP—I experiment.
Moorings of current meters, thermistors, transmissometers, and fluorometers on the Mid-Atlantic shelf, south of Long Island, suggest a seaward export of perhaps 0.20 mg Chl m−3 day−1 at depths of 75–81 m, between the 80- and 120-m isobaths during February–April 1984. Using a C/Chl ratio of 45/1, such a horizontal loss of algal carbon over the lower third of the water column would be 19–67% of the March–April 1984 primary production within the overlying euphotic zone. This possible physical carbon loss is similar to daily grazing losses to zooplankton of 32–40% of the algal fixation of carbon. Metabolic demands of the benthos could be met by just the estimated fecal pellet flux, without direct consumption of the remaining algal carbon. Similarly bacterioplankton metabolism could be fueled by excretory release of dissolved organic matter during photosynthesis, rather than by consumption of particulate carbon. Sediment traps tethered 10 and 70 m off the bottom at the 120-m isobath caught as much as 0.10–0.16 g C m−2 during March–April 1984. This presumed vertical flux is about one-third to one-half of the horizontal flux of 0.30 g C m−2 day−1 estimated over the lower 33 m of the water column at the 100-m isobath. These estimates suggest that ∼50% of the carbon export at the shelf-break might be derived from the adjacent overlying water column, with the remainder from lateral injections of near-bottom particles originating on the inner shelf.
The variability of particulate matter, chlorophyll a and zooplankton often depend strongly on physical processes. In this paper we compare distributions observed on the southeastern shelf of the United States during April 1984 and 1985. April 1984 was characterized by high freshwater runoff, vertical stratification and strong wind stress. In April 1985 all three variables were much lower. While alongshore flow was similar in both years, cross-shelf flow was more pronounced in 1984. The most noticeable effect on zooplankton was their expatriation towards offshore. The near surface, offshore and deeper Ekman flows from northeastward wind events, in conjunction with intermittent sinking, tend to trap most particulate matter that was produced nearshore in the nearshore or middle shelf region. Offshore removal was transient following upwelling wind events during stratification conditions. Zooplankton abundance on the inner and middle shelf was not only a function of near- and offshore production but also of cross-shelf and alongshore displacement. Whereas the circumglobal copepod genus Paracalanus occurs abundantly in summer, spring and autumn on the inner and middle shelf, the cosmopolitan genus Oithona which is abundant during summer is scarce during spring and appears to be affected by temperature and water column stability. The variability of particulate matter and zooplankton during early spring seems to be at least partly determined by atmosphere forcing and the amount of freshwater runoff.
Interannual variability (IAV) of net vertical heat flux (Q), potential energy anomaly or “stratification index” (V) and date of onset for water column stratification were studied using five-day averaged model output over an 11 yr (1985–1995) period for three depth zones which span water depths from less than 60 to 100 m on southern Georges Bank. Comparison between five-day averaged model Q estimates from this study and Q measurements from US-GLOBEC mooring ST1 during winter–summer 1995 shows that nearly all of the difference between uncalibrated model estimates and calibrated ST1 measurements is attributable to the difference between the sum of the latent, sensible and long-wave components (Qe+Qs+Qlw) from each data set. Furthermore, there is virtually no IAV in residual five-day averaged Qe+Qs+Qlw values relative to the long-term (1985–1995) mean seasonal cycle, suggesting strongly that the calibration of model Q estimates using measurements from mooring ST1 during 1995 may be applied to other years (1985–1994). Calibrated long-term (1985–1995) monthly mean Q values from this study show a similar seasonal cycle to climatological values determined from ship-of-opportunity data. However, values from this study are higher relative to the ship-of-opportunity long-term monthly mean climatology, with largest differences occurring during winter. IAV of monthly mean Q for zones 1–3 on southern Georges Bank is significant with the magnitude of residuals increasing from zones 1 to 3. Long-term (1985–1995) monthly mean model-derived V computed for zones 1–3 shows zone 1 (<60 m depth) remaining well-mixed throughout the year with no stratification development and therefore no detectable IAV for V. However, within zone 2 (60–80 m depth) and zone 3 (80–100 m depth) significant permanent stratification begins to develop during May and April, respectively, increasing almost linearly from May–July and reaching a maximum during July and August, respectively. IAV of V is large for zones 2 and 3 with typical standard deviations of 1000–5000 J m−2 occurring from late-spring-summer with the largest coefficient of variation during spring and fall. Results from linear regression for zones 2–3 show that IAV of V is controlled largely by IAV of Q, explaining up to ∼80% of the variance, with IAV of wind mixing being of secondary importance. Results from model-derived five-day averaged V show that temporary or “transient” stratification nearly always occurs in early-May and mid-April for zones 2 and 3, respectively, over the 11 yr study period and is associated with reduced tidal stirring during periods of neap tide. The mean date of first transient stratification for zone 3 coincides with the historical maximum abundance of early-stage (<6 mm) cod larvae and copepod nauplii for Georges Bank. In addition, the historical maximum abundance of haddock larvae occurs on Georges Bank at the same time that permanent or “seasonal” stratification is established in zone 3.
The first experiment of the ECOMARGE programme (ECOsystèmes de MARGE continentale) was initiated in 1983–1984, in the Gulf of Lions (northwestern Mediterranean Sea). The objectives of the ECOMARGE—I experiment were: to quantify the transfer of particulate matter, in general, and of organic carbon, in particular, from its introduction to and formation in the waters of the continental shelf—to its consumption or sedimentation on the shelf or its transfer to the slope and deep sea; and to understand the processes involved in that transfer, consumption and sedimentation together with their variability in space and time. The results of that experiment, from 1983 to 1988, are presented in this Special Issue. The highlights of the results are summarised in this paper.
Two regional hydrographic surveys conducted in January and July 1986, aboard the R.V. Thompson and R.V. Washington illustrate the seasonal change in water properties from winter to summer in the Yellow and East China Seas (YECS) and adjacent Kuroshio. In January 1986, water over the shelf in the YECS was locally well mixed in the vertical, and the horizontal distribution of water properties was dominated by a large tongue or plume of relatively fresh Yellow Sea Cold Water (YSCW) flowing southeastward along the Chinese margin into the East China Sea. To the east of this plume, along the Korean margin, was found the more saline Yellow Sea Warm Water (YSWW). The Kuroshio front in the East China Sea was located at the shelf break, separating the warmer, more saline Kuroshio water from the relatively well-mixed cooler, less saline coastal water. Evidence of mixing between these two water masses was observed but limited to near the shelf break. In July 1986, water over the shelf in the YECS was strongly stratified everywhere except within tidally mixed areas near the coast. The surface water distribution in the YECS was dominated by a bubble or lens of Changjiang dilute water located to the northeast of the Changjiang mouth, and the bottom YSCW intensified and extended southward to the shelf break. The relatively fresh coastal water from the East China Sea shelf extended far past the shelf break over the Kuroshio near the surface, and in turn, Kuroshio water intruded onto the shelf near the bottom. Mixing between the Kuroshio and coastal water was found over much of the mid- and outer shelf and upper slope, spanning a cross-stream distance of 75 km. The seasonal freshening due to the Changjiang discharge contributed directly to the summer increase in freshwater transport in the upper Kuroshio. In addition, evidence of deep vertical mixing within the Kuroshio itself was found near 32.0°N, 128.2°E, most likely due to a mesoscale eddy found near there and internal tidal mixing over the slope.
Water mass distribution and geostrophic circulation off Namibia are presented for April 1986, a month of relatively strong warm water intrusion from the north and west. The main result is that mesoscale processes develop in the area during the season when the coastal upwelling is relaxed, and these processes may be an alternative contribution to the fertilization of the region. The main features observed are the following.Angolan waters entered the region through the surface layer, with a maximum flow at 50 m depth.A frontal zone, centered between 20°S and 22°S separated Angolan and Benguela waters, at the surface layer.Anticyclonic eddies with a radius of 15–30 miles occurred off central and southern Namibia. Water upwelling off Walvis Bay appeared to derive from the core of the Angolan water, which laid at a depth of 50–100 m.Anticyclonic gyres might contribute to the upwelling off Walvis Bay and to the occurrence of a local sinking in its centre, about 50 miles offshore, thus allowing long residence time and recirculation.The occurrence of eddies may have been related to the general relaxation of the Benguela Upwelling System in the preceding 3 months and the penetration of the Angolan waters, in the region north of Walvis Bay, while in the south, the meanders of the oceanic front may remain most important.
Distribution and abundance of zooplankton in the North Sea during the Autumn Circulation Experiment (October 1987–March 1988) were examined. From shipboard egg production incubations and the distributions of eggs, nauplii and females, the productivity of various copepod species was described. Against the background of surface temperature, salinity and chlorophyll-a distributions, major seasonal changes in plankton biomass distributions and specific production of copepods were seen. High biomass levels in October rapidly declined into November and January, especially in the north. These changes were followed by early (January/February) production and biomass increases in the southeastern North Sea. Although lowest between November and January, depending on species and location, production continued for many copepod species throughout the winter, despite low temperatures and large predator populations. It was concluded that winter survival of herring larvae and other predators was enhanced by herbivore production in the southeastern North Sea, and that in the north, low herbivore production, competition and predation decreased the probabilities of predator survival. Copepod overwintering strategies and the implications of winter herbivore production and predator abundance for later plankton production processes are discussed.
While the basic hydrographic and circulation pattern of Santa Monica Bay (SMB) are relatively well known, the seasonal and interannual variations of these patterns are not well established nor are the mechanisms behind them well understood. We analyzed 10 years of hydrographic observations (1987–1997) in the surface layer of SMB off southern California to establish the mean seasonal cycle and to deduce patterns of sub-seasonal to interannual variability of sea surface temperature, salinity, oxygen anomaly (the difference between oxygen concentration and its solubility), and transmissivity using empirical orthogonal functions and time-lagged cross-correlation methods. All four variables exhibit distinct seasonal variations, whose spatial distributions are a consequence of the interaction of bottom topography, vertical mixing, horizontal advection, freshwater discharge, and biological processes. The seasonal anomalies of all four parameters were estimated by removing the mean seasonal cycle; they exhibit three basic patterns: (1) an inshore/offshore gradient; (2) a balance between horizontal advection of the California Current from the northwest and the Southern California Countercurrent from the south; and (3) freshwater discharge from Ballona Creek. Air/sea heat flux and vertical mixing are the main factors regulating the dynamics of sea surface temperature. Freshwater discharge centered at Ballona Creek is the principal factor creating the salinity pattern. The annual mean oxygen anomaly shows a substantial supersaturation across the entire SMB. The variations of oxygen anomalies are regulated by local biological productivity, vertical mixing, and horizontal transport. Vertical mixing influences water transparency by suspension of sediments and nutrient supply stimulating phytoplankton growth. A remarkably clear pattern of spatio-temporal variations of all parameters is observed in a relatively small basin; these variations are regulated by both local meteorological factors (air temperature, wind, atmospheric precipitation) and remote forcing, including the El Niño–Southern Oscillation cycle.
The autumn circulation on the inner- and mid-shelf of the U.S. South Atlantic Bight (SAB) is examined numerically. Using data collected in 1987 during the Fall Experiment (FLEX) the alongshore structure of the currents and the coastal sea level fluctuations were found to be correlated to local winds which were strong and persistently northeasterly. The observed inshore distribution of freshwater during FLEX, characterized by the presence of a coastal front confined to the coast inside the 25 m isobath, reflects the local autumn discharge subjected to strong and persistent downwelling winds. The freshwater signal found outside the 25 m isobath is suggested to be the previous summer's discharge advected northward by the summer winds subsequently returning south forced by the autumn winds.
A numerical modelling study of the response of the Adriatic shelf waters and the Po River plume to a bora wind event has been undertaken using the Princeton Ocean Model (POM). After a bora episode from 16 to 20 January 1987, Advanced Very High Resolution Radiometer (AVHRR) satellite image revealed a cold filament protruding from the Italian coastal area toward the open sea. To understand and numerically simulate the offshore spreading of cold water from the western coastal area, the effects of three external forcings have been examined: wind stress, heat flux and river discharge. Wind stress and surface heat flux, calculated by bulk method from the winds, air temperatures and humidities obtained by Mesoscale Model 5 (MM5) with space and time resolution of 9 km and 1 h, respectively, were used to force the oceanographic model. River discharge was introduced in the oceanographic model as a source term in the continuity equation. Moreover, it was assumed that river water had zero salinity and a 5°C lower temperature than the surrounding sea water, following some climatological estimations.
Six Argos-reported drifters drogued to a depth of 2.7 m produced eight trajectories over the Texas-Louisiana Shelf and the adjacent oceanic waters of the northwestern Gulf of Mexico from 7 March to 29 April 1989. Launched by United States Coast Guard aircraft and Texas A & M University's R.V. Gyre, the trajectories span the shelf from the vicinity of Barataria Bay, Louisiana to the southern reaches of Padre Island near Port Isabel, Texas. Two tracks demonstrate cross-slope and cross-shelf motion northward from the central western Gulf toward Louisiana. These two drifters join three others to define a coastal current flow westward from near the Mississippi delta to Galveston and then southwestward along the Texas coast. Two other trajectories indicate a relatively low-energy mid-shelf regime over the northwestern portion of the outer continental shelf. Five drifter groundings locate a convergence in the nearshore and littoral flows on the Texas coast between Matagorda Peninsula and southern Padre Island. Strong wind-driven events in the northwestern Gulf demonstrate instances of coherent shelf response over 7° of longitude and 3° of latitude. Infrared satellite imagery indicates the regional context and structure of the spatial scales of Gulf of Mexico surface circulation.
Evidence of a turbidity current sweeping through the Monterey Submarine Canyon following the October 1989 Loma Prieta earthquake was documented by the movement of bottom-deployed acoustic transponders used to navigate free-falling oceanographic instrumentation. Measuring sites located along the Canyon at distances of 55, 130 and 190 km from the Canyon head off Moss Landing, CA, all showed evidence of tectonically induced sediment transport. At the site 55 km from the Canyon head, one transponder, located in the axis of the Canyon, was carried 1.9 km down the axis and was deposited among a field of rocks. The other three transponders had been deployed on the sides of the canyon and showed evidence of sediment slumping toward the Canyon axis. Circumstantial evidence from the site 130 km down the canyon suggests that sediment deposition occurred outside the channel axis. Sediment slumping or erosional cutting moved one transponder deeper at the site 190 km from the canyon head.
The effects of wave–current interactions on ocean surface waves induced by Hurricane Hugo in and around the Charleston Harbor and its adjacent coastal waters are examined by using a three-dimensional (3D) wave–current coupled modeling system. The 3D storm surge modeling component of the coupled system is based on the Princeton Ocean Model (POM), the wave modeling component is based on the third generation wave model, Simulating WAves Nearshore (SWAN), and the inundation model is adopted from [Xie, L., Pietrafesa, L. J., Peng, M., 2004. Incorporation of a mass-conserving inundation scheme into a three-dimensional storm surge model. J. Coastal Res., 20, 1209–1223]. The results indicate that the change of water level associated with the storm surge is the primary cause for wave height changes due to wave–surge interaction. Meanwhile, waves propagating on top of surge cause a feedback effect on the surge height by modulating the surface wind stress and bottom stress. This effect is significant in shallow coastal waters, but relatively small in offshore deep waters. The influence of wave–current interaction on wave propagation is relatively insignificant, since waves generally propagate in the direction of the surface currents driven by winds. Wave–current interactions also affect the surface waves as a result of inundation and drying induced by the storm. Waves break as waters retreat in regions of drying, whereas waves are generated in flooded regions where no waves would have occurred without the flood water.
We measured dissolved inorganic carbon (DIC), combined inorganic nitrogen, O2, chlorophyll a (Chl a), particulate carbon and nitrogen (PCN) along with selected additional parameters on Georges Bank during April. Summer conditions were developing in the region as reflected by elevated biological production at the margins of the Bank (Chl a and O2 levels > 16mg m−3 and> 109% saturated respectively) and nitrate depletion on mid-Bank. Oxygen supersaturations were maintained in the regions of low NO3 concentration, suggesting that a rapid supply of NO3 was occurring horizontally from deeper waters. The productive region was propagating into deeper waters as daylight increased but was not yet associated with the tidally mixed front on the western flank. Changes in DIC and O2 suggested net community production (NCP) of 0.24-0.42 g C m−2 day−1 on the flank. A collaborative purposeful tracer experiment on mid-Bank permitted a time series of net biological activity to be made and changes in both DIC and O2 indicated similarly large net respiration was occurring there (NCP = −1.4 g C m−2 day−1). A budget of particulate carbon and nitrogen suggested the biological character of the mid-Bank seston changed from autotrophy to heterotrophy and most of the remaining suspended particulates were detrital. Thus, the prolonged residence time of shelf water on the Bank contributes to efficient respiration of much of the organic matter formed locally and diminishes the Bank's role as a source of organic matter for off-shelf or down-shelf export. Based on an analysis that accounted for mixing effects between different water masses on the Bank, the ratio of DIC to NO3 consumption was significantly greater than the Redfield ratio of 6.6. Thus, in such eutrophic regions, estimates of NCP derived from new (nitrate) production may be low by as much as 40%.
Measurements of currents and light transmission were made at bottom tripods and moorings arrayed across the northern California continental shelf along the Coastal Ocean Dynamics Experiment (CODE) “C” transect as part of the 1990–1991 Sediment Transport Events on Shelves and Slopes (STRESS) experiment. In combination with meteorological and wave data from the National Data Buoy Center Buoy 46013, these measurements provide information about the physical forcing and resultant resuspension and transport of bottom material between 21 November and 8 March. Sixteen events were identified in the wave, wind and current-meter records for this period. Only two were local storms with southerly winds, but they caused about half of the seasonal net transport. Seven were swell events that combined long-period waves generated by distant storms with local currents. At the 90-m site, swells interacted with the mean northward flow to produce northward transport. During six northerly wind events, upwelling-favorable winds often were sufficient to slow or reverse the mean northward flow and thus caused southward transport. A single current event, which produced moderate southward transport, was observed at the 130-m site. Net transport during the winter experiment was offshore at all sites, northward at the inner- and mid-shelf sites, but southward at the outer-shelf site. The results suggest that local storms with southerly winds may dominate seasonal transport, as on the Washington shelf, but significant transport also can occur during fair weather and during periods of northerly winds.
The distribution of phytoplankton abundance and species composition is described for coastal waters of the NW Adriatic Sea, for the period 1990–1999. Hydrochemical and biological properties were investigated every month, at 48 stations. This coastal system is quite unstable and heterogeneous, due to the influence of several rivers, intense water exchange with the Lagoon of Venice, and complex advective processes. In the vicinity of river plumes, dissolved nitrogen and dissolved silica appear to be in excess, rarely limiting phytoplankton growth. Examining the general trend of phytoplankton over the years, the main limiting factors for phytoplankton growth in the Adriatic coastal system are light, temperature, and the strong influence of meteorological events.The NW Adriatic coastal area was subdivided into three zones, each characterised by different trophic conditions. In each zone, phytoplankton was similar in terms of community structure and was different only in abundance. Three main periods of growth were observed: February, April and July. As expected in a nutrient-enriched system, the community structure was dominated by diatoms (both micro- and nanoplankton fractions), over most of the year. The importance of dinoflagellates in the communities was generally low, with significant abundances present only in June–July, once the spring bloom of diatoms had left ‘nutrient-poor’ conditions in the water. These low concentrations of nutrients favoured the growth of dinoflagellates, which have lower nutritional requirements. Over the 10-year study, a decrease in orthophosphate concentrations was observed. This pattern could be explained as a consequence of reduced quantities of phosphorus used in detergents, in the late 1980s. No significant change in any hydrochemical or biological property was observed.