Coastal Shelf Areas Research Articles

Crustacean resources are vulnerable to serial depletion – the multifaceted decline of crab and shrimp fisheries in the Greater Gulf of Alaska

The seas around Alaska support (or have supported) some of the most commercially significant crustacean stocks in the world, spread over an overwhelming array of extensive and diverse coastal and open shelf areas. Major resources include three species of king crab (Paralithodes spp. and Lithodes aequispina), Tanner and snow crab (Chionoecetes spp.), Dungeness crab (Cancer magister), and five species of pandalid shrimp (Pandalus spp. and Pandalopsis dispar). Excluding the Bering Sea, the resources from the Greater Gulf of Alaska (ranging from the Aleutian Chain to the state's south-eastern panhandle contiguous with British Columbia) supported rapid expansion of several crab and shrimp fisheries during the 20 year period 1960–1980. Since then, most of those fisheries have collapsed. While some of the stock declines have been well documented and discussed (most prominently the ‘dethroning’ of red king crab on the shelf around Kodiak Island), it has been less apparent that the demise of Alaskan crustace an stocks is a process on a much larger scale, and is still unfolding. Here we examine trends in catch, recruitment and abundance (when possible) and discuss existing evidence of overfishing and management options. We emphasize the importance of recognizing the multi-scale spatial structure of crustacean stocks, and suggest the need to consider spatially explicit strategies, particularly the creation of reproductive refugia

Spatial and temporal trends of pesticide residues in water and particulates in the Mississippi River plume and the northwestern Gulf of Mexico

Selected herbicides, chlorinated pesticides and polychlorinated biphenyls (PCBs) were measured using GC-MS SIM analysis in dissolved, colloidal and particulate phases of sea water samples collected seasonally over a three-year period. Sample preparation used sequential membrane filtration and ultrafiltration prior to solid phase extraction (SPE) of the dissolved phase. Chlorinated pesticides and PCBs were rarely detected. Herbicides were found primarily in the dissolved and colloidal phases. Atrazine was ubiquitous over the entire northwestern Gulf of Mexico coastal shelf area in spring, summer and fall. Highest herbicide concentrations occurred in summer and fall, except for alachlor, which was only detected in spring run-off.

Pile end-bearing capacity in crushable sands

The occurrence and formation of crushable soils is a widespread phenomenon. Warm tropical seas give rise to deep deposits of biogenic skeletal sediments in coastal shelf areas, or exposed granitic intrusions are subjected to deep weathering resulting in decomposition. The end-bearing capacity of piles in crushable soils is important for the design of foundations in these diverse soils, but the analysis of foundations in these soils is problematic due to their compressibility and highly curved Mohr–Coulomb failure envelopes. The use of the spherical cavity expansion method offers potential for a better prediction of end-bearing capacities by taking into account soil crushability and the decrease of friction angle with increasing mean normal stress. A formula relating end-bearing pressure to the pressure to expand a cavity and the soil secant friction angle has been developed, which refines and simplifies Vesic's original equations. However, the actual model pile settlement ratio at which the predicted cavity expansion values were reached increased with soil crushability and decreased with increasing relative density. A simple linear relationship has been proposed between a soil compressibility index Cp and the pile settlement ratio at which the bearing capacity predicted by the theoretical cavity expansion method is mobilized. KEYWORDS: calcareous soils: laboratory tests; model tests; offshore engineering; piles; sands. Les sols compressibles sont des formations très répandues. Its se forment dans les mers chaudes tropicales qui favorisent le dépôt de sédiments biogéniques profonds dans les zones de plate-forme côtière, ou proviennent de l'altération profonde de granites intrusifs affleurants se traduisant par une décomposition du matériau. La capacité portante en pointe des pieux devient done un paramètre primordial pour la conception de fondations dans ce type de sols. Toute analyse de fondations dans ce type de sols pose cependant des problèmes en raison de la compressibilité et des enveloppes de rupture de Mohr–Coulomb très incurvées de ces sols. La méthode par expansion d'une cavité sphérique est susceptible de permettre une meilleure prévision de la capacité portane en pointe puisqu'elle prend en compte la compressibilité du sol et al diminution de l'angle de frottement lorsque la contrainte normale moyenne augmente. Une équation, plus simple que les équations de Vesic, a été établie entre la pression en pointe, la pression d'expansion d'une cavité et l'angle de frottement tangential du sol. Cependent, avec notre modèle de pieux actuel, les valeurs de tassement, correspondant aux valeurs prévisionnelles de l'essai d'expansion de cavité, augmentent avec la compressibilité du sol et diminuent lorsque la densité relative augmente. On propose une relation linéaire simple entre l'indice de compressibilité Cpdu sol et le tassement des pieux pour lequel la capacité portante prevue par la méthode d'expansion d'une cavité théorique est mobilisée.

Zooplankton Biomass in the Southern Gulf of St. Lawrence: Spatial Patterns and the Influence of Freshwater Runoff

The role of advection of freshwater input in Zooplankton distribution was evaluated from 16 surveys carried out in the southern Gulf of St. Lawrence during 1967–71. Biomass was structured according to two main patterns: large-scale gradients (> 150 km) over the entire area and mesoscafe (≈25 km) patches. Patterns exhibited seasonal and interannual variability without any sign of recurrence through time, unlike the spatial structure of surface water temperature and salinity which was typical of large-scale dispersion gradients resulting from the seasonal input of freshwater into the area. In most cases, overall distribution patterns of biomass and hydrographic variables were not significantly correlated, indicating that biomass patterns in the southern Gulf were not related to water mass characteristics and advection of freshwater runoff over the entire area. Although mesoscale patchiness was unrelated to hydrographic processes, patch size (≈25 km) was similar to estimates obtained for the northern Gulf and for other coastal shelf areas. Large-scale gradients in biomass may correspond to species changes in relation to the salinity gradient and/or topography and bathymetric features. I conclude that biomass alone is not an adequate predictor for monitoring changes and variability of Zooplankton in response to runoff variability.

Late Tertiary subsidence history of the southern Levant Margin, eastern Mediterranean Sea, and its implications to the understanding of the Messinian Event

The late Tertiary subsidence history of the southern Levant continental margin, situated in the southeastern Mediterranean Sea, was quantitatively analyzed. Paleodepth reconstruction across the margin off Ashdod suggests the existence of a deep basin in pre‐Messinian time which resembles the present one. This implies that, the deposition of the evaporites in the study area during the Messinian desiccation of the Mediterranean Sea occurred in a deep basin. The path of the tectonic subsidence of the basement since early Tertiary is generally smooth as expected from the nature of the thermal subsidence. The unconformity beneath the Messinian indicates erosion of 50–200 m at the coastal plain. In the Pliocene, the tectonic subsidence in the coastal plain and shelf area diverts from the expected thermal path and increases from 250 m to 450 m, respectively. In the Quaternary the rate of tectonic subsidence nearly resumed the predicted thermal subsidence. Sedimentation and subsidence rates decrease but are still higher than those of the pre‐Messinian. We suggest that the evolution of the southern Levant margin is most probably influenced by three main causes: (1) the Messinian event in late Miocene, (2) the deposition of large volumes of Nile derived sediments since the Pliocene, and (3) the flexural response of the lithosphere to the load from the Nile delta and/or from the uplift of the Judea Mountains (the western shoulder of the Dead Sea Transform). We interpret the latter to be the cause of the anomalous subsidence of the southern Levant margin during the Pliocene.

Winter aspects of the ichthyoplankton community in Antarctic Peninsula waters

Ichthyoplankton was sampled from the Antarctic Peninsula area of the South Polar Ocean in early winter (May and June 1986). A total of 153 eggs from two species and 1368 larvae or juvenile stages from 12 species were obtained. These included pelagic species, and demersal species with a long pelagic larval or juvenile phase. Most abundant were larvae of Pleuragramma antarcticum and Notothenia kempi, and eggs of Notothenia neglecta. The distribution of notothenioid and paralepidid larvae was apparently unaffected by ice cover, whereas myctophid larvae were confined to ice-free waters. Areas where newly hatched Chionodraco hamatus occurred coincided with dense aggregations of Euphausia superba (Krill) furcilia larvae which is a potential food resource during winter. The hatching of icefish larvae during winter is apparently independent of the seasonal production cycle. Epipelagic eggs of Notothenia neglecta were found during the spawning season, which suggests that eggs ascend to the surface after demersal spawning and that development takes place near the sea surface during winter. Larvae of Notothenia kempi were chiefly confined to shelf and slope waters to the west of the Antarctic Peninsula, with larger larvae found in coastal shelf areas. Pleuragramma antarcticum occurred in the coastal waters off the Biscoe Islands, in the Gerlache Strait, and in the northern Bransfield Strait. The smallest larvae were found in the northern Bransfield Strait, whereas those at the Biscoe Islands and in Gerlache Strait waters were larger and of a similar size. A cyclonic gyre to the west of the Antarctic Peninsula observed in the austral summer was likely to have affected the larval drift of Pleuragramma antarcticum and Notothenia kempi. Differences in the timing of spawning and hatching and the vertical distribution of these larvae will lead to different transport and spatial distribution patterns. It is hypothesized that early winter conditions do not imply severe limitations on the year-class success of larval fish. Dispersal and increased mortality may occur during the second half of the winter.

Trace metals in the Severn estuary: a reappraisal

The salinity/concentration profiles of seven dissolved trace metals in the Severn estuary are reported. The metal concentrations observed in this study are considerably lower than those reported in previous work. Despite known inputs of metals through industrial and waste discharges, river inputs play an important part in the estuarine profiles. The profiles of cadmium, chromium and arsenic appear to be dominated by point source discharges in mid-estuary. Saline end member concentrations are consistent with ranges quoted for the European coastal shelf area.

Pollen distribution in recent marine sediments, eastern Canada

Analysis of 90 samples from coastal and continental shelf areas off eastern Canada between 42 and 69°N shows that Quaternary pollen and spores are present in recent marine muds throughout this region. Pollen and spore concentrations range from 50 000 to 100 grains per cm3 and they decrease both offshore as a function of distance from vegetation sources and northwards as a function of lower vegetation density north of the summer Arctic Frontal Zone. Isopercentage contours show that maximum relative abundances of most palynomorphs occur in marine sediments adjacent to their regions of highest vegetation representation onshore. However, Pinus and Picea pollen tend to increase in percentage abundance with distance offshore. Presence of these tree pollen in marine sediments off Baffin Island implies that wind transport is an important pathway by which bisaccate pollen types enter the ocean, in addition to fluvial transport.Q-mode factor analysis shows that four major factors account for 89% of the variance among the marine palynomorph samples, while two other factors (6% of the variance) apparently reflect selective transport of Pinus pollen or local over-representation of Gramineae. The major factor assemblages are: (1) Boreal Forest Assemblage, with high weightings of Picea and maximum loadings adjacent to the boreal forest, 48–54°N; (2) Arctic Assemblage, weighted by Rosaceae, Gramineae, and Cyperaceae, with maximum loadings off Baffin Island, 60–69°N; (3) Mixed Forest Assemblage, weighted by Betula tree pollen, Picea glauca, Pinus strobus, Tsuga, and temperate trees (e.g., Quercus, Acer), with maximum loadings in the southeast, 42–48°N; (4) Subarctic Assemblage, with high weightings of Sphagnum, Alnus, and Lycopodium, and maximum loadings adjacent to forest–tundra and low arctic tundra, 54–60°N. These regional marine pollen assemblages are empirically related to contemporary climatic parameters, thus indicating the potential value of pollen analysis for direct correlation between Quaternary paleoclimatic records onshore and in marine areas bordering eastern Canada.

Surface circulation patterns off the East Coast of Greenland as deduced from satellite photographs of ice floes

Satellite photographs for the period 13 August through 6 September 1973 show both north and south movement of ice floes off the east Greenland coast between 74° and 80°N. The paths of seven large (approximately 25 km diameter across) ice floes tracked through the entire period, and a number of smaller floes tracked through portions of the period, indicate the presence of anticyclonic surface eddies in the waters covering the coastal shelf area.