Eastern Entrance Research Articles

Sea Level Variations in the Western Mediterranean Studied by a Numerical Tidal Model of the Strait of Gibraltar

Aspects of the sea level changes in the western Mediterranean Sea are investigated using a numerical tidal model of the Strait of Gibraltar. As a prerequisite, the performance of this model, that is, a two-dimensional, nonlinear, two-layer, boundary-fitted coordinate numerical model based on the hydrostatic approximation on an f plane, is assessed in the simulation of mean and tidal circulation of the Strait of Gibraltar. The model is forced by imposing mean interface and surface displacements as well as M2, S2, O1, and K1 tidal components along the Atlantic and Mediterranean model open boundaries. Model results are compared with observations and with results obtained from a tidal inverse model for the eastern entrance of the Strait of Gibraltar. In general, good agreement is found. A sensitivity study performed by varying different model parameters shows that the model behaves reasonably well in the simulation of the averaged circulation. The model is then used to investigate the climatological sensitivity of the simulated dynamics in the Strait of Gibraltar to changes in the density difference between Atlantic and Mediterranean waters. For this purpose, given a certain density difference between Atlantic and Mediterranean waters, the authors iteratively searched for that sea level drop between the Atlantic and the Mediterranean that fulfills the mass balance of the Mediterranean. It is found that an increase of the density difference leads to an increase of the exchange flow and to an increase of the sea level drop between the two basins. A trend in the sea level drop of O(1 cm yr21), such as the one observed between 1994 and 1997, is explained by the model as the result of a trend of O(10 24 yr21) in the relative density difference between the Mediterranean and Atlantic waters. The observed north‐south asymmetry in this trend is also captured by the model, and it is found to arise from changes in the along-strait velocity. Results suggest that the dynamics within the Strait of Gibraltar cannot be neglected when sea level changes in the western Mediterranean basin are investigated.

Mesozoic sedimentation on an isolated platform at the eastern entrance to the Strait of Magellan, Tierra del Fuego (Chile)

El Bloque area Magallanes esta situado en el oceano Atlantico, dentro de la actual plataforma continental Argentina. Se ubica en la desembocadura oriental del estrecho de Magallanes y consiste de dos cordones de altos estructurales, de orientacion noroeste, que constituyen un horst, parcialmente separado por un graben central. En esta estructura se han encontrado y producido hidrocarburos desde el ano 1994 desde la Formacion Springhill, la cual se deposito durante el Valanginiano-Barremiano. Esta formacion representa una transgresion marina regional con tres subciclos de transgresion-regresion (parasecuencias) separados por superficies de inundacion marina. Las dos parasecuencias basales estan relacionadas con variaciones menores del nivel del mar o por actividad de fallas, mientras que el subciclo superior refleja una gran transgresion, que se extiende en casi toda la cuenca Austral. Las asociaciones de facies depositacionales identificadas incluyen supramareales, intermareales, playa barrera y ambientes de frente de playa, las cuales guardan una relacion clara con la paleotopografia del bloque. De esta manera, las playas-barrera se depositaron en torno a areas topograficamente elevadas, mientras que las facies supramareales e intermareales fueron depositadas hacia las zonas mas elevadas, detras de las playas-barrera. Las facies de frente de playa fueron depositadas hacia la zona baja de estas areas elevadas. Las planicies intermareales tambien se depositaron en embahiamientos internos dentro del graben central. Finalmente, al progresar la transgresion marina, el sistema depositacional evoluciono hacia una plataforma marina extensa y abierta donde se depositaron las arcillolitas de la Formacion Estratos con Favrella.

The N:Si:P molar ratio in the Strait of Gibraltar

All existing descriptions of nutrient distributions in the Strait of Gibraltar suggest that the Atlantic water brings to the Mediterranean Sea nutrients in the Redfield ratio (N:Si:P = 16:15:1). Here, the N:Si:P molar ratios (±Standard Error), obtained in April 1998, are used to show that in the Atlantic water at the western entrance of the Strait this ratio is lower (13.8(±0.5):12.1(±1.0):1) than the classical Redfield ratio; it is close to the Redfield ratio in the middle of the Strait (15.6(±0.6):10.7(±0.9):1), and increases dramatically to 23.6(±3.4):29.1(±4.5):1 at the eastern entrance of the Strait. In the Mediterranean water, the N:Si:P ratio has a quite similar trend with 31.5(±6.0):26.5(±3.6):1 in the east, 20.4(±0.2):31.5(±11.1):1 in the middle and 18.1(±0.6):17.6(±0.7):1 in the west of the Strait. The physical and biological processes that account for the observed spatial variability of the N:Si:P ratio along the Strait are identified. We estimated that in the Atlantic water entering the Mediterranean Sea, about 84% of the variability in N:Si:P molar ratio is due to biological and 16% to physical processes.

Satellite tracking of icebergs in the Weddell Sea

Small to medium sized icebergs (200 m to 10 km across) in two areas of the Weddell Sea were tracked using satellite-borne Synthetic Aperture Radar (SAR) from 22 January–21 February 1994 and 29 January–25 February 1992 respectively. The westward mass flux of icebergs in the Antarctic Coastal Current was estimated at the eastern entrance to the Weddell Sea as being 50–70 Gta−1. A large contrast was found between observations over the narrow shelf here and the off-shelf area. The latter region had very much reduced iceberg density, and the bergs moved with slower and less coherent velocities compared with the fast, but narrow (10–20 km wide) flow in the Coastal Current. This region is a promising site for the monitoring of decadal trends in iceberg fluxes. The second study area in the south-western Weddell Sea showed iceberg motion consistent with non-contemporaneous observations of tagged icebergs, with steady flow parallel to the shelf-ice edge, at significantly lower densities and speeds than in the Coastal Current but almost double that observed off-shelf in the eastern Weddell Sea. We also suggest that shipboard observations of icebergs need careful analysis to avoid their over-estimating true iceberg concentrations by a substantial amount.

Two‐layer tidal modeling of the Yellow and East China Seas with application to seasonal variability of the M2 tide

The baroclinic response of tide and tidal currents in the Yellow and East China Seas is investigated using a two‐layer numerical model. Seasonal variability in the M2 tide, especially the smaller summer amplitudes prevailing along the Korea/Tsushima Strait [Kang et al., 1995], is investigated by a series of numerical experiments with varying degrees of stratification specific to winter and summer. Model results show that the summer amplitudes of the M2 tide around the southwestern tip of the Korean peninsula and Korea/Tsusima Strait decrease, with a peak decrease of about 14 cm off the southwestern tip of the Korean peninsula, while the summer amplitudes in other coastal regions tend to increase. This seasonal variability generally coincides with the observations. These models results indicate that seasonal stratification has several noticeable effects on the tides, including varying degrees of current shear, varying frictional dissipation, and varying barotropic energy flux. In particular, it drives complicated seasonal variability in the M2 tide, with a peak amplitude modulation of nearly 5% off the southwestern tip of the Korean peninsula. The seasonal variation of barotropic M2 energy flux through the eastern entrance of the Yellow Sea is thought to induce the corresponding variability in the M2 amplitude in the Korea/Tsusima Strait, with smaller amplitudes found in the summer.

Integral measurements of mass transport and heat content in the Strait of Gibraltar from acoustic transmissions

Three View the MathML sourceacoustic transceivers were deployed on either side of the eastern entrance of the Strait of Gibraltar during April–May 1996 to determine the feasibility of using acoustic methods to make routine, rapidly repeated, horizontally integrated measurements of flow and temperature in straits. Reciprocal transmissions between the transceivers were used to test the feasibility of using traditional ray differential travel times to monitor the component of flow along the acoustic paths. Transmissions directly across the Strait were used to test the feasibility of using horizontal arrival angle fluctuations and acoustic intensity scintillations to monitor the flow perpendicular to the acoustic path. The geometry was selected to provide ray paths that only sample the lower-layer Mediterranean water, so that the feasibility of monitoring the Mediterranean outflow using the various methods could be evaluated. The acoustic scintillation method did not yield useful current estimates, but the experimental parameters were not optimized for this approach. Since the low-frequency variability in log-amplitude was found to be highly correlated at receivers View the MathML source apart, it is possible that acoustic scintillation measurements using different receiver spacings and more rapid sampling might yield better results. The horizontal deflection method gave encouraging results at the time of neap tides, but less so during spring tides. For this approach, both theoretical estimates and measured phase differences between the horizontally separated receivers suggest that internal-wave-induced horizontal arrival angle fluctuations may fundamentally limit the precision with which arrival angles can be measured. Further work is needed to determine if a smaller horizontal spacing and higher signal-to-noise ratios would yield better results. Reciprocal travel time measurements diagonally across the Strait performed the best of the three methods, giving absolute flow estimates consistent with those derived from current-meter data. The fractional uncertainty variance for the lower layer tidal transport from a single tomographic path was estimated to be 0.017 (i.e. 98% of the a priori tidal transport variance was resolved). The spatial scales of the sub-tidal flow are thought to be significantly shorter than those of the tidal flow, however, which means that a more elaborate monitoring network is required to achieve the same performance for sub-tidal variability. Finally, sum travel times from the reciprocal transmissions were found to provide good measurements of the temperature and heatcontent in the lower layer.

Transport estimates in the Strait of Gibraltar with a tidal inverse model

To estimate the volume transport through the Strait of Gibraltar and to study the spatial structure of the time‐variable flow, a varying number of current meter moorings were maintained at the eastern entrance of the strait between October 1994 and April 1998, and was complemented with intensive shipboard measurements during the European Union project Canary Island Azores Gibraltar Experiment (CANIGO). A tidal inverse model is used to merge these data sets in order to investigate the flow at the eastern entrance of the strait. The two‐dimensional structure of the tidal flow was described by simple analytical functions. Harmonics with the seven most important tidal frequencies were used as temporal functions. With this model, the tidal currents can be predicted for any time and location at the eastern entrance of the strait, and more than 92% of the variance of the lower layer flow is explained. It was used to remove the tidal currents from the individual measurements and to calculate the mean flow through the strait from the residuals. Combined with a similar inverse model for determining the depth of the interface between Mediterranean and Atlantic water, the volume transport was estimated to be 0.81±0.07 Sv for the upper layer and −0.76±0.07 Sv for the lower layer. The correlation of the tidal currents and the fluctuations of the interface accounts for ∼7% of the transport at the eastern entrance.

Tide at the eastern section of the Strait of Gibraltar

From October 1995 to April 1996, three mooring lines were deployed at the eastern entrance of the Strait of Gibraltar. The spatial coverage of the mooring array allows for a good description of the tides. They exhibit a dominant semidiurnal nature and a noticeable baroclinic structure that matches the one of the mean exchange. Tidal currents in the upper layer are irregular and usually too weak to reverse the mean upper layer flow that keeps on flowing east. Lower layer flow reverses with semidiurnal periodicity because of the smallness of the mean flow and the appreciable amplitude of the regular semidiurnal oscillation of tidal currents in this layer. Tidal transports can be satisfactorily compared with previous estimates of Bryden et al. [1994] if we allow for strong internal divergences associated with the internal tide. No significant eddy flux of water transport (tidal rectification) is observed at the eastern section, contrary to the almost 50% of the total layer transport found by Bryden et al. [1994] in Camarinal Sill section. Time‐dependent hydraulic theory provides a good scenario for interpreting these two independent sets of observations despite the fact that the composite Froude number does not reach the critical values predicted in the hydraulic models most of the time.

Biodiversity and community structure of the mega-epibenthos in the Magellan area (South America)

Epibenthic communites have been investigated at 55 stations in the Magellan region by underwater photogra-phy at water depths between 15 and 430m. Five species assemblages were identified of which the two shallow and the two deep ones in the channel system differed considerably in abundance, number of taxa, diversity and species composition. The dominant systematic group of the shallow assemblages were ascidians. In one of these assemblages decapod crustaceans, in the other mainly sessile suspension feeders e.g. sponges, anthozoans and bryozoans were the characteristic taxa. In the deeper assemblages echinoderms were most abundant. The fifth assemblage, consisting of several offshore stations south of the eastern entrance of the Beagle, was more similar to the shallow stations in the channel system in terms of dominant life forms. Various hypotheses were confirmed or rejected: The benthos at the deeper stations in the channel system was not more homogeneous than at the shallow stations. At both the channel and the offshore stations filter feeders were most dominant whereas at the bottom of the channels deposit feeders were more abundant. Differences between the wide Straits of Magellan and the narrow and steep Beagle Channel were not very distinct. The benthic assemblages inside the channel system did not differ considerably from the assemblage outside the channel system with the exception of the stations close to the continental slope of the Atlantic. Differences due to Pacific or Atlantic influences were not recognisable. The biological patterns could best be explained by the environmental parameters water depth, occurrence of soft sediment and biogenic debris.

Distribution patterns of macrozoobenthos: a comparison between the Magellan Region and the Weddell Sea (Antarctic)

The Joint Chilean-German-Italian Magellan Victor Hensen Campaign in October/November 1994 and the Polarstern expedition ANT XIII/4 (May 1996) yielded 207 quantitative multibox corer samples from 11 stations in the Paso Ancho (Magellan Straits), 10 stations in the Beagle Channel, and 15 stations of the shelf and continental slope off the eastern entrance of the Beagle Channel. Mean abundance values in the Magellan Region varied from 1591 Ind. M -2 in the Paso Ancho to 3643 Ind. m -2 in the Beagle Channel and 3983 Ind. m -2 on the shelf; the corresponding biomass values were 96.8 g, 301.6 g, and 119.0 g wet weight m -2 respectively. In terms of org. C the corresponding values are 4.8, 11.4, and 4.6 g m -2 . Abundance (3806 Ind. m -2 ) and biomass (222.6 g wet weight m-2 or 7.3 g C) data from the eastern Weddell Sea shelf (Polarstern expeditions ANT VI/3, 1987/88 and ANT VII/4, 1989) are in the same range. The composition of the fauna, however, reveals distinct differences.

New localities for Crustacea Decapoda in the Magellan region, southern South America

We report the occurrence of Lithodes turkayi (Anomura), Libidoclaea granaria, and C. edwardsii (Brachyura), in the Beagle Channel. This observation extends their range of distribution south of the previously reported limit of the Straits of Magellan. Acanthocyclus albatrossis (Brachyura) occurs south of Isla Navarino and also in the Beagle Channel, however in a particular habitat of its northern coast. The occurrence of Lithodes confundens (formerly identified as L. antarcticus) north of the eastern entrance of the Straits of Magellan is also reported. We extend its range of distribution northerly and its bathymetric distribution to the intertidal.

The California Current system in the Southern California Bight and the Santa Barbara Channel

Recent observations in the Santa Barbara Channel and over the California Cooperative Oceanic Fisheries Investigations (CalCOFI) southern California grid are used to examine seasonal circulation patterns in and near the Southern California Bight, defined as the region east of the Santa Rosa Ridge and including the Santa Barbara Channel. Poleward flow relative to 500 m is found throughout the bight, in all seasons except for spring and all subregions except the western part of the Santa Barbara Channel. In spring there is equatorward flow throughout the bight at all depths to 500 m, though it tends to be surface or midcolumn intensified. Equatorward flow offshore of the bight, present in all seasons, narrows and accelerates into a jet‐like feature and simultaneously moves close to the Santa Rosa Ridge in summer. Current meter data from the eastern entrance to the Santa Barbara Channel are consistent with the seasonal results from the CalCOFI data and further show an upward propagation of phase at annual period, with a phase speed of 1 to 2 m d−1. Comparison with the CalCOFI ship winds suggests that poleward flow in the bight may be the result of positive wind stress curl, through a Sverdrup balance, though the observed transport is only about 75% of that expected from the curl amplitude. Equatorward flow in the bight may be the result of coastal upwelling, though the transport is larger than expected from the relatively weak winds in the bight, and the offshore extent of the equatorward flow is about 10 Rossby radii, farther than might be expected for linearized coastal upwelling. Equatorward flow outside the bight cannot be explained as a Sverdrup flow because the wind stress curl is positive throughout most of the region. Upward propagation of phase and downward propagation of energy may be indicative of forcing by remote wind equatorward of the bight. Given the complexity of possible forcing mechanisms for circulation in and near the bight, a model that includes effects of remote wind forcing, Ekman pumping, and topography is needed to explain the observations in a satisfactory way.

Seafloor environments in the Long Island Sound estuarine system

Four categories of modern seafloor sedimentary environments have been identified and mapped across the large, glaciated, topographically complex Long Island Sound estuary by means of an extensive regional set of sidescan sonographs, bottom samples, and video-camera observations and supplemental marine-geologic and modeled physical-oceanographic data. (1) Environments of erosion or nondeposition contain sediments which range from boulder fields to gravelly coarse-to-medium sands and appear on the sonographs either as patterns with isolated reflections (caused by outcrops of glacial drift and bedrock) or as patterns of strong backscatter (caused by coarse lag deposits). Areas of erosion or nondeposition were found across the rugged seafloor at the eastern entrance of the Sound and atop bathymetric highs and within constricted depressions in other parts of the basin. (2) Environments of bedload transport contain mostly coarse-to-fine sand with only small amounts of mud and are depicted by sonograph patterns of sand ribbons and sand waves. Areas of bedload transport were found primarily in the eastern Sound where bottom currents have sculptured the surface of a Holocene marine delta and are moving these sediments toward the WSW into the estuary. (3) Environments of sediment sorting and reworking comprise variable amounts of fine sand and mud and are characterized either by patterns of moderate backscatter or by patterns with patches of moderate-to-weak backscatter that reflect a combination of erosion and deposition. Areas of sediment sorting and reworking were found around the periphery of the zone of bedload transport in the eastern Sound and along the southern nearshore margin. They also are located atop low knolls, on the flanks of shoal complexes, and within segments of the axial depression in the western Sound. (4) Environments of deposition are blanketed by muds and muddy fine sands that produce patterns of uniformly weak backscatter. Depositional areas occupy broad areas of the basin floor in the western part of the Sound. The regional distribution of seafloor environments reflects fundamental differences in marine-geologic conditions between the eastern and western parts of the Sound. In the funnel-shaped eastern part, a gradient of strong tidal currents coupled with the net nontidal (estuarine) bottom drift produce a westward progression of environments ranging from erosion or nondeposition at the narrow entrance to the Sound, through an extensive area of bedload transport, to a peripheral zone of sediment sorting. In the generally broader western part of the Sound, a weak tidal-current regime combined with the production of particle aggregates by biologic or chemical processes, cause large areas of deposition that are locally interrupted by a patchy distribution of various other environments where the bottom currents are enhanced by and interact with the seafloor topography.

Statistical aspects of surface drifter observations of circulation in the Santa Barbara Channel

Argos‐tracked drifters are used to study the near‐surface circulation in the Santa Barbara Channel. The mean consists of a cyclonic cell in the western Santa Barbara Channel with weaker flow in the eastern Channel. Drifter mean velocities agree well with record means from near‐surface current meters. At the eastern entrance to the channel, drifter velocities are biased toward outflow (eastward velocity) conditions. Drifter variability at synoptic and seasonal scales shows a tendency for upwelling and eastward flow in spring, a strong cyclonic circulation in summer, poleward relaxation in fall, and weak, variable circulation in winter. Drifter estimates of eddy stress divergence indicate advective terms play a secondary role in the mean surface momentum balance. Lagrangian time and space scales are about 1 day and under 10 km, respectively. The mismatch between Lagrangian and Eulerian timescales indicates advective terms are important to the fluctuating circulation.

Distribution of Epibenthic Mollusca on a Transect Through the Beagle Channel (Southern Chile)

During the Joint Chilean–German–Italian Magellan ‘Victor Hensen’ Campaign in 1994 an epibenthic sledge (EBS) was used to catch the epibenthic and benthic-boundary-layer macrofauna.Eighteen EBS samples were taken at 12 locations on a transect through the Beagle Channel. The aims of the study were to describe the epibenthic molluscan species, to quantify their abundances and to describe their distribution. In total 35,087 specimens were collected, yielding 107,223 ind 1000 nr−2. Bivalvia were most abundant 78,615 individuals (52 spp.) followed by Gastropoda (17,289, 52spp.), Aplacophora (4745, 9 spp.), Polyplacophora (4665) and Scaphopoda (1909,5 spp.). The molluscan abundance of the hauls were strikingly different, between 3 and 37,927 ind 1000 m−2 haul. Only stations off the eastern entrance showed high abundances, which was represented at all depths and sediments. Elsewhere the abundances were low. Species richness was highest in shallow water (< 100 m) with 69 species and decreased with depth. But at the deepest station 37 species were found.

Characteristic patterns of the circulation in the Santa Barbara Channel

The subtidal near‐surface circulation in the Santa Barbara Channel (SBC) and on the shelf north of Point Conception is described based on observations obtained during the 3‐year period from 1993 to 1995. Near‐surface currents in the channel are a superposition of a larger‐than‐SBC scale flow and a cyclonic circulation of variable intensity located inside the channel. On seasonal timescales the larger‐than‐SBC scale flow near the surface is equatorward in spring and poleward from summer through winter. The increase in equatorward flow in spring occurs concurrently with the increase in equatorward wind stress and the decrease in near‐surface temperatures and synthetic subsurface pressures (SSPs). The flow reverses in late spring, simultaneously with the increase in the along‐channel SSP difference and months before wind stress has reached its peak. The period during which the cyclonic circulation within the SBC is strongest coincides with the period of strongest poleward flow through the eastern entrance. A synoptic description of the circulation in the SBC is presented in terms of six characteristic patterns, labeled Upwelling, Relaxation, Cyclonic, Propagating Cyclones, Flood East, and Flood West. An analysis of the 5‐ and 45‐m currents into empirical orthogonal functions (EOFs) isolates 50% (53%) of the 5‐m (45‐m) low‐frequency current variance into three (two) modes. Combining these modes with the mean current fields, the modes have spatial patterns that correspond to the characteristic flow patterns subjectively deduced from inspection of daily averages of the near‐surface currents. From late spring through fall the two largest 5‐m current modes produce a repeating pattern of circulation in which the sequence of four states (namely, Upwelling, Cyclonic, Relaxation, and Quiescent) is traversed roughly every 16 days. In addition to the large‐scale cyclonicity in the central SBC, smaller cyclonic eddies form frequently in the eastern channel and travel toward the west with an average speed of 0.06 m s−1. The patterns described here develop in response to the wind stress and the along‐channel SSP gradient.

Abundance and diversity of peracarid taxa (Crustacea, Malacostraca) along a transect through the Beagle Channel, Patagonia

During the joint Chilean-German-Italian Magellan “Victor Hensen” Campaign in November 1994, samples were taken on a cruise of the RV Victor Hensen in order to obtain faunistic information from the Beagle Channel. Peracarida are an important fraction of the macrobenthos and were sampled in high numbers. Using an epibenthic sledge, 104,618 individuals were collected in total, comprising 62,860 Amphipoda, 14,685 Cumacea, 17,992 Isopoda, 7,168 Mysidacea and 1,893 Tanaidacea. To allow comparisons between stations, these numbers were standardized to a 1,000 m trawling distance, yielding about 368,000 individuals from all stations. Peracarida were most abundant at station 1213, southeast of Isla Picton, in the oceanic area close to the eastern entrance of the Beagle Channel (166,361 ind./1,000 m). Generally, stations off the eastern entrance were characterized by a high number of Peracarida. In the Beagle Channel itself, however, the abundance decreased from east to west with a single peak in peracarid number in the channel east of Punta Yamana. Numbers were much lower at the western entrance (792 ind./1,000 m) and even fewer Peracarida were collected in the Magdalena Channel off Punta Sanchez. Lowest numbers were recorded close to the glacier Romanche and west of Isla Picton at two locations. The composition of peracarid crustaceans was analysed in relation to the background of hydrographical and sedimentological differences, nutrient availability, and knowledge of the other associated fauna in the Beagle Channel. The available data lead us to conclude that abundance and composition of peracarid taxa in and south of the Beagle Channel (off the eastern entrance) seem to be influenced mainly by sediment composition and hydrographical characteristics as indicated above.

The ecological state of the Elefsis Bay prior to the operation of the athens sea outfall

Nutrient pollution in Elefsis Bay arises mainly from the disposal of Athens untreated wastewater at the eastern entrance of the Bay. The absence of river discharge, lack of cultivated agricultural areas providing runoff and the Bay's limited circulation provide the conditions for the Bay's eutrophication by the discharge of untreated wastewater. In the spring of 1993 this discharge was replaced by a new deep outfall system. To monitor any future changes an assessment of the present ecological state of the Bay is required. Analysis of appropriate data showed that Elefsis Bay accumulates nutrients. In summer a strong thermal stratification causes anoxic conditions to occur in the lower layer. The ecosystem exhibits reduced species diversity and strong oscillations of plankton annual cycles. Flagellates (Cryptomonas sp.) dominate the phytoplankton, followed by dinoflagellates (Gyrodinium aureolum, Prorocentum dentatum) and only in the spring diatoms (Nitzschia) predominate. The zooplankton was dominated by copepods (Acartia clausi) and cladoceran (Podon polyphemoides). Macrobenthos consists of a small number of pollution-resistant species of Polychaeta (Capitela capitata), whilst in summer only one species of Molluscs (Corbula gibba) survives.

Anatomy of an invasion: the trans-Arctic interchange

When the Bering Strait between Alaska and Siberia opened about 3.5 Ma during the early Pliocene, cool-temperate and polar marine species were able to move between the North Pacific and Arctic-Atlantic basins. In order to investigate the extent, pattern, and dynamics of this trans-Arctic interchange, I reviewed the Recent and fossil distributions of post-Miocene shell-bearing Mollusca in each of five northern regions: (1) the northeastern Atlantic (Lofoten Islands to the eastern entrance of the English Channel and the northern entrance of the Irish Sea), (2) northwestern Atlantic (southern Labrador to Cape Cod), (3) northeastern Pacific (Bering Strait to Puget Sound), (4) northwestern Pacific (Bering Strait to Hokkaido and the northern Sea of Japan), and (5) Arctic (areas north of the Lofoten Islands, southern Labrador, and Bering Strait).I have identified 295 molluscan species that either took part in the interchange or are descended from taxa that did. Of these, 261 are of Pacific origin, whereas only 34 are of Arctic-Atlantic origin. Various analyses of the pattern of invasion confirm earlier work, indicating that there is a strong bias in favor of species with a Pacific origin.A geographical analysis of invaders implies that, although trans-Arctic interchange contributed to a homogenization of the biotas of the northern oceans, significant barriers to dispersal exist and have existed for trans-Arctic invaders within the Arctic-Atlantic basin. Nevertheless, trans-Arctic invaders in the Atlantic have significantly broader geographical ranges than do taxa with a pre-Pliocene history in the Atlantic.Among the possible explanations for the asymmetry of trans-Arctic invasion, two hypotheses were explicitly tested. The null hypothesis of diversity states that the number of invaders from a biota is proportional to the total number of species in that biota. Estimates of Recent molluscan diversity show that the North Pacific is 1.5 to 2.7 times richer than is the Arctic-Atlantic, depending on how faunistic comparisons are made. This difference in diversity is much smaller than is the asymmetry of trans-Arctic invasion in favor of Pacific species. Rough estimates of regional Pliocene diversity suggest that differences in diversity during the Pliocene were smaller than they are in the Recent fauna. The null hypothesis was therefore rejected.The hypothesis of ecological opportunity states that the number of invaders to a region is proportional to the number of species that became extinct there. The post-Early Pliocene magnitude of extinction was lowest in the North Pacific, intermediate in the northeastern Atlantic, and probably highest in the northwestern Atlantic. The absolute number and faunistic importance of post-Early Pliocene invaders (including trans-Arctic species, as well as taxa previously confined to warm-temperate waters and western Atlantic species that previously occurred only in the eastern Atlantic) was lowest in the North Pacific, intermediate in the northeastern Atlantic, and highest in the northwestern Atlantic. Further support for the hypothesis of ecological opportunity comes from the finding that hard-bottom communities, especially those in the northwestern Atlantic, show a higher representation of molluscan species of Pacific origin, and are likely to have been more affected by climatic events, than were communities on unconsolidated sandy and muddy bottoms. Support for the hypothesis does not rule out other explanations for the observed asymmetry of trans-Arctic invasion.A preliminary study of species-level evolution within lineages of trans-Arctic invaders indicates that anagenesis and cladogenesis have been more frequent among groups with Pacific origins than among those with Atlantic origins, and that the regions within the Arctic-Atlantic basin with the highest absolute number and faunistic representation of invaders (western Atlantic and Arctic) are the regions in which speciation has been least common among the invaders. The asymmetry of invasion is therefore distinct from the asymmetry of species-level evolution of invaders in the various northern marine regions.

Excavations at Castle Acre Castle, Norfolk, 1975–1982 The Bridges, Lime Kilns, and Eastern Gatehouse

Limited excavations were undertaken between 1975 and 1982 in two areas: south of the upper ward gatehouse and in the area of the lower ward eastern entrance. The excavations revealed traces of four bridges, one of which had left clear evidence of the design of its timber piers. Archaeological evidence suggests that all four bridges date from no later than the middle years of the twelfth century. The eastern gatehouse was shown to be a comparatively insubstantial structure, in spite of a major rebuild, suggesting that the main defence in this area was concentrated forward on the great barbican. The latter awaits future investigation. Limited re-examination of the structure of the lower ward Westgate and finds from this area enabled a sequence of gatehouse development and use to be suggested. South of the upper ward gatehouse excavations uncovered two substantial pits; the southern one was a lime kiln, sealed by construction of the successive bridges to the upper ward, and probably associated with the building of the ‘country house’ in the 1070S. The second pit, on the northern side of the upper ward ditch to the south-west of the upper ward gatehouse, could not be fully excavated and its purpose is rather more enigmatic; it was most likely another lime kiln. Investigation of the original topsoil on the northern lip of the upper ward ditch has identified early occupation levels apparently preceeding construction of the country house.