Atlantic Shelf Research Articles

A 3‐D data‐assimilative tidal model of the northwest Atlantic

A three‐dimensional (3‐D) barotropic tidal model for the northwest Atlantic is developed for eight leading semi‐diurnal (M2, S2, N2, K2) and diurnal (K1, O1, P1, Q1) tidal constituents based on the Princeton Ocean Model (POM). Multi‐mission altimetric tidal data are assimilated into the model using a simple nudging scheme. The assimilative model results are validated against independent in situ observations and compared with a non‐assimilative run and previous tidal models. The root‐sum‐square error for the assimilative M2, S2, N2, K1 and O1 tidal elevations is 3.1 cm excluding the Bay of Fundy region and 11.1 cm otherwise. Assimilation improves the accuracy of the model tidal elevation by 40–60% and that of the tidal currents by 20–30%. The semi‐diurnal tidal currents agree better with observations than do the diurnal constituents. The model K1 and O1 tidal currents are intensified on several outer‐shelf areas, qualitatively consistent with shelf‐wave theory and moored measurements, but quantitatively overestimated. Results show that the present assimilative model reproduces the primary tidal constituents better than previous regional and inter‐regional models. In particular, the present model results are as accurate as those of Egbert and Erofeeva (2002) for the northwest Atlantic shelf seas as a whole and better if the Bay of Fundy is excluded, pointing to the importance of the high‐resolution multi‐satellite tides to partially compensate for the simple assimilation technique.

Total and phytoplankton mediated bottom-up control of bacterioplankton change with temperature in NE Atlantic shelf waters

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 58:229-239 (2010) - DOI: https://doi.org/10.3354/ame01374 Total and phytoplankton mediated bottom-up control of bacterioplankton change with temperature in NE Atlantic shelf waters Xosé Anxelu G. Morán1,2,*, Alejandra Calvo-Díaz1, Hugh W. Ducklow2 1Centro Oceanográfico de Xixón, Instituto Español de Oceanografía, Camín de L’Arbeyal, s/n, 33212 Xixón, Spain 2The Ecosystems Center, MBL, 7 MBL Street, Woods Hole, Massachusetts 02543-1015, USA *Email: xelu.moran@gi.ieo.es ABSTRACT: The regulation of heterotrophic bacterial growth by resource supply (bottom-up control) was temperature-dependent in our analysis of data obtained during 2006 in the euphotic layer of the southern Bay of Biscay (NE Atlantic) continental shelf. The dataset was split into 2 subgroups using 16°C as the boundary between warm and cool waters based on differences in associated physico-chemical conditions, e.g. inorganic nutrient limitation at higher temperatures. The linear regressions between bacterial biomass (BB) and leucine incorporation rates (LIR) were significantly positive in both temperature regimes, thus indicating similar total bottom-up control, albeit with a slightly higher slope in warm waters (0.33 vs. 0.22). However, the relationship of LIR with phytoplankton biomass (chl a), which is an indicator of bottom-up control that is mediated by phytoplankton, was only significant in waters below 16°C. The analysis of bimonthly variations in the BB-LIR and LIR-chl a correlations indicated that the strength of total bottom-up control significantly increased while the role of phytoplankton in supplying DOM to bacteria diminished with mean temperatures over the 12 to 19°C range, suggesting a seasonal switch in the major source of substrates used by bacteria. We show that the abundance of cells with relatively high nucleic acid content (HNA), which are hypothesized to be the most active ones, was positively associated with bacterial production and specific growth rates in cool but not in warm conditions. These results suggest that HNA bacteria are good predictors of bulk activity and production in temperate ecosystems only when the community relies principally on phytoplankton substrates for growth and metabolism. KEY WORDS: Bacterioplankton · Bottom-up control · Temperature · Bacterial biomass · Bacterial activity · Phytoplankton · Coastal waters Full text in pdf format PreviousNextCite this article as: Morán XAG, Calvo-Díaz A, Ducklow HW (2010) Total and phytoplankton mediated bottom-up control of bacterioplankton change with temperature in NE Atlantic shelf waters. Aquat Microb Ecol 58:229-239. https://doi.org/10.3354/ame01374 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 58, No. 3. Online publication date: January 20, 2010 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2010 Inter-Research.

Seismic stratigraphy of the Vendean-Armorican platform of the French Atlantic shelf: new insights into the history of the North Atlantic ocean

AbstractThe evolution of the North-Atlantic Ocean from its rifting stage during the Upper Jurassic until the present-day passive margin is recorded by the sedimentary wedge of eastern French-Atlantic platform. The study of a dense network of high resolution seismic profiles on the Vendean-Armorican platform (VAP) obtained during INSU-CNRS cruise “Geovend”, led to the characterization of the architecture of the sediment wedge preserved between the coast and Armorican margin shelf edge. This sediment wedge lies on a substratum composed of metamorphic and magmatic rocks of Palaeozoic age (Ub). The sediment wedge comprises six seismic units (U1-U6) bounded by regional unconformities: Jurassic marine succession (U1), Upper Cretaceous marine rocks (U2), Eocene-Oligocene marine deposits of the incipient VAP (U3), Miocene (U4) and Plio-Quaternary (U5) marine deposits overlain by the last sea-level rise ravinement deposits (U6). Above the basal unconformity at the top of Ub, the units are bounded by angular unconformities (top of U1, U2, U3), truncation with channel incision (top U4) or planar marine ravinement (top of U5) surfaces.Most of these unconformities are due to the tectonic activity of the bay of Biscay during the Mesozoic including (1) the North Atlantic rifting during the Jurassic to Early Cretaceous, (2) the propagation of the ocean crust and counterclockwise rotation of the Iberian block during the Aptian-Albian to Coniacian (magnetic anomaly 33–34) producing troughs at the top of U1 filled by downlapping U2 sediment wedges, (3) the Alpine compression at the origin of folding and faulting and the unconformable deposition of U3, and (4) the late compressive deformation during the Miocene that affected U4. The VAP acquires its actual configuration during U4. Sedimentation on the platform was then affected by climatically-controlled relative sea-level changes (U5 to U6) that forced U5 shelf margin sediment deposition above an incised unconformity and subsequently overlain by U6 transgressive sediment blanketing.One of the main interest of the VAP area is the existence of pre- to post-rift units that helps to decipher with high resolution seismics the long-lived evolution of the Armorican margin. Such units are preserved because of the specific characters of this area located on the flank of the former Aquitaine basin (near the “celtaquitaine” flexure) and the presence of the Rochebonne basement high.The VAP thus displays most of the tectonosedimentary evolution of the West Atlantic margins. This paper would however constitute a basis for comparisons to other examples around the Atlantic ocean and then contribute to strengthen the running models of passive margin evolution.

Exploring the marine biotic index (AMBI): variations on a theme by Ángel Borja

The results of manipulating the input data to AMBI are explored using various transformations of numerical species abundance (NAMBI), biomass (BAMBI) and production (PAMBI) from a variety of stations on the NE Atlantic shelf at which the pollution/disturbance status is known. There is a close agreement between the proportions of species in the five AMBI ecological groups and a phylum level meta-analysis axis of increasing environmental impact. All AMBI measures provide a better monotonic relationship with the impact axis than do traditional species diversity measures, which show higher diversity at intermediate levels of disturbance. A marginally better relationship with the impact axis of the meta-analysis is achieved by a moderate (square root) transformation of the data. ‘Production’ data (an appropriate combination of abundance and biomass information) provide only a marginal improvement on abundance data, but are ecologically and functionally much more relevant. Severe transformation of the data, culminating in presence/absence, degrades the relationship with the impact axis, but if only simple species lists are available then these may still be useable in making an environmental assessment.

Fresh from the Ocean

Geology Freshwater springs have been found along the Atlantic coastal shelves of North America and Europe even at depths of several hundred meters below sea level, and several hundred kilometers offshore. In some areas (for example, the southern United States), springs are emanating from coherent aquifers that are recharged high enough above sea level to provide the gravitational potential to push the fresh water out into the denser saltwater. Elsewhere though, the fresh water seems to have been trapped during glacial cycles when sea level was considerably lower. Cohen et al. provide an overview of these settings and explore their formation and resource potential through a series of numerical models. Their models suggest that extensive recharging occurred beneath Pleistocene ice sheets, which extended out onto the shelves or along submarine canyons. They estimate that some submarine aquifers off New England may contain as much as 10,000 km3 of fresh water. Globally, there may be as much as 300,000 km3 of fresh water, though it is nonrenewable. Groundwater 10.1111/j.1745-6584.2009.00627.x (2009).

Seasonal and geographic variations in phytoplankton losses from the mixed layer on the Northwest Atlantic Shelf

The total daily phytoplankton loss from the mixed layer is estimated as the difference between the primary production and the realized change of phytoplankton carbon biomass. A Monte Carlo procedure is used to recover the total loss rates for ten geographic locations on the Northwest Atlantic continental shelf. The strong seasonal and geographic variations in mixed-layer loss rates of phytoplankton are tied closely to the primary production. The daily, mixed-layer, total loss ranges from 50 to 1000 mg C m − 2 d − 1 , which is compared with the output of process models, the closure error being generally less than 10% of the total loss. The model results show that the annual respiration is generally greater than losses due to zooplankton grazing and sinking, except that zooplankton grazing dominates other loss terms on the west Greenland shelf.

Marine climate in the Irish Sea: analysis of a 489-year marine master chronology derived from growth increments in the shell of the clam Arctica islandica

We demonstrate here that the growth increment variability in the shell of the long-lived bivalve mollusc Arctica islandica can be interpreted as an indicator of marine environmental change in the climatically important North Atlantic shelf seas. Multi-centennial (up to 489-year) chronologies were constructed using five detrending techniques and their characteristics compared. The strength of the common environmental signal expressed in the chronologies was found to be fully comparable with equivalent statistics for tree-ring chronologies. The negative exponential function using truncated increment-width series from which the first thirty years have been removed was chosen as the optimal detrending technique. Chronology indices were compared with the Central England Temperature record and with seawater temperature records from stations close to the study site in the Irish Sea. Statistically significant correlations were found between the chronology indices and (a) mean air temperature for the 14-month period beginning in the January preceding the year of growth, (b) mean seawater temperatures for February–October in the year preceding the year of growth (c) late summer and autumn air temperatures and sea surface temperatures for the year of growth and (d) the timing of the autumn decline in SST. Changes through time in the correlations with air and seawater temperatures and changes towards a deeper water origin for the shells in the chronology were interpreted as an indication that shell growth may respond to stratification dynamics.

Extent and deglacial chronology of the last British–Irish Ice Sheet: implications of exposure dating using cosmogenic isotopes

AbstractOver 160 10Be and 36Cl exposure ages pertaining to the extent and chronology of the last British–Irish Ice Sheet (BIIS) are critically reviewed. Despite uncertainties inherent in the use of exposure ages, this approach has demonstrated that at the Last Glacial Maximum (LGM; ca. 26–21 ka) the BIIS extended over all low ground in Scotland and all (or almost all) of Ireland, consistent with recent models depicting extension of the last BIIS to the Atlantic shelf edge. All exposure ages obtained for high‐level sites above trimlines on the mountains of northwestern Scotland and Ireland pre‐date the LGM. This finding confirms that high plateaux in these areas escaped significant glacial erosion, probably under a protective cover of cold‐based glacier ice that remained frozen to the underlying substrate. In terms of deglaciation chronology, the southern Irish Sea basin was probably deglaciated by 19–18 ka, with extensive deglaciation of low ground in Ireland and northeastern Scotland prior to ca. 14.5 ka. Ice cover on low ground in northwestern Scotland apparently persisted after 14.0 ka, suggesting survival of ice in favourable locations during the Lateglacial Interstade (ca. 14.5–12.9 ka). Attempts to date readvances have had mixed success. The Wester Ross Readvance in northwestern Scotland was initially dated to ca. 16.3 ka, but more recent research implies a much younger age (ca. 14.0–13.5 ka). Exposure ages indicating a prolonged stillstand during ice sheet retreat in the Cairngorms at ca. 14.5–14.0 ka conflict with ages obtained on postglacial rockslides and basal radiocarbon dates, which indicate deglaciation before ca. 15.5–15.0 ka. Exposure dating has been successfully employed to confirm a Loch Lomond Stadial (ca. 12.9–11.5 ka) age for moraines in several mountain areas, and to constrain the minimum altitude of the ice cap that extended over the Western Grampians at that time. Copyright © 2009 John Wiley & Sons, Ltd.

Basin-Scale Coherence in Phenology of Shrimps and Phytoplankton in the North Atlantic Ocean

Climate change could lead to mismatches between the reproductive cycles of marine organisms and their planktonic food. We tested this hypothesis by comparing shrimp (Pandalus borealis) egg hatching times and satellite-derived phytoplankton bloom dynamics throughout the North Atlantic. At large spatial and long temporal (10 years or longer) scales, hatching was correlated with the timing of the spring phytoplankton bloom. Annual egg development and hatching times were determined locally by bottom water temperature. We conclude that different populations of P. borealis have adapted to local temperatures and bloom timing, matching egg hatching to food availability under average conditions. This strategy is vulnerable to interannual oceanographic variability and long-term climatic changes.

Compensation and recovery of feeding guilds in a northwest Atlantic shelf fish community

Disturbance of marine ecosystems from fishing has resulted in significant shifts in the composition of fish communities. Such changes can result in shifts in dominance of functional (trophic) guilds within communities and the role that guilds play in ecosystem functioning. In partic- ular, studies in coral reef and kelp forest fish communities have demonstrated a low level of func- tional redundancy, yet few studies have examined fish communities from temperate large marine ecosystems. We compared the abundance and composition of feeding guilds (i.e. planktivores, ben- thivores, amphipod-shrimp feeders, crabivores, echinoderm feeders, shrimp-fish feeders, and pisci- vores) on the continental shelf of the northeast US across 4 decades (i.e. 1970s to the present), exam- ining changes in guild structure to determine if: (1) there have been significant changes in the abundance of functional guilds within the fish community, (2) there have been significant changes within guilds, and (3) there are commonalities in responses with studies from other habitats. We found that 5 of the 7 guilds exhibited remarkable stability in abundance over time despite extreme levels of exploitation and shifts in the abundance of individual species. We suggest there are compen- satory mechanisms within those guilds that resulted in, or are an outcome of, their functional role within the shelf fish community. Comparing our results with studies in more spatially discrete habi- tats such as coral reefs or kelp forests suggests that erosion of functionally redundancies may be related to the resolution involved, be it spatial extent of observations, movement patterns of fishes, taxonomic precision of functional role, or variations in the diversity of species interactions within these ecosystems. Regardless, to prevent any potential for erosion of functional roles, we recommend that guild structure be considered as a unit for monitoring and resource management.

Effect of random hydrodynamic inhomogeneities on low-frequency sound propagation loss in shallow water.

Low‐frequency (100–500 Hz) sound propagation loss on the US Atlantic continental shelf and in the Barents Sea in the presence of stochastic surface waves, and for the US Atlantic shelf also in the presence of internal waves, is studied for the range of up to 150 km by means of numerical simulations. Surface waves simulations are carried out with Pierson–Neumann spectrum. Internal wave field realizations are modeled according to frequency spectrum of thermocline displacements that were recorded in Shallow Water’06 experiment. Qualitative difference between sound propagation loss behavior on the US Atlantic shelf and in the Barents Sea is demonstrated for summertime conditions even without random inhomogeneities. It is shown that whereas internal waves, on average, have a weak effect on propagation loss, surface waves result in its considerable increase in both areas under wintertime conditions with a wind speed of more than 9 m/s. [The authors wish to acknowledge and thank Dr. J. F. Lynch for discussing this work. The work was supported by the Russian Foundation for Basic Research Project 08‐02‐00283 and by the Civilian Research and Development Foundation Project No. REC 010.]

A numerical study of the Southwestern Atlantic Shelf circulation: Stratified ocean response to local and offshore forcing

This article discusses the results of a suite of numerical simulations of the oceanic circulation in the Southwestern Atlantic Shelf region that are aimed to characterize its mean circulation and seasonal variability and to determine the dynamical mechanisms controlling them. Our experiments indicate that south of 40°S the mean circulation is dominated by a general northeastward flow in the southern portion of the shelf, which is controlled by the discharges from the Magellan Straits, tidal mixing, wind forcing, and the offshore influence of the Malvinas Current farther north. The region from 40°S to 33°S presents the highest seasonal variability, with intrusions of cold sub‐Antarctic waters and the northward expansion of mixtures of the Río de la Plata waters in late fall and a slower retraction of the plume during spring‐summer. Wind stress variability seems to be the primarily forcing mechanism for the plume dynamics. These model results are in reasonable agreement with observations and previous model results. The present solutions also reveal important additional features of the shelf response. The along‐shelf circulation, for example, is largely driven by the western boundary currents in the middle and outer shelf, with induced transports that are 3 times larger than in experiments forced by winds and tides. The analysis also indicates that the upstream influence of the Malvinas Current is felt well beyond its retroflection point in the form of a northward middle‐shelf current and that the interaction of the Brazil Current with the Brazilian shelf topography is primarily responsible for inducing steady shelf break upwelling.

Pliocene climate and seasonality in North Atlantic shelf seas

This paper reviews North Atlantic shelf seas palaeoclimate during the interval 4-3Ma, prior to and incorporating the 'Mid-Pliocene warm period' (ca 3.29-2.97Ma). Fossil assemblages and stable isotope data demonstrate northwards extension of subtropical faunas along the coast of the Carolinas-Virginia (Yorktown and Duplin Formations) relative to the present day, suggesting a more vigorous Florida Current, with reduced seasonality and warm water extending north of Cape Hatteras (reconstructed annual range for Virginia 12-30 degrees C). This interpretation supports conceptual models of increased meridional heat transport for the Pliocene. Sea temperatures for Florida (Lower Pinecrest Beds) were similar to or slightly cooler than (summers 25-27 degrees C) today, and were probably influenced by seasonal upwelling of cold deep water. Reduced seasonality is also apparent in the Coralline Crag Formation of the southern North Sea, with ostracods suggesting winter sea temperatures of 10 degrees C (modern 4 degrees C). However, estimates from Pliocene bivalves (3.6-16.6 degrees C) are similar to or cooler than the present day. This 'mixed' signal is problematic given warmer seas in the Carolinas-Virginia, and climate model and oceanographic data that show warmer seas in the 'Mid-Pliocene' eastern North Atlantic. This may be because the Coralline Crag Formation was deposited prior to peak Mid-Pliocene warmth.

The altitude of the last ice sheet in Caithness and east Sutherland, northern Scotland

Synopsis On the mountains of southern Caithness and east Sutherland, geomorphological evidence for the upper limit of glacial erosion implies that, at the last glacial maximum (LGM), warm-based erosive ice extended up to altitudes of 410–600 m. Above the upper limit of glacial erosion is a cover of frost-weathered detritus, with tors on conglomerate bedrock. Kaolinite and gibbsite in soils on such terrain imply that it escaped significant glacial erosion at the LGM. This is confirmed by cosmogenic 10 Be exposure ages of (>) 146.6±15.1 ka and (>) 178.2 ± 18.1 ka obtained for summit tors; sites below the upper limit of glacial erosion yielded deglacial ages of 16.6 ±2.1 to 18.5 ± 2.4 ka. It is unlikely that glacially unmodified summits represent LGM nunataks, as the implied ice thickness is inconsistent with an ice sheet that extended across high ground on Orkney onto the Atlantic shelf; under conservative assumptions at least 70 m of ice must have covered the highest ground in southern Caithness at the LGM. We infer that the absence of evidence for glacial erosion on some summits and plateaux reflects cover by cold-based ice, frozen to the underlying substrate, that preserved the underlying tors and blockfields. This inference calls into question the use of similar evidence in NW Scotland to identify the maximum altitude of the last ice sheet.

Is there a signal of sea‐level rise in Chesapeake Bay salinity?

We evaluate the hypothesis that sea‐level rise over the second half of the 20th century has led to detectable increases in Chesapeake Bay salinity. We exploit a simple, statistical model that predicts monthly mean salinity as a function of Susquehanna River flow in 23 segments of the main stem Chesapeake Bay. The residual (observed minus modeled) salinity exhibits statistically significant linear (p < 0.05) trends between 1949 and 2006 in 13 of the 23 segments of the bay. The salinity change estimated from the trend line over this period varies from −2.0 to 2.2, with 10 of the 13 cells showing positive changes. The mean and median salinity changes over all 23 cells are 0.47 and 0.72; over the 13 cells with significant trends they are 0.71 and 1.1. We ran a hydrodynamic model of the bay under present‐day and reduced sea level conditions and found a bay‐average salinity increase of about 0.5, which supports the hypothesis that the salinity residual trends have a significant component due to sea‐level rise. Uncertainties remain, however, due to the spatial and temporal extent of historical salinity data and the infilling of the bay due to sedimentation. The salinity residuals also exhibit interannual variability, with peaks occurring at intervals of roughly 7 to 9 years, which are partially explained by Atlantic Shelf salinity, Potomac River flow and the meridional component of wind stress.

Structure of a northwest Atlantic Shelf macrofaunal assemblage with respect to seasonal variation in sediment nutritional quality

We examined temporal variation in the relationship between benthic macrofaunal assemblage structure and sediment nutritional quality using core samples taken seasonally from a 232-m deep site in Wilkinson Basin, Gulf of Maine, from October 2003 through August 2004. The benthic assemblage was dominated by deposit-feeding polychaetes of the families Cirratulidae, Paraonidae, and Cossuridae. Assemblage composition and abundance remained relatively constant over the course of the study, despite seasonal changes in sediment nutritional quality. Constant seawater temperatures and/or relatively long species generation times may account for this pattern. Sediment depth-frequency distributions of cirratulid and paraonid polychaetes varied temporally and exhibited subsurface abundance peaks; depth-frequency distributions of cossurid polychaetes, in contrast, were temporally stable. Subsurface peaks of plant pigment concentrations matched those of the cirratulid and cossurid polychaetes, suggesting that these groups transport and cache recently deposited phytodetritus below the sediment surface. This subsurface caching may ameliorate the effects of a seasonally variable food supply, damping any seasonal response of the fauna.

Cross-front variations in adult abundance and recruitment of Patagonian scallop (Zygochlamys patagonica) at the SW Atlantic Shelf Break Front

Abstract Mauna, A. C., Franco, B. C., Baldoni, A., Acha, E. M., Lasta, M. L., and Iribarne, O. O. 2008. Cross-front variations in adult abundance and recruitment of Patagonian scallop (Zygochlamys patagonica) at the SW Atlantic Shelf Break Front. – ICES Journal of Marine Science, 65: 1184–1190. We studied cross-front variations in adult abundance and recruitment of Patagonian scallop (Zygochlamys patagonica) and their relationship with the Shelf Break Front and satellite-derived chlorophyll a in the Southwest Atlantic between 38°S and 39°30′S. Integrated data from commercial fleets (CF, 1996–2005), research cruises (RC, 1998–2005), sea surface temperature (SST; 1985–1997), and satellite-derived chlorophyll a (CSAT; 1998–2004) were included in the analysis. One-way ANOVA was used to assess differences in recruitment and scallop abundance in a cross-front direction. The abundance of scallops was greatest (RC > 200 kg h−1) closer to the onshore side of the front or beneath it, and the East–West extension of scallop beds (∼40 km) matched the seasonal zonal displacement of the front (∼37 km). The highest CSAT gradients were west of or matched the position of the front. The annual mean position of the front correlated with the spatial variability in recruitment within areas where the species recruited successfully (RC > 2000 h−1). The spatial variability in adult abundance and recruitment of scallops is strongly related to the spatial variability in the position of the front and with the high CSAT concentrations associated with it.

Cohort splitting in bluefish, Pomatomus saltatrix, in the US mid‐Atlantic Bight

AbstractAtlantic bluefish exhibit cohort splitting, whereby two modes of juvenile recruits originate from spatially distinct spring‐ and summer‐spawning regions in US Atlantic shelf waters. We evaluate the pattern of cohort splitting in a transition area (US Maryland coastal region and Chesapeake Bay) between the two major spawning regions. Spring and summer cohorts were differentially represented in Maryland estuarine (Chesapeake Bay) and coastal waters. The spring cohort was dominant in Chesapeake Bay, but was not well represented in the ocean environment, and the converse true for the summer cohort. We hypothesized that ocean temperatures control the bimodal spawning behavior and extent of cohort splitting. As evidence, we observed an intervening early summer cohort produced in years when shelf temperatures during early summer were suitably warm for spawning. In most years however, two dominant cohorts were evident. We propose that vernal warming dynamics in the mid‐Atlantic Bight influence spawning behavior and the resultant bimodal pattern of seasonal juvenile cohort production commonly observed along the US east coast.

Global zoogeography of fragile macrozooplankton in the upper 100–1000 m inferred from the underwater video profiler

AbstractStemmann, L., Youngbluth, M., Robert, K., Hosia, A., Picheral, M., Paterson, H., Ibanez, F., Guidi, L., Lombard, F., and Gorsky, G. 2008. Global zoogeography of fragile macrozooplankton in the upper 100–1000 m inferred from the underwater video profiler. – ICES Journal of Marine Science, 65: 433–442. Mesopelagic gelatinous zooplankton fauna are insufficiently known because of inappropriate and infrequent sampling, but may have important trophic roles. In situ imaging systems and undersea vehicles have been used to investigate their diversity, distribution, and abundance. The use of different platforms, however, restricts the comparison of data from different regions. Starting in 2001, the underwater video profiler (UVP) was deployed during 12 cruises in six oceanic regimes (Mediterranean Sea, North Atlantic shelves, Mid-Atlantic Ridge, tropical Pacific Ocean, eastern Indian Ocean, and Subantarctic Ocean) to determine the vertical distribution of organisms in the upper 1000 m. Nine oceanic regions were identified based on the hydrological properties of the water column. They correspond to nine of the biogeochemical provinces defined by Longhurst. In all, 21 morphotypes were recognized: sarcodines (eight groups), ctenophores (two groups), siphonophores, medusae (five groups), crustaceans (one group), chaetognaths, appendicularians, salps, and fish. The similarity in the community assemblages of zooplankton in the 100–1000 m layer was significantly greater within regions than between regions, in most cases. The regions with comparable composition were located in the North Atlantic with adjacent water masses, suggesting that the assemblages were either mixed by advective transport or that environmental conditions were similar in mesopelagic layers. The data suggest that the spatial structuring of mesopelagic macrozooplankton occurs on large scales (e.g. basin scales) but not necessarily on smaller scales (e.g. oceanic front).

Phylogeography of the common shrimp, Crangon crangon (L.) across its distribution range

The common or brown shrimp Crangon crangon (L.) is a highly abundant and important taxon, both ecologically and commercially, yet knowledge on its population structure and historical biogeography is limited. We studied population genetic structure across the distribution range of this species by sequencing a 388 bp fragment of the cytochrome- c-oxidase I gene for 140 individuals from 25 locations. Strong population structuring and high levels of genetic diversity were observed. Four main phylogroups were uncovered: northeastern Atlantic, western Mediterranean, Adriatic Sea and Black Sea. Gene flow of these shrimp across known oceanographical barriers (e.g., the Strait of Gibraltar and/or Oran-Almeria front, Sicilian Straits, and Turkish Straits) is severely restricted. The oldest and most variable populations currently inhabit the western Mediterranean. The observed absence of structure across the entire northeastern Atlantic shelf is proposed not to be due to gene flow, but to relatively recent colonization following the glacial cycles of the late Pleistocene. Black Sea shrimp are currently disconnected from Mediterranean populations, and colonization is inferred, on the basis of coalescent analysis, to have happened relatively recently, but possibly earlier than 7000 years ago. We postulate the hypothesis that C. crangon survived the last brackish-water (<7‰) period inside the Black Sea and/or one of the adjacent inland seas. We conclude that (1) common shrimp populations from different basins are strongly differentiated, (2) gene flow across basins is probably very limited, and (3) the biogeographic history of the taxon is largely in accordance with the geographic history of its distribution range. This study provides further evidence that high population connectivity of marine species (e.g., by policy makers) should not be assumed.