Nova Scotian Shelf Research Articles

Temporal Variability of the Labrador Current Pathways Around the Tail of the Grand Banks at Intermediate Depths in a High‐Resolution Ocean Circulation Model

AbstractThe Northwest Atlantic Shelf and Slope have warmed dramatically in the past decade, changing marine life and challenging fisheries management. A rapid warming event in 2009/2010, linked to a reduced supply of cold water from the Labrador Sea, pushed this region to a new state of unprecedentedly high temperatures that persists today. However, a mechanistic understanding of how the Labrador Current connectivity is reduced at the Tail of the Grand Banks of Newfoundland has been lacking. Here, we present the results of a 25‐year (1993–2017) Lagrangian analysis using the HYbrid Coordinate Ocean Model. Synthetic particles were released in the vicinity of the Labrador Current upstream of the Grand Banks and tracked in a 2‐D velocity field. We found that the Labrador Current can be completely blocked by Gulf Stream eddies and meanders that impinge on the shelf break along the Grand Banks. This blocking can occur in many different locations at, upstream, or downstream of the Tail of the Grand Banks, since the Labrador Current needs a clear passage over a long distance to continue its path. In the simulation, the Labrador Current has been blocked more often since 2008, which led to the warming of the Northwest Atlantic Shelf and Slope. These results, which are consistent with satellite observations, can provide predictability for the New England and Nova Scotia shelf environments potentially helpful for ecosystem management.

Exploring the potential of Sentinel-3 delay Doppler altimetry for enhanced detection of coastal currents along the Northwest Atlantic shelf

In this study, we evaluate Sentinel-3A satellite synthetic aperture radar (SAR) altimeter observations along the Northwest Atlantic coast, spanning the Nova Scotian Shelf, Gulf of Maine, and Mid-Atlantic Bight. Comparisons are made of altimeter sea surface height (SSH) measurements from three different altimeter data processing approaches: fully-focused synthetic aperture radar (FFSAR), un-focused SAR (UFSAR), and conventional low-resolution mode (LRM). Results show that fully-focused SAR data always outperform LRM data and are comparable or slightly better than the nominal un-focused SAR product. SSH measurement noise in both SAR-mode datasets is significantly reduced compared to LRM. FFSAR SSH 20-Hz noise levels, derived from 80-Hz FFSAR data, are lower than 20-Hz UFSAR SSH with 25% noise reduction offshore of 5 km, and 55–70% within 5 km of the coast. The offshore noise improvement is most likely due to the higher native along-track data posting rate (80 Hz for FFSAR, and 20 Hz for UFSAR), while the large coastal improvement indicates an apparent FFSAR data processing advantage approaching the coastlines. FFSAR-derived geostrophic ocean current estimates exhibit the lowest bias and noise when compared to in situ buoy-measured currents. Assessment at short spatial scales of 5–20 km reveals that Sentinel-3A SAR data provide sharper and more realistic measurement of small-scale sea surface slopes associated with expected nearshore coastal currents and small-scale gyre features that are much less well resolved in conventional altimetric LRM data.

Ocean circulation changes drive shifts in Calanus abundance in North Atlantic right whale foraging habitat: A model comparison of cool and warm year scenarios

Starting ~2010, declines in Calanus finmarchicus and Calanus hyperboreus, the biomass dominant copepods on the northwest Atlantic shelf, coincide with notable ocean warming and a northward shift in endangered North Atlantic right whale (Eubalena glacialis) foraging habitat from the Bay of Fundy/Western Scotian Shelf to the southern Gulf of St. Lawrence (sGSL). Abundance changes in right whales’ prey, lipid-rich Calanus late stages, are known to relate to shifts in Calanus demography and changes in Calanus transport. Our objective was to investigate how these mechanistic drivers shaped potential whale preyscapes during the relatively cooler, high-abundance 2000′s vs the warmer, lower abundance 2010′s. Here, bio-physical Calanus model “Cool Year” and “Warm Year” simulations are used to quantify relative effects of the physical environment vs population level. Analysis is focused on the western sGSL, where right whales have recently been observed foraging in high numbers. Simulations indicate that Calanus variability is most strongly influenced by changes in advection from upstream areas, linked to river runoff and winds on the coastal Gaspé Current in late spring and summer. In the simulated Cool Year (2008), high runoff is associated with a strong Gaspé Current hugging the coastline and advection of Calanus into the western sGSL. Conversely, in the simulated Warm Year (2012), lower runoff led to a separation of the Gaspé Current from the coast, transporting Calanus eastward along the Laurentian Channel, away from Shediac Valley. Variation in population size drives changes in late stage Calanus delivery to the western sGSL in late summer and fall. Complementary analyses on the Western Scotian Shelf indicate that the source of Calanus to Roseway Basin can shift between the coastal Nova Scotia Current and Scotian Shelf slope water, but large declines in upstream population drive lower Warm Year abundance. Our findings inform marine managers about how Calanus high abundance areas may continue to change in the future.

Ocean Turbulence and Mixing Near the Shelf Break South-East of Nova Scotia

Formation of coastal fog was observed near the southern tip of Nova Scotia when warm, humid air was advected towards the shore over an area of colder water. The sea-surface temperature in the colder and higher salinity patch near the coast was below 14 °C compared to that of the surrounding sea of ≈ 17–18 °C. Measurements of stratification, currents, and the dissipation rate of turbulence kinetic energy (e) on the Nova Scotia shelf, shelf break, and continental slope revealed the frequent occurrence of shear instability in the pycnocline. The probability of the gradient Richardson number $$(Ri)$$ being less than a critical value of $$Ri_{cr} = 0.25$$ in the depth range 30–90 m exceeded 50% on the shelf and over the slope, and it was above 75% along the shelf break, while in the open ocean pycnocline the probability of $$Ri < 0.25$$ is below 5%. The cumulative distribution functions of the dissipation rate in the pycnocline south-east of Nova Scotia followed the Burr probability model with median values from $$7.9 \times 10^{ - 9}$$ W kg−1 on the slope to $$1.9 \times 10^{ - 8}$$ W kg−1 at the shelf break. The eddy diffusivity ( $$K_{N}$$ ) estimates follow a generalized extreme value distribution with a high median value of $$\left\langle {K_{N} } \right\rangle = 5.8 \times 10^{ - 5}$$ m2 s−1 at the shelf break. The diffusivity $$K_{N}$$ depended on $$Ri$$ in general agreement with the parametrization previously suggested by Lozovatsky and Fernando (Philos Trans R Soc A 371:20120213, 2013) but with a higher level of mixing at $$Ri > 1$$ . This could be relevant to the nature of turbulence generation near the shelf break by random wave instabilities in the pycnocline at high Richardson and high Reynolds numbers.

Geology and prospects of oil and gas bearing of east — Canadian continental margins

At the limit of the East-Canadian continental margin there are three oil and gas regions from north to south: Labrador Sea shelves, margins of the Great Newfoundland Bank and the continental margin of Nova Scotia. In each of these distinguishing regions are a number of sedimentary basins completely plunging under the water. At the shelf of Labrador Sea distinguishing the following large sedimentary basins: Saglek, Hopdale and Havke, at the margin of Newfoundland Bank it is known the basins: Jeanne d’Arc, Flemish Pass and Orphan. At the Nova Scotia shelf there are Nova Scotian and Sable basins. It is remarkable at the lofty latitude like of Labrador Sea region the age of the productive sediments beginning from more ancient rocks (Paleozoic), than in basins situated in law latitude (Mesozoic). In consequence of this the stratigraphy diapason of oil and gas bearing of the north latitude is considerably wide. The prospect of oil and gas bearing in all region is related principally with continental slopes and turbidites sediments in its. Late Jurassic and early Cretaceous reservoirs would be the aim for deep drilling sediments. Wide distribution of late Cretaceous and early Tertiary prospects reservoirs of oil and gas is quite really so far as they are bedded in the shallow horizons. Also the prospect of oil and gas bearing at the margin of the basin is related to late Cretaceous and Tertiary sediments, to deposits of fan and diapirs salt.

Slope-shelf faunal link and unreported diversity off Nova Scotia: Evidence from polychaete data

Deep-water sedimentary habitats off Nova Scotia have only rarely been explored. The topographically and oceanographically complex shelf of Nova Scotia harbours two interesting topographic features, Emerald Basin, a sedimentary habitat reaching greater depths (max of 270 m) than the surrounding shelf and the Gully, the largest canyon in NW Atlantic. Emerald Basin is exposed to upwellings of slope water and harbours predominantly deep-sea hexactinellid sponges. Such distributional pattern resembles “deep-water emergence”. In this study an abundant benthic group, the polychaetes, were selected to test for such deep-water faunal link. Qualitative boxcores were collected from Emerald Basin (180 m depth, N = 5) and the adjacent Gully Canyon (1600 m, N = 3). At species level, there was no overlap in distribution between Emerald Basin (N = 73, S=29) and Gully Canyon (N = 351, S = 60) fauna based on morphological assessment of all specimens and molecular analysis (COI and 16S markers) of selected morphotypes. In an alternative approach, Multivariate analysis (nMDS, Cluster Analysis) of incidence data for polychaete genera (N = 179) from 24 Atlantic sites (5–1600 m) was carried out. These results showed a greater similarity of Emerald Basin polychaetes to bathyal sites (400–1000 m), particularly the 680 m site off Nova Scotia rather than shelf sites (5–80 m), including those on the Nova Scotia shelf. Thus, at 1600 m, the Gully Canyon samples were likely “too deep” for our comparative purposes and depths of < 1000 m should be targeted in the future.Our data also provide the first published assessment of polychaete diversity from the Gully Canyon, suggesting the presence of a diverse assemblage (S = 60). Unusually for a deep-sea site, the Gully Canyon polychaetes are mostly known taxa with wider distribution across bathyal NW Atlantic. Additionally, our molecular data provide an interesting insights into the distribution of several polychaete species commonly found in deep-sea (e.g Aurospio dibranchiata Maciolek, 1981; Ophelina abranchiata Støp-Bowitz, 1948) suggesting wide geographical distribution for some but revealing species complexes for others.

Assessment of the effect of mass-transport deposits on fault propagation in Penobscot area, offshore Nova Scotia

Abstract Three-dimensional (3D) seismic data and well logs from the Penobscot area, located within the Scotian Basin offshore Nova Scotia, are used to assess the role of mass-transport deposits (MTDs) on fault propagation. Four MTDs characterized by chaotic seismic facies were mapped, with the earliest hosted by the Late Cretaceous–Recent Dawson Canyon Formation and latest three hosted by the Banquereau Formation. Two types of faults were also mapped. R-faults are regional faults that cut across all the interpreted MTDs in the study area, while P-faults are polygonal faults that cut across MTDs 2 and 3 but tip out at the basal surfaces of MTDs 4 and 2. Representative seismic profiles and isochron maps of the MTDs and throw–depth ( T–z ) and throw–distance ( T–x ) plots allows us to distinguish the families and propagation history of the faults. Our results show that fault propagation is not affected by the presence or thickness variation of MTDs, and is also unaffected by lithological contrast in the Penobscot area of the Nova Scotian Shelf.

Basin%Ro: A vitrinite reflectance model derived from basin and laboratory data

AbstractThe combination of vitrinite reflectance, apatite fission track and present‐day borehole temperature data is very useful when performing tectonic and thermal reconstructions in sedimentary basins which, in turn, are essential for assessing risk in hydrocarbon exploration and for testing hypotheses of basin evolution. Releasing the full potential of the combined data set requires that the predictive models are accurate in themselves. Here, we calibrate a new kinetic vitrinite reflectance model ‘basin%Ro’ using borehole data from a number of sedimentary basins and vitrinite reflectance data from laboratory maturation experiments. The entire data set is inverted for the kinetic parameters of the reflectance model under consideration of uncertainty in the temperature histories of the calibration samples. The method is not sensitive to inconsistent calibration data, which are revealed by significant corrections to the temperature histories. The performance of the model is tested on independent well data from the East China Sea and the Nova Scotian Shelf. The widely used easy%Ro‐model overestimates vitrinite reflectance in the interval 0.5–1.7% Ro by up to 0.35%. Delimiting of oil generating intervals by prediction of vitrinite reflectance may lead to significant underestimation of the generative potential, which may call for a revision of some petroleum systems. The overestimation by easy%Ro may have fuelled the idea of pressure retardation of vitrinite reflectance evolution under sedimentary basin conditions, where pressures in fact are too low for this to be important.

Water Masses and Nutrient Sources to the Gulf of Maine.

The Gulf of Maine, a semi-enclosed basin on the continental shelf of the northwest Atlantic Ocean, is fed by surface and deep water flows from outside the Gulf: Scotian Shelf Water from the Nova Scotian shelf that enters the Gulf at the surface, and Slope Water that enters at depth and along the bottom through the Northeast Channel. There are two types of Slope Water, Labrador Slope Water (LSW) and Warm Slope Water (WSW); it is these deep water masses that are the major source of dissolved inorganic nutrients to the Gulf. It has been known for some time that the volume inflow of Slope Waters of either type that enters the Gulf of Maine is variable, that it co-varies with the magnitude of inflowing Scotian Shelf Water, and that periods of greater inflows of Scotian Shelf Water have become more frequent in recent years, accompanied by reduced Slope Water inflows. We present here analyses of a ten-year record of data collected by moored sensors in Jordan Basin, in the interior Gulf of Maine, and in the Northeast Channel, along with recent and historical hydrographic and nutrient data, that help reveal the nature of Scotian Shelf Water and Slope Water inflows. Proportional inflows of nutrient-rich Slope Waters and nutrient-poor Scotian Shelf Waters alternate episodically with one another on time scales of months to several years, creating a variable nutrient field upon which the biological productivities of the Gulf of Maine and Georges Bank depend. Unlike decades past, the inflows of Slope Waters of either type do not appear to be correlated with the North Atlantic Oscillation, which had been shown earlier to influence the relative proportions of the two Slope Waters, WSW and LSW, that enter the Gulf. We suggest that of greater importance in recent years are more frequent, episodic influxes of colder, fresher, less dense, and low-nutrient Scotian Shelf Water into the Gulf of Maine, and concomitant reductions in the inflow of deep, nutrient-rich Slope Waters. We also discuss evidence of modified Gulf Stream ring water that penetrated to Jordan Basin in summer of 2013.

Spatio-temporal patterns of stratification on the Northwest Atlantic shelf

A spatially explicit stratification climatology is constructed for the Northwest Atlantic continental shelf using daily averaged hydrographic fields from a 33-year high-resolution, data-assimilated reanalysis dataset. The high-resolution climatology reveals considerable spatio-temporal heterogeneity in seasonal variability with strong interplay between thermal and haline processes. Regional differences in the magnitude and phasing of the seasonal cycle feature earlier development/breakdown in the Middle Atlantic Bight (MAB) and larger peaks on the shelf than in the Gulf of Maine (GoM). The relative contribution of the thermal and haline components to the overall stratification is quantified using a novel diagram composed of two key ratios. The first relates the vertical temperature gradient to the vertical salinity gradient, and the second relates the thermal expansion coefficient to the haline contraction coefficient. Two distinct regimes are identified: the MAB region is thermally-dominated through a larger portion of the year, whereas the Nova Scotian Shelf and the eastern GoM have a tendency towards haline control during the year. The timing of peak stratification and the beginning/end of thermally-positive and thermally-dominant states are examined. Their spatial distributions indicate a prominent latitudinal shift and regionality, having implications for the seasonal cycle of ecosystem dynamics and its interannual variability.

Temporal variability of temperature-nitrate relationship in a coastal region

The inverse relationship between nitrate and temperature (N-T relationship) has been used to estimate new production from remotely sensed sea surface temperature at the regional or global scale of oceans. This study aimed to develop a time-series model of the N-T relationship from automated, continuous hourly observations over two years on the coast of Halifax, Canada. The model demonstrated time-series variability of the N-T relationship at a coastal station on the Nova Scotia Shelf, with adjusted R-2=0.999 4 and RMSE=0.025 7. The maximum residual value was 0.077. The annual temperature variations described a sine curve, and daily, weekly, and monthly variations fluctuated within the normal ranges, controlled by the local climate. The annual variation of nitrate concentration formed nearly a sine curve. Heavy or long-lasting rainfall increased nitrate concentration by 4 to 30-fold in 24 h, and then the increased nitrogen was quickly depleted by phytoplankton growth in 10 to 48 h. In general, biological activity was a key factor in causing nitrate concentration change, dependent mainly on seawater temperature. The power function of the N-T relationship observed in our study area could be used to quickly estimate sea surface nitrate concentration, in combination with temperature data obtained by remote sensing.

Wind-induced interannual variability of sea level slope, along-shelf flow, and surface salinity on the Northwest Atlantic shelf

In this study, we examine the importance of regional wind forcing in modulating advective processes and hydrographic properties along the Northwest Atlantic shelf, with a focus on the Nova Scotian Shelf (NSS)-Gulf of Maine (GoM) region. Long-term observational data of alongshore wind stress, sea level slope, and along-shelf flow are analyzed to quantify the relationship between wind forcing and hydrodynamic responses on interannual time scales. Additionally, a simplified momentum balance model is used to examine the underlying mechanisms. Our results show significant correlation among the observed interannual variability of sea level slope, along-shelf flow, and alongshore wind stress in the NSS-GoM region. A mechanism is suggested to elucidate the role of wind in modulating the sea level slope and along-shelf flow: stronger southwesterly (northeastward) winds tend to weaken the prevailing southwestward flow over the shelf, building sea level in the upstream Newfoundland Shelf region, whereas weaker southwesterly winds allow stronger southwestward flow to develop, raising sea level in the GoM region. The wind-induced flow variability can influence the transport of low-salinity water from the Gulf of St. Lawrence to the GoM, explaining interannual variations in surface salinity distributions within the region. Hence, our results offer a viable mechanism, besides the freshening of remote upstream sources, to explain interannual patterns of freshening in the GoM.

Nutrients and water masses in the Gulf of Maine–Georges Bank region: Variability and importance to blooms of the toxic dinoflagellate Alexandrium fundyense

We report here the results of ten oceanographic survey cruises carried out in the Gulf of Maine–Georges Bank region of the Northwest Atlantic during the late spring to summer period in 2007, 2008 and 2010, for which we examine and characterize relationships among dissolved inorganic nutrient fields, water mass dynamics and cell densities of the toxic dinoflagellate Alexandrium fundyense. Nutrients are supplied to continental shelf waters of the Gulf of Maine–Georges Bank region by inflows of deep offshore water masses; once in the Gulf they are transported with the residual circulation and mix with surface waters, both in the Gulf and on the Bank. Those fluxes of offshore water masses and their nutrient loads are the major source of nutrients for phytoplankton production in the region, including annual blooms of A. fundyense in the Gulf and on Georges Bank. This much is already known. We suggest here that the locations and magnitude of A. fundyense blooms are controlled in part by variable nutrient fluxes to the interior Gulf of Maine from offshore, and, those interior Gulf of Maine waters are, in turn, the main nutrient source to Georges Bank, which are brought onto the Bank by tidal pumping on the Northern Flank. We present evidence that nitrate is the initial form of nitrogenous nutrient for A. fundyense blooms, but it is quickly depleted to limiting concentrations of less than 0.5µM, at which time continued growth and maintenance of the population is likely fueled by recycled ammonium. We also show that phosphate may be the limiting nutrient over much of Georges Bank in summer, allowing recycled ammonium concentrations to increase. Our temperature–salinity analyses reveal spatial and temporal (seasonal and interannual) variability in the relative proportions of two deep source waters that enter the Gulf of Maine at depth through the Northeast Channel: Warm Slope Water (WSW) and Labrador Slope Water (LSW). Those two source waters are known to vary in their nutrient loads, with nitrate concentrations about 50% higher in WSW than LSW, for example, and as such the proportions of these two water masses to one another are important determinants of the overall nutrient loads in the interior Gulf. In addition to these deep slope water fluxes, we show evidence here of episodic fluxes of relatively fresh and low-nutrient shelf waters from the Nova Scotian Shelf, which enter the Gulf in pulses at depths between the surface and approximately 150m, displacing deep slope waters, and consequently they significantly dilute the Gulf's interior waters, reducing nutrient concentrations and, in turn, affect the magnitude of A. fundyense blooms.

Interannual variability in phytoplankton blooms and plankton productivity over the Nova Scotian Shelf and in the Gulf of Maine

A 1D ecosystem model, driven by surface heat and wind forcing and relaxed toward observed salinity profiles, was applied to simulate the interannual and decadal scale variability of phytoplankton blooms and plankton production from 1984 to 2007 in the Nova Scotian Shelf (NSS) and Gulf of Maine (GoM) region. The model captured the mean observed timing and magnitude of the spring (SPB) and fall phytoplankton bloom (FPB) in both systems, as well as observed interannual variations in SPB peak timing. Model simulations for both the GoM and NSS exhibited marked inter- annual variability in SPB and FPB timing (±2 to 3 wk) and magnitude (up to ~1 mg chlorophyll m -3 ). Earlier SPBs and delayed FPBs are linked to enhanced water column stability generated by less saline surface water or sharper salinity gradients over the top 50 m of the water column. The modeled variation in annual primary productivity, mesozooplankton productivity, and particle export flux was modest (<10% of the mean). Years with high primary production were weakly associated with early SPBs (GoM: r = -0.205; NSS: r = -0.51), but there was no significant relationship with water column stability. This suggests that variation in annual productivity in the GoM and NSS reflects a combina- tion of variation in light limitation (which is alleviated by increased water column stability) and nutri- ent limitation (which is exacerbated by increased water column stability) that offset and are of near equal importance when averaged over the year. Interannual variations in fisheries production due to changes in annual productivity are thus likely secondary to profound shifts in fisheries recruitment and production that have been linked to variations in SPB and FPB timing.

Phenology of phytoplankton blooms in the Nova Scotian Shelf-Gulf of Maine region: remote sensing and modeling analysis

Remotely sensed ocean color data and numerical modeling have been used to study the phenology of both spring and fall phytoplankton blooms (FPBs) in the Nova Scotian Shelf (NSS)-Gulf of Maine (GoM) region. The ocean color data reveal a general pattern of westward progression of the spring phytoplankton bloom (SPB), and an eastward progression of the FPB in the NSS-GoM region. The spatial pattern of mean chlorophyll concentration in spring is similar to that in fall, with a lower concentration in the NSS and higher in the GoM. Interannually, there is a weak but significant tendency for years with earlier (delayed) SPBs to be followed by delayed (earlier) FPBs, but the mean chlorophyll concentrations during SPBs are not correlated with those during FPBs. The inter-annual variability of SPB timing is significantly correlated with sea surface salinity (SSS), but the FPB timing is correlated with both SSS and sea surface temperature. The process-oriented numerical modeling experiments suggest that (i) salinity is the main factor influencing the bloom timing and magnitude in the NSS-GoM region, especially for the timing of SPBs; (ii) compared to buoyancy forcing induced by vertical salinity gradients, the impact of surface heating and surface wind stress on the blooms variability is much weaker; and (iii) the nutrient level controls the bloom magnitude, but only has a minor effect on bloom timing. This study provides a quantitative estimation of relationship between changes in local/ remote environmental forcing and phytoplankton phenological shifts, thus improving our understanding on the possible impact of climate change on coastal/shelf ecosystems.

Modeling the connectivity between sea scallop populations in the Middle Atlantic Bight and over Georges Bank

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 380:147-160 (2009) - DOI: https://doi.org/10.3354/meps07916 Modeling the connectivity between sea scallop populations in the Middle Atlantic Bight and over Georges Bank Rucheng C. Tian1,*, Changsheng Chen1,2, Kevin D. E. Stokesbury1, Brian J. Rothschild1, Geoffrey W. Cowles1, Qichun Xu1, Song Hu1, Bradley P. Harris1, Michael C. Marino II1 1Department of Fisheries Oceanography, School for Marine Science and Technology, University of Massachusetts Dartmouth, New Bedford, Massachusetts 02744, USA 2Marine Ecosystem and Environmental Laboratory, Shanghai Ocean University, 999 Hucheng Huan Road, 201306 Shanghai, China *Email: rtian@umassd.edu ABSTRACT: The dispersion and settlement of sea scallop larvae spawned on Georges Bank (GB) and in the Great Southern Channel (GSC) were explored using an individual-based population dynamics model. The model consisted of 4 pelagic life stages (egg, trochophore, veliger, and pediveliger) and 3 benthic life stages (juvenile, young adult, and adult). It was driven by the 1995 to 2005 hindcast flow field predicted by the Finite Volume Coastal Ocean Model (FVCOM) in the Gulf of Maine (GOM), with spawning stocks specified by field survey data. In 1998, 2001, 2004, and 2005, a large amount of larvae drifted southward along the shelf break to the Middle Atlantic Bight (MAB). The potential for long-distance southward transport of larvae was dependent on the upstream flow conditions on the Nova Scotian Shelf, climate forcing, and the timing and location of spawning on GB. The model also predicts considerable larval exchange between the GB and the GSC subpopulations, with 83% of larvae settled in the GSC being spawned on GB, and 46% of larvae settled on GB being spawned in the GSC on average from 1995 to 2005. KEY WORDS: Sea scallop · Individual-based model · Lagrangian simulation · Georges Bank Full text in pdf format PreviousNextCite this article as: Tian RC, Chen C, Stokesbury KDE, Rothschild B and others (2009) Modeling the connectivity between sea scallop populations in the Middle Atlantic Bight and over Georges Bank. Mar Ecol Prog Ser 380:147-160. https://doi.org/10.3354/meps07916 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 380. Online publication date: April 07, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Modeling the influence of low-salinity water inflow on winter-spring phytoplankton dynamics in the Nova Scotian Shelf–Gulf of Maine region

Previous analyses of SeaWiFS ocean color and hydrographic data [fi, R., Davis, C. S., Chen, C. et al. (2007) Influence of ocean freshening on shelf phytoplankton dynamics. Geophys. Res. Lett., 34, L24607. doi: 10.1029/2007GL032010] have suggested that changes in the intensity of low-salinity Scotian Shelf Water (SSW) inflow can significantly influence the winter-spring phytoplankton dynamics in the Scotian Shelf and Gulf of Maine region. In the present study, a 3-D coupled biological-physical model was applied to examine underlying mechanisms through a quantitative comparison of circulation pattern, water column stability, nutrient/ phytoplankton concentrations and net primary productivity between low and high freshening scenarios. The model results revealed that observed levels of surface freshening can significantly change water column stability and therefore control the winter-spring phytoplankton bloom dynamics in different regions. Freshening caused earlier blooms both on the Scotian Shelf and in the eastern Gulf of Maine, agreeing with inferences drawn from prior empirical analyses. Earlier phytoplankton blooms in the low surface salinity case were followed by earlier depletion of nutrients at the surface along with earlier decline of blooms compared with the high surface salinity case. The model results also suggested that surface water freshening can impede vertical nutrient exchange between surface and deep waters, thus reducing the overall spring primary productivity throughout the region. The coupled model described in this study provides a unique tool to quantify the response of lower trophic level production to climate-scale environmental disturbances in a complex but ecologically important shelf ecosystem that has a history of such alternations in surface salinity.

Influence of ocean freshening on shelf phytoplankton dynamics

Climate change‐induced freshening of the ocean can enhance vertical stratification and alter circulation patterns in ways that influence phytoplankton dynamics. We examined the timing of spring phytoplankton blooms and the magnitude of net primary productivity in the Nova Scotian Shelf (NSS) ‐ Gulf of Maine (GoM) region with respect to seasonal and interannual changes in surface water freshening from 1998 to 2006. The general pattern of temporal westward progression of the phytoplankton bloom corresponds with the gradient of increasing sea surface salinity from the NSS in the east to the western GoM. Increased freshening enhances the spatial gradients in bloom timing by stimulating earlier blooms upstream (NSS), but it has less impact downstream (the western GoM). Strong spatial gradients (increasing westward) of mean chlorophyll concentration and net primary productivity during post‐bloom months (May–June) indicate that lower sea surface salinity upstream can likely impede nutrient fluxes from deep water and therefore affect overall productivity.

Dependence of light-saturated photosynthesis on temperature and community structure

Using data collected in the Arabian Sea and on the Nova Scotian Shelf we examined the relationships between temperature, phytoplankton taxonomic composition (indexed by pigment composition) and the size structure (indexed by chlorophyll-specific absorption coefficient at 676 nm), Chlorophyll- a biomass and the light-saturated photosynthesis parameter P m B . Our results show that changes in phytoplankton community structure for the Scotian Shelf closely follow changes in temperature. In the Arabian Sea the relationship between phytoplankton community structure and temperature was less striking and fluctuations in phytoplankton pigment composition were strongly related to Chlorophyll- a biomass. The light-saturated photosynthesis parameter P m B was correlated with temperature for the Scotian Shelf. When we compared data collected on the Scotian Shelf with the temperature-dependent function of Eppley (1972. Fishery Bulletin 70, 1063–1085), the model fitted the data remarkably well. For the Arabian Sea data, however, only a weak temperature dependence in P m B was observed, which may be a result of a strong negative correlation between ambient nitrate concentration and temperature. Our results also show that for both the Scotian Shelf and the Arabian Sea, diatom-dominated waters had similarly low P m B values even though the temperatures of the two systems were markedly different. The absorptive characteristics of phytoplankton on the Scotian Shelf were well correlated with P m B , thus providing another potential predictor of P m B that may, in the future, be routinely accessible by remote sensing. The contrasting relationships among community structure, temperature, and primary production observed in the two study regions suggest that primary production algorithms incorporating such information could be developed and applied on a domain-specific basis. These algorithms should be centred on knowledge of the relationships between physical forcing, community structure and phytoplankton photophysiology.

Natural Gas Hydrate Stability in the East Coast Offshore-Canada

The methane hydrate stability zone beneath the Canadian East Coast oceanic margin has developed to a depth of more than 600 meters beneath the deep water column in the area of the deep shelf and the slope. This zone is continuous spreading from the Labrador continental shelf in the north to the slope of the Nova Scotia shelf in the south. Gas hydrates within the methane hydrate stability zone are detected only in one situation, however, they are numerous in the deeper zone in which type II gas hydrates are present through the whole area at water depths as low as 100-200 m. Well-log indications of gas hydrate situated deeper than the base of the methane hydrate stability zone may be an indication of wetter, compositionally more complicated hydrates that probably are not of bacterial only origin. This could indicate a deep thermogenic source of gas in hydrates. The presence of hydrates in the upper 1000 m of sediments also can be considered as an indicator of deeper hydrocarbon sources.