Scotian Shelf Water Research Articles

Warming waters of the Gulf of Maine: The role of Shelf, Slope and Gulf Stream Water masses

We present an analysis of a 19-year time series of mooring data (T, S,) collected since 2003 in Jordan Basin in the eastern Gulf of Maine and beginning in 2016, in situ nitrate; we also include hydrographic station data collected by NOAA’s ECOMON Program. Analyses support, in part, conclusions drawn by others that the Gulf of Maine has warmed and continues to warm as a result of long-term and shorter term air-sea heat fluxes. We show that warming is to a large extent a result of changes in advective heat fluxes via lateral exchanges with water masses in neighboring shelf and slope regions (Warm Slope Water and Scotian Shelf Water). Water properties of the source waters and their inflows to the Gulf have been shown to have been changing over time, and now reveal a mix that includes warm, salty, low nutrient Gulf Stream Water. The result is a new baseline in the Gulf of Maine, of warmer temperatures and higher salinities, which appears to have become established after 2010. Annual cycles of spring-to summer water column warming and fall-to-winter cooling and convective mixing are presented and discussed, showing that the maximum depth of convective mixing in the eastern interior Gulf of Maine does not usually exceed 150 m, a depth constrained by the presence a strong vertical density gradient created by layer of high salinity bottom waters. Approximately-six months after the annual late-summer influx to the Gulf of warm and salty water masses at depth and along the bottom, a mass of cold, low salinity Scotian Shelf Water arrives in the upper water layers in winter – these two major water mass fluxes are a half year out of phase with one another, every year. We suggest that these two water mass intrusion events are linked – that the winter-spring inflow of Scotian Shelf Water creates a barotropic pressure gradient in the Gulf that persists into the summer, thereby impeding the inflow of warm and salty Slope, and/or Gulf Stream Water, until that Scotian Shelf Water mass has become sufficiently dispersed in late summer, thereby effectively opening a valve that allows warm and salty deep waters from outside the Gulf to penetrate.

Sources and sinks of bottom water oxygen in a seasonally hypoxic fjord

Deoxygenation of the ocean has been occurring over the last half century, particularly in poorly ventilated coastal waters. In coastal and estuarine environments, both the water column and sediments play key roles in controlling oxygen variability. In this study, we focus on controls of oxygen concentration in Bedford Basin (BB), a 70 m deep, seasonally hypoxic semi-enclosed fjord on the West Atlantic coast in Nova Scotia. The basin is connected to the Scotian Shelf via a narrow 20 m deep sill that restricts the resupply of bottom water. Hypoxia was recorded seasonally in 2018, 2019 and 2021 with minimum oxygen concentration of 5, 6.7 and 2.7 μM, respectively. Using a 1-D benthic-pelagic coupled model we investigate oxygen consumption and resupply processes during these years. The model was constrained with weekly water column measurements of temperature, salinity, chlorophyll-a fluorescence and dissolved oxygen from a monitoring station in the central basin together with seasonal measurements of benthic diffusive oxygen uptake. Our model suggested that 29-81%, and up to 36% of bottom water re-oxygenation occurred during the winter mixing period and through summer/fall intrusions of Scotian Shelf water, respectively. Occasional shelf water intrusions occurred rapidly, on a timescale of a few hours, and delivered equivalent amounts of oxygen as winter mixing and were sufficient to end bottom water hypoxia. Collectively, these mechanisms supplied the majority of the oxygen delivered to the bottom water. Oxygen supply to bottom waters during periods of water column stratification accounted for 19-36% of the annual flux. The mean benthic uptake was 12 ± 8 mmol m-2 d-1 and contributed ~20% of the total oxygen consumption below the sill depth. In 2021, sea surface temperature (SST) was unusually high and likely resulted in 50% less bottom water oxygenation compared to 2018 and 2019 due to increased stratification; SST in BB was found to be increasing at a rate of 0.11 ± 0.02 °C/year. Climate control on water column stratification are discussed and numerical experiments are used to compare the effects of different water column mixing scenarios on bottom water oxygenation.

Remote silicate supply regulates spring phytoplankton bloom magnitude in the Gulf of Maine

AbstractSpring phytoplankton blooms in the Gulf of Maine (GoM) are sensitive to climate‐related local and remote forcing. Nutrient supply through the slope water intrusion has been viewed as critical in regulating the GoM spring blooms, with an assumption that nitrogen is the primary limiting nutrient. In recent years, this paradigm has been challenged, with silicate being recognized as another potential limiting nutrient, but the source of silicate and its associated water mass remain difficult to be determined. In this study, a time series of spring bloom magnitude was constructed using a self‐organizing map algorithm, and then correlated with the fluctuation of water composition in the deep Northeast Channel. The results reveal the importance of silicate supply from previously less‐recognized deep Scotian Shelf Water inflow. This study offers a new hypothesis for spring bloom regulation, providing a better understanding of mechanisms controlling the spring bloom magnitude in the GoM.

Seasonal Variations and Driving Factors of the Eastern Maine Coastal Current

AbstractTo investigate the coastal current in the Gulf of Maine (GoME) and its relation to forcing from outside of the gulf, a high‐resolution circulation model was developed and validated. Our model shows that the Eastern Maine Coastal Current (EMCC) possesses two cores, an offshore and a nearshore core that peak in summer and spring, respectively. The two cores can be traced back to outflows from the Bay of Fundy from opposite sides of Grand Manan Island, and both cores are deeper and slightly more onshore in summer and fall in response to tidal mixing, surface thermal stratification and wind. The two cores merge south of Pleasant Bay, then split into two branches again east of Mount Desert Rock, where the nearshore branch flows along the coast, while the offshore branch turns southward to recirculate in the eastern GoME. Subject to variations of Scotian Shelf Water and Slope Water (SW) inflows, the offshore veering occurs further upstream (northeastward) in late winter and summer, but gradually shifts downstream (southwestward) from summer to winter. Diagnosis of momentum balance demonstrates that the EMCC is primarily driven by the pressure gradient force (PG), of which the barotropic PG is dominant and offshore, while the baroclinic PG is onshore and increases with depth. The large baroclinic PG at depths, modulated by SW, that is, blended by tidal mixing, offsets the barotropic PG. Near the surface, the barotropic PG is nearly balanced by the Coriolis force, forming the geostrophic EMCC.

Recent warming and decadal variability of Gulf of Maine and Slope Water

AbstractThe oceanographic conditions in the Gulf of Maine, Scotian Shelf, Slope Sea, and surroundings are determined by interplay of two major circulation systems—the Gulf Stream and Labrador Current. This study aims to better understand regional long‐term climate trends caused by the Gulf Stream decadal variability. The in situ data analysis confirms a continuous slow warming within all three areas over the last five decades. It is shown that the warming accelerated in the recent 10 years coinciding with a strengthened northward incursion of warm water in the summer months. Such strong northward migration of warm water was not seen in the four preceding decades, making the current rapid warming different from previous ones. We argue that the recent decadal‐scale warming is unique and may signal that the shift of the thermal regime in this region may be at least partially caused by a changed pattern of the Gulf Stream extension zone's long‐term variability. We found that the Scotian Shelf and Slope Water region has recently been warming much faster than the Gulf of Maine, both in the subsurface (the upper ~ 50 m) and in deeper layers, indicating that the probable cause of the faster warming in the most recent decade is due to the regime change in the Gulf Stream extension region.

Anomalous 1992 spring and summer right whale (Eubalaena glacialis) distributions in the Gulf of Maine

No right whales (Eubalaena glacialis) were sighted during aerial surveys in May-July 1992 in the Great South Channel region of thesouthwestern Gulf of Maine. This was the first year that spring surveys failed to detect right whales in this region. During the latespring/early summer season when right whales would normally be expected in the Great South Channel, a few were sighted in the centralGulf of Maine, none were found in their usual late summer/early autumn feeding areas near Nova Scotia and a few were seen inMassachusetts Bay. The absence of right whales in the Great South Channel in 1992 can be attributed to a shift in the regional zooplanktoncommunity. The usual spring zooplankton of the region is strongly dominated by the calanoid copepod Calanus finmarchicus, verticallyand horizontally aggregated into dense patches which are the preferred foraging areas of right whales. The 1992 zooplankton was dominatedby pteropods, distributed evenly throughout the water column. It is possible, although unlikely, that pteropods are unacceptable prey forright whales. A more likely explanation is that their local densities within small-scale patches were below the energetic threshold requiredfor successful right whale feeding. The shift in zooplankton dominance in 1992 is likely related to significantly reduced water temperaturesand a delay in the development of the usual hydrographic structure of the region. The 1992 temperature and hydrographic anomalies, inturn, can be attributed principally to an unusually large influx of colder and fresher Scotian Shelf Water, and may have been enhanced bywidespread cooling of the Northern Hemisphere caused by sulphuric acid haze in the atmosphere from the June 1991 eruption of Pinatubovolcano in the Philippines.

A 250‐Year, Decadally Resolved, Radiocarbon Time History in the Gulf of Maine Reveals a Hydrographic Regime Shift at the End of the Little Ice Age

AbstractIn order to document relative changes in water mass contributions in the Gulf of Maine (GoM), we used the shell material of the long‐lived ocean quahog (Arctica islandica). A multicentury, crossdated master shell growth chronology facilitated the reconstruction of a radiocarbon Δ14C history prior to the radiocarbon bomb‐pulse of the 1950s. This reconstruction reveals a highly variable Δ14C series (mean = −56.6 ± 8.0‰ (1σ); N = 34) from CE 1685 to 1935. Δ14C values indicate a rapid shift ca. 1860 CE in source waters to the GoM. From CE 1685 to 1860, GoM waters were dominated by an admixture of Warm Slope Water primarily composed of tropical Atlantic surface waters/Gulf Stream Waters, and Scotian Shelf Water. This water regime was followed by a rapid Δ14C transition to a Labrador Slope Water endmember after CE 1860, with an apparent decrease in Scotian Shelf Water. Together, this shift is likely related to broader changes in the Arctic and the Labrador Sea, and a short‐term strengthening of the Atlantic meridional overturning circulation. Labrador Slope Water dominating GoM hydrography in the 1900s is verified by the similarities between this record and other coral‐ and shell‐derived Δ14C records influenced by waters with Labrador Sea origin. This suggests that GoM radiocarbon variability broadly reflects large‐scale ocean circulation processes in the Northwestern Atlantic. The lack of Δ14C values much below the Labrador Slope Water endmember suggests that the interior GoM gets very little to no Antarctic Intermediate Water as other studies had previously suggested.

Oceanic Environmental Effects on American Eel Recruitment to the East River, Chester, Nova Scotia

AbstractThe effect of oceanic environmental conditions on the recruitment of American Eel Anguilla rostrata to coastal waters is poorly understood. This study examined correlations between the annual elver count index of the East River, Chester, Nova Scotia, seasonal oceanic environmental conditions associated with the larval and glass eel stages, and larval and glass eel migration times between the spawning area and continental recruitment. In recent years, the elver fishery of the East River has started earlier in conjunction with increasing inshore, continental shelf, and Gulf Stream water temperatures, with the strength of the correlation decreasing with distance from shore. A northward shift of the Gulf Stream may also have contributed to earlier arrival dates. Annual elver index values increased over the study period but were not significantly correlated with any atmospheric or oceanic environmental conditions, perhaps due to a relatively short time series, although effect sizes (correlation coefficient r) were often medium, a level potentially considered of biological importance. Elver mean lengths varied annually with no trend and were not significantly correlated with the elver index. Significant correlations occurred between the North Atlantic Oscillation and Sargasso Sea primary productivity (lag of 0) and Emerald Basin (Scotian Shelf) water temperatures (lag of –5), while Scotian Shelf chlorophyll values decreased with increasing water temperatures (lag of –3). The mean migration period from spawning to stream entrance at the East River was estimated at 1.2 years, mostly spent growing in and ultimately leaving the Sargasso Sea, then crossing oceanic and continental shelf waters between the Gulf Stream and coastal streams, with a short period of transport along the Gulf Stream. The estimate of total migration period based on observed recruitment times at various sites and the mean of the estimated spawning period averaged 45.5% greater than those estimated from otolith methods, perhaps due to the effect on otolith daily ring deposition during glass eel migration in oceanic water temperatures <10ºC between the Gulf Stream and coast.

The effects of source water masses and internal recycling on concentrations of dissolved inorganic nutrients in the Gulf of Maine

It has been historically documented that deep, nutrient-rich, offshore slope waters that enter the Gulf of Maine through the Northeast Channel are the principal source of nutrients to the region. Silicate (Si(OH)4) and nitrate (NO3) in particular are potentially limiting nutrients for phytoplankton production. We examine here the extent to which nutrient variability in the region may be caused by internal recycling of organic material (i.e., chemical breakdown and dissolution of diatoms) versus variability in water mass sources, which can be identified by temperature and salinity properties. We present here the results from a gulf-wide survey conducted in October 2016. Nutrient samples were collected at 44 stations and compared to temperature, salinity, and beam attenuation profiles. Additionally, suspended particulate material in near-bottom waters was collected on filters at all stations and analyzed for biogenic silica. The results show that after being supplied via a slope water source, nitrate is likely to become depleted and silicate is likely to become enriched at any location in the gulf. We suspect that most of the nutrient variability is due to internal recycling, but there is evidence for an input of Scotian Shelf Water to the Jordan Basin region contributing nutrients at mid-depths and mixing with the deeper slope waters.

Drivers of Regional Bacterial Community Structure and Diversity in the Northwest Atlantic Ocean.

The fundamental role of bacteria in global biogeochemical cycles warrants a thorough understanding of the factors controlling bacterial community structure. In this study, the integrated effect of seasonal differences and spatial distribution on bacterial community structure and diversity were investigated at the regional scale. We conducted a comprehensive bacterial survey, with 451 samples of the Scotian Shelf sector of the Northwest Atlantic Ocean during spring and fall of 2014 and 2016, to analyze the effects of physicochemical gradients on bacterial community structure. Throughout the region, Pelagibacteraceae and Rhodobacteraceae were the most common in the free-living fraction, while Flavobacteriia and Deltaproteobacteria were more abundant in the particle-associated fraction. Overall, there was strong covariation of the microbial community diversity from the two size fractions. This relationship existed despite the statistically significant difference in community structure between the free-living and particle-associated size fractions. In both size fractions, distribution patterns of bacterial taxa, and species within taxa, displayed temporal and spatial preferences. Distinct bacterial assemblages specific to season and depth in the water column were identified. These distinct assemblages, consistent for both 2014 and 2016, suggested replicable patterns in microbial communities for spring and fall in this region. Over all sites, temperature and oxygen values were highly correlated with community similarity, and salinity and oxygen values were the most strongly positively- and negatively correlated with alpha diversity, respectively. However, the strengths of these correlations depended on the depth and season sampled. The bathymetry of the Scotian Shelf, the abrupt shelf break to the Scotian Slope and the major ocean currents dominating in the region led to the formation of distinct on-shelf and off-shelf bacterial communities both in spring and fall. The highest species richness was observed at the shelf break, where water masses from the two major currents meet. Our study establishes the baseline for assessing future changes in the bacterial community of the Scotian Shelf waters, a rapidly changing sector of the Atlantic Ocean.

Diagnosing transit times on the northwestern North Atlantic continental shelf

Abstract. The circulation in the northwestern North Atlantic Ocean is highly complex, characterized by the confluence of two major western boundary current systems and several shelf currents. Here we present the first comprehensive analysis of transport paths and timescales for the northwestern North Atlantic shelf, which is useful for estimating ventilation rates, describing circulation and mixing, characterizing the composition of water masses with respect to different source regions, and elucidating rates and patterns of biogeochemical processing, species dispersal, and genetic connectivity. Our analysis uses dye and age tracers within a high-resolution circulation model of the region, divided into nine subregions, to diagnose retention times, transport pathways, and transit times. Retention times are shortest on the Scotian Shelf (∼ 3 months), where the inshore and shelf-break branches of the coastal current system result in high along-shelf transport to the southwest, and on the Grand Banks (∼ 3 months). Larger retention times are simulated in the Gulf of St. Lawrence (∼ 12 months) and the Gulf of Maine (∼ 6 months). Source water analysis shows that Scotian Shelf water is primarily comprised of waters from the Grand Banks and Gulf of St. Lawrence, with varying composition across the shelf. Contributions from the Gulf of St. Lawrence are larger at near-shore locations, whereas locations near the shelf break have larger contributions from the Grand Banks and slope waters. Waters from the deep slope have little connectivity with the shelf, because the shelf-break current inhibits transport across the shelf break. Grand Banks and Gulf of St. Lawrence waters are therefore dominant controls on biogeochemical properties, and on setting and sustaining planktonic communities on the Scotian Shelf.

Circulation, dispersion and hydrodynamic connectivity over the Scotian Shelf and adjacent waters

A nested-grid ocean circulation modelling system is used in this study to examine the circulation of surfacewaters over the Scotian Shelf and its adjacent coastal water bodies. The modelling system consists of acoarse-resolution (1/12°) barotropic storm surge (outer) model covering the northwest Atlantic Ocean, and afine-resolution (1/16°) baroclinic (inner) model covering the Gulf of St. Lawrence, Scotian Shelf, and Gulf of Maine.The three-dimensional model currents are used to track trajectories of particles using a Lagrangian particle-trackingmodel. The simulated particle movements and distributions are used to examine the dispersion, retention, andhydrodynamic connectivity of surface waters over the study region. The near-surface dispersion is relatively high overwestern Cabot Strait, the inner Scotian Shelf, and the shelf break of the Scotian Shelf. The dispersion is relatively low inNorthumberland Strait. A process study is conducted to examine the physical processes affecting the surface dispersion,including tidal forcing and local wind forcing. The model results show that the tidal currents significantly influence thedispersion of surface waters in the Bay of Fundy.

Spatial and temporal variability in the δ18Ow and salinity compositions of Gulf of Maine coastal surface waters

Hydrographic variability and dynamics in the Gulf of Maine are examined through the investigation of δ18Ow and salinity properties of coastal surface waters. Data from Gulf of Maine waters sampled over a decade, from 2003 to 2015, including a suite of samples that were collected monthly from April 2014 to March 2015, are presented. These water samples fall on a mixing line between Maine River Water (MRW) and Scotian Shelf Water (SSW). However, slope waters likely also contribute to these surface waters. The seasonal variability in water samples collected during 2014 and 2015 indicates the strong influence of river runoff on coastal Gulf of Maine surface water properties. The coastal Gulf of Maine mixing line presented in this paper is a needed baseline for reconstructing hydrographic variability in bicarbonates using oxygen isotopes.

Impacts of the North Atlantic Oscillation on sea surface temperature on the Northeast US Continental Shelf

The North Atlantic Oscillation (NAO) is the dominant large-scale atmospheric oscillation in the North Atlantic and has profound effects on water temperatures in the North Atlantic. In this paper we diagnosed the effects of the NAO on sea surface temperature (SST) in the Northeast US Continental Shelf (NES). Waters in the Gulf of Maine (GOM) originate primarily from Scotian Shelf Water (SSW) at the surface and Labrador Slope Water (LSW) and Warm Slope Water (WSW) at depth through the Northeast Channel. By using a high-resolution SST dataset, we found that the correlation between the NAO and annual mean SST in the GOM is significant and negative at lag of four years. Further spatial correlation analysis shows that the NAO influences SST in the GOM primarily through advection of SSW or shelf water at the surface from the Labrador Sea. Cross-correlation analysis was also applied between the NAO and SSTs in other subregions of the NES (Georges Bank, Southern New England, and Mid-Atlantic Bight), but no statistically significant relationships were found at any lags. Different from temperature at depth in the GOM that is positively influenced by the NAO with a lag of two years, we concluded that the NAO has a significant negative effect on SST in the GOM four years later, while its effects on SSTs in the other three subregions of the NES are negligible. The four-year lagged relationship we found between the NAO and annual mean SST in the GOM provides a robust empirical method to predict the effect of the NAO on annual mean SST in the GOM four years in advance.

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.

Spatial and temporal variability of SST and ocean color in the Gulf of Maine based on cloud-free SST and chlorophyll reconstructions in 2003–2012

The spatial and temporal variability of sea surface temperature (SST) and Chlorophyll-a (Chl-a) in the Gulf of Maine (GOM) is examined using daily, cloud-free Data INterpolating Empirical Orthogonal Function (DINEOF) reconstructions during 2003–2012. The utility of the DINEOF SST and Chl-a is demonstrated through direct comparisons with buoy- and ship-based observations. EOF analyses of cloud-free products are further used to quantify the SST and Chl-a variability on seasonal to inter-annual timescales. The first mode of SST is dominated by an annual cycle in response to net surface heat flux, with SST lagging surface flux by ~57days. The second mode of SST underscores interactions between GOM, the Scotian Shelf, and the slope sea in response to the basin scale atmospheric forcing represented by the North Atlantic Oscillation. The third mode correlates well with the evolution of Scotian Shelf-slope frontal displacement. The first EOF mode of Chl-a is dominated by a winter–spring bloom and a fall bloom, with a spatial distribution modified by the tidal mixing that facilitates nutrient delivery from the deep ocean. The second EOF mode is likely associated with a winter bloom in the warm slope sea, where the low-frequency variations of second modes of SST and Chl-a are in phase, suggesting a possible coupling between physical and biological responses to atmospheric forcing. The third mode of the Chl-a is likely associated with freshening events associated with advection of the Scotian Shelf Water, which enhance stratifications in the eastern GOM.

Seasonal and interannual variability in Gulf of Maine hydrodynamics: 2002–2011

In situ observations including long-term moored meteorological and oceanographic measurements and multi-year gulf-wide ship survey data are used to quantify interannual variability of surface wind, river runoff, and hydrographic conditions in the Gulf of Maine during summers 2002–2011. The cumulative upwelling index shows that upwelling (downwelling)-favorable wind conditions were most persistent in 2010 (2005) over the 10-year study period. River discharge was highest in 2005; peak runoff occurred in early April in 2010 as opposed to late April to middle May in other years. Moored time series show that coastal water temperature was 0.5–2°C warmer than average in summer 2010, and about 2°C colder than average in 2004. Coastal salinity in April 2010 was the lowest in the 10-year study period. Both moored Acoustic Doppler Current Profiler (ADCP) current measurements and dynamic height/geostrophic velocity calculations based on gulf-wide ship survey data show May–June 2010 had one of the weakest alongshore transports in the western Gulf of Maine during the 10-year study period, likely associated with intrusions of warm slope water and fresher-than-usual Scotian Shelf water. Comparisons of coastal currents to the Paralytic Shellfish Poisoning (PSP) closure maps resulting from Alexandrium fundyense blooms suggest a linkage between alongshore transport and the downstream extent of toxicity.

BEACHED BIRD SURVEYS ON SABLE ISLAND, NOVA SCOTIA, 1993 TO 2009, SHOW A DECLINE IN THE INCIDENCE OF OILING

Sable Island, located about 160 km southeast of the landmass of NovaScotia, Canada, is far offshore and provides a platform for beach surveysto monitor oil pollution in Scotian Shelf waters. Sporadic beach surveysconducted there during the 1970s and 1980s indicated that oiled birds andbeached tar were common occurrences. During a survey program fromJanuary 1993 to December 2009, more than 10,800 bird corpses werefound in 171 surveys covering a total of >13,500 km of shoreline. Sixtyfourspecies were recorded, of which 52 were seabirds and waterfowl.The numbers of beached birds and species composition exhibited largefluctuations, which reflected both the seasonal distribution of species andthe effects of weather and beach conditions. The oiling rate of corpsesfor all seabirds and waterfowl combined was 28.6%, and ranged from ahigh of 69.9% in 1996 to a low of 1.4% in 2009. Alcids had the highestrates of oiling (averaging 54.3%), while lower rates were observed forshearwaters (1.9%) and Larus gulls (2.4%). The results of the 1993-2009surveys, as well as those of earlier studies in the 1970s and 1980s, indicatea declining trend in the oiling rate of beached birds on Sable Island.Keywords: oil pollution, marine pollution, seabird oiling rate, beachedbird survey, Sable Island

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.

Transport and retention of dormant copepods in the Gulf of Maine

Abstract Variability in the availability of dormant copepods to seed productive shelf areas has been hypothesized to influence the abundance of the dominant copepod species Calanus finmarchicus in several regions of the North Atlantic. One source of this variability is advection of dormant copepods in deep water. Using Lagrangian particle simulations, we examined the influence of environmental forcing and copepod behavior on transport and retention of dormant C. finmarchicus in the deep Gulf of Maine, in the northwestern Atlantic. Retention in the Gulf of Maine was relatively high, >40% over 6 months, under all conditions simulated. Transport within the Gulf of Maine was high, resulting in shifts of eastern copepods into the western Gulf and of upstream copepods, from slope and Scotian Shelf waters, into the eastern Gulf. Copepod behavior during dormancy was a major source of uncertainty, but it is probably not a major source of interannual variability in retention. Retention increased with the initial depth of dormant copepods, and vertical positioning behavior had a strong influence on retention for simulations started at depths greater than 150 m, because copepods that can stay below basin sill depths are retained. Mean cross-shore winds reduced retention slightly (