Gulf Of Maine Research Articles

The demography of Calanus finmarchicus (Copepoda: Calanoida) in the Middle Atlantic Bight, USA, 1977–2001

The spatial distribution and annual cycle of abundance for the late-stage copepodites of the calanoid copepod Calanus finmarchicus are described for the shelf and slope waters of the Middle Atlantic Bight (MAB), a subregion of the US Northeast large marine ecosystem. Samples were collected with bongo nets from 1977 to 2001 on broad-scale surveys of the region and from a continuous plankton recorder (CPR) towed at 10-m depth along a route that traversed the area. The copepod’s abundance in shelf waters increased rapidly in early spring and reached maximum levels in May‐June. It declined slowly thereafter, until a minor secondary pulse occurred in late autumn, which was followed by the annual winter low. Slope water abundance of C. finmarchicus peaked in April and was virtually absent there at 10-m depth from July through the following February. The two samplers, both portrayed similar high interannual abundance variability with no long-term trend evident for the 25-year period. Cluster analysis pinpointed three shelf areas with similar abundance patterns, one of which was a region of high density located in the northeastern offshore waters. Evidence is presented that indicates the source of this high abundance is likely from the populations that overwinter in deep water basins of the Gulf of Maine (GOM). The copepod’s interannual abundance variability was found to be negatively correlated with water temperature and unrelated to fluctuations of the North Atlantic oscillation (NAO) index.

Toxic Alexandrium blooms in the western Gulf of Maine: The plume advection hypothesis revisited

The plume advection hypothesis links blooms of the toxic dinoflagellate Alexandrium fundyensein the western Gulf of Maine (GOM) to a buoyant plume derived from river outflows. This hypothesis was examined with cruise and moored-instrument observations in 1993 when levels of paralytic shellfish poisoning (PSP) toxins were high, and in 1994 when toxicity was low. A coupled physical‐biological model simulated hydrography and A. fundyense distributions. Initial A. fundyense populations were restricted to low-salinity nearshore waters near Casco Bay, but also occurred in higher salinity waters along the plume boundary. This suggests two sources of cells—those from shallow-water cyst populations and those transported to shore from offshore blooms in the eastern segment of the Maine coastal current (EMCC). Observations confirm the role of the plume inA. fundyense transport and growth. Downwelling-favorable winds in 1993 transported the plume and its cells rapidly alongshore, enhancing toxicity and propagating PSP to the south. In 1994, sustained upwelling moved the plume offshore, resulting in low toxicity in intertidal shellfish. A. fundyense blooms were likely nutrient limited, leading to low growth rates and moderate cell abundances. These observations and mechanisms were reproduced by coupled physical‐biological model simulations. The plume advection hypothesis provides a viable explanation for outbreaks of PSP in the western GOM, but should be refined to include two sources for cells that populate the plume and two major pathways for transport: one within the low-salinity plume and another where A. fundyense cells originating in the EMCC are transported along the outer boundary of the plume front with the western segment of the Maine coastal current.

Bluefin tuna (Thunnus thynnus) distribution in relation to sea surface temperature fronts in the Gulf of Maine (1994–96)

AbstractFishery‐linked aerial surveys for bluefin tuna (Thunnus thynnus) were conducted in the Gulf of Maine (GOM) from July through October, 1994–96. Each year, from 507 to 890 surface schools were detected and their locations examined in relation to oceanographic conditions. Correlations between bluefin tuna presence and environmental variables were explored for sea surface temperature (SST), distance to a SST front, frontal density (relative density of all SST fronts seen in a given 1 km area for 2 weeks prior to each tuna sighting), and bottom depth and slope. Mean SST associated with bluefin schools was 18.1°C (±2.8). Schools were located at a mean distance of 19.7 km (±19.6) from SST fronts, and in water masses with an average frontal density of 28.2 m km−2 (±35.7). Mean bottom depth of detected schools was 139.0 m (±70.3), and mean bottom slope was 0.7% rise (±0.7). A binomial generalized linear model fit to these variables indicated that bluefin are seen closer to fronts than locations in which no tuna were seen. Using simple and partial Mantel tests, we investigated the spatial correlation between bluefin tuna presence and the environmental variables, controlling for spatial autocorrelation. For each day that schools were sighted, we performed 24 Mantel tests, on a combination of response and predictor variables. The spatial relationship between bluefin tuna and SST fronts was inconsistent. Our analysis identified significant spatial structure in the bluefin school locations that had no significant correlation with any of the measured environmental features, suggesting that other untested features, such as prey density, may be important predictors of bluefin distribution in the GOM.

MASSIVE PREY RECRUITMENT AND THE CONTROL OF ROCKY SUBTIDAL COMMUNITIES ON LARGE SPATIAL SCALES

What drives community change on large spatial scales? An opportunity to address this fundamental question was provided by a massive subtidal recruitment of the mussel, Mytilus edulis, across the southwest Gulf of Maine (GOM), USA, in 1995. Since M. edulis is consumed by many predator species, we hypothesized that the episodic increase in food resources had a bottom-up effect on populations of mussel consumers. The mussel recruitment event was unprecedented in its large spatial extent, high coverage of the sea floor, and depth of penetration into the rocky subtidal zone. During 1995-1997 we tracked mussels and their consumer populations in the area to document what happened. Surveys across a 120-km marine landscape revealed that juvenile mussels covered 33-91% of the bottom (8-12 m depth) at 13 of 17 sites between October 1995 and June 1996. The cover of mussel prey in June 1996 explained significant variation in the density and biomass of mussel predators, the sea stars Asterias spp. and rock crabs Cancer spp., at 16-17 sites 10, 13, and 23 months after mussel recruitment. These results are consistent with the hypothesis that the extensive mussel recruitment had a bottom-up effect on the subtidal food web. Levels of Asterias spp. recruitment, biomass, and density following mussel recruitment were the highest recorded at two monitored sites in 16-18 years. One year after the large pulse of mussel food resources, Asterias spp. recruitment was positively related to the percent cover of mussels, suggesting a feedback between food supply and consumer re- cruitment on a large spatial scale. Crab size-structure data supported the interpretation that recruitment contributed to the high densities of rock crabs recorded in July 1997. We suggest that the bottom-up effects observed in consumer populations were produced by increased growth, elevated reproduction and recruitment, and possibly by immigration. Coupling between bottom-up and top-down control was observed after a lag of 12-14 months post-mussel recruitment when predation by sea stars, rock crabs, and sea urchins eliminated extensive beds of juvenile mussels across the study region. The elimination of Mytilus prey triggered cannibalism in the sea star Asterias vulgaris, which contributed to density-dependent population declines in sea stars between 1996 and 1997. A key as- sumption of a consumer reproduction and recruitment response—that some of the larvae produced by local adult stocks of consumers could be retained in the study region—was tested by investigating the potential larval dispersal of sea stars and crabs in a simulated three-dimensional flow field of the southwest GOM. The simulation supported the as- sumption as 15-75% of particles released below 1-m depth were retained in the region over 2-5 week periods approximating the larval life-span of the consumers. Furthermore, the degree of retention increased dramatically between the surface (1 m) and 15-m depth, implying that the extent of large-scale connectivity by passive larval dispersal is highly dependent on depth. In this study, massive prey recruitment stimulated coupling between bottom-up and top-down forces, resulting in broad-scale changes in local consumer pop- ulations that persisted after the prey-recruitment signal was removed, underscoring the importance of episodic events in marine community dynamics.

Spatial and temporal abundance patterns for the late stage copepodites of Metridia lucens (Copepoda: Calanoida) in the US northeast continental shelf ecosystem

The annual cycle of abundance for the calanoid copepod Metridia lucens and its interannual variability are described from 17 611 samples collected on 193 broad-scale plankton surveys of the US northeast continental shelf region from 1977 to 1999. The copepod's seasonal distribution is illustrated with bi-monthly geographic abundance plots. Abundance of M. lucens is compared with various environmental variables, abundance of the copepod Calanus finmarchicus, density of potential predators and the North Atlantic Oscillation (NAO) index. Metridia lucens abundance increased throughout the ecosystem during spring warming and usually reached peak levels in late spring The highest mean densities were measured in the Gulf of Maine (GOM), where the copepod is usually found year round and exhibits a weak, decreasing west to east abundance gradient. The less dense populations located on Georges Bank and within the Middle Atlantic Bight are characterized by year-round onshore to offshore gradients that strengthen during the summer months when the copepod concentrates offshore where there is a cool layer of subthermocline water, the region often referred to as the cold pool. Interannual variability in abundance showed two persistent periods: above average values from 1979 to 1982, followed by very low levels from 1983 to at least 1987. Metridia lucens abundance in the 1990s has usually been at or above time series means. None of the environmental variables measured during the surveys could be correlated to this variability. The copepod's abundance in the GOM was not related to variation in the MAO index or to changes in predator abundance found in the region. The seasonal abundance cycle and interannual variability of M. lucens in the GOM were found to be very similar to those found for the dominant copepod, Calanus finmarchicus. Reasons for this coupling are discussed.

The Gulf of Maine biogeographical information system project: developing a spatial data management framework in support of OBIS

Central to the development of an inventory of marine life and improved conceptual understanding of the mechanisms that dynamically shape species distribution patterns is the implementation of strategies aimed at enhancing assimilation and access to existing biogeographical information. Using the Internet as a medium, the Gulf of Maine Biogeographic Information System (GMBIS) project provides a framework and a set of reusable tools for the integration, visualization, analysis and dissemination of diverse types of biogeographical and oceanographic information. End-to-end viability of this approach is demonstrated in the context of a series of scientific storylines and a pilot application for the Gulf of Maine (GoM), a well-studied ecosystem that has been subject to large-scale perturbation due to overfishing. Databases at the core of the information system include those of the DFO Bedford Institution of Oceanography and Atlantic Reference Center, which are the product of multidisciplinary research efforts over the last several decades. Development of GMBIS may serve not only as a model for OBIS, but it may also provide a tool supporting new international and Canadian directives for integrated marine resource management. This paper summarizes the status of the GMBIS project, currently in its final phase, and outlines possible future directions in information system development for the CoML.

A simulation study to evaluate impacts of uncertainty on the assessment of American lobster fishery in the Gulf of Maine

The status of the American lobster (Homarus americanus) in the Gulf of Maine (GOM) has been assessed for the last two decades by comparing an estimated current fishing mortality rate for females (Fcur) with a deterministically estimated biological reference point, F10%. The most recent assessment determined that GOM had exceeded the F10% for every year calculated, although in this time landings and abundance have doubled. The current policy does not consider uncertainty in the assessment. This study evaluates the impacts of uncertainty in F10% and Fcur on the status assessment of lobster fishery. Using a Monte Carlo simulation approach, we found that the status assessment would be influenced by uncertainties in estimating F10% and Fcur, and by the choice of decision confidence level reflecting the level of risk managers would like to take. A large uncertainty in Fcur and F10% and a high decision confidence level reduce the likelihood of defining the stock as overfished, and vice versa. Our results suggest that the probability of lobster overfishing may be less than previously thought, and that uncertainty in Fcur and F10% should be quantified and considered in determining the status of the GOM lobster stock.

Climatological phytoplankton chlorophyll and sea surface temperature patterns in continental shelf and slope waters off the northeast U.S. coast

Satellite‐derived chlorophyll estimates from the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) and Coastal Zone Color Scanner (CZCS), a large archive of in situ near‐surface chlorophyll data, and satellite sea surface temperature (SST) measurements were used to quantify spatial and seasonal variability of near‐surface chlorophyll and SST in middle shelf to slope waters off the coast of the U.S. Northeast. The results of empirical orthogonal function (EOF) analysis on normalized monthly fields (after temporal and spatial means were removed) show that all three chlorophyll climatologies have similar mode 1 temporal and spatial patterns in these waters. Mode 1, which explains about half of the total variability in monthly climatological images, shows that shelf waters in the Gulf of Maine (GOM) are out of phase with the mid‐Atlantic bight (MAB), with seasonally high chlorophyll concentrations in winter in the MAB and in summer in the GOM. The three chlorophyll climatologies begin to differ at higher modes (modes 2 and 3), although SeaWiFS and in situ climatologies keep similar features through mode 3. Higher modes in both SST and chlorophyll are related to the effects of tidal mixing on Georges Bank, differences in seasonal stratification between the southwestern and northeastern GOM, and the importance of the spring bloom in MAB outer shelf waters and western GOM. SST patterns during the CZCS and SeaWiFS eras are very similar, and this indicates that the observed differences between results obtained with these two sensors are probably not caused by differences in physical processes during the two satellite eras.

Mitochondrial DNA sequence variation and phylogeographic patterns in harbour porpoises (Phocoena phocoena) from the North Atlantic

The harbour porpoise (Phocoena phocoena)experiences high rates of incidental mortalityin commercial fisheries, and in some areasthese rates are sufficiently high to justifyconcern over population sustainability. Giventhe high incidental mortality, the resolutionof population structure will be important toconservation and management, but in the NorthAtlantic the relationships among many of theputative populations remain unclear. Aprevious genetic study demonstrated substantialgenetic differences between eastern and westernNorth Atlantic populations, however thelocation of this break remained unresolved. Inthe present study, we addressed this issue byincluding new samples from Iceland. Toinvestigate population structure, variation inthe mitochondrial DNA of 370 porpoises wascompared among six locations corresponding toseveral of the putative populations (Gulf ofMaine, Gulf of St. Lawrence, Newfoundland, WestGreenland, Iceland, Norway). The first 342base pairs of the control region were sequencedand genetic variation investigated by analysisof molecular variance (F ST andΦ ST ) and χ2 withpermutation. Although some fine scalepopulation structure was detected, porpoisesfrom Iceland were found to be more similar tothe western populations (W. Greenland, Gulf ofSt. Lawrence, Newfoundland, Gulf of Maine) thanto Norway. Furthermore, porpoises from Norwaywere different from all other regions. Thesepatterns suggest the existence of adiscontinuity between Iceland and Norway,possibly the result of isolating events causedby repeated range contractions and expansionsthroughout Quaternary glaciation events withinthe North Atlantic. These results suggest thatharbour porpoise populations within the NorthAtlantic are distinguishable, but patterns mustbe interpreted in light of their historicalbiogeography.

A 3-D prognostic numerical model study of the Georges bank ecosystem. Part II: biological–physical model

A three-dimensional ecosystem-physical model is presented for the Gulf of Maine (GOM), including Georges Bank (GB) and the Nantucket Shoals (NS). The coupling of a simple nutrient-phytoplankton–zooplankton model to a detailed physical model forced by the M 2 tides generated patterns of plankton and nutrients that agreed closely with data. High phytoplankton biomass developed in regions of strong vertical mixing, particularly on GB and the eastern flank of the NS. Low concentrations were seen in the GOM, where a subsurface chlorophyll maximum developed at 20 m, coincident with the top of the nutricline. High surface phytoplankton concentrations in the GOM, and low concentrations on the top of GB led us to reject several parameterizations of the biological model. A horizontally dependent euphotic depth was required for an accurate simulation of GB and the GOM with a single biological parameter set. High f-ratios developed in the frontal regions, implying a vertical flux of nutrients from below the euphotic zone. Biological patterns were dominated by vertical mixing induced by the tidal forcing, though removing advection from the simulations led to a decrease in the regions of high f-ratio. A surprising feature of the model was the robustness of the simulated patterns of phytoplankton and nutrients, in spite of large changes in biological parameters and the removal of advection from the model. This reinforces the hypothesis of the overall dominance of vertical mixing in structuring the plankton in and around GB.

Interannual variability of boundary fluxes and water mass properties in the Gulf of Maine and on Georges Bank: 1993–1997

Analysis of three years (October 1993–September 1996) of monthly mean current, temperature and salinity observations from moorings in the major Gulf of Maine (GOM) inflows off southwest Nova Scotia (C2) and in Northeast Channel (NECE) reveals some new features of the annual NECE cycles, including (1) a peak in near-surface (<75 m) inflow in spring (vs. late summer at depth), suggesting that dynamic control of shallow and deep layers may be different, (2) a maximum near-surface cross-channel flow toward Georges Bank (GB) in late winter, suggesting a climatological tendency for Scotian Shelf “cross-overs” in that season, and (3) the absence of a significant salinity cycle over most of the water column. Deviations from the annual cycle indicate that the first part of the observation period was characterized by enhanced warm, saline, deep (>75 m) NECE inflow, followed by later episodes of enhanced cold, fresh inflow at C2 and shallow NECE. Generally, the flow rates at C2 and deep NECE were out of phase, with increased inflow at C2 (deep NECE) associated with reduced inflow at deep NECE (C2) and cooler, fresher (warmer,saltier) conditions at both sites. Freshwater transport anomalies to the Gulf are maximum in the surface layers and largely negative(positive) over the first(last) half of the measurements. The timing of these freshwater inflow variations is consistent with observed fluctuations in hydrographic measurements in the GOM and GB, which reached peak salinities in late 1994, then declined through 1995–1996. Oxygen isotope analysis suggests that almost all of the fresh water present on the central cap of GB in 1996 and early 1997 is of northern (Scotian Shelf) origin as opposed to 1994 and 1995 when Maine River Waters contributed 38 and 26%, respectively, to the freshwater (relative to 34.8) on the cap. A simple box model driven by observed changes in the boundary fluxes indicates that over the last half of the measurement period (April 1995–September 1996), the volumetric flow rate through the GOM increased by 10 5 m 3 s −1 (roughly 17% of the total transport, 5.83×10 5 m 3 s −1), and that increased freshwater fluxes in the surface layers at C2 and NECE produced a net decrease of 0.73 in the salinity of the outflow waters. Average volumetric transports at C2(NECE) were roughly twice(half) those observed in the late 1970s, but the total is consistent with climtological estimates. The net change in the freshwater fluxes exceeds the total climatological mean estimate. Examination of possible local and remote sources confirms that the origin of the 1996–1997 freshwater anomaly is in the northern Labrador Sea/Baffin Bay and results from exceptionally cold winters in the early 1990s. Analysis of a similar event in the early 1980s suggests their occurrence is part of a quasi-decadal climate signal which follows the North Atlantic Oscillation (NAO).

Seasonal record for alkenones in sedimentary particles from the Gulf of Maine

C 37–39 alkenones and C 36 alkenoates, biomarkers of haptophyte origin, were measured in a 10-month sediment trap times series from the Wilkinson Basin in the Gulf of Maine (GOM). Highest biomarker flux to the seabed was observed in summertime, the period when surface waters are stratified and a persistent, subsurface chlorophyll maximum (SCM) exists within the upper thermocline and at the base of the euphotic zone throughout the GOM. Comparison of biomarker content and composition of sediment trap particles and underlying surface sediments indicates significant loss (>50%) of signal due to the impact of early diagenesis. Despite such loss, however, C 37 alkenone unsaturation patterns ( U 37 K′) are not altered. Estimates of algal growth temperature made from analysis of U 37 K′ in these sedimentary materials correspond with water temperature measured at the SCM, identifying this biological oceanographic feature as a key site of alkenone export production to the GOM sediment record. Given the common occurrence of SCM in surface waters of the world ocean, particularly the expansive oligotrophic regions of the subtropical to temperate ocean, export of alkenones produced within such features is a potentially widespread biological oceanographic phenomenon which shapes the sediment record for these biomarkers.

Interpopulation differences in growth rates and food conversion efficiencies of young Grand Banks and Gulf of Maine Atlantic cod (Gadus morhua)

Geographically separated Atlantic cod (Gadus morhua) stocks in the northwest Atlantic exhibit life history variation and have been shown to differ genetically. The genetic and environmental contributions to phenotypic differences, however, have not yet been measured. We used common environment experiments to evaluate the importance of temperature on the observed growth variation between Grand Banks (GB) and Gulf of Maine (GOM) cod stocks. Larvae from the GB grew faster than GOM larvae at both 7 and 12°C. Growth rates of juveniles were not different, but GB juveniles had higher food conversion efficiencies than those from the GOM (at both ambient and warm temperatures). The results indicate that faster growth of GOM cod in the wild is not due to a higher genetic capacity for growth rate in GOM than in GB fish. The findings give evidence of genetically based phenotypic variation, which is in agreement with molecular studies on population differentiation in cod, and support the theory of countergradient variation in growth rates of larval fish.

Size-fractionated plutonium isotopes in a coastal environment

We have examined the distribution of individual Pu isotopes ( 239 Pu, 240 Pu , and 241 Pu ) in seawater from the Gulf of Maine (GOM). Samples were size-fractionated with a 1 kD cross-flow ultrafiltration (CFF) membrane. Subfractioned samples were radiochemically purified and Pu isotopes were analyzed using a three-stage thermal ionization mass spectrometer (TIMS). To our knowledge, this is the first time that both size class and Pu isotopic data have been obtained for seawater samples. Within measurement uncertainties a single 240 Pu/ 239 Pu atom ratio of 0.18 was found for all sample collection depths and sample size fractions. This signifies a current, single Pu source in GOM waters, namely global fallout, and suggests that no measurable isotopic fractionation occurred during CFF processing. The majority of Pu was found in the low molecular weight fraction (<1 kD). Colloidal Pu varied from 8% of the total in surface waters to <1% in the deepest (250 m) seawater sample. Evidence suggests that the vertical distribution of Pu in GOM is primarily controlled by conservative mixing processes. The high Pu fraction found in the low molecular size fraction implies that most of the Pu is in the non-particle-reactive oxidized fraction, and is consistent with the conservative Pu behavior. The activity levels are in agreement with other studies which show a slow decrease in Pu with time due to continued mixing and relatively slow particle removal.

Interactive effects of flow speed and particle concentration on growth rates of an active suspension feeder

Suspension-feeding organisms dominate subtidal habitats in the Gulf of Maine (GOM) at depths of 25‐40 m. Parasmittina jeffreysi is an encrusting cheilostome bryozoan found in this zone throughout the GOM. Regional variation between coastal and offshore locations in both flow speed and particle concentration makes the GOM an ideal location to study the effect of these factors on the growth of P. jeffreysi. Particle flux measurements at two offshore and two coastal sites indicated that flow speeds were .3-fold greater offshore than at coastal sites. Chlorophyll a concentrations and particulate quantity and quality were greater at offshore sites, but sedimentation rates were greater in the coastal region. The growth rate of P. jeffreysi was measured at each of the sites using unobstructed colonies free from spatial competition and colonies that remained within their natural community. Both unobstructed colonies and those in quadrats within the natural community displayed no regional difference in growth rates, despite higher particle flux in the offshore region. The interactive effects of flow speed and particle concentration are acting upon growth rates in opposite directions, resulting in the net effect of no regional differences in growth. Growth of unobstructed colonies at one of the coastal sites (Halfway Rock) was less than that at any of the other three sites and may have resulted from the combination of high sedimentation and extremely low flow at this site. A comparison between unobstructed colonies and those in natural quadrats suggested that growth was limited by spatial competition in the natural community. A significant site interaction effect indicates that this response did not occur at all four sites and was absent at Halfway Rock because of poor growth of unobstructed colonies. Any differences in growth due strictly to differential food acquisition (e.g., unobstructed colonies) appear to be dampened by competitive interactions in the natural community.

Spatial and seasonal patterns in abundance and age-composition of Calanus finmarchicus in the Gulf of Maine and on Georges Bank: 1977–1987

The mean seasonal cycle and distribution of various life history stages of C. finmarchicus throughout the Georges Bank (GB)-Gulf of Maine (GOM) region were characterized based on 5966 MARMAP zooplankton samples collected during 106 surveys over a 10-year period (autumn 1977–autumn 1987). A high degree of seasonal and spatial variability in C. finmarchicus abundance throughout the region was evident in contoured portrayals of data, grouped into standard stations and 2-month “seasons”.Eight subareas of the Gulf of Maine-Georges Bank region were identified through cluster analysis of standard stations having similar seasonal patterns in mean abundance of C. finmarchicus stages C3, C4, C5 and adults. These were the northern Gulf of Maine (Northern GOM); southern Gulf of Maine (Southern GOM); Scotian Shelf-coastal Gulf of Maine (Scotian-Coastal GOM); Mass Bay; tidally mixed Georges Bank (Mixed GB); tidal front on the Bank separating mixed from seasonally stratified water (Tidal Front GB); seasonally stratified water on the Bank (Stratified GB) and the Continental Slope adjacent to Georges Bank (SLOPE).A distinct seasonal abundance cycle was present in all subareas, but, the magnitude and timing of annual maxima varied greatly among subareas. Peak abundance was reached early (March–April) in Mixed GB, Tidal Front GB and Mass Bay, and late (July–August) in Northern GOM and Scotian-Coastal GOM. Remaining subareas had maxima in May–June. Abundance increased 10-fold from January–February to March–April and decreased sharply from July–August to September–October in all areas except southern GOM and northern GOM. The amplitude of the annual cycle was weakest in northern GOM and southern GOM, where high concentrations of C. finmarchicus persisted year-round, and strongest in the tidally mixed shallow water on GB, where the sparsest densities of C. finmarchicus occurred most of the year. Abundance curves for the various areas converged in March–April, when C. finmarchicus was ubiquitously very abundant (> 104/10 m2), and diverged from September to December.C. finmarchicus stage distribution in the GB-GOM area was highly negatively correlated with mean water column temperature during the stratified season. This seemed more related to the hydrography of the region, which isolates warmer well mixed Georges Bank from the Gulf of Maine and the stratified areas on the Bank, than to temperature, because Calanus abundances decline on the Bank before water temperatures exceed their preferences.A large part of the spatial and seasonal variation in C. finmarchicus abundance and age structure appears to be tightly coupled to major hydrographic regimes and to major circulation patterns in the region. There was a sharp ecotone between well-mixed Georges Bank and the Gulf of Maine as defined by C. finmarchicus abundance patterns and life history distributions. The ecotone is present year-round but is most apparent during the stratified season (May–October), when thermohaline density gradients and the near-surface current jet along the northern flank are generally strongest. The Gulf of Maine had the highest abundances of C. finmarchicus, and lowest spatial and seasonal variation in the region, while tidally mixed Georges Banks displayed the opposite pattern. This indication of stable population centers in the Gulf of Maine would make it a major source of Calanus in the region, particularly during March–April. Distributional patterns also suggest a strong Calanus influence from Scotian Shelf water in northern Gulf of Maine and on the southern flank of Georges Bank.

Springtime nutrient and chlorophyll a concentrations in the southwestern Gulf of Maine

We report nutrient and phytoplankton pigment concentrations during three hydrographic surveys in the southwestern Gulf of Maine (GOM) during the South Channel Ocean Productivity Experiment (SCOPEX). During the first survey in mid-March 1988, winter conditions prevailed in the deeper central waters of the survey area, with a deep mixed layer, high surface nutrient concentrations (NO 3 > 8 μM), and low Chl a concentrations (<0.5 μg l −1). In shallower areas along the western flank of the southwestern GOM and over Georges Bank, the spring bloom had begun with Chl a concentrations >2 μg 1 −1. During the second survey in late April 1988, the spring bloom was declining in the southwestern GOM, and considerable spatial heterogeneity in the stage of the bloom and the levels of nutrients occurred. Near the sill of the Great South Channel, and east of Cape Cod where a low salinity surface plume had stabilized the water column, nutrient and phytoplankton concentrations were low (NO 3 < 0.5 μM, Chl a 1–2 μg 1 −1. In the deeper offshore waters in some areas, the bloom still continued (NO 3 1–2 μM, Chl a > 4 μg 1 −1), while in the northeastern part of the study area it had just begun (NO 3 > 5 μM, Chl a 1–2 μg l −1 The spring bloom had terminated throughout the southwestern GOM during the third survey in early June 1989. A low salinity surface plume that entered the study area along the western flank extended east from Cape Cod to Georges Bank, resulting in a strong pycnocline at 15–20 m. Nutrient and phytoplankton concentrations in this surface plume were low in the deeper central region of the survey area (NO 3 <0.25 μM, Chl a < 1 μg 1 −1 Beneath this surface plume, subsurface maxima of ammonia and Chl a occurred (NH 3 > 2 μM, Chl a > 1 μg 1 −1). Over Nantucket Shoals and Georges Bank, the water column was well mixed due to enhanced tidal dissipation, resulting in higher nutrient and phytoplankton concentrations. The nutrient and phytoplankton data collected in these hydrographic surveys suggest that phytoplankton abundance during spring is controlled by seasonal hydrographic changes. These observations do not support the SCOPEX hypothesis that dense surface aggregations of the copepod Calanus finmarchicus in this region in late spring are due to unusually high phytoplankton productivity.

Response of water‐leaving radiance to particulate calcite and chlorophyll a concentrations: A model for Gulf of Maine coccolithophore blooms

A coupled atmosphere and ocean radiative transfer model, the Gulf of Maine (GOM) model, was developed to simulate water‐leaving radiance from a vertically stratified ocean containing a bloom of the coccolithophore Emiliania huxleyi. The model is based largely on atmospheric and ocean data representing the Gulf of Maine. The atmospheric submodel simulates direct sunlight and diffuse skylight illuminating the sea surface and is adjusted to account for seasonal changes in atmospheric aerosols. The optical properties of E. huxleyi, required by the ocean submodel, are derived from measurements collected in Gulf of Maine coccolithophore blooms occurring in 1989 and 1990. The modeled response of volume reflectance to the combined effects of chlorophyll and particulate calcite compares favorably with field measurements of E. huxleyi cell abundance and coastal zone color scanner (CZCS)‐derived volume reflectance representing a coccolithophore bloom in the northeast Atlantic Ocean. The GOM model was used to investigate the response of normalized water‐leaving radiance, modeled for visible CZCS and sea viewing wide field of view sensor (SeaWiFS) bands, to particulate calcite and chlorophyll a. Ranges in the concentrations of particulate calcite, chlorophyll a, and colored dissolved organic material (CDOM) are selected to represent conditions reported for coccolithophore blooms. Water‐leaving radiance increases with increasing particulate calcite concentration, primarily because of a disproportionately large amount of backscatter from detached coccoliths (about an order of magnitude larger than is predicted using Mie theory). As a result, CZCS plant pigment algorithms based upon radiance ratios may be corrupted more severely than previously estimated. As an alternative to radiance ratio‐based algorithms, an iterative procedure (also referred to as optimization) is used to invert the GOM model in order to simultaneously retrieve particulate calcite and chlorophyll a concentrations. The approach uses normalized water‐leaving radiance computed for all visible CZCS or SeaWiFS bands. Tests of the approach suggest that independent of errors associated with instrument calibration and atmospheric correction, errors in the retrieved concentrations are small, even when high concentrations of CDOM and vertical structure within the water column are neglected, i.e., with the assumptions that CDOM concentration is small and the water is vertically homogeneous. However, since there are no data sets of contemporaneous chlorophyll a concentration, particulate calcite concentration, and CZCS imagery, a rigorous test of the model and inversion technique must wait for the launch of new ocean color scanners such as the NASA SeaWiFS.

The origin of continental shelf and slope water in the New York Bight and Gulf of Maine: Evidence from H218O/H216O ratio measurements

The H218O/H216O ratio of meteoric water in eastern North America decreases with increasing latitude. The annual weighted average δ18O content of the Middle Atlantic Bight (MAB) and Gulf of Maine (GOM) rivers are −9.33‰ and −10.89‰, respectively. The δ18O compositions of the major east coast rivers are 2‰ enriched during summer months. By using H218O and salinity measurements, both conservative properties of sea water, the geographic origin of continental shelf water can be identified. The isotope‐salinity tracer method, which has been used so successfully in polar regions, is also an effective method in the temperate MAB because subpolar waters are exported to this region. The apparent dilutent of slope water is −22‰ δ18O relative to standard mean ocean water. Water of this composition is presently forming in the Labrador Sea. Melt water from sea ice has nearly the same isotopic composition as the sea water from which it formed and thus may be distinguished from meteoric water. In March 1977, the New York Bight cold pool contained as much as 2.5% sea ice melt water. The Gulf of St. Lawrence is the nearest source of sea ice. The New York Bight ‘cold pool’ was renewed from the north between March and July, which was the extent of our sampling period.