Photosynthetic production of organic matter by microscopic oceanic phytoplankton fuels ocean ecosystems and contributes roughly half of the Earth's net primary production. For 13years, the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) mission provided the first consistent, synoptic observations of global ocean ecosystems. Changes in the surface chlorophyll concentration, the primary biological property retrieved from SeaWiFS, have traditionally been used as a metric for phytoplankton abundance and its distribution largely reflects patterns in vertical nutrient transport. On regional to global scales, chlorophyll concentrations covary with sea surface temperature (SST) because SST changes reflect light and nutrient conditions. However, the ocean may be too complex to be well characterized using a single index such as the chlorophyll concentration. A semi-analytical bio-optical algorithm is used to help interpret regional to global SeaWiFS chlorophyll observations from using three independent, well-validated ocean color data products; the chlorophyll a concentration, absorption by CDM and particulate backscattering. First, we show that observed long-term, global-scale trends in standard chlorophyll retrievals are likely compromised by coincident changes in CDM. Second, we partition the chlorophyll signal into a component due to phytoplankton biomass changes and a component caused by physiological adjustments in intracellular chlorophyll concentrations to changes in mixed layer light levels. We show that biomass changes dominate chlorophyll signals for the high latitude seas and where persistent vertical upwelling is known to occur, while physiological processes dominate chlorophyll variability over much of the tropical and subtropical oceans. The SeaWiFS data set demonstrates complexity in the interpretation of changes in regional to global phytoplankton distributions and illustrates limitations for the assessment of phytoplankton dynamics using chlorophyll retrievals alone.

The temporal and spatial distributions of SST and surface chlorophyll in northeastern coastal areas (46.0°N, 74.0°W to 40.0°N, 62.0°W) were examined using a year-long time series of composite SST and chlorophyll images during 1998. Five study areas were identified in these images: Gulf of Maine, Nantucket shoals/Georges Bank, Scotian shelf waters, slope water and coastal waters (depth <100 m) from Cape Cod to Nova Scotia. The mean SST and surface chlorophyll for each region was compared to the mean for the entire area. The coldest water (2.5 °C) was observed on the Scotian shelf in late March (YD 89). Throughout the year, coastal waters were cooler than the mean for the entire area. In the Gulf of Maine during a single day in summer (YD 208) SST ranged from 10 °C south of Nova Scotia to 20 °C in the central Gulf with a variance of over 8 °C. The Maine Coastal Current and its offshore extension were responsible for both these observations. Slope water was consistently warmer than the mean for the entire area. Several warm core Gulf Stream rings were observed in the SST images during 1998. These rings were partially responsible for the warm SST and high variances in slope water. Surface chlorophyll levels were higher than the mean in the Gulf of Maine, Nantucket shoals/Georges Bank and coastal waters; levels were below the mean on the Scotian shelf and in slope water. Using the SST and chlorophyll composite images a transect line along 67.6°W was extracted and surface contour plots for the entire year were made. The SST contour plot showed a strong surface manifestation of the Maine Coastal Current first appearing in late spring (YD 150) and continuing until the end of October (YD 300). High chlorophyll on Georges Bank throughout the year was the dominant feature in the surface contour plot of chlorophyll.

We developed an absorption-based primary production model that includes the effects of phytoplankton community size structure for the continental margin and adjoining Gulf Stream waters of the Middle Atlantic Bight (MAB). The model uses seasonal cycles of phytoplankton community size structure from previously published results, representative absorption spectra, remotely sensed chlorophyll concentration, sea surface temperature, photosynthetically active radiation, in situ determination of mixed layer dynamics, and previously determined nitrate concentration. The model allows for both light- and nutrient-limitation during the MAB seasonal cycle. Primary production was calculated every month for 5 yr for study areas representing shelf, shelf break, slope, and Gulf Stream waters. Two main approaches were taken to calculate production: using satellite observations integrating to the depth of the mixed layer and using profile observations integrating to the depth of the euphotic zone. The profile euphotic zone production estimates were greater than the satellite mixed layer estimates. Additionally, the timing of production peaks and troughs was largely related to the depth of integration, with profile euphotic zone peak production occurring generally 2 months after the satellite mixed layer estimates. Relative to cell size and seasonality, primary production was regulated more by biomass than light acquisition capability. Comparison of remotely based production estimates and estimates made with in situ depth-dependent data revealed that approximately 30% of daily water column photosynthesis was missed by satellite-based estimates.

During the first 4 years of the SeaWiFS mission (September 1997 through August 2001), the spring blooms in the Slope Sea increased in magnitude. During the same time, the mean path of the Gulf Stream shifted northward. The northward trend of the Gulf Stream is evidenced in satellite sea-surface temperature imagery as well as in situ temperature, salinity, and current vector information collected by the merchant vessel Oleander on its weekly trip between New Jersey and Bermuda. Surface temperature and salinity increased in the Slope Sea over the 4 years. It surprised us to find a collective increase in phytoplankton chlorophyll, temperature, and salinity, contrary to the commonly observed inverse relationship between temperature and chlorophyll in surface waters of this region. While the Gulf Stream surface waters are depleted of nutrients and low in biomass content, waters at depth are rich in nutrients. Although the Gulf Stream usually serves as a barrier between Sargasso waters to the south and Slope waters to the north, cross-stream exchange occurs when there is upward flow along isopycnals toward the surface waters of the Slope Sea. Under certain conditions, warm-core rings and associated streamers shed by the Gulf Stream may also bring nutrient-rich water up into the euphotic zone. The outflow of surface water from the Labrador Sea appears to influence both the size and horizontal transport of the Slope Sea. When the Slope waters are warm and saline there is less Labrador water present, resulting in less dilution of the Gulf Stream waters leaking into the Slope Sea and less horizontal advection within its cyclonic gyre. While the intensity of the spring blooms during this period has dramatic interannual variability, we found the total surface chlorophyll concentration integrated over the Slope Sea remains nearly unchanged. This suggests that the Labrador is not the major supplier of nutrients, but rather that the Slope Sea receives a steady nutrient supply from the sub-surface Gulf Stream waters.

Ocean margin waters contain a host of dissolved and particulate materials of terrestrial and marine origin. The presence of these materials can confound the chlorophyll a (chl a) estimates retrieved by ocean-color satellites’ empirical algorithms. We apply edge detection software to chlorophyll a and water-leaving radiance (Lwn) data from 1998 sea-viewing wide field-of-view sensor (SeaWiFS) imagery to examine this problem within ocean margin waters off the southeastern continental United States (SEC) and South Atlantic bight (SAB). We identify the location of boundaries differentiating waters containing different backscattering components. Identifying those areas where apparent chl a gradients may be caused by differential backscattering helps to determine the location of those gradients caused by real changes in chl a concentrations. An onshore/offshore phytoplankton gradient and seasonal signal not previously detected in SEC waters was revealed from examination of cross-shelf transect data for the months of the study. Phytoplankton concentrations and associated gradients or fronts were connected with the inner, middle or outer shelf based on the biological response to local physical and atmospheric forcings. River flow and wind stress affect inner shelf chl a distributions, while offshore chl a distributions are controlled by Gulf Stream meanders. Carolina Capes’ oceanography influenced chl a frontal variability in that local region. We also explore the possibility of utilizing the edge detection algorithm to delineate boundaries between waters dominated by different algal classes.

The interannual variability of the spatial extent of the spring bloom in the central Sargasso Sea was quantified by remote-sensing approaches. Proxy measurements employing satellite-derived sea-surface temperature and chlorophyll a concentrations were used to estimate the amount of new production derived from inorganic nitrate supplied by convective winter mixing. Nitrate supply and new productivity were estimated from temperature–nitrate and chlorophyll a–euphotic-zone depth relationships derived from in situ measurements made at the US JGOFS Bermuda Atlantic Time-series Study (BATS) site near Bermuda (31°40′ N, 64°10′ W). Proxy estimates of springtime new production at the BATS latitude fall within the range of previous estimates from geochemical approaches, numerical modeling, and analysis of hydrographic measurements. However, estimates of new production made at BATS are poor predictors of the regional-scale new production due to large interannual differences in the spatial gradients of the controlling factors. Moreover, the BATS latitude is in a region of local maxima of meridional gradients in sea-surface temperature and thereby new production. The BATS site lies in a boundary region between the dynamic, spatially variable and productive northern Sargasso Sea and the spring-bloom-free region of the southern Sargasso. Hence, interpretations of spring bloom-forced regional scale biogeochemical processes from point observations made at BATS should be made with caution.

In its typical use for the study of large scale and relatively slow variability of phytoplankton biomass, ocean-color imagery is often binned in space and in time, and variability within the bin is discarded as noise. Since small- to mesoscale processes at time scales as short as a day may play a significant role in global cycles of carbon and nutrients, characterizing variability at these scales is necessary. With the first four years of nearly continuous daily imagery from the SeaWiFS instrument, we investigated patterns of variability at the mesoscales, operationally defined as that within a 2×2-degree neighborhood. We show that mesoscale variability of chlorophyll concentration (Chl) is high near the coasts, in dynamically active areas, and at the oligotrophic centers of subtropical gyres. High apparent variability over the oligotrophic ocean is a surprising contrast to the low variability in composite imagery at the same locations and may be due to increased relative noise at low mean Chl. Low correlation between pairs of images as little as 1 day apart in the oligotrophic ocean is consistent with a noise artifact, or alternatively may indicate that the observed variability is due to high-frequency phenomena. Spatial patterns of variability observed when data are binned into narrow ranges of mean Chl, suggest oceanographic origins. Patterns of variability in Chl and in sea-surface height have little correlation, suggesting that eddy pumping or turbulent diffusion along temporarily slanted isopycnal surfaces are not the major sources of Chl variability. The correlation between mesoscale anomalies of Chl and sea-surface temperature is not always negative as would have been the case if anomalies were produced mainly by the entrainment of colder, nutrient-rich thermocline waters into the euphotic layer. Instead, we find roughly zonal bands of alternating negative and positive correlations determined by the relative directions of the background gradients of Chl and SST. Thus the most obvious influence of mesoscale motion on the distribution of Chl is advection of the existing gradients. Both long-term means and local anomalies of scatterometric winds from QuikSCAT are also correlated with mean Chl. Much of this correlation appears to be due to changes in the relationship between surface roughness and wind speed, brought on by factors like surface films, thermal stability of the air column, and surface currents. Our analyses show the feasibility of using ocean-color imagery to study mesoscale variability but also identify areas where there is room for major improvements. Minimization of speckling due to imperfect atmospheric correction, in particular, would significantly enhance the utility of SeaWiFS data at mesoscales.

The 4‐year, calibrated SeaWiFS data set provides a means to determine seasonal and other sources of phytoplankton variability on global scales, which is an important component of the total variability associated with ocean biological and biogeochemical processes. We used empirical orthogonal function (EOF) analysis on a 4‐year time series of global SeaWiFS chlorophyll a measurements to quantify the major seasonal (as well as the late El Niño and La Niña phase of the 1997–1998 ENSO) signals in phytoplankton biomass between 50°S and 50°N, and then a second analysis to quantify summer patterns at higher latitudes. Our results help place regional satellite chlorophyll variability within a global perspective. Among the effects we resolved are a 6‐month phase shift in maximum chlorophyll a concentrations between subtropical (winter peaks) and subpolar (spring‐summer peaks) waters, greater seasonal range at high latitudes in the Atlantic compared to the Pacific, an interesting phasing between spring and fall biomass peaks at high latitudes in both hemispheres, and the effects of the 1998 portion of the 1997–1998 ENSO cycle in the tropics. Our EOF results show that dominant seasonal and ENSO effects are captured in the first six of a possible 184 modes, which explain 67% of the total temporal variability associated with the global mean phytoplankton chlorophyll pattern in our smoothed data set. The results also show that the time (seasonal)/space (zonal) patterns between the ocean basins and between the hemispheres are similar, albeit with some key differences. Finally, the dominant global patterns are consistent with the results of ocean models of seasonal dynamics based on seasonal changes to the heating and cooling (stratification/destratification) cycles of the upper ocean.

The influence of two dominant aerosol species on satellite ocean color retrievals is examined off the U.S. East Coast in the western Sargasso subtropical gyre. Waters of very low chlorophyll concentration have normalized water‐leaving radiance (nLw) spectra highest in blue and decreasing monotonically with increasing wavelength. For water with chlorophyll concentrations less than 0.13 mg m−3 we compared the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) nLw spectra for different aerosol conditions (clear, dust‐dominated, and sulfate‐dominated) over 1 year between March 1999 and March 2000. With appropriate atmospheric correction, satellite‐derived nLw spectra should be insensitive to the presence of atmospheric aerosols, but we found the SeaWiFS spectra to be sensitive to the species and optical thickness of aerosols. The SeaWiFS bio‐optical chlorophyll algorithms use nLw(λ)/nLw(555) band ratios (where λ = 443, 490, or 510 nm): any biases in the band ratios result in incorrect chlorophyll concentration estimates. When aerosols were negligible, nLw spectra were consistent with in situ reference data between 443 and 510 nm. With increasing aerosol optical thickness during dust events, most common during summer, the nLw spectra were lowered between 412 and 510 nm, decreasing nLw(λ)/nLw(555) band ratios and resulting in artificially high chlorophyll a estimates. Sulfate‐dominated pixels were associated with elevated nLw spectra between 412 and 555 nm. Increasing sulfate optical thickness corresponded to decreases of the bio‐optical band ratios because of the increase at 555 nm, which also biased the chlorophyll a estimates high. The effect of sulfate upon ocean color retrievals is more problematic than dust off the U.S. east coast because of the nearly constant presence of sulfate along with commonly colocated pollutant aerosols that confound atmospheric correction algorithms.

Satellite observations of global ocean chlorophyll span more than two decades. However, incompatibilities between processing algorithms prevent us from quantifying natural variability. We applied a comprehensive reanalysis to the Coastal Zone Color Scanner (CZCS) archive, called the National Oceanic and Atmospheric Administration and National Aeronautics and Space Administration (NOAA-NASA) CZCS reanalysis (NCR) effort. NCR consisted of (1) algorithm improvement (AI), where CZCS processing algorithms were improved with modernized atmospheric correction and bio-optical algorithms and (2) blending where in situ data were incorporated into the CZCS AI to minimize residual errors. Global spatial and seasonal patterns of NCR chlorophyll indicated remarkable correspondence with modern sensors, suggesting compatibility. The NCR permits quantitative analyses of interannual and interdecadal trends in global ocean chlorophyll.

More than 50 years ago, Harald Sverdrup developed a simple model for the necessary conditions leading to the spring bloom of phytoplankton. Although this model has been used extensively across a variety of aquatic ecosystems, its application requires knowledge of community compensation irradiance ( I C ), the light level where photosynthetic and ecosystem community loss processes balance. However, reported I C values have varied by an order of magnitude. Here, I C estimates are determined using satellite and hydrographic data sets consistent with the assumptions in Sverdrup's 1953 critical depth hypothesis. Retrieved values of I C are approximately uniform throughout much of the North Atlantic with a mean value of 1.3 mol photons meter −2 day −1 . These community-based I C determinations are roughly twice typical values found for phytoplankton alone indicating that phytoplankton account for approximately one-half of community ecosystem losses. This work also suggests that important aspects of heterotrophic community dynamics can be assessed using satellite observations.

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.

One indicator of health in estuarine and coastal ecosystems is the ability of local waters to transmit sunlight to planktonic, macrophytic, and other submerged vegetation for photosynthesis. The concentration of coloured dissolved organic matter (CDOM) is a primary factor affecting the absorption of incident sunlight in coastal and estuarine waters. In estuaries, CDOM concentrations vary due to changes in salinity gradients, inflows of industrial and domestic effluents, and the production of new dissolved organic matter from marine biologic activity. CDOM absorption data have been collected from a variety of waters. However, there are a limited number of measurements along the US east coast and a general lack of data from New England waters.This study characterized the temporal and spatial variability of CDOM absorption over an annual cycle in Narragansett Bay and Block Island Sound (Rhode Island). Results suggested that, in Narragansett Bay, the magnitude of CDOM absorption is related to the seasonal variability of freshwater input from surrounding watersheds and new CDOM production from in situ biologic activity. The data show that the average CDOM absorption coefficient at 412 nm was 0.45 m(-1) and the average spectral slope was 0.020 nm(-1). (C) 2002 Elsevier Science Ltd. All rights reserved.

During the lifetime of the Coastal Zone Color Scanner, there were 21 instances in which both satellite-derived ocean color and sea-surface temperature are simultaneously available over large areas of the Sargasso Sea. These images reveal close correspondence between mesoscale structures observed in temperature and pigment fields. In general, higher (lower) pigment biomass occurs in mesoscale features consisting of cold (warm) temperature anomalies. This relationship is consistent with the idea that upward displacement of isopycnals at the base of the euphotic zone by mesoscale eddies is an important mechanism of nutrient supply in the region.

During late spring 1997, a Gulf Stream warm-core ring (WCR) strongly influenced water mass and chlorophyll distributions along the southern flank of Georges Bank. Entrainment of Georges Bank shelf water by the WCR, centered at the central southern flank, persisted through May. By late May, shelf water encircled nearly the entire WCR, and horizontal convergence toward the central southern flank was evident. Scotian Shelf water (SSW) extended across Northeast Channel, onto Georges Bank near the northeast peak, and along the southern flank between the 60-m isobath and the shelfbreak front. SSW extended furthest southwest along the southern flank shelfbreak. Satellite imagery from the ocean color and temperature sensor (OCTS) showed that by late May, in addition to the high chlorophyll concentrations within the entrained shelf water encircling the WCR, pigment rich bands of chlorophyll developed along the 60- and 100-m isobaths. Where this biological enhancement developed along >100 km of the shelfbreak (100-m isobath), shelf waters extended furthest seaward due to entrainment by the WCR. The enhanced shelfbreak chlorophyll was sampled in situ along two transects separated by ≈40 km along shelf. Along both transects, the enhanced chlorophyll coincided with divergent cross-shelf flow, maximum along-shelf flow and minimum surface temperature. Coincidence of the highest surface chlorophyll and lowest surface temperature with divergent cross-shelf flow is consistent with upwelling. Coincidence with along-shelf jets and their associated vertical shear is consistent with turbulent vertical mixing. The importance of turbulent vertical mixing was supported. Maximum velocity at the shelfbreak coincided with the subsurface temperature minimum of SSW near 30-m depth. Estimated gradient Richardson numbers (Ri) above the SSW were below critical (<0.25) within the shelfbreak chlorophyll maximum along both transects, and surface chlorophyll was significantly, inversely correlated with Ri along the southern flank. WCR entrainment of shelf water continued into June as the WCR propagated southwestward. On June 11, high chlorophyll concentrations along the shelfbreak north of the WCR were observed by the OCTS. Satellite sea-surface temperature showed that in addition to WCR entrainment, breaking waves (λ≈30 km) of the shelf-slope front coincided directly with the locally enhanced chlorophyll. The waves propagated west at ≈12kmday−1. Vertical mixing due to these breaking frontal waves may have contributed to local nutrient enrichment of near-surface waters at the shelfbreak.

The purpose of our study was to use the 7.5-year coastal zone color scanner (CZCS) image time series (Oct. 1978 to July, 1986) to study general patterns in near-surface phytoplankton chlorophyll concentrations in ocean margin waters off the US East Coast. We defined 21 relatively large study areas (>100km2) within the MAB and SAB to set boundaries for averaging and subsequent analyses. Our objective was to partition the observed CZCS-derived chlorophyll concentration (CSAT, mgm−3) variability of these 21 study areas within three general categories based on time scale: daily (i.e. day–week), seasonal and interannual. An additional objective was to determine relations between the temporal patterns in the 21 study areas. All available CZCS imagery (more than 3500 scenes of Level 1 imagery, i.e. top-of-the-atmosphere radiance in satellite swath coordinates) covering some or all of our area of interest (northwest Atlantic off the US East Coast) were obtained at full resolution, processed to Level 2 (water-leaving radiance, chlorophyll concentration and other derived products in satellite swath coordinates) and mapped to two different study regions located off the southeast and northeast coasts of the US. Satellite-derived estimates of near-surface chlorophyll concentrations (CSAT) were extracted on a pixel-by-pixel basis from each of the 21 study areas (chosen based on oceanographic criteria) from each of the daily composite CSAT images. For each image and when satellite coverage permitted, CSAT values were averaged to yield a time series of daily mean values for each of the 21 study areas. We used three basic approaches to quantify temporal and spatial patterns in the 21 time series: (1) multiple linear correlation, (2) structure functions (semi-variance calculations) and (3) empirical orthogonal functions (EOF). Our results show:(1)a simple annual CSAT cycle common to all ocean margin waters along the entire US East Coast, consisting of a broad peak in CSAT concentration during winter and minimum concentrations during the summer;(2)relatively subtle across- and along-shelf changes to the timing and relative magnitude of the winter CSAT maxima and summer minima, as well as the presence of secondary seasonal peaks in some regions;(3)high variability at time scales of days to weeks superimposed on the seasonal pattern;(4)high spatial coherence of the seasonal component between all 21 study areas;(5)high coherence of the days-to-weeks component between adjacent study areas, but generally low or no coherence for study areas not adjacent or near each other; and(6)detectable, but low interannual variability.

Meanders of the shelf break front in the Mid‐Atlantic Bight (MAB) during April and May of 1997 were associated with chlorophyll enhancement along a hydrographic and a topographic feature. The hydrographic feature was the surface outcrop of the front, which ranged from ∼10 to >100 km seaward of the shelf break owing to the meanders. The topographic feature was the shelf break (100‐m isobath). Chlorophyll enhancement was observed by a satellite instrument, the ocean color and temperature sensor, and by a fluorometer in situ. It developed in near‐surface waters typically nutrient depleted during late spring, thus local nutrient enrichment of near‐surface waters was probable. Observations of sufficient resolution to define processes were available only for the region of shelf break chlorophyll enhancement. Along two meander troughs (shoreward extremities near the shelf break), we observed shoaling of cold shelf water. Shelf water shoaled >20 m along frontal isopycnals, and phytoplankton absorption maxima coincided directly with the shoaled water. Thus local nutrient enrichment by along‐isopycnal upwelling was the supported mechanism of chlorophyll enhancement at the shelf break. The basis for along‐isopycnal upwelling was seaward flow of shelf water forced by meander circulation near the shelf break. Strong cross‐isobath flow and mixing developed as these meanders propagated along the shelf break front of the MAB at a relatively constant rate of ∼9 km day−1.

In 8 yr (1979–1986) of Coastal Zone Color Scanner (CZCS) imagery, we find annual enhancement of chlorophyll at the shelfbreak of the Mid‐Atlantic Bight (MAB) and Georges Bank during the spring transition from well‐mixed to stratified conditions. Spatial and temporal extents of enhancement vary interannually, and expression is intermittent intraannually. This feature can span the entire MAB and southern flank of Georges Bank (∼1,100 km) and can be expressed for as long as 10 weeks (mid‐April to late June). Pigment concentrations within the feature average more than two times that of adjacent shelf and slope waters. Enhanced shelfbreak chlorophyll consistently coincided with the shelf‐slope front and often extended inshore of the surface outcrop of the front a few to ∼10 km. In all years except 1986, it coincided with seaward entrainment of shelf water by Gulf Stream warm‐core rings (WCRs) or meanders. Shelfbreak chlorophyll enhancement was most pronounced during 1980. Using satellite and in situ observations, we found that during 1980, it coincided with the shelf‐slope front for 10 weeks, and, unlike the spring bloom, it was dominated by the nanophytoplankton (<20 um) size fraction. During the peak of the 1980 occurrence, four WCRs simultaneously interacted with shelf water, and chlorophyll enhancement inshore of one WCR coincided with a slope‐water intrusion onto the shelf. Empirical orthogonal function (EOF) decomposition of CZCS images for late March–June 1980 showed that shelfbreak enhancement was strongly pronounced in an EOF that accounted for >10% of the variance about the mean. This annual biological feature, brought to light in satellite ocean color imagery, is an important aspect of the shelf‐slope ecology of the MAB and Georges Bank.

Spatial and temporal variability of sea-surface temperature (SST) and surface chlorophyll derived from the AVHRR and CZCS sensors was examined using a 5-year (1981–1986) time series of 866 SST and 372 CZCS images averaged into monthly mean fields. Five-year mean CZCS-Chl and SST mean fields showed similar patterns, which closely followed the shelf and slope water bathymetry. Along-shelf surface pigment variability is greatest in the near-shore waters and decreases offshore. Monthly mean CZCS-Chl and SST showed consistent cross-shelf gradients with near-shore surface waters cooler and more pigment-laden than offshore waters and with summer months warmer and with lower CZCS-Chl concentrations compared to winter. Harmonic regression analysis of monthly (59 months) CZCS-Chl and SST vs time showed that the seasonal cycle explains the highest proportion (> 80%) of CZCS-Chl variability in shelf waters from Cape Fear, S.C. to Cape Lookout, N.C., with highest concentrations observed during colder months of the year (ca November through March). The seasonal signal also dominates SST in this region explaining approximately 80–90% of the variability in the 5-year time series. Seasonal cycles of CZCS-Chl and SST are inversely related with the highest correlation observed south of Cape Hatteras and relatively poor correlation north of Cape Hatteras. In general, the seasonal cycle explained more of the variability in near-shore and mid shelf waters than in waters near the shelf-break. © 1997 Elsevier Science Ltd