Oceanography

Coast of Maine

Condensed from Townsend, D.W.,A.C. Thomas, L.M. Mayer, M. Thomas and J. Quinlan. 2006. Oceanography of the Northwest Atlantic Continental Shelf. pp. 119-168. In: Robinson, A.R. and K.H. Brink (eds). The Sea, Volume 14, Harvard University Press.

Several important features and processes act together to control the oceanography, including nutrient fluxes and biological productivity, of the Gulf of Maine. These features and processes include: vertical mixing by tides (Garrett et al., 1978); the seasonal cycle of heating and cooling that leads to winter convection and vertical stratification in summer; pressure gradients from density contrasts set up by deep water inflows and lower salinity waters (Brooks, 1985); and influxes of the cold, but fresher waters associated with Scotian Shelf Water (Smith, 1983).

Tidal ranges in the Gulf of Maine are among the highest in the world ocean, and consequently generate swift tidal currents. Tides in the Gulf decrease from northeast to southwest (Fig. 5.12) and the resulting differences in intensity of tidal mixing (e.g., Loder and Greenberg, 1986) exert a strong influence on the spatial pattern of hydrographic structure in the Gulf, nutrient delivery to the euphotic zone, benthic-pelagic coupling (Townsend et al., 1992a), and, ultimately, upon biological productivity.

The mean circulation in the Gulf of Maine-Georges Bank region is generally cyclonic, driven by density contrasts between dense Slope Water-derived bottom waters residing in the three offshore basins, and fresher waters along the coast that are fed by discharges from the St. John, Penobscot, Kennebec/Androscoggin, and Merrimac Rivers (Brooks, 1985; Xue et al., 2000). River discharges account for only about half of the freshwater budget for the Gulf of Maine; the remaining half enters the Gulf as a surface flow of relatively cold, low salinity Scotian Shelf Waters (Smith, 1983). Beardsley et al. (1997) and Lynch et al. (1997) described the generalized surface circulation in the Gulf as dominated by a buoyancy-driven coastal current system that flows counter-clockwise around its edges. In addition, topographically rectified tidal currents are important in the Gulf (e.g., Loder, 1980), and contribute to clockwise circulation patterns around Browns and Georges Banks, and Nantucket Shoals south of Cape Cod.

The Eastern Maine Coastal Current (EMCC) system is arguably essential to the overall nutrient budget and biological oceanography of the Gulf (Townsend et al., 1987; Brooks and Townsend, 1989; Townsend, 1998). Pettigrew et al. (1998) describe the Eastern Maine Coastal Current (EMCC) as the cold band of tidally-mixed water that originates on the southwest Nova Scotian shelf, crosses the mouth of the Bay of Fundy, and continues along the coast of eastern Maine to the offing of Penobscot Bay. Vertical nutrient fluxes driven by vigorous tidal mixing create summertime surface nitrate concentrations >7 μM NO3 in the northeastern part of the EMCC.

As the EMCC flows to the southwest its waters become increasingly vertically stratified, and nutrient depleted, concomitant with an increase in phytoplankton biomass and developmental stages of copepods. This pattern of an offshore patch of high phytoplankton biomass at the distal end of the plume was noted as early as Bigelow (1926). Townsend et al. (1987) estimated that the EMCC contributes approximately 44% of the inorganic nutrient flux (to surface waters) required to meet estimated levels of new primary production for the entire Gulf of Maine.

Before it reaches Penobscot Bay, the EMCC is often directed away from the coastline and out over the central Gulf of Maine as a plume-like feature of colder water, which is clearly visible in satellite images of sea surface temperature. A portion of the offshore-directed plume may be entrained in the cyclonic gyre over Jordan Basin, with the remaining portion entering an anticyclonic eddy at the distal end, bringing EMCC-plume waters back toward the Maine coast where it continues as part of the western Maine coastal current (Pettigrew et al., 1998).

The Gulf of Maine’s high rate of primary production was first documented by Henry Bryant Bigelow (Bigelow, 1926, 1927; Bigelow et al., 1940). Levels of primary production in offshore waters, the least productive areas in the Gulf of Maine, average about 270 gC m-2 yr-l (O’Reilly and Busch, 1984; O’Reilly et al., 1987). The principal source of nutrients that support this offshore primary production is generally thought to be the influx into the Gulf of nutrient-rich deep Slope Water through the Northeast Channel (Ramp et al., 1985; Schlitz and Cohen, 1984; Townsend, 1991; Townsend, 1998). The major mechanisms that supply these bottom water nutrients to surface waters include: vertical mixing by tides and upwelling in the eastern Gulf and on the southwest Nova Scotian Shelf; fluxes via the EMCC-plume system discussed above; vertical fluxes across the seasonal pycnocline; and, winter convection, which supplies the standing stock of nutrients that fuels the spring phytoplankton bloom (Townsend, 1991). Additional vertical nutrient fluxes throughout much of the year in offshore waters are driven by processes associated with internal waves (Brickley, 2000).

Recent analyses have shown that nutrients that enter the eastern Gulf of Maine at the surface via Scotian Shelf Water may be as important to production as those that enter via the deep Slope Water that comes through the Northeast Channel (Townsend, 1997; 1998). While the gross nutrient influx via Slope Water is much greater, only about 23% of that nutrient load reaches the surface layer (euphotic zone) where it becomes available to phytoplankton.

The spring phytoplankton bloom – in the Gulf of Maine as well as throughout the northwest Atlantic shelf region – is one of the most important biological oceanographic events of the year. The annual bloom in temperate and high latitude continental shelf waters may represent more than half the total input of organic matter to deep water and the benthos (Parsons et al., 1984; Smetacek et al., 1978; Smetacek, 1980; Peinert et al., 1982).

The spring bloom throughout the northwest Atlantic shelf region begins first in shallow inshore areas (Hitchcock and Smayda, 1977; Townsend, 1984; Townsend and Spinrad, 1986), when the critical depth (i.e., Sverdrup, 1953) exceeds the bottom depth. In deeper offshore waters the spring bloom is fueled primarily by nutrients mixed upward by winter convection.

Variations in the surface water nutrient load available for the spring bloom are driven by the relative proportions of Labrador Slope Water and Warm Slope Water entering the gulf and the degree of winter convective mixing (Brown and Beardsley, 1978). Nutrient concentrations are greatest for Warm Slope Water, which carries nitrate concentrations >23 μM and least for cold Labrador Slope Water, which has on the order of 15-16 μM. Silicate concentrations in both water masses, on the other hand, are on the order of 10-14 μM, which means that nitrate concentrations are greater than silicate. Because diatoms, which dominate the spring bloom period, take up nitrate and silicate in roughly equal proportions, silicate, not nitrate, normally limits the spring bloom in the Gulf of Maine. On the other hand, nearer the coast and the influence of riverine sources of silicate, which can exceed 200 μM (Schoudel, 1996), the bloom is limited first by nitrate.

Current thinking holds that oceanographic conditions for the initiation and evolution of the spring phytoplankton bloom in the deeper offshore waters of the Gulf will conform to one of three scenarios:

First, the bloom may be set up according to the classical Sverdrup (1953) model. When deepening light penetration reaches a critical light intensity and the thermocline is established, net planktonic production commences. Riley (1957; 1967) suggested that the value of the critical light intensity is reached when the depth-averaged solar irradiance within the mixed layer increases to ca. 20.9 W m-2; this has been corroborated by a number of subsequent reports from around the world (Gieskes and Kraay, 1975; Pingree et al., 1976; Hitchcock and Smayda, 1977) including studies in the Gulf of Maine (Townsend and Spinrad, 1986; Townsend et al., 1992b, 1994b).

Second, it has been shown that the spring bloom in the Gulf of Maine may develop in the absence of any stratification (Townsend et al., 1992b, 1994b) as long as the water column is not actively mixing. If wind speeds are below a threshold of about 20 kts even a well-mixed water column remains stable (Townsend et al., 1994b). In such cases, if the growth rates of the phytoplankton exceed losses from sinking or grazing, bloom production may commence. In this scenario, the bloom may not exhaust the supply of nutrients prior to the development of the seasonal thermocline. Rather, there may be several spring bloom pulses, each interrupted by self-shading light limitation or vertical mixing events. The possibility of a succession of episodic blooms means that the spring bloom period may be significantly more productive, result in more export production, and be more important to higher trophic level production than has been generally assumed.

Third, the spring bloom in the eastern Gulf of Maine and on the Nova Scotian Shelf may result from the presence of Scotian Shelf Water. As those cold (0-3°C), low salinity waters flow to the southwest, with the major portion flowing into the Gulf of Maine and some crossing the Northeast Channel to Georges Bank, they appear to bring with them sufficient buoyancy and inorganic nutrients such that, depending on solar irradiance levels (e.g., weather), initiation of a phytoplankton bloom can occur. The potential for highly efficient benthic-pelagic coupling of a phytoplankton bloom in such cold water is intriguing (e.g., Rivkin et al., 1996). For example, Pomeroy and Deibel (1986) showed that during early spring blooms in waters near 0°C off the coast of Newfoundland, the metabolism of heterotrophic activity is slowed greatly relative to the rate of photosynthesis by autotrophs, thus allowing fresh organic material to be delivered to the benthos relatively unrespired. Townsend and Cammen (1988) argued that such early blooms in cold waters could benefit recruitment of juvenile demersal fishes in the Gulf of Maine region by stimulating benthic productivity prior to the arrival of newly metamorphosed juveniles in late spring.

The Bay of Fundy is the site of the world’s highest tides, which can exceed 16 m (during spring tides) in the upper reaches of Minas Basin. Gordon and Baretta (1982) have described the system in detail. The upper Bay has extensive tidal flats and salt marshes, and its high benthic productivity supports large populations of sea birds. The swift tidal currents in the inner Bay, however, keep even surface waters highly turbid, creating a light-limited environment that impedes planktonic primary production (Hargrave et al., 1983). An exception is the waters over Grand Manan Basin just inside the mouth of the Bay, where waters are sufficiently deep to allow summertime thermal stratification, but shallow enough that the tidally mixed layer rises above the critical depth (Townsend et al., 2001). This part of the Gulf of Maine is important to seasonal aggregations of migratory pelagic fishes, especially herring, and its prey, Calanus finmarchicus, and are an important feeding ground for several species of great whales (Mate et al., 1997; Kenney et al., 2001).

Georges Bank, measuring 150 km by 200 km, is one of the most prominent features of the northwest Atlantic continental shelf region. Georges Bank has long been well known for its high rates of biological productivity and bountiful fisheries (reviewed in Backus, 1987). Shallow enough that tidal currents keep the overlying waters well mixed year round, Georges Bank is encircled with by an anticyclonic current resulting from the rectification of tidal flow that was first described by Bigelow (1927). Loder (1980), Butman et al. (1982), Butman and Beardsley (1987), Limeburner and Beardsley (1996) and others have provided much more detail, showing the tidally-rectified “jet” current along the steep northern flank of the Bank, the springtime increase in residual current speeds around the remainder of the Bank, and the closed, but “leaky” recirculation of the flow at the southwestern edge.

Primary productivity of Georges Bank is thought to be among the highest of any continental shelf sea, with rates reported to exceed 400 gC m-2 y-1 in the central portion of the Bank (O’Reilly et al., 1987). The year-round production cycle is highly variable, marked by pronounced late winter-early spring phytoplankton blooms (Riley, 1941; Walsh et al., 1987; Cura, 1987; Townsend and Pettigrew, 1997; Townsend and Thomas, 2001; 2002).

The spring bloom on Georges Bank can begin over the central shallow (<60 m) regions as early as January, whenever the critical depth exceeds the water depth (Riley, 1941; Townsend et al., 1994b; Townsend and Thomas, 2001; 2002). Dominated by diatoms (Cura, 1987; Kemper, 2000; Townsend and Thomas, 2001; 2002), the winter-spring bloom on Georges Bank becomes silicate-limited as early as February when surface concentrations approach diatom half saturation constants (2-4μM Si(OH)4; Paasch, 1973). It is only after silicate has become depleted that dissolved inorganic nitrogen concentrations are reduced to levels that would limit phytoplankton production, which usually occurs in April (Townsend and Thomas, 2001; 2002). For the remainder of the year, primary production is thought to be fueled largely by recycled nitrogen (Horne et al., 1989).

During the six-month winter-spring period from January to June of 1999, Townsend and Thomas (2002) observed a steady increase in overall plankton biomass and an increase in planktonic food quality, as indicated by lower particulate C:N ratios. Much of this early summer plankton community would be dependent on recycled nutrients, especially nitrogen; in keeping with this scenario, Townsend and Thomas (2002) showed that the heterotrophic component of plankton increases in May and June, possibly facilitating the recycling of nitrogen, which drives the majority of planktonic primary production following the winter-spring bloom.

The nature of how and where nutrients make their way onto Georges Bank and are transported across its area is as important to the banks’ biological oceanography as the presence and relative ratios (N: Si) of nutrients. Cross-isobath, cross-frontal mixing and nutrient injections onto Georges Bank appear to be concentrated along the Northern Flank of the Bank (Pastuszak et al., 1982; Townsend and Pettigrew, 1997; Houghton and Ho, 2001), which is closest to the nutrient-rich Slope Water resident in Georges Basin. This nutrient flux (Pastuszak et al., 1982; Townsend and Pettigrew, 1997) is utilized by phytoplankton before dispersing across the top of the Bank (Cura, 1987). The phytoplankton population itself is then spread across the northeastern portion of the Bank and circulates to the Southern Flank via the anticyclonic residual tidal current. Along the way, it fuels higher trophic level production. The highest zooplankton biomass is, on average, observed on the southern half of Georges Bank (Davis, 1984).

Loder et al. (1992) used 15N tracer techniques to measure f-ratios, the percentage of primary production fueled by new nitrogen (NO3) fluxes to that fueled by NO3 plus recycled NH4 (Dugdale and Goering, 1967; Eppley and Peterson, 1979), along a transect running from deep waters just north of Georges Bank to the tidally well-mixed waters on the Northeast Peak. F-ratios were as high as 0.7 in regions where nitrate is mixed upward and onto the Bank and on the order of 0.1 to 0.2 in the central regions on top of the Bank (Loder et al., 1992). Thus, nitrate fluxes appear to support about 70% of primary production along the Bank’s edges, but just 10-20% of primary production on the Bank itself. This means that despite the high measured rates of primary production throughout the majority of the Bank’s area (O’Reilly et al., 1987) the particulate nitrogen so formed is principally recycled primary production. The limited exchange between newly upwelled waters along the Bank edges and waters on the shallow central portion of the Bank can be seen in the satellite image of sea surface temperature. The edges of the Bank are noticeably cooler, reflecting a mix of newly upwelled and tidally-mixed waters, while the central portions of the Bank are distinctly warmer, reflecting a relatively prolonged residence time and limited exchanges with waters from the edges of the Bank. Consequently, Townsend and Pettigrew (1997) concluded that the center of Georges Bank is an area of predominantly nitrogen limited, recycled primary production.

The idea of nitrogen limited secondary production on Georges Bank is supported by observations of anomalously low zooplankton production compared with the high rates of planktonic primary production (Sherman et al., 1987). Total zooplankton production (sum of microzooplankton and macrozooplankton) on the Bank has been estimated at 18% of phytoplankton production, while in waters of the adjacent Gulf of Maine it is 26% (Cohen and Grosslein, 1987). The temporal and spatial patterns of annual zooplankton abundance in each area are also divergent. The Georges Bank copepod population drops abruptly to low summertime levels following a late spring peak, while in the Gulf of Maine the decline in summer is far more gradual (Sherman et al., 1987). Zooplankton abundance after the spring bloom is generally greatest on the Southern Flank (Davis, 1984), downstream of the area of maximal nitrate fluxes and high “new” phytoplankton production on the Northern Flank and Northeast Peak (Cura, 1987). Such a conceptual framework also fits with observed fish spawning strategies on Georges Bank. Cod and haddock generally spawn on the northern and northeastern parts of the Bank where the residual circulation can then carry the developing larvae to the Southern Flank (Mountain and Schlitz, 1987) where prey (zooplankton) abundances are greatest.

The commercial fisheries in the Gulf of Maine-Georges Bank region have a long history dating back more than 200 years, but starting in the middle of the last century, stocks were experiencing significant reductions (Cohen and Langton, 1992; Sinclair, 1996). Landings in this region have not only declined severely in recent decades, with concomitant species flip-flops (Cohen and Langton, 1992; Sherman et al., 1987) but, with few exceptions, many important stocks remain at low levels (O’Bannon, 2002).