Ecology of the Northeast US Continental Shelf
Oceanography and Hydrography
The oceanography of the Northeast U.S. Continental Shelf Large Marine Ecosystem (NES LME) is shaped by a number of factors including the flow of water from the north into our region, the influence of major river systems, winds, and tidal forces. The physical oceanography of the region is strongly influenced by two major current systems, the equatorward flowing Labrador Current from the north and the poleward flowing Gulf Stream (see Figure 1). Water mass characteristics such as temperature and salinity and oceanographic features such as circulation patterns and the position of frontal zones affect every aspect of the ecology of the system, including the distribution patterns of species at all levels of the food web, the basic biology of individual species, and dispersal and migration pathways.
Water mass characteristics of the Gulf of Maine are strongly influenced by input of Scotian Shelf water at the surface and continental slope water entering the Gulf through the deep Northeast Channel. Three distinctive water mass units have been identified in the Gulf. The influx of relatively warm, salty slope water through the channel forms the distinctive Maine Bottom Water layer below approximately 100m depth. This layer is relatively stable with respect to temperature (6-8°) and salinity (34-35 parts per thousand, ppt) characteristics. Overlying this layer is the colder Maine Intermediate Water (MIW) characterized by relative fresh waters (31-32 ppt). The temperature minimum generally occurs in the MIW layer except in the winter months when convective overturn results in mixing from the surface to the bottom water layer or below. The relatively fresh (31-33ppt) Maine Surface Water in the upper 50m or so of the water column undergoes wide seasonal temperature excursions (from 1-15° C) as a result of atmospheric influences. The relative contribution of the Scotian Shelf Water to fresh water inputs to the Gulf is approximately equal to that of the major river systems.
On Georges Bank, strong tidal forces keep the water on the shallow crest of the bank (<60m) well mixed and isothermal throughout the year. Recent evidence suggests the importance of cross-over events from the Scotian Shelf onto Georges Bank, particularly in winter and short-circuiting the 'typical' pathway of water exchange from the shelf to the bank. The salinity on the bank is relatively stable and slightly higher than the Maine Surface Water, suggesting an influence from slope waters or deeper waters in the Gulf of Maine.
Seasonal warming of surface water of the Mid-Atlantic Bight (MAB) results in the establishment of a strong thermocline and the isolation of a cooler subsurface water layer between the warmer surface waters and the foot of the shelf-slope front near the shelf-break. This 'Cold Pool' is a persistent and distinctive characteristic of the Mid-Atlantic region. The Mid-Atlantic Bight exhibits strong seasonal cycles in temperature and salinity. The annual temperature range in the Bight is the most extreme within the region with surface temperatures spanning 5-30°C. Freshwater inputs from the Hudson River and through Delaware and Chesapeake Bays strongly influence the salinity characteristics of the Bight.
Warm, saline continental slope water extends seaward from the MAB shelf water with a sharp discontinuity of these water masses at the shelf-slope front throughout the Bight and extending northward to Georges Bank.
The surface circulation in the Gulf of Maine is cyclonic (counterclockwise), driven by buoyancy-driven flow resulting from the contrast between freshwater inputs from river systems and higher density water over the central gulf (Figure 2). The eastern Maine coastal current (EMCC), originating on the Scotian Shelf and flowing along the coast, is an important pathway for the transport of nutrients and planktonic organisms in the gulf.
Tidal forces also play an important role in the circulation patterns in the gulf. Tides within the Gulf of Maine are among the strongest in the world ocean with the Bay of Fundy having the highest tidal amplitude. Smaller-scale circulation patterns may form over several of the features of the Gulf of Maine including some of its deep-water basins.
Tides and topographic features of the Georges Bank region result in the establishment of an anticyclonic (clockwise) circulation pattern, particularly during the stratified period on the bank (see Figure 2). This semi-closed gyre holds important implications for the retention of planktonic organisms on the bank. A strong tidal circulation 'jet' forms on the steep northern edge of the bank and continues in more diffuse form around the northern edge and its southern flank. In the general flow, some water exits over the Great South Channel while the remainder recirculates on the bank. It has been estimated that the average retention time of a parcel of water (and associated organisms) is approximately 5 months during the stratified season and on the order of two months in the remainder of the year.
Water entering the northern Mid-Atlantic Bight from the Gulf of Maine and Georges Bank flows equatorward (Figure 2). This generally southwesterly flow regime parallels the isobaths on the shelf. However, the flow highly variable and may reverse direction at times, notably during the summer months.
The Gulf Stream is a classic western boundary current system, driven by wind fields and serving as a major mechanism of heat redistribution in the North Atlantic. The Gulf Stream exerts important influences on the NES LME, particularly through the formation of meanders and eddies. Warm core rings - meanders that separate from the Gulf Stream and form a clockwise rotation pattern - can draw large volumes of water off the shelf, along with the phytoplankton and zooplankton in that water (see animation).
The NES LME experiences the highest seasonal temperature range in the North Atlantic. This steep seasonal amplitude holds important implications for the ecology of the region and resulting in high diversity in fish populations over a relatively narrow latitudinal range. The annual temperature range approaches 20oC in the mid-Atlantic Bight with narrower seasonal temperature amplitude On Georges Bank and in the Gulf of Maine (Figure 3). The temperature range is least in the most tidally energetic regions.
The seasonal pattern of sea surface temperature (SST) is depicted in Figure 4. Sharp latitudinal differences in SST emerge in spring and intensify through the summer months. The Gulf of Maine and Georges Bank regions remain substantially cooler than the Mid-Atlantic Bight. More homogeneous latitudinal temperature patterns prevail during the winter months.
Seasonal changes in temperature and the salinity on the shelf strongly control stratification-the layering within the water column. Water that is colder and saltier is more dense than warmer, fresher water and forms the lower layer. Less dense water sits on top unless mixed by winds and tides. Stratification affects the turn-over of nutrients that support the base of the food web. Once this stratification becomes established each year (in late spring-early summer), the mixing of nutrient-rich bottom water is impeded, and nutrients are rapidly depleted in surface waters in some areas.
Strong geographical differences in stratification in summer are evident in the NES LME (Figure 5). With the rapid increase in water temperatures in the central to southern Mid-Atlantic Bight, and the input of fresh water from the major estuaries, this area is strongly stratified. This can have important effects on oxygen levels in this region, since the turn-over of bottom waters is critical in replenishing the oxygen supply. In years of strong stratification, oxygen depletion has been observed, particularly in the mid-Atlantic region, leading to high mortality of bottom-dwelling animals and changes in the distribution patterns of more mobile animals. In contrast, the shallow waters on the central crest of Georges Bank remain well-mixed throughout the year, because of very strong tidal forces and the influence of winds. Intermediate levels of stratification are found in the northern Mid-Atlantic Bight and in the western Gulf of Maine (Figure 5).
Other important hydrographic features on the shelf with direct implications for its ecology include frontal zones - areas of sharp discontinuities in water mass characteristics (Figure 6). Areas where water masses driven by tidal forces converge are often important feeding locations for many species because small plankton prey items are often concentrated there by physical forces. Similarly, a frontal zone develops between the cooler, fresher water over the continental shelf and the warmer, saltier water over the continental slope. The shelf-slope front also tends to be an area where predators concentrate seeking their prey. These include marine mammals and the top fish predators.
On the shelf itself, primary production is strongly influenced by oceanographic processes, which govern the availability of nutrients. The central crest of Georges Bank stands out as an example of these processes. Nutrient-rich bottom water reaches the Bank through upwelling and other mechanisms, and the strong tidal mixing in the shallow central region of the Bank ensures that the nutrients can be distributed throughout the water column.
During stratified conditions, bacteria become an important factor in primary production as they function to mineralize or recycle organic matter into inorganic nutrients such as nitrate, phosphate and silicate which may then be made available to stimulate more primary production. As noted earlier, stratification limits the nutrient exchange from bottom-waters. However, bacterial activity releases nutrients from dead plants and other material. This recycling of nutrients dominates the primary production processes at this point, and involves a fundamentally different pathway for energy flow and a very different community of primary producers.
During summer, temperature, salinity and the concentration of major plant nutrients such as nitrate, phosphate and silicate, are vertically uniform and generally low in shallow areas that are vertically mixed by strong tides, such as Nantucket Shoals, central Georges Bank, eastern Gulf of Maine. Other areas such as the southern flank of Georges Bank, the MAB and western Gulf of Maine, which are vertically stratified, typically have a vertical profile of low concentrations of nutrients in the upper mixed layer (above the seasonal thermocline), a gradient of increasing nutrients in the thermocline layer, and a reservoir of relatively high nutrient concentrations in the lowermost layer between the thermocline and the sea bed.
In some areas such as the coastal waters off New Jersey, summer upwelling is an important process which can stimulate phytoplankton production by bringing up cold water rich in nutrients such as nitrate from depth into the euphotic layer and stimulating new primary production (see animation). During some years it appears that extensive summer blooms of phytoplankton are responding to these upwelling events.