Ecosystem Status Report for the Northeast Large Marine Ecosystem
3. Physical Pressures
Oceanographically, the NES LME is located on the western boundary of two large oceanic gyres, which span the North Atlantic Basin. The source waters feeding the NES LME include contrasting water masses carried by the converging currents from these two gyres: the Gulf Stream carrying warm and salty water from the south and the Labrador Current carrying cold and fresh water from the north. Climate oscillations (e.g. NAO, AMO) and long-term trends (e.g. warming, acidification) can lead to changes in the intensity of these currents, their position relative to the NES LME, and the water masses that they carry (Curry and McCartney 2001; Häkkinen and Rhines 2004; Joyce et al. 2000; Marsh 2000), ultimately influencing the physical environment of the NES region.
Climate drivers impact the physical environment of the NES LME through a combination of external pressures at its boundaries and direct effects on internal conditions. External influences on the NES include the Gulf Stream at the southern and offshore boundary, the Labrador Current at the northern boundary, river discharges at the coastal boundary and winds and atmospheric fluxes at the sea surface. In addition to these external pressures, climate processes also directly influence the internal physical environment of the NES, altering the horizontal and vertical distribution of temperature and salinity. The combination of these physical pressures can cause significant ecosystem changes, which are discussed in sections 4-6.
The Gulf Stream system is an important component of global climate and an important physical pressure on ecosystems in the North Atlantic. The Gulf Stream and its extension transport a significant amount of heat from the tropics to higher latitudes. Vigorous cooling along the Gulf Stream's path returns a considerable amount of this heat to the atmosphere, influencing storm tracks in the North Atlantic and resulting in milder climates in Europe compared to similar latitudes in North America (e.g. Ireland vs. Labrador). At high latitudes, the cooled water sinks and ultimately returns southward in deep-reaching currents beneath the warmer tropical and subtropical waters. This so-called Atlantic Meridional Overturning Circulation (AMOC) plays an important role in regulating earth's climate and the Gulf Stream is a dominant component of its vertical circulation.
Measurements of the Atlantic Meridional Overturning Circulation have only recently become available from estimates of the meridional flow integrated across the width of the North Atlantic (Cunningham et al. 2007). However, studies suggest that fluctuations in the strength of the AMOC are associated with changes in the basin-scale circulation in the North Atlantic, including shifts in the Gulf Stream path (Joyce and Zhang 2010). Furthermore, shifts in the north-south position of the Gulf Stream are strongly correlated with temperature changes in the slope region offshore of the NES (Peña-Molino and Joyce 2008) and are a reliable indicator of bottom water temperature on the shelf (Nye et al. 2011): a northward shift in the Gulf Stream is associated with warmer shelf temperatures. Shifts in the position of the north wall of the Gulf Stream are a leading indicator of conditions on the shelf and indirectly related to the distribution of some commercially important fish species (Nye et al. 2011) as well as changes in plankton community composition (Taylor, 1995).
Interannual shifts in the position of the Gulf Stream are correlated with atmospheric fluctuations over the North Atlantic, including the changes in wind stress and buoyancy forcing that are associated with the NAO. The latitude of the Gulf Stream north wall is positively correlated with the NAO with a lag of 1-2 years (Frankignoul et al. 2001). An index of the position of the North Wall of the Gulf Stream, available since 1966, reveals a shift in the early 1980s from low to high index values (Figure 3.1), reaching a peak in the early 1990s, and characterized by subsequent multiyear reversals related to changes in the NAO index. Interestingly, the relationship between NAO and Gulf Stream position is not as clear after year 2000. Around this time, the character of the NAO changes, shifting away from prolonged periods of high or low toward a weaker higher-frequency oscillation.
The northeast U.S. shelf ecosystem is located at the downstream end of an extensive interconnected coastal boundary current system that carries a combination of cold/fresh arctic-origin water, accumulated coastal discharge, and ice melt thousands of kilometers around the boundary of the subpolar North Atlantic. The Labrador Current is one regional component of this boundary current system that flows southward along the western boundary of the Labrador Sea and whose shallow and deep branches are parts of the larger basin-wide gyre circulation in the northern North Atlantic. Together with the southward-flowing Deep Western Boundary Current, the deeper Labrador Current is also considered part of the returning cold/fresh half of the northern AMOC. Ultimately, a portion of these cold/fresh waters carried by the Labrador Current feeds into the northeast U.S. shelf ecosystem via the Gulf of Maine.
The Labrador Current provides two of the three main sources of water entering the NES ecosystem: Labrador Shelf Water is the coldest and freshest water and is confined to the shelf, while Labrador-Subarctic Slope Water (LSSW) is a deeper cold/fresh water mass that arrives along the continental slope. These northern-source waters combine with the deep warm/salty southern-origin Atlantic Temperate Slope Water (ATSW) to define the temperature, salinity, stratification and nutrient content of the shelf water within the NES ecosystem. Variations in the properties and/or relative proportion of the source waters can lead to significant changes in stratification, nutrient loads, and community composition within the ecosystem.There is compelling evidence that variations in the composition of the slope water in the Gulf of Maine are correlated with basin-scale atmospheric forcing in the North Atlantic (specifically the NAO). When the NAO is in a positive state, the volume transport of Labrador-Subarctic Slope Water (LSSW) is relatively low and does not penetrate much beyond the Gulf of St. Lawrence basin (Drinkwater et al. 2002). When the NAO is in a negative state, volume transport of the Labrador Current is high and the LSSW penetrates to the Mid-Atlantic Bight, displacing Atlantic Temperate Slope Water (ATSW) further offshore (Greene and Pershing 2007). During these low NAO conditions, the amount of LSSW entering the Gulf of Maine through the Northeast Channel is high and bottom temperatures are colder and fresher (Petrie 2007).
Recently, investigators have shown that decadal shifts in the position of the Gulf Stream are closely tied to changes in slope water temperature and to the composition of slope water entering the Gulf of Maine (pers. comm., T. Joyce and Y-O. Kwon.) Cooling in the slope water offshore is accompanied by a southward shift in the Gulf Stream and a predominance of LSSW in the deep layers of the Northeast Channel.
The amount of freshwater entering the ocean is another important pressure that responds to climatic drivers. Freshwater run-off transports pollutants and nutrients to the continental shelf, which can affect coastal ecosystems. Nutrient over-enrichment – termed eutrophication – is a major problem in many coastal systems and has been linked to increased algal biomass, including harmful algae species, hypoxia/anoxia, and increased water turbidity. Increased freshwater run-off can also affect coastal circulation through the influx of less dense water on the continental shelves. Most freshwater enters marine systems through rivers, rather than direct precipitation or runoff. River flow is tightly correlated in the Gulf of Maine and Southern New England regions, resulting in coherent freshwater forcing in the northern portion of the region. River flow into the Mid-Atlantic region is somewhat different than for the Gulf of Maine and southern New England (Figure 3.4). Complex long-term patterns have been identified for river flow in the region. Tootle et al. (2005) found interactions among different climate drivers affecting river flow; for example, the AMO and ENSO (El Ninõ Southern Oscillation) signals combine to affect river flow in the Mid-Atlantic part of the NES LME. Earlier work by Visbeck et al. (2001) found links between river flow in the northeast and the NAO. A time series of river flow suggests the effect of multiple factors (Figure 3.4). Prior to 1970, river flow appears to fluctuate on longer time scales, while the period decreases later in the record. In general, stream flow into all three regions has increased over the past decade, with the largest increases occurring in the Middle Atlantic and Gulf of Maine regions. However, in 2012 riverflows were somewhat lower resulting from decreased precipitation and warm temperatures leading to increased evaporation.
Winds are an important pressure on shelf ecosystems. Wind stress (the force of the wind on the surface of the ocean) acts to vertically mix the water column and drive horizontal currents. The greater the wind stress, the more vertical mixing and the more force for driving horizontal currents. In the NES LME, winds are responsible for breaking down seasonal stratification in the fall and for causing reversals in the generally southwestward surface currents during summer. In addition, winds blowing along a coastal boundary will drive a vertical circulation in the water column near shore, drawing deeper waters upwards toward the surface where they are carried away from the coast (upwelling) or driving surface waters downward toward the bottom where they are carried offshore (downwelling). Because of the shape of the coastline in the NES, north-south winds have the greatest impact on the along shelf flow in the southern Middle Atlantic Bight, while east-west winds are more important along the inner shelf of New England. Long-term records from NOAA Pacific Fisheries Environmental Group indicate substantial inter-annual variability in the magnitude and direction of the wind stress over the NES LME (Figure 3.5). Total annual wind stress has been variable over the period with relatively high winds in the late 1970s-early 1980s and again in the late 1990s-early 2000s. In recent years total annual wind stress has been low but increasing in 2010 and 2012. Winds over the NES are consistently directed out of the northwest (blowing eastward and southward). There is considerable inter-annual variability in the magnitude of the winds. As an example winds near Cape Hatteras in 2011 were northward while in 2012 they were strongly southward. Mean winds have remained fairly consistent over time, but there has been a shift from northwest winds (southeastward) to more west winds (eastward), particularly in the vicinity of Cape Hatteras. These changes in wind stress may be linked to the NAO, as well as a northward shift in the location of the jet stream (Archer and Caldeira 2008). Despite the uncertainty as to the cause of the inter-annual variability, these changes in wind will impact local physical conditions and local marine resources.
Temperature is one of the most important governing environmental factors for marine organisms. Marine organisms have minimum and maximum temperatures beyond which they cannot survive. Additionally, they have preferred temperature ranges and within these bounds, temperature influences many processes including metabolism, growth, consumption, and maturity. Thus, changes in temperature will have far-reaching impacts on species in the ecosystem and on the ecosystem itself. Temperature in the NES LME has varied substantially over the past 150 years (Figure 3.6). The late 1800s and early 1900s were the coolest in the 150 year record. This relatively cool period was followed by a period of warm temperatures from 1945-1955. There was a rapid drop in temperatures through the 1960s followed by a steady increase to the present. Summer temperatures over the past 5 years are comparable to the warm period in the late-1940s/early 1950s and the summer 2012 surface temperature was the highest in the 158-year record. Winter temperatures in recent years, however, remain near the long-term mean indicating that the seasonal range in temperature has increased (Friedland and Hare, 2007).
Regional water column temperatures measured by the Northeast Fisheries Science Center (NEFSC) give spatial context to the shelf-wide trends in sea-surface temperature (Figure 3.7). Surveys began in the late 1970s, so the time series are shorter than sea-surface temperature records shown in Figure 3.6. Time series constructed within each region reveal interannual temperature fluctuations larger than 2°C near the surface and bottom. Long-term warming trends are observed at the surface and bottom in the Mid-Atlantic Bight, Gulf of Maine, and Georges Bank regions and at the surface in the Scotian Shelf region,with waters warming by 1°-1.5°C over the length of the records. Even larger warming trends have been observed in recent years, with the surface and bottom waters warming by more than 2 degrees since 2004 within all regions except the Mid-Atlantic Bight. Perhaps most notable, 2012 temperatures were the warmest observed in the 35-year record at the surface and bottom over all regions of the NES, exceeding long-term annual mean values by up to 2 degrees at the surface and 1 degree at the bottom.
Most aquatic organisms are also affected by salinity – the amount of salt in the water. Organisms in nearshore environments are adapted to wide ranging salinities owing to the interaction between freshwater (salinities of 0) and oceanic-water (salinities greater than 30). However, many organisms found on the continental shelf, slope and deep-sea are sensitive to small changes in salinity because they are adapted to more constant conditions. The NEFSC measures salinity in combination with the temperature measurements described above (Figure 3.8). Regionally, time series show interannual salinity fluctuations as large as 1.6 salinity units near the surface and 1.5 units near the bottom, with the largest fluctuations in the Mid-Atlantic Bight. A trend of long-term freshening is observed at both the surface and at depth in the Georges Bank region, equivalent to roughly 0.3 units of freshening over the length of the record. However, during the last 2-3 years waters were in fact saltier everywhere on the NES relative to the long-term annual mean. Interannual fluctuations occur coherently between the surface and bottom in both the Mid-Atlantic Bight and Georges Bank regions, having a distinct 5-year cycle in the latter. By contrast, fluctuations in the Gulf of Maine and Scotian Shelf regions are not as coherent between the surface and bottom. This is likely due to the fact that deep and shallow layers in the Gulf of Maine are fed by different sources whose properties and volume may vary independently from one another: Bottom waters are fed by deep slope waters entering through the Northeast Channel, varying in response to the relative proportion of LSSW to ATSW, while surface waters are fed by northern shelf waters and the discharge of local rivers. The Scotian Shelf records indicate that the salinity of near-bottom waters has remained higher than the long term mean during the past 6 years. This ecological production unit encompasses the eastern Gulf of Maine, including the deep Northeast Channel. The salinification of near-bottom waters here reflects the dominance of ATSW in the slope water mixture entering the Gulf of Maine during recent years (Figure 3.2).
During much of the year, portions of the northeast U.S. shelf are stratified. Stratification refers to the vertical stacking of layers of water having different densities due to changes in temperature, salinity, etc. at different depths within the water column. If there is no stratification, density is uniform throughout the water column and mixing is achieved with little work. When dense water overlays less dense water, the denser water sinks, thereby mixing the water column (this occurs as the surface ocean cools in the fall). However, when less dense water overlays denser water, the water column is considered stable and it takes more energy to mix. The greater the stratification, the greater the density difference from the surface to depth and the more energy is required to mix the water column. The issue of stratification is important because deeper waters are often nutrient rich. Increased stratification makes it harder for these nutrient rich waters to be brought to the surface where they are available to primary producers.
Density is determined by the temperature and salinity of the water: Warm/fresh water is less dense than cold/salty water. Therefore, the observations of temperature and salinity collected by the NEFSC (Figure 3.7 and Figure 3.8) can be used to determine density, and hence stratification. Stratification is strongest in the Mid-Atlantic Bight with large fluctuations about the record-long average, although there is no significant trend over time in this region (Figure 3.9). By contrast, stratification in the Georges Bank region increased between 1995-2007 and has remained at or above normal throughout the current decade. While temperature fluctuations are vertically coherent in this region, warming is larger at the surface making waters less dense than at depth (Figure 3.8 and Figure 3.9). In general, vertical temperature differences (warm water overlaying cold water) tend to be more important than vertical salinity difference (fresh water over salty water) in determining stratification almost everywhere on the NEUS shelf. The exception is the Scotian Shelf region where the coldest/freshest Labrador Shelf water first enters the Gulf of Maine and temperature and salinity contribute equally to the stratification.