Ecology of the Northeast US Continental Shelf
Climate and weather patterns over the North Atlantic are strongly influenced by the relative strengths of two large-scale atmospheric pressure cells - the Icelandic Low and a high pressure system generally centered over the Azores in the eastern Atlantic. A deepening of the Icelandic Low is typically accompanied by a strengthening of the Azores High and vice versa. This characteristic pattern is called the North Atlantic Oscillation (NAO) and a simple index of its state is given by the difference in sea level pressure in the vicinity of the Azores and Iceland in winter (December- February).
When the NAO index is positive, we see a northward shift and increase in westerly winds, and an increase in precipitation over southeastern Canada, the eastern seaboard of the United States, and northwestern Europe. We also see increased storm activity tracking toward Europe. Water temperatures are markedly lower off Labrador and northern Newfoundland, influencing the formation of Deep Labrador Slope water, and warmer off the United States (Figure 1). Conversely, when the NAO index is negative, we have a southward shift and decrease in westerly winds, decreased storminess, and drier conditions over southeastern, the eastern United States, and northwestern Europe. Water temperatures are warmer off Labrador and Newfoundland, but cooler off the eastern United States (Figure 1). These changes in the state of the North Atlantic Oscillation tend to persist over decadal time scales. Changes in winds, precipitation and temperature associated with the North Atlantic Oscillation can have far reaching effects on the oceanography of our region.
Over the last several decades, the NAO has primarily been in a positive state (Figure 2). We have generally experienced warm water temperatures during this period, particularly in nearshore areas. This temperature increase closely tracks the change in the NAO index.
Multidecadal patterns in sea surface temperature (SST) in the North Atlantic are represented by the Atlantic Multidecadal Oscillation (AMO) index. The AMO signal is based on spatial patterns in SST variability after removing the effects of anthropogenic forcing on temperature, revealing natural long term patterns in SST. The AMO is characterized by warm and cool phases with periods of approximately 20-40 years. The AMO index is related to air temperatures and rainfall over North America and Europe and is associated with changes in the frequency of droughts in North America and the frequency of severe hurricane events. The AMO is thought to be related to the North Atlantic branch of the deep thermohaline circulation
The AMO index shows a relatively cool period starting in the early 1960’s and extending through the mid 1990’s. Since 1997, the AMO has been in a warm phase (Figure 3). If past patterns continue to hold, the warm phase will potentially continue for the next several decades.
Changes in the NAO index have also been associated with shifts in the position of the Gulf Stream. The north wall of the Gulf Stream shifts to the north under positive NAO states (Figure 4). In addition, when the NAO is in a positive state, the volume transport of Labrador-Subarctic Slope Water (LSSW) is relatively low, and it does not reach our area. When the NAO is in a negative state however, the LSSW penetrates to the Mid-Atlantic Bight, displacing Atlantic Temperate Slope Water (ATSW) further offshore. The NAO index was low during the mid-1950s to early 1970s and we have seen two major drops in the NAO index over the last decade. These resulted in the penetration of cool, fresh, low-nutrient Labrador Subarctic Slope water off the eastern United States after a lag of about 18 months - the time it takes for the LSSW to reach our area from northern Canadian waters (Figure 5).
Long-term Water Temperature Trends
Long-term sea surface temperature records at shore stations in Woods Hole, Massachusetts and in Boothbay Harbor, Maine indicate a general warming trend over the last century with superimposed decadal-scale fluctuations. A shorter time series available for Virginia indicates a steady increase in temperature over the last four decades. Markedly warmer conditions were observed during the 1950s and over the last decade (Figure 6). Cold periods were observed during the early years of the last century, during the period 1914-1918, during the late 1930's, and again during the mid-1960s. The periods correspond to periods of negative NAO anomalies which in all likelihood reflect enhanced southward advection of cold Labrador-SubArctic Slope Water.
Satellite and buoy data for offshore regions of the Gulf of Maine, Georges Bank and southern New England/Mid-Atlantic Bight waters have shorter histories and are useful for a variety of purposes over shorter periods of time. There is an extensive data base of temperature, salinity and density profiles and other water column properties, but these also are best suited to focused questions concerning spatial or shorter temporal patterns.
Effects on Marine Ecosystems
Shifts in distribution of marine populations in our region can be expected with increasing water temperatures. Most marine species exhibit distinct thermal preferences with well defined optimal temperatures. Populations of marine animals at the high end of their thermal range will be adversely affected under current climate change scenarios if redistribution to more favorable conditions is not possible. Temperature preferences of species and overall habitat requirements (for example, substrate type, prey and predator abundances, etc.) will determine the extent of potential distributional changes and adaptation by marine organisms. In general, poleward shifts in distribution can be anticipated, although compensatory changes in depth distribution may also occur. However, other habitat requirements may prevent or limit movement for some species, requiring them to accommodate to higher temperatures. Because growth, survival, and reproduction function most efficiently within fairly narrow temperature ranges, energetic costs associated with living at unfavorable temperatures may result in loss or decline of regional populations.
Collectively these changes in distribution with respect to latitude or depth will affect the availability of fish and invertebrate species to regional fisheries, in some instances changing the character of these fisheries and the communities they support.
An example of changes in distribution for red hake is provided in Figure 7. The distribution and abundance of this species exhibits an overall diminution in population size in the Middle-Atlantic Bight over time and a relative increase in the Gulf of Maine.
Temperature change may also affect the relative timing of the production cycles of the base of the food chain and consumers thus affecting their growth and survival. During the early life history stages of many fish and invertebrate species there are critical timing relationships between the seasonal primary production cycle and their spawning cycle. As the timing of the primary production cycle is changed by shifting thermal conditions, fish species may not be able to respond to these changes and suffer reduced growth and survival because food resources were not available at the right time of year.
Temperature plays a direct role in the physiology of fishes and marine invertebrates, controlling rates of growth and other processes with important implications for survival. Optimal temperatures for growth are critical for organisms to transition through vulnerable periods of their life history, thus temperature change will upset the growth strategies species use in a particular habitat.
Regional changes in salinity are also expected under climate change. Decreased salinity is expected in coastal areas affected by high precipitation and runoff. Increased runoff will intensify buoyancy-driven coastal currents and the effect these currents have on a range of ecosystem properties including organism transport and primary productivity. Increased salinity is anticipated in offshore areas where higher temperatures will lead to higher evaporation rates. Many marine organisms exhibit distinct salinity tolerance levels and it is anticipated that these changes will contribute to overall changes in distribution patterns of marine species. Changes in salinity will also affect the density of sea water and hence stratification.
Increases in water temperatures and in precipitation under global climate change will result in enhanced stratification of the water column with important implications for productivity as noted above. The overall effect will be to increase the energy required for mixing in the water column, resulting in less turnover and a reduction in the mixed layer depth. Replenishment of nutrients in marine ecosystems is dependent on enrichment of the water column from bottom waters, which will be directly affected by changes in stratification. The consequences of these changes can be expected to vary regionally
A reduction in wind-driven forcing in the major current systems such as the Gulf Stream will affect transport and can also be expected to reduce the formation of meanders and rings which can affect advective loss of continental shelf biota. For example, the frequency of warm core ring formation from the Gulf Stream has been related to recruitment success of a number of fish populations. In years in which larger numbers of ring events occur, recruitment is reduced, presumably due to advection as the rings entrain water from the continental shelf and slope regions.
Wind mixing increases turbulence levels in the water column. It has been shown that turbulent mixing can increase the contact rates between zooplankton and their prey. As turbulence increases, however, the probability of successful prey capture declines. The probability of feeding success therefore is dome-shaped with a maximum at intermediate levels of wind-speed and turbulence. The impact of changes in wind intensity must therefore be evaluated with respect to the optimal wind speeds and levels of turbulence.
One of the most dramatic examples of the sweeping nature of these changes can be seen in the relationship of the copepod Calanus finmarchicus and changes in the physical oceanography of the NW Atlantic related to the NAO. Calanus finmarchicus is the dominant copepod species during the spring and summer and is a critical link in the marine food chain. Variations in the abundance of this zooplankton species, has been linked to NAO-associated changes in the physical oceanography; positive NAO states result in higher Calanus abundance and negative NAO to lower abundance. Change in the abundance of C. finmarchicus cascade up the food chain and have, been linked to reproduction in the endangered northern right whale, Eubalaena glacialis.
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For more information, contact Kevin Friedland