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Chapter 4 - Physical OceanographyHatteras, between the Mid-Atlantic and South-Atlantic bights (Townsend et al. 2006).The circulation patterns of this region are influencedby the North Atlantic Oscillation(NAO), an atmospheric phenomenon thatchanges the strength of major wind patterns(Longhurst 2007). During periods of negativeNAO, the Gulf Stream shifts to the south andthe Labrador Current increases in volume, penetratingfarther down the coast (Townsend etal. 2006; Longhurst 2007). In contrast, duringperiods of positive NAO, the Gulf Stream shiftsto the north and the Labrador Current weakens(Townsend et al. 2006; Longhurst 2007). Theshifting balance between these two currents hassignificant implications for biological communities becauseit can expose those communities to very differenttemperature and salinity regimes (Townsend et al. 2006).Wind forcing on seasonal and shorter time scales also affectscirculation of the northwestern Atlantic shelf. Windsfrom the north, which are common in winter, push thecolder shelf water to the inner shelf (Longhurst 2007).Winds from the south push surface water out away fromthe coast, which in some places causes upwelling of deepwater near the coast (Longhurst 2007). The Mid-AtlanticBight is frequently in the path of cyclones, which causevertical mixing and thus resupply nutrients to the surfacelayer (Townsend et al. 2006).Tidal InfluenceTides have an obvious effect at the shoreline, where marineorganisms must adapt to exposure to air, but theyalso influence ecosystem processes further offshore. Forexample, tides can prevent stratification (i.e., layering ofdifferent water masses). Strong tidal mixing in the Gulfof Maine prevents stratification over shallow areas such asGeorges Bank (Longhurst 2007). Such areas often serveas spawning and nursery grounds for fish because theytend to be characterized by high biological productivityand have recirculating currents that retain larvae (Mannand Lazier 2006). When stratification persists over theshelf, the tide interacts with the sharp change in topographyat the shelf break to generate internal waves whichhelp mix nutrients through the water column and are onemechanism for transporting larvae across the shelf (Mannand Lazier 2006).Tides can also play a major role in sediment mobility,which affects bottom communities. If energy from tidesexceeds a certain threshold, it disturbs sediments on thesea floor. Tidal energy varies with the range in tidal heightand the amount of constriction by bottom topography.The energy needed to move sediment depends on the sedimentgrain size and density, seabed roughness, and howwell the sediment grains are cemented together (Porter-Smith et al. 2004).It is important to note that tides are just one process formobilizing sediment. Storm waves, for example, also cancause rapid sediment transport that exceeds the amount oftransport caused by normal wave and tidal energy over thecourse of months (Porter-Smith et al. 2004). Sedimentmobility has implications for the types of benthic communitiesfound in a given area and the persistence or stabilityof these communities over time. Mobile sediments tendto be dominated by a single opportunistic species that canquickly recolonize following a disturbance, while stablesediments, such as gravel, tend to support greater speciesdiversity (Newell et al. 1998).Northwest Atlantic Marine Ecoregional Assessment • Phase 1 Report 4-