Full report - Conservation Gateway

Chapter 4 - Physical Oceanography• Stratification increases the stability of the watercolumn, providing conditions for seasonalaccumulation of high density patches of phytoplankton, which may provide a rich food sourcefor higher trophic levels (McManus et al. 2003).However, if stratification extends too long, thewater masses become depleted of nutrients.Fortunately, winter winds cause stratification tobreak down. This has the advantage of enablingnutrients from deeper, colder waters to come tothe surface.• Stratification controls the development ofphytoplankton blooms. Because the surface layeris well mixed down to the pycnocline (the depthof maximum change in density), phytoplanktonare physically mixed throughout the layer (Mannand Lazier 2006). If the surface layer is muchthicker than the euphotic zone (the verticalzone where light intensity is high enough forphotosynthesis to occur), phytoplanktonpopulations cannot grow. Conversely, if thesurface layer is thin enough relative to theeuphotic zone, phytoplankton populations cangrow rapidly, forming a bloom. This is themechanism responsible for the springphytoplankton bloom in the North AtlanticOcean (Mann and Lazier 2006).• Stratification also increases the potential forhypoxia by preventing deep water fromexchanging with the atmosphere (Rabalais et al.2002). Hypoxia causes the exclusion of fish andother mobile organisms and mortality of manybenthic organisms (Rabalais et al. 2002).MethodsStratification was calculated by subtracting the density at50 m from the surface density. Where the seafloor is shallowerthan 50 m, stratification was calculated as the densitydifference between the seafloor and the surface. Theresulting stratification values were then interpolated usingordinary kriging in ArcGIS 9.1, creating a smooth datagrid representing the average degree of density stratificationfor 1980 – 2007 for each season.ResultsThe maps produced by this analysis agree with observedseasonal patterns of stratification. In the winter (Figure4-8a), the water column was mixed by winds from thesurface down to 50-100 m (Longhurst 2007). The waterover the shelf was nearly completely mixed, except for anarrow band of stratification near the coast. The higheststratification during these months (dark red) occurredat the Hudson outflow, in Delaware Bay, and betweenChesapeake Bay and Cape Hatteras. In the spring (Figure4-8b), the water column becomes stratified and formed athinner surface layer due to solar heating and increasedfreshwater inputs (Longhurst 2007). The map of springstratification showed stratified conditions throughout theecoregion, except north of Penobscot Bay in the Gulf ofMaine. The broadest extent of stratification occurred inthe Mid-Atlantic Bight, extending to the 50 m isobaththroughout.In the summer (Figure 4-8c), stratification greatly intensifiedand extended throughout the Gulf of Maine, but noton Georges Bank or Bay of Fundy. In the Southern NewEngland subregion, only parts of the Great South Channelremained mixed. All of the Mid-Atlantic Bight subregionwas very stratified, with the exception of the southeasternend of the region. As water over the shelf stratifies in thesummer, a pool of cold, higher nutrient deep water remainedisolated (Townsend et al. 2006). This cold poolis distinctly colder and fresher than the water mass overthe Continental Slope. The cold pool flows to the south,bringing a supply of nutrients to the southern end of theMid-Atlantic Bight (Townsend et al. 2006).In the fall (Figure 4-8d), increased wind events causemixing throughout the Gulf of Maine, Georges Bank, bythe Block Island Delta (Southern New England subregion)and the outer shelf. The inner shelf of the SouthernNew England subregion from Narragansett Bay south andthe Mid-Atlantic Bight remained moderately stratified(yellow); the coast just south of Chesapeake Bay showed asmall patch of increased stratification (red).4-12Northwest Atlantic Marine Ecoregional Assessment • Phase 1 Report