Other Physical Drivers

Northeast ecosystem component

Overview

Salinity

There is strong geographic variation in salinities across the northwest Atlantic. Input from fresh, cold Scotian shelf water and warmer, saltier slope water flowing through the Northeast Channel at depth result in three distinct water masses in Gulf of Maine. The saltiest and deepest is Maine Bottom Water, while the overlying Maine Intermediate Waters and Maine Surface Waters are fresher and warmer1. Salinities on Georges Bank are relatively stable and slightly more saline than Maine Surface Water, which could indicate interaction with deeper, saltier Maine Bottom Water2. In the southern and central Mid-Atlantic Bight, nearshore salinities tend to be lower due to freshwater input from rivers and estuaries, becoming saltier towards the shelf-break. In the northern Mid-Atlantic, freshwater input lessens, but fresher water lies inshore of more saline water as in the south3. This contrast is accentuated at the shelf break, where a sharp temperature and salinity front separates cool and fresh coastal waters from warm and salty slope waters offshore.

Annual mean surface (red) and bottom (blue) salinities from the NEFSC survey programs from the four Ecological Production Units.
Annual mean surface (red) and bottom (blue) salinities from the NEFSC survey programs from the four Ecological Production Units.

Freshwater Discharge

The fresh continental runoff entering the shelf from rivers and tributaries in the Mid-Atlantic Bight is several times larger than the Gulf of Maine and the majority of this input is centered in the southern Mid-Atlantic Bight, provided by the Hudson, Susquehanna, and Delaware Rivers. Peak freshening from local freshwater runoff to the Northeast Shelf occurs within a 3-month period between March and May. This seasonal freshening is responsible for increasing the stratification in near shore waters and enhancing cross-shelf salinity gradients and flow along the inner shelf, particularly inshore of the 60 m isobath3,4. However, because most of the cold/fresh shelf water feeding the NES arrives from the north, the entire shelf experiences a freshening several months after the period of local run-off, with inter-annual variations that often swamp local annual variability.

Climate-change induced freshening of surface waters flowing into eastern Gulf of Maine is increasing stratification and affects changing primary productivity patterns as a result. The inputs of rivers and other continental sources provide nutrients to the NES which stimulate phytoplankton and primary productivity. While the overall contribution of rivers is less than offshore sources, continental sources provide a locally important contribution in river plumes and are supplied directly into the biologically productive surface zone. While nitrate is the primary limiting nutrient within the NES, dissolved silicate is required by diatoms to sustain intense blooms. In contrast to nitrate, silicate concentrations in river water are significantly higher than those offshore and continental sources provide approximately equal quantities of silicate as offshore and deepwater sources5.

 

Stratification

The magnitude and timing of stratification in the northwest Atlantic are key determinants of phytoplankton bloom phenology and strength, although stratification exhibits high seasonal and spatial variability throughout the region6. As stratified layers in the water column form and break down, nutrients are mixed vertically and phytoplankton are moved into or out of the euphotic zone. Salinity gradients drive stratification during the winter and early-spring when thermal gradients are minimized and stratification is at its weakest. In the spring and summer months, thermally driven stratification increases in association with surface heating.

Following a peak period, surface cooling and wind-induced mixing cause the breakdown of the seasonal thermocline in fall3,6. Georges Bank is dominated by tidal mixing currents across most of its area but especially within the 60 m isobath, where the water column remains vertically mixed throughout the year1. In the Gulf of Maine, variations in the seasonal cycle of temperature and salinity result in the western Gulf being more highly stratified in summer and more vertically uniform in winter than the eastern GOM7.

Winds

Winds are an important pressure on the Northeast Shelf that can alter regional circulation patterns, drive vertical currents near the boundary, and weaken vertical property gradients (stratification) by enhancing vertical mixing. In the Mid-Atlantic Bight, wind-forcing plays an important role in coastal upwelling, where northerly winds can drive upwelling events during summer months. Episodic surface winds also tend to dictate the direction and strength of short-term along-shore flow in the region8. Winds are important in facilitating the development of the halocline in southern New England, where freshwater outflow is driven offshore by eastward blowing winds9. In the fall, westward blowing winds play an important role in breaking down thermally stratified waters in southern New England, resulting in vertical mixing of the water column9. There is evidence that wind fluctuations associated with the passage of atmospheric low-pressure systems may play an important role in the transport and retention of particles and dissolved nutrients across the tidal mixing front on Georges Bank10.

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    1. 1. Townsend DW, Thomas AC, Mayerm LM, Thomsas MA (2004) Oceanography of the northweast Atlantic continental shelf. In: The Sea: The Global Coastal Ocean: Interdisciplinary Regional Studies and Syntheses. Harvard University Press.

    2. 2. NEFSC Ecology of the Northeast US Continental Shelf: Oceanography. https://www.nefsc.noaa.gov/ecosys/ecosystem-ecology/oceanography.html.

    3.  Castelao R, Glenn S, Schofield O (2010) Temperature, salinity, and density variability in the central Middle Atlantic Bight. J Geophys Res Ocean 115.
    4. Lentz, S. J. (2008) Seasonal variations in the circulation over the Middle Atlantic Bight continental shelf. Journal of Physical Oceanography, 38(7): 1486-1500.

    5.  Rebuck, N. D. (2011), Nutrient Distribution in the Gulf of Maine: An Analysis of Spatial and Temporal Patterns of Dissolved Inorganic Nitrate and Silicate, University of Maine, Orono, ME.

    6.  Li Y, Fratantoni P, Chen C, Hare J, Sun Y, Beardsley R, Ji R (2015) Spatio-temporal patterns of stratification on the Northwest Atlantic shelf. Prog. in Oceanography 134: 123-137.

    7. Mountain D, Manning J (1994) Seasonal and interannual variability in the properties of the surface waters of the Gulf of Maine. Cont Shelf Res 14: 1555–1581.Go here for current publications associated with salinity.
    8.  Glenn S, Arnone R, Bergmann T, Bissett WP, Crowley M, Cullen J, Gryzmski J, Haidvogel D, Kohut J, Moline M, Oliver M, Orrico C, Sherrell R, Song T, Weidemann A, Chant R, Schofield O (2004) Biogeochemical impact of summertime coastal upwelling on the New Jersey Shelf. J Geophys Res C Ocean 109:1–15.
    9. Lentz S (2003) Evolution of stratification over the New England shelf during the Coastal Mixing and Optics study, August 1996–June 1997. J Geophys Res 108:1–14.
    10. 3. Chen C, Schlitz R, Lough R, Smith K, Beardsley R, Manning J (2003) Wind-induced, cross‐frontal exchange on Georges Bank: A mechanism for early summer on-bank biological particle transport. J Geophys Res Oceans 108:C11.
Stratification in the NE-LME
The spatial pattern of timing associated with the development (a,b) and breakdown (c,d) of stratification on the Northwest Atlantic shelf based on daily climatology of surface to 50 m stratification constructed from a high-resolution data-assimilated reanalysis dataset (From Li et al. 2015).