Abstract
Georges Bank is a large, shallow feature separating the Gulf of Maine from the Atlantic Ocean. Previous studies demonstrated a strong tidal-mixing front during the warm season on the northern bank margin between thermally stratified water in the Gulf of Maine and mixed water on the bank. Tides transport warm water off the bank during flood tide and cool gulf water onto the bank during ebb tide. During 10 days in August 2009, we mapped frontal temperatures in five study areas along ∼100 km of the bank margin. The seabed “frontal zone”, where temperature changed with frontal movment, experienced semidiurnal temperature maxima and minima. The tidal excursion of the frontal boundary between stratified and mixed water ranged 6 to 10 km. This “frontal boundary zone” was narrower than the frontal zone. Along transects perpendicular to the bank margin, seabed temperature change at individual sites ranged from 7.0°C in the frontal zone to 0.0°C in mixed bank water. At time series in frontal zone stations, changes during tidal cycles ranged from 1.2 to 6.1°C. The greatest rate of change (−2.48°C hr−1) occurred at mid-ebb. Geographic plots of seabed temperature change allowed the mapping of up to 8 subareas in each study area. The magnitude of temperature change in a subarea depended on its location in the frontal zone. Frontal movement had the greatest effect on seabed temperature in the 40 to 80 m depth interval. Subareas experiencing maximum temperature change in the frontal zone were not in the frontal boundary zone, but rather several km gulfward (off-bank) of the frontal boundary zone. These results provide a new ecological framework for examining the effect of tidally-driven temperature variability on the distribution, food resources, and reproductive success of benthic invertebrate and demersal fish species living in tidal front habitats.
Highlights
Georges Bank is a large, relatively shallow offshore continental shelf feature that extends eastward from the southeastern Massachusetts coast toward the southern tip of Nova Scotia and forms the southern boundary of the much deeper Gulf of Maine (Fig. 1)
Individual isotherms that intersected the seabed within the frontal boundary zone moved 6 (Fig. 7A, B) to 10 km (Fig. 7D) between high and low tides
Water column temperature data collected along 6 transects across the bank margin in 5 study areas showed that the frontal boundary moved 6 to 10 km between high and low tides in August 2009 (Fig. 7A-F), in line with observations in July 1988 by Loder et al ([18]: Fig. 7A-F)
Summary
Georges Bank is a large, relatively shallow offshore continental shelf feature that extends eastward from the southeastern Massachusetts coast toward the southern tip of Nova Scotia and forms the southern boundary of the much deeper Gulf of Maine (Fig. 1). It is one of the world’s most productive marine ecosystems [1]. The hydrographic setting for the benthic habitats of Georges Bank results from the interaction of water masses, tides, topography, and atmospheric warming and cooling. Shallow tidally-mixed Georges Bank Water (GBW) is bounded on the north by a 3-layer system of water masses in the. High primary productivity on the bank results when nutrient-rich water from the Gulf of Maine is transported up the steep northern slope of the bank and pumped into the euphotic zone on the bank’s northern margin [9,10,11] where it enters the gyre
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