Light absorption and scattering by particles and CDOM at the New England shelfbreak front

A fine-scale survey of physical, chemical, and optical properties was done across the shelfbreak front in the Middle Atlantic Bight region south of Martha's Vineyard, Cape Cod (70.5 degrees W) during late summer 2002. The front displayed physical and biological structure typical for this region and season: a density-compensating front with strong horizontal T and S and optical gradients, and a chlorophyll maximum layer that deepened in the seaward direction across the front, became detached beneath the shelfbreak jet, reappeared on a deeper and greater density surface on the seaward side of the shelfbreak front, and then shoaled again in the seaward direction. At times the shoreward leading edge of the chlorophyll maximum layer seaward of the front was situated beneath the chlorophyll maximum layer shoreward of the front. Hydrography on the shoreward side of the front was dominated by a bolus of cold pool water of shelf origin with minimum salinity and temperature of 33 and 10 degrees C, respectively. The cold pool was also a pool of high CDOM absorption whose distribution coincided with the temperature and salinity across the front. The persistence of sharp horizontal T, S, and CDOM gradients indicated that advective processes exceed diffusive mixing processes at the front. CDOM in the euphotic zone showed a strong inverse relationship to temperature and, below the euphotic zone, a strong inverse relationship to salinity, indicating a terrestrial CDOM source within the cold pool and a photodegradation sink at the surface. Cross-frontal exchange of shelf CDOM and upper slope waters is inferred from the T-S properties of the CDOM pool and previously reported measurements of the mixing and circulation at this site. The chlorophyll specific absorption coefficients of discrete samples collected within the chlorophyll maximum layers, along with LPS surveys of dissolved and particulate absorption and scattering, indicate that the variability of optics at the shelfbreak front is influenced mostly by particle optics. Due to the presence of the cold pool with its high CDOM signature, the waters masses across the shelfbreak front can be considered as case 1 for particles but case 2 for dissolved materials.