High-frequency sampling of the 1984 spring bloom within the Mid-Atlantic Bight: Synoptic shipboard, aircraft, and in situ perspectives of the SEEP—I experiment

Abstract

Moorings of current meters, thermistors, transmissometers, and fluorometers on the Mid-Atlantic shelf, south of Long Island, suggest a seaward export of perhaps 0.20 mg Chl m −3 day −1 at depths of 75–81 m, between the 80- and 120-m isobaths during February–April 1984. Using a C/Chl ratio of 45/1, such a horizontal loss of algal carbon over the lower third of the water column would be 19–67% of the March–April 1984 primary production within the overlying euphotic zone. This possible physical carbon loss is similar to daily grazing losses to zooplankton of 32–40% of the algal fixation of carbon. Metabolic demands of the benthos could be met by just the estimated fecal pellet flux, without direct consumption of the remaining algal carbon. Similarly bacterioplankton metabolism could be fueled by excretory release of dissolved organic matter during photosynthesis, rather than by consumption of particulate carbon. Sediment traps tethered 10 and 70 m off the bottom at the 120-m isobath caught as much as 0.10–0.16 g C m −2 during March–April 1984. This presumed vertical flux is about one-third to one-half of the horizontal flux of 0.30 g C m −2 day −1 estimated over the lower 33 m of the water column at the 100-m isobath. These estimates suggest that ∼50% of the carbon export at the shelf-break might be derived from the adjacent overlying water column, with the remainder from lateral injections of near-bottom particles originating on the inner shelf.