The 1983–1984 shelf edge exchange processes (SEEP)—I experiment: hypotheses and highlights

Abstract

A multidisciplinary SEEP—I study of particle formation, transformation, transport, sedimentation, and storage was conducted across the shelf-break south of Martha's Vineyard and Long Island during July 1983–October 1984. Moored arrays of current meters, thermistors, transmissometers, fluorometers, and sediment traps provided time-series data on the distributions of currents, temperature, light transmission, fluorescence, organic carbon, carbonate, and particulate 210Pb within shelf and slope waters of the Mid-Atlantic Bight. Shipboard and aircraft measurements focused on the birth and death processes of plankton during the spring bloom, when seasonal fluxes of carbon, carbonate, and 210Pb to slope waters and sediments were maximal. Simulation models of nitrogen, carbon, and oxygen transfers during the 1979 and 1984 spring blooms suggested that 50–60% of the daily primary production was consumed by the rest of the shelf food web. Surviving shelf particles enter a near-bottom physical regime on the continental slope which favors their downslope transport and deposition. On an annual basis, 210Pb budgets of the slope sediments suggest that only 20–60% of organic debris from the adjacent shelf ecosystem may exit the shelf-break in the SEEP—I region, with the remainder entrained downstream towards Cape Hatteras. After spring export, continued oxidation of organic matter within slope sediments of the SEEP—I region allows carbon accumulation rates here of <1% of the annual primary production on the shelf. The SEEP—I study area represents a region of minimal export of particles from the Mid-Atlantic Bight, however. For example, 250 km farther south on the 1000-m isobath, the annual fluxes of organic carbon and 210Pb at 25 m above bottom in 1985–1986 were two- to three-fold greater than those at equivalent depths in the SEEP—I experiment.