The fate of a spring phytoplankton bloom: export or oxidation?

Abstract

The hypothesis that a large fraction of the primary production on continental shelves is exported to the deep ocean basins was investigated during the spring of 1984 off the coast of Long Island, New York. Using data collected aboard ship and from moored instruments, we constructed a carbon budget to account for the production and utilization of the spring bloom. We observed a strong cross-shelf gradient in the species composition of the phytoplankton and in chlorophyll a throughout the spring. Shelf waters were separated from slope water by a front, which was located between the 80- and 100-m isobaths. Between mid-February and mid-April chlorophyll concentrations increased from <2 to >10 μg l −1 in waters shallower than 70 m. This increase is typical of a spring bloom on the shelf. However, records from moored fluorometers near the shelf-slope front did not document an increase in chlorophyll, and it appears that very little of the bloom from the shelf was transported across the front, even during storms. Grazing pressure by zooplankton on the shelf increases throughout the spring, and copepods cropped about 34% of the daily production in early spring. About 51% of the daily production sank and formed a nepheloid layer which was oxidized on the shelf. Despite the lack of vertical density stratification on the shelf, there was a vertical gradient in dissolved oxygen. The oxidation and decomposition of organic matter regenerated nutrients that sustained high productivity until the onset of stratification in late spring. We conclude that the export of shelf-derived production to the deep ocean is small, averaging only 10–20% of the spring bloom.