The relationship between primary production and the vertical export of particulate organic matter in a river-impacted coastal ecosystem

Abstract

As part of the National Oceanic and Atmospheric Administration’s (NOAA) Nutrient Enhanced Coastal Ocean Productivity program, we have conducted four research cruises, July–August 1990, March 1991, September 1991, and May 1992, in the Mississippi River plume and adjacent shelf regions. Over this time period, photic-zone-integrated primary production varied significantly in both the river plume and shelf study regions, with greatest variability observed in the river plume region. In the river plume and the adjacent shelf, highest production occurred during July–August 1990 (8.17 g C m⁻² d⁻¹ for the plume and 1.89–3.02 g C m⁻² d⁻¹ for the shelf) and the lowest during March 1991 (0.40–0.69 g C m⁻² d⁻¹ for the plume and 0.12–0.45 g C m⁻² d⁻¹ for the shelf). The vertical export of POC from the euphotic zone, determined with free-floating MULTITRAP sediment trap systems, also varied temporally in both study regions, with highest values occurring in May 1992 (1.80±0.04 g C m⁻² d⁻¹ for the plume and 0.40±0.02 g C m⁻² d⁻¹ for the shelf) and the lowest values occurring during July–August 1990 (0.29±0.02 g C m⁻² d⁻¹ for the plume and 0.18±0.01 g C m⁻² d⁻¹ for the shelf). The fraction of production exported out of the photic zone was highly variable and was dependent, in part, on phytoplankton species composition and on the grazing activities of microzooplankton and mesozooplankton. The lowest ratio of export to production coincided with the time when production was greated and the highest ratios occurred when production was the lowest.