Summer nutrient dynamics in the Middle Atlantic Bight: primary production and utilization of phytoplankton carbon

Abstract

In late summer, production and utilization of carbon in the stratified water of the Middle Atlantic Bight appears to approach steady-state conditions. In the euphotic zone there is a paniculate organic carbon (POC) pool of ˜6000 mg Cm⁻². Primary production adds ˜350 mg C m⁻² d⁻¹, while zooplankton ingestion removes ˜450 mg C m⁻² d⁻¹, of which ˜135 mg C m⁻² d⁻¹ are returned to the POC pool as feces. Sinking of POC averages ˜240 mg C m ⁻² d⁻¹. Thus, there is a net loss from the euphotic zone of ˜200 mg C m⁻² d⁻¹, which represents a removal rate from the POC pool of ˜3%d⁻¹. However, sinking losses of phytoplankton carbon from the POC pool were small (˜12 mg C m⁻² d⁻¹), which suggests that most of the primary production enters the pelagic food chain. This is in sharp contrast to the conditions in early spring, when the POC pool increases substantially, and the main loss is sinking rather than heterotrophic consumption.