Water column dynamics of dissolved inorganic carbon (DIC), nitrogen and O 2 on Georges Bank during April, 1990

Abstract

We measured dissolved inorganic carbon (DIC), combined inorganic nitrogen, O 2, chlorophyll a (Chl a), particulate carbon and nitrogen (PCN) along with selected additional parameters on Georges Bank during April. Summer conditions were developing in the region as reflected by elevated biological production at the margins of the Bank (Chl a and O 2 levels > 16mg m −3 and> 109% saturated respectively) and nitrate depletion on mid-Bank. Oxygen supersaturations were maintained in the regions of low NO 3 concentration, suggesting that a rapid supply of NO 3 was occurring horizontally from deeper waters. The productive region was propagating into deeper waters as daylight increased but was not yet associated with the tidally mixed front on the western flank. Changes in DIC and O 2 suggested net community production (NCP) of 0.24-0.42 g C m −2 day −1 on the flank. A collaborative purposeful tracer experiment on mid-Bank permitted a time series of net biological activity to be made and changes in both DIC and O 2 indicated similarly large net respiration was occurring there (NCP = −1.4 g C m −2 day −1). A budget of particulate carbon and nitrogen suggested the biological character of the mid-Bank seston changed from autotrophy to heterotrophy and most of the remaining suspended particulates were detrital. Thus, the prolonged residence time of shelf water on the Bank contributes to efficient respiration of much of the organic matter formed locally and diminishes the Bank's role as a source of organic matter for off-shelf or down-shelf export. Based on an analysis that accounted for mixing effects between different water masses on the Bank, the ratio of DIC to NO 3 consumption was significantly greater than the Redfield ratio of 6.6. Thus, in such eutrophic regions, estimates of NCP derived from new (nitrate) production may be low by as much as 40%.