Respiration and the carbon-to-nitrogen ratio of a phytoplankton bloom

Abstract

During the 1995 spring bloom in Bedford Basin, Nova Scotia, Canada, dissolved organlc carbon (DOC) and nitrogen (DON) in surface waters were separated by cross-flow ultrafiltration into low molecular weight and colloidal size fractions. In order to obtain realistic estimates of the total standing stock of organic carbon and nitrogen during the bloom, measurements of DOC and DON in the iiltrafiltered size fractions were combined with particulate organic carbon (POC) and nltrogen ( P O N ) measurements. The combined C.N ratio of the total standing stock of organlc matter ( T O M ) ~ncreased from near-Redfield values to a maxinlum duling the early bloom as carbon-r~ch diatom exudates accumulated in the dissolved (primarily the colloidal) size fraction. Even though the colloidal exudates were only a small frdctlon (<12%) of the TOM, they had a pronounced effect on the C:N ratio. In distinct contrast, the C:N of part~culate organlc matter (POhl) remdined almost invariant, contr~butlng little to the change in the comb~ned C::N ratio. Respiration was closely associated w ~t h the C : N of TOM and colloidal organic matter ( C O M ) , but appeared to have little effect on one of the most commonly utilized indices of ocean productivitythe C : N of P O M . Instead, by the end of the bloom, respiration had returned the C : N ratio of the TOM to near-Redfield values by selectively degrading the colloidal and low molecular weight organic carbon associated with the production of diatom exudates.