ABSTRACT: During the 1995 spring bloom in Bedford Basin, Nova Scotia, Canada, dissolved organic 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 ultrafiltered size fractions were combined with particulate organic carbon (POC) and nitrogen (PON) measurements. The combined C:N ratio of the total standing stock of organic matter (TOM) increased from near-Redfield values to a maximum during the early bloom as carbon-rich diatom exudates accumulated in the dissolved (primarily the colloidal) size fraction. Even though the colloidal exudates were only a small fraction (<12%) of the TOM, they had a pronounced effect on the C:N ratio. In distinct contrast, the C:N of particulate organic matter (POM) remained almost invariant, contributing little to the change in the combined C:N ratio. Respiration was closely associated with the C:N of TOM and colloidal organic matter (COM), but appeared to have little effect on one of the most commonly utilized indices of ocean productivity -- the C:N of POM. 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.

KEY WORDS: Microbial respiration · Coastal bloom · C:N ratio

Published in MEPS Vol. 150 (1997) on April 30
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1997