Production and partitioning of organic matter during simulated phytoplankton blooms

Few studies have examined the partitioning of organic matter in upwelling systems, despite the fact that these systems play a key role in carbon and nitrogen budgets in the ocean. We examined the production and partitioning of phytoplankton-derived organic matter in deck incubations off Oregon during the upwelling season. During exponential growth of the phytoplankton, $78% of total accumulated organic matter was in particulate (POM) form. This suggests that dissolved organic matter (DOM) is a small fraction of primary production during the exponential growth of coastal phytoplankton blooms. After nitrate depletion, carbon-rich (C : N $ 16) DOM accumulated in incubations dominated by the diatom Chaetoceros sp., accounting for 38% (68.5%) of accumulated total organic carbon (TOC) and 24% (68%) of accumulated total organic nitrogen (TON). However, in a bloom dominated by the diatom Leptocylindrus minimus, a relatively smaller amount of DOM accumulated, accounting for only 15% of accumulated TOC and 7% of accumulated TON. On the basis of measured concentrations of nitrate and accumulated TOC, ;70%–157% more carbon was fixed than would be predicted by Redfield stoichiometry (referred to as ‘‘excess carbon fixation’’), with 20%–69% of the excess carbon fixation occurring after nitrate depletion. The accumulation of carbon-rich DOM and excess carbon fixation suggests that nitrate assimilation (i.e., new production) might not equate to net production of POM in coastal upwelling systems. Coastal upwelling systems are among the most productive marine ecosystems in the world. Although accounting for only 1% of total ocean surface area, coastal upwelling regions account for $10% of global new production (Chavez and Toggweiler 1995). New production is defined as primary production that is based on newly available nitrogen (Dugdale and Goering 1967) and is commonly determined from regional observations of nitrate-based primary production, because nitrate is globally the main source of new nitrogen to the euphotic zone. In coastal regions, the fate of new production is thought to be export through sinking of particulate organic matter (POM) or through the accumulation of higher trophic level biomass (e.g., fish production) (Eppley and Peterson 1979; Walsh 1991). However, earlier paradigms are now being revisited, because numerous studies have shown an accumulation of dissolved organic matter (DOM) after phytoplankton blooms and over the course of the growing season in a variety of marine systems (Ittekkot et al. 1981; Bronk et al. 1994; Williams 1995). Additionally, elevated carbon fixation relative to nitrogen assimilation has been observed in the surface ocean (Sambrotto et al. 1993). The fate of this excess fixed carbon is largely unknown. Mechanisms, such as aggregate formation resulting from transparent exopolymer (TEP) production by phytoplankton, are now being elucidated, and these might facilitate rapid export of the excess fixed carbon from the euphotic zone (Engel et al. 2002).