Temporal decoupling of carbon and nitrogen dynamics in a mesocosm diatom bloom

Flows of the major biogeochemical elements (C, N, P, Si) and of transparent exopolymer particles (TEP) were traced during a bloom of a natural assemblage of marine diatoms in a mesocosm (1 m 3 ) to determine whether the exudation and subsequent gelation of carbon-rich phytoplankton exopolymers can account for the formation and potential export of carbon in excess of that predicted by Redfield ratios. Exponential growth of the phytoplankton community in the mesocosm extended for 10 d until nitrate concentration fell below detection and concentrations of dissolved inorganic and particulate organic nitrogen and phosphorus remained stable. Tight covariation of particulate organic elements occurred as long as nutrients were replete. But, after nitrate depletion, decoupling of carbon dynamics from that of nitrogen and phosphorus was observed, with a large flow of carbon into TEP. An uptake of 72% more dissolved inorganic carbon (DIC) than inferred from nitrate supply and Redfield stoichiometry (referred to as carbon overconsumption) occurred during the study, largely during the postbloom phase, and was almost entirely traced to the particulate organic matter (POM) pool. Marine snow (aggregates .0.5 mm) appeared at the onset of nitrate depletion and coincided with rapid increase in TEP concentrations. Elemental composition of marine snow differed from the Redfield ratio by an enrichment in carbon and a depletion in phosphorus relative to nitrogen. It is suggested that sinking of TEP-rich marine snow could be a possible mechanism for export of carbon above calculations that are based on the Redfield stoichiometry. An essential tool for the calculation of oceanic carbon fluxes in biological models is the Redfield ratio (Redfield et al. 1963), since it describes a fixed covariation of the elements C, N, and P in a ratio of 106 : 16 : 1. The Redfield ratio implies a reciprocal interaction between the elemental composition of marine biota and their dissolved nutrition resources. Organisms or, more generally, particulate organic matter (POM) leaving the upper mixed layer should export this ratio to the deep ocean and leave its imprint on the stoichiometry of remineralization products. Indeed, in the deep ocean the covariation of the major nutrients nitrate and phosphate and the amount of oxygen used (AOU) for oxidation of organic carbon seem to reflect this ratio almost perfectly. The theoretical framework for the calculation of carbon export from nitrate fluxes was introduced by Dugdale and Goering (1967), who differentiated between new biological production based on nitrate and regenerated production based on the degradation products of organic nitrogen compounds. Eppley and Peterson (1979) later concluded that the amount of carbon being exported to the deep sea should be equal to new production on long temporal scales. Recent investigations, however, demonstrated that the net uptake of dissolved inorganic carbon (DIC) exceeds the amount expected from nitrate removal and Redfield stoichiometry at various marine sites (Sambrotto et al. 1993; Michaels et al. 1994; Marchal et al. 1996). Possibilities that may cause this discrepancy include underestimation of the nitrogen avail1