Net accumulation and flux of dissolved organic carbon and dissolved organic nitrogen in marine plankton communities

Abstract

Marine mesocosms were manipulated with inorganic nutrients over a period of 22 d to investigate organic carbon partitioning under a variety of nutrient regimes. The chemical analyses and biotic measurements included inorganic nutrients, pigment signatures, particulate and dissolved organic species, bacterial production, and community respiration. The biodegradability of dissolved organic carbon (DOC) was investigated with in vitro decomposition experiments. The net particulate organic carbon (POC) production was 50% of the total organic production during the initial 6 d of nutrient‐replete growth and during a major diatom bloom. In all other situations the carbon partitioning was strongly in favor of DOC, which accounted for 82 to 111% of the total production. The production of new DOC preceded new DON by about 1 week. Thus, the new dissolved organic matter (DOM) initially had an infinite C:N ratio, which fell to 11‐20 when DON started to accumulate. The highest C:N ratio was measured during a nutrient‐replete diatom bloom. Dissolved polysaccharides accounted for 50 to 70% of the new DOC, and the lowest relative amount was produced during a diatom bloom. The chemical analyses unequivocally demonstrated that carbon partitioning in favor of carbon‐rich DOM can take place during an active diatom bloom and not only during the decay of a bloom. The DOC‐producing mechanisms cannot be fully identified. However, during the different growth phases the DOC production varied, as did the speciation of DOM with respect to the C:N ratios. When net production of dissolved organic nitrogen (DON) was detected after 11 d, the DON production accounted for 25 to 50% of the daily added and assimilated inorganic nitrogen. The measurements of community respiration made it possible to calculate the maximum carbon recycling by bacteria and bacterial net DOC assimilation. These calculations showed the estimates of carbon partitioning to be very sensitive to bacterial growth yield values and the factors used to convert leucine and thymidine isotope incorporation to bacterial production. Decomposition experiments showed that at least 35% of the new DOC was biodegradable over 10–12 d and that inorganic nutrients only marginally affected use. The calculated turnover times of new DOC were between 15 and 25 d. The semilabile nature of new DOC with respect to microbial attack is suggested as the main reason for the medium‐term accumulation of new DOC.

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