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

Marine mesocosms were manipulated with inorganic nutrients over a period of 22 d to investigate organic carbon partitioning under a variety of nutrien t 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 di ssolved organic carbon (DOC) was investigated with in vitro decomposition e xperiments. The net particulate organic carbon (POC) production was 50% of the total or ganic production during the initial 6 d of nutrient-replete growth and duri ng a major diatom bloom. In all other situations the carbon partitioning wa s strongly in favor of DOC, which accounted for 82 to 111% of the total pro duction. 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 polysacchari des accounted for 50 to 70% of the new DOC, and the lowest relative amount was produced during a diatom bloom. The chemical analyses unequivocally dem onstrated that carbon partitioning in favor of carbon-rich DOM can take pla ce during an active diatom bloom and not only during the decay of a bloom. The DOC-producing mechanisms cannot be fully identified. However, during th e 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 org anic nitrogen (DON) was detected after 11 d, the DON production accounted f or 25 to 50% of the daily added and assimilated inorganic nitrogen. The mea surements of community respiration made it possible to calculate the maximu m carbon recycling by bacteria and bacterial net DOC assimilation. These ca lculations showed the estimates of carbon partitioning to be very sensitive to bacterial growth yield values and the factors used to convert leucine a nd thymidine isotope incorporation to bacterial production. Decomposition experiments showed that at least 35% of the new DOC was biode gradable 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. Th e semilabile nature of new DOC with respect to microbial attack is suggeste d as the main reason for the medium-term accumulation of new DOC.