Simulation of upper-ocean biogeochemistry with a flexible-composition phytoplankton model: C, N and Si cycling in the western Sargasso Sea |
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| Authors: | Mathieu Mongin David M Nelson Philippe Pondaven Mark A Brzezinski Paul Trguer |
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| Institution: | a College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Admien Building, Corvallis, OR 97311, USA;b Institut Universitaire Européen de la Mer—UMR CNRS 6539, Technopole Brest-Iroise, Place Nicolas Copernic, F-29280, Plouzané, France;c Department of Ecology Evolution and Marine Biology and the Marine Science Institute, University of California, Santa Barbara, CA 93106, USA |
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| Abstract: | We report the first application of a biogeochemical model in which the major elemental composition of the phytoplankton is flexible, and responds to changing light and nutrient conditions. The model includes two phytoplankton groups: diatoms and non-siliceous picoplankton. Both fix C in accordance with photosynthesis-irradiance relationships used in other models and take up NO3− and NH4+ (and Si(OH)4 for diatoms) following Michaelis-Menten kinetics. The model allows for light dependence of photosynthesis and NO3− uptake, and for the observed near-total light independence of NH4+ uptake and Si(OH)4 uptake. It tracks the resulting C/N ratios of both phytoplankton groups and Si/N ratio of diatoms, and permits uptake of C, N and Si to proceed independently of one another when those ratios are close to those of nutrient-replete phytoplankton. When the C/N or Si/N ratio of either phytoplankton group indicates that its growth is limited by N, Si or light, uptake of non-limiting elements is controlled by the content of the limiting element in accordance with the cell-quota formulation of Droop (J. Mar. Biol. Ass. U.K 54 (1974) 825).We applied this model to the Bermuda Atlantic Time-series Study (BATS) site in the western Sargasso Sea. The model was tuned to produce vertical profiles and time courses of NO3−], NH4+] and Si(OH)4] that are consistent with the data, by adjusting the kinetic parameters for N and Si uptake and the rate of nitrification. The model then reproduces the observed time courses of chlorophyll-a, particulate organic carbon and nitrogen, biogenic silica, primary productivity, biogenic silica production and POC export with no further tuning. Simulated C/N and Si/N ratios of the phytoplankton indicate that N is the main growth-limiting nutrient throughout the thermally stratified period and that Si(OH)4], although always limiting to the rate of Si uptake by diatoms, seldom limits their growth rate. The model requires significant nitrification in the upper 200 m to yield realistic time courses and vertical profiles of NH4+] and NO3−], suggesting that NO3− is not supplied to the upper water column entirely by physical processes. A nitrification-corrected f-ratio (fNC), calculated for the upper 200 m as: (NO3− uptake—nitrification)/(NO3− uptake+NH4+ uptake) has annual values ranging from only 0.05–0.09, implying that 90–95% of the N taken up annually by phytoplankton is supplied by biological regeneration (including nitrification) in the upper 200 m. Reported discrepancies between estimates of organic C export based on seasonal chemical changes and POC export measured at the BATS site can be almost completely resolved if there is significant regeneration of NO3− via organic-matter decomposition in the upper 200 m. |
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| Keywords: | Phytoplankton Nutrient limitation Elemental ratio Model BATS Nitrogen cycle Silicon cycle Redfield ratio |
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