Inverse modeling of carbon and nitrogen flows in the pelagic food web of the northeast subarctic Pacific |
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| Institution: | 1. LEGOS, Laboratoire d''Etudes en Géophysique et Océanographie Spatiales (Université de Toulouse, CNRS/CNES/IRD/UPS), Observatoire Midi Pyrénées, 14 Avenue Edouard Belin, 31400 Toulouse, France;2. Division of Marine Science, University of Southern Mississippi, Stennis Space Center, MS 39529, USA;3. Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA;1. CAS Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, People''s Republic of China;2. Laboratory for Marine Ecology and Environmental Science, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266237, People''s Republic of China;3. University of Chinese Academy of Sciences, Beijing 100049, People''s Republic of China;4. Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, People''s Republic of China |
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| Abstract: | We use inverse analysis to model carbon and nitrogen flows in the upper ocean food web at Ocean Station Papa (OSP; 50°N, 145°W) for winter, spring, and late summer. The seasonal variability in basic physical, chemical, and biological characteristics is low, and the particulate carbon and nitrogen flux at 200 m is remarkably constant. Despite this apparent uniformity, the food web structure undergoes significant seasonal changes. The diversity of trophic pathways is higher during late summer than during the other two periods. The spring ecosystem is not in steady state and undergoes net phytoplankton growth and macronutrient consumption. The microbial loop is well developed only during late summer. Nevertheless, ammonium regeneration by the food web seems insufficient to meet demand by the primary producers. The difference may be due to recycling of dissolved organic nitrogen (urea+free amino acids), a process not represented in the model. The winter food web is the closest to steady state, with nitrate utilisation approximately in balance with export of particulate nitrogen. The inverse analysis suggests two main seasonally invariant features of the NE Pacific ecosystem. First, the major trophic pathway is always from picophytoplankton (0.2–5 μm) to microzooplankton (heterotrophic dinoflagellates and ciliates) to mesozooplankton. This supports the idea of a strong coupling between the microbial and metazoan food webs. Second, much of the primary production (and bacterial production in late summer) is not grazed and is recycled through the detrital pool. Both these features seem to arise from the requirement to conserve nitrogen as well as carbon in the food web. More complete measurements on the microzooplankton 20–200 μm in size, including the small metazoans like nauplii larvae, are required to improve the models presented here. |
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