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A biophysical NPZ model with iron for the Gulf of Alaska: Reproducing the differences between an oceanic HNLC ecosystem and a classical northern temperate shelf ecosystem
Affiliation:1. College of Marine Science, University of South Florida, St. Petersburg, FL 33701, United States;2. Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University, Norfolk, VA 23529, United States;3. Department of Ecology, Evolution and Marine Biology, University of California Santa Barbara, Santa Barbara, CA 93106, United States;1. Institute of Low Temperature Science, Hokkaido University, Kita-19, Nishi-8, Kita-ku, Sapporo 060-0819, Japan;2. Graduate School of Environmental Earth Science, Hokkaido University, Kita-10, Nishi-5, Kita-ku, Sapporo 060-0810, Japan;3. Arctic Research Center, Hokkaido University, Kita-21, Nishi-11, Kita-ku, Sapporo 001-0021, Japan;1. Department of Crop Production Ecology, Swedish University of Agricultural Sciences, Uppsala, Sweden;2. ASTRO Agrosystèmes Tropicaux, INRA, 97170 Petit-Bourg, Guadeloupe, France;3. UMR951 Innovation, INRA, Univ Montpellier, F-34060 Montpellier, France
Abstract:Modeling the coastal Gulf of Alaska (CGOA) is complicated by the highly diverse physical and biological features influencing productivity and energy flow through the region. The GOA consists of the offshore oceanic environment, characterized by iron limitation, high-nutrients and low-chlorophyll. The coastal environment is consistently downwelling, with high iron levels from glacial melt water and runoff, but lower concentrations of macronutrients, and with a spring bloom, nutrient depletion cycle (low-nutrient, high-chlorophyll). Cross-shelf movement of water masses mixes coastal and oceanic ecosystem elements.Simulations and field data indicate that the minimum model complexity necessary to characterize lower trophic-level production and biomass in the offshore and coastal regions includes 10 boxes: iron, nitrate, ammonium, small phytoplankton, large phytoplankton, small microzooplankton, large microzooplankton, small copepods, large oceanic copepods and detritus, with copepod mortality as a model closure term. We present the model structure, equations required (and initial parameters used) to simulate onshore and offshore lower trophic-level production in the Gulf of Alaska, along with the information from field data and simulations used to construct the model. We show the results of simulations with and without iron, and with and without two size classes of phytoplankton. These simulations indicate that our method of inclusion of iron works well to distinguish the coastal and the oceanic ecosystems, and that the inclusion of two size categories of phytoplankton is also necessary to generate the differences between these two ecosystems.
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