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Abstract
The ocean’s ability to take up and store carbon is largely influenced by marine food web structure and population dynamics. Zooplankton in particular play a major role as trophic links between primary producers and higher trophic levels, like fish. Grazing activity on lower trophic levels and the subsequent repacking of smaller material into dense, faster sinking fecal pellets, can contribute significantly to the vertical movement of carbon away from the surface ocean. The contribution of fecal pellets to the overall flux of particulate organic carbon is variable depending on location, season, depth and is influenced by the zooplankton community structure (the sizes and dominant zooplankton species) and behavior, such as diel vertical migration (DVM). To investigate the role of varying zooplankton taxa and behavior on the export of fecal pellet carbon in the ocean, we built a new agent-based model that was used in three oceanic locations with different nutrient regimes and plankton communities: a western subarctic North Pacific site, an eastern subarctic North Pacific site, and a subtropical North Pacific site. We looked at the impact of overall zooplankton abundance and vertical distribution of zooplankton individuals, in addition to diet preference and DVM behavior. We started with a simple model of copepods and expanded the model to include amphipods and salps (a gelatinous tunicate). In the first iteration of the model, which contained only copepods, DVM behavior in the model resulted in fecal pellet fluxes that were closer to that of field data, compared to scenarios with no DVM. When comparing a copepod community that was dominated by omnivores to a community dominated by carnivores or detritivores, we found that the omnivore-dominated community resulted in higher fecal pellet carbon flux and lower POC attenuation than other communities. Validation with field data showed a relatively simple model that uses trait-based relationships for physiological rates can be used to give insight into particle flux variability in the deep ocean. In the expanded model of copepods, amphipods, and salps, model outputs highlighted the outsized role that salps play in the eastern subarctic North Pacific. This study also looked at seasonal behavior of diapause and the importance of looking at individual variability, rather than just abundance, in modeling zooplankton in the biological pump. Finally, this study also highlighted the importance of looking at sequestration depths when considering comparing the role of different zooplankton taxa in the overall long-term storage of carbon. We also provided a review of agent-based models used in biogeochemistry and in marine food-web modeling.