Abstract
Pelagic fish communities are shaped by bottom-up and top-down processes, transport by currents, and active swimming. However, the interaction of these processes remains poorly understood. Here, we use a regional implementation of the APex ECOSystem Model (APECOSM), a mechanistic model of the pelagic food web, to investigate these processes in the California Current, a highly productive upwelling system characterized by vigorous mesoscale circulation. The model is coupled with an eddy-resolving representation of ocean currents and lower trophic levels, and is tuned to reproduce observed fish biomass from fisheries independent trawls. Several emergent properties of the model compare realistically with observations. First, the epipelagic community accounts for one order of magnitude less biomass than the vertically migratory community, and is composed by smaller species. Second, the abundance of small fish decreases from the coast to the open ocean, while the abundance of large fish remains relatively uniform. This in turn leads to flattening of biomass size-spectra away from the coast for both communities. Third, the model reproduces a cross-shore succession of small to large sizes moving offshore, consistent with observations of species occurrence. These cross-shore variations emerge in the model from a combination of: (1) passive offshore advection by the mean current, (2) active swimming toward coastal productive regions to counterbalance this transport, and (3) mesoscale heterogeneity that reduces the ability of organisms to return to coastal waters. Our results highlight the importance of passive and active movement in structuring the pelagic food web, and suggest that a representation of these processes can help to improve the realism in simulations with marine ecosystem models.
Highlights
Marine ecosystems provide a variety of services to humans, including food provision by fisheries (Reid, 2005; Beaumont et al, 2007; Barbier, 2017)
While the influence of food and temperature are generally accounted for when assessing the impacts of climate change in marine ecosystem models, impacts of changes in ocean currents and movement have received far less attention (Watson et al, 2015; Heneghan et al, 2021)
By comparing the ensemble of APex ECOSystem Model (APECOSM)-1D simulations to observations, we identified 6 best plausible parameter sets
Summary
Marine ecosystems provide a variety of services to humans, including food provision by fisheries (Reid, 2005; Beaumont et al, 2007; Barbier, 2017). The dynamics of marine ecosystems reflect the interplay of physical, biological, and ecological processes, as well as direct human impacts through, e.g., fishing. Understanding these processes is key to adequately represent and project the state of fisheries. Size-dependent trophic interactions connect prey to predators (Estes et al, 2016) and are modulated by the environment Factors such as temperature and food abundance affect metabolism, growth, and reproduction, in turn altering the accumulation of biomass in the food web (Free et al, 2019). While the influence of food and temperature are generally accounted for when assessing the impacts of climate change in marine ecosystem models, impacts of changes in ocean currents and movement have received far less attention (Watson et al, 2015; Heneghan et al, 2021)
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