Award: OCE-1259057

Objectives and Intellectual Merit: A major feature distinguishing shelf ecosystems is the physical context that defines rates of nutrient import, nutrient recycling, and plankton exchange between coastal and oceanic waters. When comparing dynamics of marine ecosystems involving multiple species, researchers have focused upon studying differences in food web structures. Food web models integrate our knowledge of community composition, ecosystem productivity, and species interactions. This allows investigation of temporal variability among species that are primarily driven through predator-prey interactions and fishing pressure. However, to understand ecosystem dynamics and resilience to natural and anthropogenic perturbations, researchers must also consider the physical context of the ecosystem. The two major goals of this project were: 1) to develop a common, intermediate complexity physical/biological model platform that may be applied to study diverse ecosystems, and 2) to understand how food web structure and physical context each control the dynamics of coastal ecosystems. Approach: Our first major activity was the development of a standardized multi-species ecosystem model platform, ECOTRAN, that could be applied to diverse shelf ecosystems. It considers multiple trophic levels, includes nutrient and detritus cycling pathways, and is built upon a 2-dimensional 5-box geometry to incorporate physical advection and mixing across the shelf. We considered four food web models within four physical settings representing diverse coastal ecosystems: the Northern California Current upwelling ecosystem, the Coastal Gulf of Alaska downwelling ecosystem, Georges Bank shallow bank ecosystem, and the North Sea semi-enclosed basin ecosystem. Our second major activity was to apply the models to investigate the role of food web structure and the role of physical context in determining ecosystem dynamics. We examined response times of coupled physical - food web models following perturbations to nutrient input rates, and we ran different food web models within each of the four physical settings. ECOTRAN was also applied to the Northern California Current upwelling ecosystem to examine the effects of changes to upwelling characteristics expected to occur into the future as a result of global warming. Results and Impact: ECOTRAN simulations provide information about ecosystem resilience and response to environmental change and resource management strategies, and these are of further value to economic and social sciences. Response times of different species following changes to plankton production are related to the time required for the plankton production rate to balance physical transport rates, the rate of detritus recycling back into the food web, and growth rates of higher trophic level consumers. Detritus recycling plays a major role in system dynamics since it affects both "bottom-up" fluxes through the food web and "top-down" pressures by predators. When detritus recycling is low, as in an upwelling system, response times are short and less sensitive to differences in physical transport rates. But when detritus recycling is high, as in a semi-enclosed basin, more production is recycled through longer-lived, higher trophic level consumers whose slow growth impose long response times. Comparative analyses of different food webs within different physical settings tested the hypothesis that when nutrient supply and physical transport rates are standardized, there are no substantial differences in the dynamics within diverse food webs. With few specific exceptions, physical context played the greater role controlling dynamics. Exchange between shelf and ocean affected not only nutrient recycling but also trophic transfer efficiencies and the importance of pelagic vs. benthic components of the food web. Differences in plankton transport rates leads to apparent decoupling of lower trophic and upper trophic level production - reducing upper trophic production relative to plankton production (upwelling) or enhancing upper trophic level and benthos production (downwelling). Food web structure further affects dynamics within different physical contexts. Food webs with high intrinsic detritus production have especially productive benthos where physical conditions provide plankton subsidies. Application of ECOTRAN to the Northern California Current allowed us to look at response to changes in upwelling. Dome-shaped relationships exist between upwelling strength and productivity across all trophic levels. Phytoplankton production is a balance between nutrient supply and physical export off the shelf. Productivity increases as phytoplankton take advantage of increasing nutrient rate, but as plankton export begins to exceed the ability of the phytoplankton community to utilize the increased nutrient supply, production declines with increasing upwelling intensity. Other researchers have proposed that global warming will lead to steeper temperature gradients between the ocean and the land, leading to greater alongshore wind-stress and increased upwelling intensity. ECOTRAN simulations suggest that as upwelling intensifies, we should expect to see increasing ecosystem productivity in the future. However, the broad distribution of upwelling event intensities over the past 50 years suggests that during the strongest events, the Northern California Current already realizes the negative effects of high rates of plankton export to the ocean. As the proportion of strong upwelling events increases beyond current conditions, the historically observed correlations between upwelling and overall ecosystem productivity may break down. Last Modified: 05/22/2017 Submitted by: James J Ruzicka