Objectives:

A major feature distinguishing shelf ecosystems is the physical context that defines rates of nutrient import, nutrient recycling, and plankton exchange between the coastal waters and the ocean. When comparing the dynamics of marine ecosystems involving multiple species, researchers have focused upon studying the fundamental differences in food web structures. This allows investigation of temporal variability among functional guilds that are primarily driven through predator-prey interactions and fishing pressure. However, in order to understanding 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 continental shelf ecosystems, and 2) to understand how food web structure and physical context each control the dynamics of continental shelf ocean ecosystems.

Approach:

Our first major activity was the development of a standardized multi-species ecosystem model platform that could be applied to diverse shelf ecosystems. This model platform is called ECOTRAN; it considers multiple trophic levels, includes nutrient and detritus cycling pathways, and is built upon a simple 2-dimensional 5-box geometry to incorporate physical advection and mixing between regions across the shelf. We considered four food web models within four physical settings representing diverse coastal ecosystems. These are: 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. The model platform was also applied specifically 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.

Significant Results:

Response times of different food web components following a change in nutrient availability are related to the time required for the new plankton production rate to balance physical transport rates, the rate that detritus is recycled back into the food web, and the individual growth rates of higher trophic level consumers. Detritus recycling plays a major role in controlling system dynamics since it affects the importance of both “bottom-up” fluxes through the food web and “top-down” pressures by predators. When detritus recycling is low, response times are short and less sensitive to differences in plankton transport rates. But when detritus recycling is high, more system production is recycled through longer-lived, higher trophic level consumers whose slower growth impose longer response times.

Comparative analyses of four different food webs within each physical setting tested the null hypothesis that when nutrient input and physical factors are standardized, there are no substantial differences in the productivity of similarly defined guilds 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 relative 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 within sub-regions where physical conditions provide plankton subsidies.

Application of ECOTRAN to the Northern California Current allowed us to look at response to changes in upwelling. Simulations showed dome-shaped relationships between upwelling strength and productivity across all trophic levels. Phytoplankton production is a balance between nutrient supply and physical export of plankton off the shelf. Productivity increases as phytoplankton take advantage of the increasing nutrient supply rate, but as plankton export rates begin to exceed the ability of the phytoplankton community to utilize the increased nutrient supply rate, production rates decline 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 within eastern boundary current ecosystems. Our model simulations then 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 NCC 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, correlations between upwelling and overall ecosystem productivity may break down.

Last Modified: 05/19/2017
Modified by: Kenneth H Brink