Affect of ecosystem variables on an estuarine food web

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Food web models, such as those by J. Dunne and N. Martinez, provide increasingly accurate descriptions of the feeding relationships and population dynamics of ecological communities. However, these models assume that the population growth rate of producers, the "bottom nodes" of plants and phytoplankton, is independent of environmental conditions. In reality, the growth rate of photosynthetic producers is mainly a function of light energy and nutrient availability. We are building or altering a model of an estuarine food web, ideally of the Chesapeake Bay ecological community. Estuaries are regions where fresh water empties into the sea, mixing with salt water and carrying sediments and nutrients such as nitrogen and phosphorous. We will tie the growth rate, "r," of phytoplankton (plant-like plankton) to the level of light, silica, nitrogen, and so on in a species-specific manner. We will then vary these parameters and observe the effect on the food web as a whole. We hypothesize that, in general, reducing energy and key limiting nutrients will reduce populations of all organisms in the web. Current studies indicate that oceanic communities experience "top-down control," in which animals at higher trophic levels ("higher on the food chain") have a greater affect on food web dynamics than the food-limiting affects of producers. Given the high productivity, or biomass produced per unit time, of estuaries, we expect our model will show a pronounced "bottom-up effect," consistent with the large seasonal changes in the composition and population dynamics of estuarine food webs.

Group members: Sandra Arndt, Bill Burnside