Document: DEN-3-52-54

Food web complexity and the response of aquatic systems to stress.

BREITBURG, D.L.* ¹, C.E.RICHMOND ¹ and S.M.BARTELL ²

The Academy of Natural Sciences, St. Leonard, MD 20678, USA ¹
The Cadmus Group ²

Abstract:

Understanding how the complexity of food webs affects responses of aquatic systems is critical to both our basic understanding of the ecology of these systems and to determining the complexity of experimental food webs necessary to generate sound information for management decisions. As part of the COASTES program we have been conducting a series of mesocosm, large field enclosure, and small-scale laboratory experiments to examine how trophic complexity affects the response of a representative coastal system, the Patuxent River, to nutrient enrichment and trace elements. Our results indicate that the effect of increasing food chain length can be quite different than the effect of increasing the number of species that feed at the same trophic level. Thus the geometry of food webs as well as the diversity of species within an assemblage is important. For example, increasing food chain length increased the magnitude and variability of the response of phytoplankton, bacterioplankton, and heterotrophic nannoflagellates to nutrient additions, while increasing the number and kinds of taxa feeding on the microbial food web (i.e., increasing the complexity of the food web) dampened the absolute and proportional responses. The effect of food web complexity on responses to stressors was independent of the strength of the direct effects of consumers on prey. In addition, increasing both food chain length and food web complexity increased the temporal stability of the system. A food web simulation model was used to test alternate mechanisms that could explain the pattern we have seen, i.e., maximum response to stressors in long, relatively simple food webs and dampening of responses in more complex webs. These mechanisms are not mutually exclusive and include increased numbers of pathways in complex food webs, differences in total consumption with different numbers of grazers, and the dominance of species-specific characteristics of grazers.

Keywords: food web complexity, estuary, plankton, multiple stressors

This abstract is being presented at: 2:30 PM in session:
Oral Session #11: Trophic Cascades.