Food Webs and Rivers: Importance of Contiguous Habitat Interactions

In recent years, ecological research in the study of trophic interactions between contiguous ecosystems and microhabitats has begun to develop; however, this topic remains not entirely understood. These interactions are important because they contribute to healthy ecosystem structure and function. The focus of this project was to more fully understand and document the dynamic interactions and flow of energy between stream and riparian zones in wetland, cedar (Cedrus spp.), and alder (Alnus spp.) dominated reaches. We hypothesized the base of the aquatic food web would be autotrophic in the wetlands with energy derived mainly from in-stream processes; whereas the cedar and alder dominated reaches would be more heterotrophic due to riparian shading and limited sunlight. In these reaches, energy that sustains the food web should be largely derived from riparian leaf litter inputs. Because leaf litter enters the stream in the fall, there is a time delay (termed a reciprocal subsidy) such that energy transfers between stream and riparian zones are asynchronous. We predicted that there would be a stronger energy flux in the cedar and alder reach due to the increased area conferred by the larger and more complex tree structure relative to the wetlands. We used stable isotopes of carbon and nitrogen to measure food web structure and to estimate energy transfer rates. In addition, we measured organic matter content of various trophic levels, the physical and chemical habitat between reaches, and an assessment of aquatic macrophytes, macroinvertebrates, and fish communities, with an emphasis on the latter two. Habitat comparison and characterization show that many of the chemical characteristics of each reach (pH, conductivity, temperature , etc.) are fairly uniform throughout each reach and the entire stream. The increased riparian canopy and terrestrial vegetation of the alder and cedar reaches, however, acts to increase both allochthonous input and in-stream OM. Amongst all three sites, strong relationships as indicated by stable isotope analysis showed trophic interactions between organic matter to terrestrial macroinvertebrates and then to fish populations. Isotopic signatures from algae were not measured. However, inferences from typical algae isotope signatures in similar streams, along with data on aquatic macroinvertebrates, suggested significant trophic interaction from algae up through aquatic macroinvertebrates to fish. Although our results did not support our initial hypotheses concerning the differences between study reaches, we did find evidence of strong interactions between the aquatic and terrestrial riparian zone. Specifically, fish populations in all three reaches were feeding on a mixture of terrestrial and aquatic insects. Terrestrial insects were indirectly receiving energy from in-stream organic matter. Thus the link between aquatic and terrestrial energy sources was shown to be quite strong in all three reaches, including the wetland-dominated reach. Management implications include the importance of protecting and restoring both in-stream and riparian habitat. The two are inextricably linked by the exchange of organisms and energy