Quantitative Predictions of the Responses of Aquatic Ecosystems to Disturbance III
Presiding: C P Hawkins, Utah State University; L Yuan, U.S. Environmental Protection Agency; A I Pollard, U.S. Environmental Protection Agency
NB42E-01 10:30h
The Land Cover Cascade: Cause-Effect Couples Between Watersheds and Stream Biota
A cascade may be defined as the propagation of a stimulus through a series of interdependent cause - effect relationships that generate an ultimate response. The cascade concept has been applied to biotic systems ranging in scope from molecules to ecosystems. The approach has been useful in organizing complex, multivariate, biotic relationships, and in identifying mechanisms associated with the transformation of stimuli in biotic systems. We propose the Land Cover Cascade (LCC) to summarize the translation of anthropogenic disturbance through physical ecosystem elements to ultimate influences on stream biota. Much is known about how individual or groups of stream elements respond to disturbance, but how element responses are coupled, and how these couples translate disturbance implications, are largely unknown. To quantify the LCC, we estimated land-cover and physical ecosystem elements in streams influenced by agricultural land-uses. Path analysis identified cascades that organized direct and indirect effects of elements on biotic responses, and assessed the predictive ability of each cascade. Cascade models of 3-4 links explained 50-94 percent of variation observed in thirteen fish and macroinvertebrate assemblage responses. This approach moves us beyond traditional bivariate relationships, and brings us closer to identifying the mechanisms and pathways involved in ecosystem responses to disturbance.
NB42E-02 10:45h
Appraisal of Multiple-Scale Land Cover and Site-Level Physical Factors on the Contribution of Terrestrial Invertebrates to Stream Ecosystems
There is a growing body of literature emphasizing the importance of terrestrial invertebrates to the structure and function of stream ecosystems, mainly as an energy subsidy for stream fish. The input of terrestrial invertebrates from the riparian area of streams may be linked to the vegetation type and land use of the riparian area. To test this, terrestrial invertebrates falling into streams were collected with floating pan traps. These data were combined with a GIS model of surrounding land use composition, assessed at multiple spatial scales in 12 northeast Ohio streams. A site-level index of habitat quality was used to evaluate local stream characteristics to determine the scale at which factors influencing terrestrial invertebrate input are most prominent. Correlation analysis showed the strongest relationships between the amount of shrub/scrub vegetation near the stream and invertebrate diversity and the ratio of terrestrial-derived to aquatic-derived invertebrates caught in the pan traps. These relationships were significant at multiple spatial scales up to 5000 m. Site-level physical features that correlated with terrestrial invertebrate input included stream gradient and substrate quality. The results show that the contribution of terrestrial invertebrates to streams is related to land use beyond the immediate riparian area.
NB42E-03 11:00h
The impact of solar radiation on the use of dissolved organic matter in two Ozark Streams: Possible role of photo-enhanced humification
The effect of solar radiation on dissolved organic matter (DOM) utilization was studied in two contrasting streams from June 2002 through October 2004. Moores Creek is an agricultural stream with elevated nutrient and dissolved organic carbon (DOC) concentrations. Huey Hollow is a forested stream with low nutrient and DOC concentrations. A series of experiments were conducted seasonally to assess how solar radiation influenced DOM utilization. Exposure of DOM to solar radiation significantly decreased its utilization during most seasons in both streams. Each stream experienced one seasonal period when exposure of DOM significantly increased bacterial production; during these periods, DOM appeared to be the least bioavailable and most photochemically reactive. Interestingly, in spring when bioavailability of DOM was lowest in Moores Creek solar radiation exposure further reduced DOM bioavailability. Elevated ammonium concentrations during this spring experiment suggest photochemically-enhanced humification may have been an important mechanism influencing DOM cycling. Bioassays using 15N-labeled ammonium indicated no significant effect of elevated ammonium on the utilization of DOM in either stream in fall 2004. Detection of elevated 15N in the DOM fractions, however, would reveal light stimulated humification under elevated ammonium concentrations not detected with the bioassay.
NB42E-04 11:15h
Ecological Status of the Botic Stream - Effect of Longitudinal Connectivity Disturbance by a Reservoir on Benthic Community Structure
Reservoirs impose a lentic habitat within a lotic system and thus disrupt stream longitudinal connectivity. We studied a small urban stream affected by a large reservoir (1.310 million m3) in order to determine changes in benthic community structure (BCS) and in the drift and distances upon which they manifest themselves. Benthic invertebrate were collected by Surber sampler at one site above the reservoir and three sites below it (200, 500 and 1000 m). At the same sites drift was measured by drift nets at dawn. Immediately below the reservoir the BCS changed significantly (similarity index 52%). No cased Trichoptera, Gastropoda or Coleoptera were found, whereas lentic species (Daphnidae, Cyclopidae, Hydrida or Ostracoda) appeared. 1000 m downstream the BCS similarity increased to 58%. Benthic Index of Biotic Integrity deteriorated from fair conditions above the reservoir to poor conditions below it. 1000 m downstream it improved to fair conditions again. Drift density dropped from 400 individuals per 100 m3 filtered water above the reservoir to 189 individuals per 100 m3. Further downstream it gradually increased to 789 individuals per 100 m3. Discussion on the uncertainty of the drift measurement due to the quantification of the discharge through the drift net is included.
NB42E-05 11:30h
The Effects of Agricultural Land-use on Stream Fish and Invertebrate Communities and Food-web Structure.
Incorporating knowledge of the surrounding landscape can further the understanding of stream processes. This is particularly true in areas like the Midwest where human alteration of the landscape, such as conversion of natural cover types into cultivated row crops, is widespread. When assessing stream health, the composition and structure of biological communities themselves often are the best indicators of water quality. Previous work in Hurricane Creek (Coles and Cumberland Counties, IL) demonstrated significant differences in water chemistry and community metabolism between sites subject to differing intensities of farming in the upstream watershed. Our objective was to examine differences in fish and invertebrate communities at four sites along the stream representing varying degrees of agricultural land-use. Fish were sampled using electroseining techniques and invertebrates were collected using the 20-jab method in each of four seasons. Sites were compared using fish and invertebrate community metrics, including indices of biotic integrity (IBI, MBI). Stable isotope analyses were also performed to quantify differences in food-web structure in streams draining watersheds characterized by different degrees of agricultural land-use. This study improves understanding of how landscape alteration impacts stream biota and will facilitate more informed decisions concerning stream rehabilitation.