The effect of anthropogenic alteration on large river structure and function measured by algal response to nutrient regime, ecosystem metabolism, carbon cycling, and energy flow

The objective of this research was to measure and model various components of ecosystem structure and function in a large, regulated system in the Pacific Northwest. Hypotheses tested were based on the physical habitat templet, which consisted of a progression from canyon, to braid, to meander, all within a relatively short distance. Measures of ecosystem structure included benthic and sestonic concentrations of chlorophyll a and organic matter, ambient nutrient concentration, and macroinvertebrate community structure. Measures of ecosystem function were analyzed by (1) using periphyton response to nutrient addition to determine the type and extent of nutrient limitation, (2) measuring river metabolism rates at the scales of both individual reaches and individual compartments (i.e. benthos, seston, & macrophyte), and (3) estimating organic matter mass balance and spiraling rates. Results of a nutrient addition study (Chapter 1) conducted on 4 large river systems suggest that ultimate factors, such as drainage basin characteristics and longitudinal location within the same basin, are more important at determining the potential benthic diatom assemblage than small scale, proximate variables provided by artificial nutrient-diffusing substrata. Ecosystem-level metabolism rates (Chapter 2) in the Kootenai River, Idaho (U.S.A.) were generally heterotrophic, with longitudinal shifts explained only in the context of anthropogenic alteration. Transported organic matter (seston) declined longitudinally—an unexpected pattern based on contemporary theory, but not unexpected given anthropogenic alteration of the system (Chapter 3). An energetic budget (Chapter 4), used to examine the potential for autotrophic and detrital food sources to limit higher trophic levels (such as the endangered Kootenai white sturgeon), suggested that autotrophic production was rarely enough to support higher trophic levels. All reaches tended to be losing organic matter and hence in the short-term, were carbon-limited. Thus higher trophic levels appear to be food limited. In initial modeling results (Chapter 5), discharge, ambient nutrients, and ambient periphyton chlorophyll- a were relatively well simulated. The Kootenai River appears to be a system still coming into a new dynamic equilibrium dictated by anthropogenic alterations (dams and levy construction). As expected, patterns in ecosystem structure and function are best explained when these alterations are considered.