PARENT SESSION
Oral Session #16: Disturbance in diverse systems: Flooding and other. Presiding: T. Stohlgren.
Monday, August 6, 2001. 1:00 PM to 5:00 PM. Hall of Ideas G.
Direct and indirect effects of disturbance and nutrients on stream communities of the Midwestern U.S.
Riseng, Catherine1, Wiley, Michael1, Stevenson, Jan2, 1 2
ABSTRACT- The influence of hydrologic disturbance and nutrient levels on algal and benthic invertebrate standing stock and taxa richness was examined in streams of the Midwestern United States. Biota and nutrient chemistry were sampled and channel morphology, benthic substrate and discharge were measured in streams representing both nutrient and hydrologic disturbance gradients during the summers of 1996 and 1997. Metrics of flood and drought flow intensity, frequency and variability were developed based on measured morphological characters and long-term hydrologic gauge data. Direct and indirect effects of hydrologic and hydraulic disturbance on trophic transfer of nutrients to invertebrate biomass were examined using covariance structure models. These models suggest that both flood and drought flow have strong negative direct effects on benthic herbivore standing stock. Flood flow had a negative direct effect and drought flow had a positive direct effect on algal biomass. Nutrients had a strong positive direct effect on algal biomass. Algae had a positive but weak effect on grazers while grazers had reciprocal negative effects on algal standing stock. This pattern suggests that flood flows sufficient to move benthic substrate and drought flows sufficient to support ponded pools have a positive indirect effect on algal biomass by reducing the invertebrate grazing community and temporarily releasing periphyton from grazing pressure. Covariance structure models relating disturbance, standing stock and taxa richness of invertebrates suggest that disturbance negatively influences taxa richness but only though indirect effects on invertebrate biomass.
KEY WORDS: hydrologic disturbance, benthic community structure, stream ecology, invertebrates