e t a l .S t r e a m m a c r o i n v e r t e b r a t e s o v e r 1 0 y e a r s f o l l o wi n g w i l d f i r e W F 0 1 0 1 8 G . W a y n e M i n s h a l l , C h r i s t o p h e r T . R o b i n s o n , D e r o n E . L a w r e n c e , D o u g l a s A . A n d r e w s a n d J a m e s T . B r o c kAbstract. The effects of wildfire on benthic macroinvertebrate assemblages of streams in mixed-conifer forest were examined for 10 successive years following the Mortar Creek Fire of 1979. Changes in burned-catchment streams were evaluated relative to a paired set of reference-catchment streams. Taxa richness and total abundance tended to be lower in burn than in reference streams but to converge near the end of the study; increases in the final years in both burn and reference streams were associated with reduced flows due to drought. Total biomass and that of the scraper, filterer, and miner functional groups usually were greater in the burn streams. Lack of a strong relationship of macroinvertebrate metrics with weather conditions showed that factors specific to each stream also were influencing the biotic community. Mean among-year Jaccard similarity was lower for burn than for reference streams. Specific taxa responded differently to the effects of fire. Densities of disturbance-adapted forms (e.g. Chironomidae, Baetis) increased after the fire but not during the drought period of more stable flows at the end of the study; many other taxa showed the opposite response. Adverse effects of wildfire on the biotic community were largely the result of physical changes in habitat due to increased runoff. Timing and magnitude of effects differed widely among streams as a result of differences in stream size, burn severity, and specific storm or snowmelt events. Though major effects of the fire on the macroinvertebrates dissipated within 7 years, adjustment in the habitat and biotic conditions still were taking place at the end of 10 year and normal recovery patterns may have been obscured by the drought.

The Mortar Creek Fire burned 26 000 ha of mixed-conifer Rocky Mountain forest in July-August 1979. Changes in burn stream conditions were examined relative to reference streams for various ecological factors on two to six occasions, from October 1979 to August 1980. Factors included major ions and nutrients, suspended and benthic particulate matter, periphyton (algae), and macroinvertebrates. Elevated levels of most dissolved chemicals in the burn streams were evident soon after the fire and again during spring runoff. However, there were no major disruptions in the relative composition of cations (and presumably of anions also) in the burn streams during the study. Concentration (mg/L) and load (g/s) of some constituents were higher (e.g. NO 3 -N) and of others (e.g. Ca) were lower in the burn than in the reference streams during spring runoff, depending on whether they were normally under biological or geological control, respectively. Suspended sediment and particulate organic matter generally were higher in burn streams, especially during snow-melt runoff or following heavy rain storms. Benthic organic matter was higher in burn streams and was mainly charcoal, compared to the usual leaf litter found in the reference streams. Fine sediments increased and periphyton decreased in the burn streams. The fire increased the sensitivity of the burn streams to more routine smaller-scale disturbances, such as rainstorms, which had major impacts on the burn streams but not on the reference streams. The macroinvertebrate assemblage showed little direct effect from the fire but was severely altered in composition and abundance by the subsequent runoff, scouring, and channel alteration initiated by spring runoff. The burn streams showed considerable individuality in their response to fire depending on the particular set of conditions to which they were exposed. s o f I d a h o s t r e a ms t h e f i r s t y e a r a f t e r w i l d f i r e WF 0 1 0 1 7 G . W a y n e M i n s h a l lA , J a m e s T . B r o c k , D o u g l a s A . A n d r e w s , a n d C h r i s t o p h e r T . R o b i n s o n

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Summary The Portneuf River (mean monthly discharge near mouth 1.4‐15.4 m3/s) in southeastern Idaho, U.S.A., flows through an area of reduced precipitation (approx. 33 cm/year) and is dependent on underground aquifers and snowmelt runoff from the surrounding mountains for its water. The stream was examined at ten locations, distributed over its 156‐km course, during the period 1967‐1971. The Portneuf River is shown to have undergone a number of changes from its natural state as evidenced by alterations in water quality and the distribution of benthic invertebrates along the stream course. Of particular interest are changes brought about by the use of the stream for irrigation and by runoff from agricultural lands, factors whose effects are magnified by the semiarid conditions of the region and by poor soil‐conservation practices. However, the stream also is affected by wastes from a sewage‐treatment plant, phosphate‐processing operations, and an assortment of scattered urban sources. Benthic invertebrates were collected during all four seasons by means of artificial substratum samplers and during summer and autumn by a qualitative dip‐net technique. In general, the samplers were more effective in obtaining a representative picture ofthe fauna. However, neither procedure alone gave as much information as the combined results. The artificial substratum collections are not believed to be representative of the usual effects of stream devvatering by irrigation withdrawal in as much as the samplers provide refugia for the benthos during the periods of reduced habitat.