Driftwood: an alternative habitat for macroinvertebrates in a large desert river

Abstract

Driftwood was an important substratum for macroinvertebrates in the Colorado River, a desert river in southwestern U.S.A. with high suspended sediments and limited cobble substrata. Higher light availability and reduced abrasion on driftwood substrata resulted in a significantly higher ash-free dry mass (AFDM) of biofilm when compared to cobbles. Overall mean mass of biofilm on driftwood was 3.76 g m-2 AFDM (SE ±0.19) compared to 2.45 g m-2 AFDM (±0.15) on cobbles. Total macroinvertebrate AFDM was not significantly different between cobble and driftwood substrata. However, there were differences in the Ephemeroptera assemblage on the two substrata which were attributed to the type of food resources available in each habitat. Driftwood was dominated by the scraper/collector Heptagenia elegantula (Eaton), while the filterer/collector, Traverella albertana (McDunnough) dominated cobbles. Twenty taxa were found on driftwood substrata. This habitat expands the types of niches available to macroinvertebrates in lotic systems with high suspended sediments. We calculated that 4.4 m2 of driftwood substratum passed our sample station each minute during a 12-h sampling period on the rising limb of the hydrograph. River impoundments limit the supply and transport of driftwood, which may have negative implications on macroinvertebrate communities in desert rivers with high suspended sediment. Studies in turbid desert rivers that do not sample driftwood may underestimate both the total standing mass/energy of the system and taxon richness.