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.
Disturbance-habitat complexity interactions and effects on colonisation of hard substrata by a stream alga.
Downes, Barbara1, Street, Jodie1, 1
ABSTRACT- Differences between rivers in disturbance frequency may explain large-scale differences in abundances of different macroalgae. In the Steavenson River, southeastern Australia however, there is high variance in cover of the red alga Audouinella hermannii at individual sites, with stones with 100% cover lying next to stones with zero cover. Disturbance by floods causes patchy mosaics - some rocks are turned over and abraded and others not. Additionally, stone surface texture has profound effects on the alga - rough-textured surfaces are colonised more quickly. We hypothesized that there is an interaction between disturbance by floods and roughness of rock surfaces that differentially affect colonisation of alga. We carried out a field experiment that mimicked disturbance by overturning and abrasion, using both rough- and smooth-surfaced stones. A. hermannii recovered very quickly from abrasion, probably by regrowing from basal cells, but very poorly from overturning of stones, which presumably must be colonised anew by spores. Rough and smooth surfaces were initially similar in cover, but cover on rough stones gradually fell below that seen on smooth stones. We suggest that big floods that overturn many substrata may allow grazing insects to keep surfaces clear of A. hermannii for some time, whereas small, abrasive floods may represent only a small setback for the alga. Substantial within-river variability suggests models pitched at the river level will only explain a limited percentage of variation in algal abundance.
KEY WORDS: disturbance, habitat complexity, freshwater algae