MECHANISMS OF LOTIC MICROALGAL COLONIZATION FOLLOWING SPACE-CLEARING DISTURBANCES ACTING AT DIFFERENT SPATIAL SCALES

Diel drift/colonization studies were conducted in Sycamore Creek, Ariz ona, U.S.A. in late June, 87 d post-spate and, again, 4 d after recess ion of a large mid-August spate to examine differences in benthic alga l colonization of space cleared by small-scale, localized disturbance versus large-scale, system-wide disturbance. Cell densities and specie s composition in the algal drift pool were quantified with hourly samp les, composited every 3 h for 36 h. Diel changes in colonization activ ity were assessed by sampling ceramic tiles incubated during each of t he twelve 3-h periods (instantaneous colonization). Cumulative changes in algal communities were assessed by collecting tiles, introduced at the start of each study, at 3-h intervals for 36 h, then again at 48 and 96 h. Consecutive 3-h measures from instantaneous-colonization sam ples were summed to compute ''expected'' cell densities of taxa within different algal divisions, based on short-term colonization alone, af ter 6 to 36 h, and compared to actual cell densities on cumulative til es. Tiles introduced during the June interflood period were colonized much more slowly than in August, and supported assemblages dominated b y bluegreen algae. Comparisons of cell densities and changes in taxono mic structure in instantaneous- and cumulative-colonization assemblage s indicated that, in June. algal reproductive activity was low and rat es of emigration and death were high. In August, diatom and green alga l densities in drift and colonization assemblages were significantly h igher than in June, and accrual of these taxa on cumulative-colonizati on tiles exceeded that expected, indicating rapid reproduction. August drift and colonization assemblages exhibited clear diel changes in bo th cell densities (with mid-day maxima) and taxonomic structure, sugge sting that rates of immigration, emigration, and reproduction varied a mong taxa. In June, mid-day peaks in algal drift activity were caused by passive entrainment of cells into the water column, likely by oxyge n bubbles produced by photosynthesis. In August, live diatom cells exh ibited higher drift peaks than dead cells, indicating reproductive act ivity and, likely, changes in cell buoyancy was influential in generat ing drift maxima. Many diatom taxa displayed clear mid-day minima in c olonization efficiency, corresponding to periods of peak drift, sugges ting diel changes in cell bouyancy. No such patterns were detected in bluegreen or green algal taxa. Rapid recovery of diatom assemblages in Sycamore Creek after spates appears to be driven by rapid reproductio n, emigration, and reimmigration of early successional diatom taxa. Th e rate and pattern of benthic algal colonization of open space in Syca more Creek, and the mechanisms controlling this process, varied consid erably depending on the circumstances under which substrata became ava ilable.