MECHANISMS OF BENTHIC ALGAL RECOVERY FOLLOWING SPATES - COMPARISON OFSIMULATED AND NATURAL EVENTS

We conducted a manipulative field experiment to examine individual and interactive effects of scour and short-term nutrient enrichment (4 h exposure) on post spate recovery of benthic algae in a desert stream. We then compared recovery from these simulated-spate conditions to alg al recovery patterns following a natural spate that increased water-co lumn nutrient levers for 2 weeks. That event differentially scoured co mmunities on artificial substrata in place for a long-term experiment, significantly reducing biomass in 49-day-old communities but causing no significant reduction of biomass in older, 133-day-old communities. Thus, we were able to examine recovery of scoured and non-scoured ben thic algal communities under natural post-spate conditions. Both natur al and simulated spates reduced actual and relative abundances of diat oms within communities. In the manipulative experiment, scoured commun ities accrued biomass more rapidly than those not subjected to scour, but short-term enrichment had no effect. Accrual of diatoms and green algae was stimulated by the scour manipulation, while cyanobacteria ma intained equal rates of growth in all treatments. Following the natura l spate, diatom and green-algal densities increased in scoured communi ties, but recovery of algal biomass was slow on both scoured and non-s coured substrata, primarily because cyanobacteria, the dominant algal group on all tiles, did not increase under exposure to highly nitrate- enriched waters. Rates of algal cell accrual were inversely correlated with the amount of algal biomass present at the start of a recovery s equence. Algal immigration rates measured immediately after the natura l spate and during an interflood period in the same season did not dif fer, indicating that the algal drift pool was not augmented by disturb ance. Benthic algal recovery following spates is strongly influenced b y the degree of scour generated by the event, but recovery patterns ar e also affected by the length of post-spate enrichment and the taxonom ic composition of the affected community.