TEMPORAL VARIATION IN ENRICHMENT EFFECTS DURING PERIPHYTON SUCCESSION IN A NITROGEN-LIMITED DESERT STREAM ECOSYSTEM

Periphyton succession was studied over 89 d in longitudinally adjacent reaches (a riffle and run) in Sycamore Creek, a spatially intermittent desert stream. Effects of nitrogen limitation were assessed by comparing algal development on clay saucers containing either nitrate-enriched or unenriched agar. We evaluated effects of grazing on periphyton accrual by amending agar in half the substrata from each enrichment condition with an insecticide (Malathion). Early successional (25-d) communities on unenriched substrata were dominated by Epithemia sorex, a diatom capable of N2 fixation; non-fixing diatoms dominated enriched communities. NO3-N enrichment increased algal diversity (H') and delayed late-successional dominance by Calothrix, a heterocystous cyanobacterium. Replacement of diatoms by cyanobacteria was likely facilitated by autogenic changes in nutrient and light conditions within the periphyton and temporal increases in water temperature. Three measures of algal biomass exhibited nearly linear increases in all treatments over the 3-mo study. Enrichment enhanced standing crops of chlorophyll a and ash-free dry mass (AFDM) in both reaches, an effect most pronounced within the first 3-4 wk. Algal populations unable to fix N2 were stimulated by enrichment but remained in low abundance on unenriched substrata whereas biovolume of N2-fixing populations was lower on enriched substrata relative to controls. Malathion reduced the density of only one common grazer, the moth larva Petrophila jaliscalis, which was abundant only in the riffle. Addition of Malathion to enriched substrata in this habitat had no significant effect on net primary productivity or total algal biovolume but resulted in increased chlorophyll a and AFDM, suggesting that Petrophila did influence algal communities, but by a mechanism that remains unclear. Spatial heterogeneity of nutrient supply in this system contributes to maintenance of algal diversity; but over long successional series diversity declines regardless of enrichment owing to autogenic changes that occur during succession. Our results confirm earlier observations that low nitrogen availability limits accrual of algal biomass following spates and constrains algal community structure in Sycamore Creek. However, highly significant early successional differences in algal standing crop and community structure among enrichment treatments were eliminated or much reduced in mid to late stages of succession. We propose that this temporal change in enrichment response was caused by development of a thick periphyton mat that reduced availability of both water-column or substratum-derived nutrients to algae, and increased reliance on internal nutrient recycling.