BENTHIC DIATOM DISTRIBUTION IN A PENNSYLVANIA STREAM: ROLE OF pH AND NUTRIENTS1

Abstract

ABSTRACTBuffalo Creek is in a forested watershed in eastern Pennsylvania and is relatively acid in upstream reaches (pH∼6), becoming alkaline downstream (pH∼8). Temperature, nitrogen (NO3‐N) and phosphorus (O‐PO4) increase significantly downstream whereas N/P declines. Nutrient‐diffusing substrata were deployed in triplicate at an upstream and downstream site. Six treatments included two concentrations of nitrate, two concentrations of phosphate, nitrogen + phosphate, and a control. Substrata were collected after 18 days, scraped and analyzed for accrual of chlorophyll a and algal community structure. Chlorophyll a and algal biovolume were greatest downstream across all nutrient treatments. At the community level, accrual appeared to be limited by phosphorus at upstream sites. Downstream accrual also may have been phosphorus‐limited, but the results were equivocal. Benthic algae on all treatments at both sites were ∼96% diatoms. Minimal overlap in species composition was observed between upstream and downstream sites. Of the 75 species of diatoms encountered in the study, 58 species did not occur at the upstream site and 10 species did not occur at the downstream site. The upstream site was depauperate in species and dominated by Eunotia exigua (Bréb. ex Kütz.) Rabh., which showed a positive response to phosphorus and accounted for over 50% of the biomass across treatments. The downstream site showed a four‐fold increase in species richness. Communities at this site contained some species that appeared to be phosphorus‐limited, e.g. Melosira varians Ag., and others that seemed to be nitrogen‐limited, e.g. Diatoma vulgare Bory and Navicula seminulum Grun. We conclude that extreme conditions upstream (low pH, high N/P) result in a species‐poor community dominated by acidophilous phosphorus‐limited diatoms. Increases in downstream nutrients and pH result in a relatively rich and diverse community.