STRUCTURAL AND FUNCTIONAL APPROACHES TO ALGAL BIOASSESSMENT: THE IMPORTANCE OF SESTON-PERIPHYTON RELATIONS WITH AUTOGENIC ORGANIC ENRICHMENT IN PRODUCTIVE STREAMS AND RIVERS.

S.D. Porter. U.S. Geological Survey, Box 25046, MS406, DFC, Denver, CO 80225

The ecological condition of streams can be influenced more by natural landscape factors than land use intensity. The importance of these factors varies spatially because of regional differences in hydrologic landscape characteristics, differences in rainfall-runoff conditions, and the importance of ground and surface water relations. Structural (algal community composition and biomass indicators) and functional (e.g. rates of stream metabolism) approaches to algal bioassessment are applied to data collected by the USGS National Water-Quality Assessment (NAWQA) Program in agricultural, urban, and rangeland streams. Streams dominated by sestonic algae (eutrophic seston chlorophyll a concentrations (CHLa); periphyton communities dominated by lentic taxa) were characterized by high rates of productivity and respiration in agricultural streams. Seston CHLa concentrations increased significantly with concentrations of total organic nitrogen, phosphorus, and carbon, suggesting that organic enrichment is associated with autogenic processes. Streams dominated by periphyton (above-average periphyton CHLa; periphyton dominated by lotic taxa) were moderately productive, however rates of respiration were relatively low. Macroinvertebrate indicators of organic enrichment were significantly higher in seston than periphyton dominated streams. Algal community indicators responded to a gradient of specific conductance and phosphorus concentrations in urban streams. The availability of nitrogen appeared to limit periphyton biomass and productivity in rangeland systems.