Linking Benthic Algal Biomass to Stream Substratum Topography

The physical properties of substrata significantly influence benthic algal development. We explored the relationships among substratum surface texture and orientation with epilithic microphytobenthic biomass accumulation at the whole-substratum and micrometer scales. Unglazed clay tiles set at three orientations (horizontal, vertical, and 45 ), and six substrata of varying surface roughness were deployed in a prairie stream for 3 weeks. Substrata were analyzed for loosely attached, adnate, and total benthic algal biomass as chl a, and confocal laser scanning microscopy was used to measure substrata microtopography (i.e., roughness, microscale slope angles, and three-dimensional surface area). At the whole-substratum level, vertical substrata collected significantly (P < 0.05) less algal biomass, averaging 34% and 36% less than horizontal and 45 substrata, respectively. Benthic algal biomass was also significantly less on smoother surfaces; glass averaged 29% less biomass than stream rocks. At the microscale level, benthic algal biomass was the greatest at intermediate values, peaking at a mean roughness of approximately 17 lm, a mean microscale slope of 50 , and a projected ⁄ areal surface area ratio of 2:1. The proportion of adnate algae increased with surface roughness (26% and 67% for glass and brick, respectively), suggesting that substratum type changes the efficiency of algal removal by brushing. Individual substrata and microsubstrata characteristics can have a strong effect on benthic algae development and potentially affect reach scale algal variability as mediated by geomorphology.