For many terrestrial (prairie, forest) and some aquatic (pond, lake) ecosystems, we have well-developed, but sometimes contentious hypotheses about primary ecological succession. Stream and river systems have received much less attention than terrestrial systems primarily because they are often old, carving through the same landscape for millennia. Studies in systems undergoing glacial retreat have provided us with testable hypotheses in terms of algal and macroinvertebrate community development through time. In this study, we surveyed four replicate streams at Mount St. Helens (WA, USA) that were created de novo by the massive eruption in 1980. Drainage networks on the north-flank of the mountain were buried by up to 100 m volcanic deposits. Community assembly was predicted to take many decades to centuries, but stream surveys 35 years post-eruption have found significant riparian, periphyton and insect community development and differences in a variety of physio-chemical variables.
Results/Conclusions
Despite similarities in parent material, slope, and aspect, the four streams vary in terms of physio-chemical traits like: temperature, channel stability, dissolved oxygen, discharge, pH, dissolved organic carbon, and canopy cover. Reach-scale surveys (n=20) in the four watersheds (June-Sept. 2015; 35-years post-eruption) revealed up to 10-fold differences in many physical and chemical variables across the four study streams. Temperatures ranged from 3.7 to 25.5°C, specific conductivity ranged from 60 to 861 µS/cm, and canopy cover ranged from 0 to 98%. Riparian plant (A=0.058, p<0.0001) and periphyton communities (A = 0.2500, p = 0.0037) differed significantly among the four streams. Diatoms dominated cold sites and filamentous algae/cyanobacteria dominated warm sites. Although nutrient concentrations were uniformly low among streams (<0.01 mg/L ammonium-N, <0.07 mg/L nitrate-N, and <0.06 mg/L phosphate-P), algal biomass was positively correlated with both nitrate and phosphate (p<0.0004, p=0.0304, respectively). Periphyton communities were primarily influenced by differences in slope, bankfull width, bed shear stress and riparian canopy cover.