Early periphyton accumulation and composition in a wastewater effluent dominated stream: effects of season, distance from discharge, and flow regime

Abstract

Municipal wastewater effluent can alter the receiving stream's algal production and community structure by affecting natural hydrologic patterns and nutrient availability. In this thesis, early succession periphyton assemblages were studied to determine if seasonal and spatial colonization trends existed in a stream that had multiple locations of wastewater discharge. Four colonization experiments were conducted, one during each season, at six sites in Carter Creek, Brazos County, Texas. In each experiment, PVC pipe samplers were used to collect periphyton samples on five consecutive days at each site, beginning 48 hours after substrate deployment. Samples were analyzed to determine trends in accumulation rate, chlorophyll a concentrations (biomass), and assemblage composition. In addition to this, the creek's physiochemical properties and hydrologic regime were used to uncover their roles in periphyton development. Early periphyton biomass showed a significant linear trend in the colonization experiments (P < .0001), with a significant season*station interaction effect (P < .0001). Upstream stations closest to the WWTP outfalls had a maximum biomass in the summer, and a minimum in the winter, with downstream sites having a similar seasonal biomass. Periphyton assemblage composition did not display a spatial or temporal trend. Early succession periphyton dynamics were not correlated with any of the nutrients or water quality parameters measured. Biomass levels during the first week of growth exceeded reported periphyton nuisance levels of 100 mg chl a m⁻² and reached a 30-day steady state biomass of over 3500 mg chl a m⁻². However, assemblages rarely reached peak biomass. Periphyton assemblages in this system were mainly regulated by the hydrologic regime of creek. The frequent and high storm flows caused by the highly developed watershed commonly scoured all visible periphyton from the substrate and reset the succession process. This frequent flushing of accumulated biomass may have kept the stream healthy by inhibiting harmful levels of periphyton, as well as hindering the dominance of unfavorable late successional cyanobacteria.