Interactions between flow, periphyton, and nutrients in a heavily impacted urban stream: implications for stream restoration effectiveness

Abstract

Urban stream restoration is a very complex task due largely to the interactions between the physical, chemical, and biological stream components. Because of these interactions, restoring only a single component to a more natural state could have a negative affect on stream health. We studied pre-restoration interactions between hydrology, nutrients, and periphyton in a stream where wastewater effluent and a highly developed urban watershed dominated stream flow. Floods capable of scouring all visible periphyton from the stream were produced from rainfall events as small as 1.3 cm and created 47 periphyton biomass reset events during our 22-month study period. Despite these disturbances, periphyton biomass rapidly accumulated throughout the stream and reached nuisance levels after 5 days of growth during every season. Floods did, however, severely limit the occurrence of steady-state assemblages, which attained biomass levels 30 times the nuisance level. Although the high frequency of floods did not prevent nuisance levels of periphyton, it did allow more edible early stage periphyton assemblages to become far more common than late-stage, less edible assemblages. In the case of the stream studied, a successful restoration strategy must consider coupled processes relating to hydrology, chemistry, and biota.