Catchment urbanization increases benthic microalgal biomass in streams under controlled light conditions

Stormwater from urban land degrades aquatic ecosystems. Nutrients, light and flow regime affect the development of benthic microalgae (microphytobenthos), and all are affected by urban stormwater. The relative influence of these factors on microphytobenthos is unknown and is largely untested. This study investigated the effect of urbanization, controlling for irradiance, on the development of stream microphytobenthos assemblages. Three light levels were achieved (two were comparable) in four streams of different catchment urbanization. Microphytobenthos assemblages were sampled fortnightly from each stream over 79 days in winter. Biomass (chlorophyll a, pheophytin and cell density) increased with catchment urbanization. Light only affected biomass in the more urban streams and scour may have affected microphytobenthos assemblages in the most urban stream. Each stream had distinct assemblages, although time and light had no apparent effect on their composition. Physiological analysis suggested that the microphytobenthos was potentially light-limited in all four streams. However, light limitation was overridden by nutrient limitation in the least urbanized streams. The alleviation of nutrient limitation in one stream under the highest light treatment was attributed to microphytobenthos having sufficient energy to support active uptake of nutrients. Light did not drive differences in microphytobenthos biomass among the four study streams; differences were due to other factors affected by urbanization, most likely nutrient enrichment. To minimize the risk of algal blooms in urban waterways, reducing eutrophication should be a higher management priority than limiting irradiance.