Flood events can reduce key fatty acid content of early-stage benthic algal assemblages in an urban stream

Abstract

Effects of urbanization on stream chemistry and biota have been widely examined. However, few studies quantify the effect of urban stream characteristics on the biochemical properties of basal food resources, such as benthic algae, which can affect aquatic consumers and food webs. A common feature of urban streams is the high frequency of short floods, which may disrupt aquatic communities and their biotic controls. These disturbances can create algal assemblages low in biomass and which remain in early successional stages. This study examined the effects of frequent flood events and macroinvertebrate grazing on biomass, elemental stoichiometry, and essential fatty acid (EFA) concentration of early colonizing benthic algal assemblages in an urban stream. Over a 3-week period in which two flood events occurred, algal biomass (chlorophyll- a ) and EFA profiles were each affected by floods, but the responses were weakly correlated. Concentrations of EFA molecules essential for aquatic consumers, such as arachidonic acid (ARA) 20:4ω6), eicosapentaenoic acid (EPA) 20:5ω3), and docosahexaenoic acid (DHA) 22:6ω3) were greatly reduced after floods, potentially lessening algal nutritional quality. There were no statistically significant differences in biomass or stoichiometry of benthic algae in assemblages under different grazing regimes, suggesting hydrological factors played a greater role in their production and nutritional quality. This study provides evidence that flood-prone streams in urban landscapes may be susceptible to greatly altered biochemical profiles in early colonizing benthic algal assemblages. Results also suggest that biochemically important compounds, such as EFAs, offer valuable information on the quality of benthic algal assemblages under anthropogenic stress.