The effects of storm-induced events on the seasonal dynamics of epilithic algal biomass in subtropical mountain streams

Abstract

Information concerning the drivers of seasonal variation in algal biomass in subtropical mountain streams is limited. To identify the drivers of biomass dynamics for epilithic algae, a 20-month study was conducted in mountain streams in Taiwan, an area characterised by different levels of riparian vegetation coverage and agricultural activity, in which a process-based model was optimally fit to field data. We found that episodic typhoon-induced floods were the major drivers shaping the seasonal variations in algal biomass. Flow-induced detachment was frequently observed in periods of higher algal biomass. In contrast, an increased flow stimulated algal growth during periods with slower flow rates. Increased temperature stimulated algal growth at sites with an open canopy cover and higher light availability but constrained biomass at sites with dense canopy shading. Overall, scraper biomass exerted less influence on algal biomass than did environmental factors. The effects of grazing were visible only at the pristine, low-stream-order site in winter. The effects of minimal algal biomass required for recovery was comparable to environmental factors only at sites with intermediate canopy cover, moderate discharge, and higher nutrient concentrations. We suggest that agricultural activity and riparian vegetation can affect epilithic algal biomass in subtropical mountain streams.