The Effects of Being Stranded After Flooding (Hydraulic Disturbance) on Cased Caddisfly Larvae

The distribution of species of caddisfly larvae within the study stretch of the Whiteadder Water was attributed to three main environmental factors: near-bed water velocity, depth and the amount of Coarse Particulate Organic Matter and species distribution was consistent with previous research. Species of caddisfly that occur within riffles generally have unsynchronised lilecycles, compared to a more synchronised lifecycle in species that occurred in pools, and this more pronounced in the case-less caddisfly larvae. The incidence of cased caddisfly larvae being stranded in bank-sidebank-side quadrats increased • with discharge once a flooding threshold had been exceeded. The incidence of being stranded was dependant on adjacent bank-sidebank-side microhabitat, local topography, and the larval case type. The likelihood of being stranded was not homogeneous, with more individuals being stranded adjacent to riffles than pools, and on gravel substrates than turf substrates. Cased caddisfly larvae with fine mineral cases were the most abundant type found stranded, while species with vegetative cases were rarely found, although they were present in the benthos. Flume experiments indicated that cased caddisfly larvae were passively deposited on inundated bank-sides, and that they did not actively seek this refuge. The ability of some cased caddisfly larvae to regain the water once stranded varied with case type, and the topography (substrate and slope) of the bank in artificial arenas. Time taken to regain the water was species/case specific with vegetative cased larvae regaining the water fastest and fine mineral cased larvae the slowest. Time taken to regain the water increased with increasing substrate complexity and distance from the water, but decreased with increasing slope. The incidence of cased caddisfly larvae being stranded by flood disturbance within the Whiteadder Water, while it may be important, did not have a significant negative impact on the assemblage of cased caddisfly larvae. Only (an estimated) 12% of the