Distributional Responses to Flow Disturbance by a Stream-Dwelling Snail

Abstract

This study examines how flow disturbance influences distributions of the snail Potamopyrgus antipodarum, a common grazer in New Zealand streams, at the drainage basin, substratum, and rock face scales. We analyzed survey data from 48 streams across New Zealand to relate snail densities to hydrological patterns and conducted experiments in a laboratory flow tank and in artificial streams in field conditions to evaluate refugium effects of different substrate types in high flows. The survey showed that local densities were significantly inversely related to flood frequency, suggesting flow disturbance influenced regional distributions of P. antipodarum. The survey also revealed that lake-fed streams and low-gradient streams in foothill regions tended to have fewer, less severe floods and higher snail densities than high-gradient (>1%) streams in mountain and foothill regions. Results from both the flow tank and artificial stream experiments showed that finer-grained substrates provided better refugium from dislodgment conditions than larger substrates (i.e., cobbles) when bed sediments were artificially stabilized. In the flow tank where velocities were incrementally increased, dislodgment rates of snails were significantly lower on artificial gravels than on artificial cobbles or bedrock. Snails moved to low-velocity patches (i.e., crevices) on all substrates as current velocities increased. However, more snails accumulated in crevices between gravels and pebbles than between cobbles; the relatively high turbulence between cobbles, in part, contributed to high dislodgment rates from this substrate. Similar differences in dislodgment vulnerability and flow refugium effects were observed in experiments using real substrates in artificial streams in field conditions. A substrate size-flood duration experiment showed that significantly more snails were dislodged from cobbles and pebbles than from gravels and that more were dislodged in longer-lasting high discharge events (30 min) than in brief ones (1 min). Another experiment in which snails were transported downstream (1, 3, 6 or 9 m) from non-stabilized substrate patches similarly showed that snail dislodgment and mortality rates were generally higher from cobbles than from gravels and pebbles. Like other studies, we found that larger substrate particles were less likely to move than smaller ones (gravels) under similar hydraulic conditions. However, we discovered that the best refugium conditions for resisting dislodgment are not necessarily linked to larger substrate size. Our findings also suggest that flooding may alter local densities of snails by moving and relocating snails rather than by killing them. The observed patterns of distributions of these snails appear to result from complex interactions between hydrological. hydraulic features and substrate architecture.