Processing of willow leaves in two suburban streams in Chrisrtchurch, New Zealand

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Abstract
Breakdown of willow leaves (Salix babylonica) enclosed in mesh bags (3 and 0.2 mm mesh) was investigated in Cashmere Stream and Avon River, Christchurch, New Zealand. Leaves lost weight faster in 3 mm than in 0.2 mm mesh bags and weight losses in 3 mm mesh bags were faster in Cashmere Stream. Calculated decay coefficients (‐k), for 3 and 0.2 mm mesh bags respectively, were 0.0154 and 0.0114 (Avon River) and 0.0173 and 0.0117 (Cashmere Stream). Faster breakdown in 3 mm mesh bags could be in part the result of feeding activities of macroinvertebrates, especially Potamopyrgus antipodarum and Physa acuta. The significantly faster leaf processing in Cashmere Stream can be attributed to the presence of larvae of the shredder Triplectides obsoleta which were found in small numbers. Densities of invertebrates (predominantly P. antipodarum) per gram of leaf (dry weight) peaked in bags which had been immersed for 41 days (Cashmere Stream) and 58 days (Avon River). At these times the amount of initial leaf biomass left was 45 and 38% respectively. Results of this and overseas studies on willow leaf breakdown suggest that leaves of Salix spp. are most palatable to stream invertebrates when 35–45% of initial leaf biomass remains.