A field experiment on the value of allochthonous straw as food and substratum for lake macro-invertebrates.

The relative importance of habitat structure and food availability to lake macro-invertebrates was investigated experimentally. A gravel-pit lake of uniform depth was partitioned into two parts and allochthonous material (straw) added to one half. Replicate litter bags of 0.3-mm mesh size containing sisal string, straw, wood-wool or polypropylene twine were added to each halt ol the lake in May 1980. After 2 months they were removed and the invertebrates within them identified and counted. Data were analysed statistically to determine the relative importance of bag Material and the level of allochthonous detritus in the vicinity of the bags (referred to as Food). Of the abundant taxa (> 5 individuals per bag mean density) only Chironomus was distributed independently of Material. The other abundant taxa (Gammarus, Asellus, Cryptochirnomus and Helobdella stagnalis) were all more strongly influenced by Material than Food. The effect of Material on total numbers was 14 times more important than that of Food. Of the 17 taxa analysed, only Pisidium was distributed Independently of either factor.

The relative abundance of total taxa was lowest in the sisal bags and greatest in the polypropylene and wood-wool bags. This is presumably related to structural properties of these materials with high abundance coinciding with an open buoyant substratum similar to that provided by aquatic macrophytes. The importance of Material for most taxa indicates that the structural properties of the habitat may be very significant in determining animal distribution.