A new quantitative air-lift sampler for collecting invertebrates designed for operation in deep, fast-flowing gravel bed rivers
Pehofer, Harald E.
Conventional methods for collecting benthos from gravel substratum usually fail when quantitative samples have to be taken in deep and large fast-flowing rivers. As a purchased air-lift sampler did not perform satisfactorily under the special conditions encountered in alpine gravelbed rivers, a new sampler was designed. This sampler was used successfully on the armed gravel substratum of rivers with current velocities of >2 m/s and depths of 1.5 to 6 m. To operate the sampler a stable floating platform or a boat large enough to accommodate sampler, generator, compressor and the crew needed for handling (3-5 people, depending on the conditions in the river) is needed. Sampling is possible without anchoring, provided the sampling craft is able to hold its position in the current for the time needed to retrieve a sample (normally 10 to 20 seconds). Quantitative samples can be collected from sediments containing predominantly sand (> 100µm the mesh size of the collecting net) to cobble nearly the diameter of the suction pipe (10 cm). In fine muddy sediment the collecting cylinder penetrates very fast and the collecting nets tend to clog rapidly and to rupture by the abruptly growing pressure. Therefore, in fine mud an adequate core sampler normally is preferable. Penetration depth into the substratum is controlled by the time the sampler is left in operation. In our studies mean penetration depths usually lay around 20-25 cm. Sampling efficiency of the new air-lift is excellent: When compared to other deep water samplers (Petersen grab, slurp gun, deep water freeze corer) the air-lift samples regularly yielded by far the highest benthos numbers. Regarding sampler precision, when stratified sampling techniques were employed, the sample variances with the air-lift samples usually lay in the range of those achieved with stratified core samples from lakes or with stratified HESS-samples from gravel bars. A relative measure of precision (95 % confidence intervals expressed as a proportion of the mean values), calculated for the air-lift samples from two impoundments, showed values between 42 and 99 % for the abundance of total macrobenthos. Comparison of a deep mid-channel site and an adjacent shallow gravel bar, while yielding similar overall benthos densities, showed significant differences in the faunal composition of the two sites, emphasizing the importance of the deep main-channel areas for quantitative assessment of the benthic biota of a large river.