Jämförande undersökning av två bottenfaunaprovtagare i Vänern: jämförelse av Ekman och van Veen-hämtare

Two different types of benthic fauna samplers have been compared at a sampling event in Storvänern. Since the start in 1973, a slightly modified Ekman-type sam-pler has been used in the national environmental monitoring of the lake. The sam-pler is extra weighted to ensure that sampling can take place vertically at great wa-ter depths and the closing mechanism is modified so that the sampler is closed when it is pulled up from the bottom. An ordinary Ekman sampler, which is a com-mon sampler type in comparatively soft lake sediments, instead requires a weight to be sent down to trigger the closing mechanism. The other type of sampler tested is of the van Veen type that is a common sampler in more marine environments where the sediments often are "sandier" and more compact. A total of 10 Ekman samples and five van Veen samples were taken at each of the two regular monitor-ing sites, which in total per site covered a bottom area of 0.25 m2 with the Ekman sampler and 0.5 m2 with the van Veen.A total of eight different benthic fauna taxa were caught with the van Veen sampler at Megrundet, while the Ekman sampler caught six taxa. The difference was a cou-ple of taxa that only occurred in single specimens and where the larger total sam-pling area with van Veen increases the possibility to catch less common taxa. At Tärnan, both samplers caught four taxa each.For the most common taxa, the amphipod Monoporeia affinis, there was a statisti-cally significant difference in both abundancies and biomasses with significantly higher average abundancies and biomasses for the individual subsamples taken with the Ekman sampler at both sites. For the second common group of benthic fauna oligochaetes, there was only a statistically significant difference between the biomasses at Megrundet, although there were consistently slightly lower abundan-cies and biomasses for the van Veen sampler. However, the variation between the different samples was not sufficiently large enough to be able to statistically ensure the differences in all aspects. For other less common benthic fauna, the result was more variable, which probably reflect the larger role that pure chance plays when the organisms occur more sparsely.Similar results are obtained by comparing the results from the changes that have taken place in the national environmental monitoring of benthic fauna in Lake Vät-tern, where in 2004 a change from Ekman samplers to van Veen was made.In all, this means that the recommendation for the continued monitoring of the benthic fauna in Lake Vänern is to continue to use the Ekman sampler. This is partly not to break important time series that have been created during many years of monitoring, but also because it is not possible to see any obvious benefits of changing the sampler to the van Veen type.