Sampling performance of Ekman grabs — in situ observations and design improvements

The performance of five types of Ekman grab: the Birge-Ekman, the Kebo Lab-Ekman, the Hydro-Bios-Ekman, the Winnipeg-Ekmanand the Håkanson-Ekman, were examined in situ by SCUBA diving. Trials and observations are presented, and photographic documentation provided. Each type of grab often produced unrepresentative samples due to operation deficiencies. Tilting, brimming, and entry of sediment from outside the sample chamber, as well as redistribution, resuspension and loss of enclosed matter are the main biasing factors. Since the grabs under study are models in general use, these findings suggest that caution must be exercised when evaluating studies based on Ekman grabs of the usual design. Sampling reliability can be significantly improved by furnishing the grabs with a supporting stand and a mechanism which secures the lids during ascent. Tested designs for such devices are presented. A properly operated Ekman grab of adequate design is a suitable instrument for collecting certain types of samples such as benthic macrofauna, and infauna in particular.