Diurnal and seasonal variations in the near-bottom fauna -the hyperbenthos- in one of the deeper channels of the Kieler Bucht (Western Baltic)

• Main Author: Hesthagen, Ivar H.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Institut für Meereskunde 1973
• Subjects: Kattegat; Rinne; Thysanoessa raschii
• Online Access: https://oceanrep.geomar.de/id/eprint/55976/; https://oceanrep.geomar.de/id/eprint/55976/1/Hesthagen_I_H_1973.pdf

This work is based on 63 samples obtained by Beyer's 50 cm epibenthic closing net and 43 vertical hauls with a 70 cm Hensen-net from December 1969, and March, June and November 1970. The depth in the sampling locality, Vejsnäs Rinne, varied between 28 and 32 meters. Only abundant and particularly interesting species and animal groups have been treated here. Seasonal variations were studied in the cumacean Diastylis rathkei, some mysids, the euphausid Thysanoessa raschii, the decapod Crangon crangon, the chaetognath Sagitta elegans and in some fishes. During winter there are, relatively, a higher number of species (including some rare ones), as well as of individuals. This may partly be ascribed to a stronger inflow of water from the Kattegat in winter, and partly to seasonal horizontal migrations in the animals. It is furthermore apparent that by the present sampling method a larger fraction of the more vagile near-bottom fauna is collected than by more conventional sampling techniques. The diurnal variations in the hyperbenthos were studied in benthic as well as some pelagic groups of animals. Based on the results, it is possible to recognise at least three main patterns of vertical migrations in the hyperbenthos: A. The relatively few pelagic animals found stay just above the bottom during the day. They leavethe region sampled by the epibenthic closing net (i.e. from the bottom to about 80 cm above it) about sunset and stay away until the next morning when they reappear at this level. B. Some of the benthic animals at certain seasons or developmental stages are sparsely represented in the epibenthic closing net samples during day. The catches are increased at dusk, but decrease again later in the evening. In the morning the animals appear once more in the hyperbenthos, only to disappear again after sunrise. The reduced number collected around midnight is either due to midnight sinking, i.e. the animals burrow in the sediment and thus avoid the epibenthic net, or the animals ascend above the level sampled by ...