Composition and vertical distribution of the motile epifauna on a vertical rock wall in Gullmarsfjorden, western Sweden, using an alternative sampling approach

Abstract

The main purpose of this study was to evaluate the efficiency of the alternative quantitative sampling approach and if it was possible, by using this method, to elucidate temporal variations concerning the composition, vertical distribution, and abundance of the motile epifaunal community. The study was performed on a vertical rocky bottom in Gullmarsfjorden on the Swedish west coast. Quantitative sampling was performed at four depths (2, 6, 10 and 15 m) on three occasions (19 June, 23–24 July and 23 September 1996) by means of a water jet powered suction sampler (venturi principle). In total, 53 macrofaunal taxa were found, of which 29 taxa occurred on all sampling dates. Significant vertical zonation patterns were recorded in July and September, but not in June, with amphipods, polychaetes, and echinoderms (mainly Asterias rubens) as the most important contributors to these patterns. A temporal variation in the faunal assemblage also occurred. Species abundance was significantly different between the sampling dates at 2, 6 and 10 m, but not at 15 m. The abundance was higher in July and September than in June. Species with the highest observed differences in abundance were juvenile Asterias rubens, abundant in July, while juvenile shrimp, especially Thoralus cranchii, and some amphipods, especially Microdeutopus spp., increased in abundance in September. This sampling approach was highly effective, and the high abundance and diversity of the motile epifauna indicate that these species play an important role in hard-bottom communities.

Keywords:

• Epifauna
• hard bottoms
• sublittoral
• Sweden
• vertical distribution
• zonation

Acknowledgments

This paper is Smithsonian Marine Station at Fort Pierce contribution number 607. The authors would like to thank Jarl-Ove Strömberg for his valuable support, advice, and critical review of the manuscript. We are grateful to Peter Adolfsson and Rocío Fernández for invaluable help during the field work, and to Sherry Reed for a critical review of the revised manuscript. We thank Stefan Agrenius for valuable help with the species identification. Thanks are also due to Michelle Kush for linguistic corrections.

Notes

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Taxonomical remark: there exists some uncertainty whether the amphipods Microdeutopus anomalus and M. propinquus are separate species or if they should be regarded as one species. Sars (1890) treated them, with some hesitation, as two species, as did Oldevig (1933) and Dahl (1948). Myers (1969) and Hansson (1994) dealt with them as synonyms, while Enequist (1949) did not express a definite opinion. Some consistent differences in character were observed from the material of this study and they were therefore treated as two species here. Adult females with eggs (from both species) have occurred at the same time and have shown a distinct difference in size and in the number of joints on the accessory flagellum. The collected males also differed in size, in the appearance of the anterior gnathopod and in the number of joints on the accessory flagellum.