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JOURNAL OF PLANKTON RESEARCH | VOLUME 18 | NUMBER 6 | PAGES 923-939 | 1996
© Oxford University Press
research-article |
Temporal variability in the night-time distribution of epipelagic siphonophores in the North Atlantic Ocean at Bermuda
Texas A&M University, Department of Oceanography, College Station TX 77843-3146, USA 1Present address: National Sun Yat-Sen University, Department of Marine Resources Kaohsiung, Taiwan, 80424, ROC
Received on September 22, 1995; accepted on January 16, 1996 The temporal variability of siphonophores was compared on night-to-night, month-to-month and year-to-year scales in open-net collections made at or near the Bermuda Atlantic Time Series (BATS) site. These collections were made as oblique tows to 100 m on (i) one summer night in each of 1986, 1989 and 1991, (ii) one night per month on 13 cruises between April 1991 and August 1992, and (iii) three consecutive summer nights of 1992. The mean wet displacement volume (WDV) of sipho nophores in the these collections was 1.4 ± 0.6 ml per 100 m3 or 20 ± 10% of the total catch biovolume. The most common species belonged to the Calycophorae families Diphyidae and Abylidae: the combined numerical abundance of Chelophyes appendiculala, Eudoxoides mitra, Lensia subtilis. Eudox-oides spiralis, Bassia bassensis and Abytopsis eschscholtzi averaged 300–400 per 100 m3. Combined numbers of these common species varied little when sampled hour by hour at the same location during consecutive summer nights or at different night-time sampling locations 101–102 km apart when these were sampled within 3–5 h. In contrast, most of these six species exhibited 2- to 3-fold differences in night-time abundance between summers of different years and between seasons. Three seasonal groups were detected, with a winter group being the most distinct from the others. Changes in the abundance of several species were correlated with seasonal changes in the depth of the mixed layer.