TEMPORAL VARIABILITY IN THE NIGHTTIME DISTRIBUTION OF EPIPELAGIC SIPHONOPHORES IN THE NORTH-ATLANTIC OCEAN AT BERMUDA

The temporal variability of siphonophores was compared on night-to-nig ht, month-to;month and year-to-year scales in open-net collections mad e at or near the Bermuda Atlantic Time Series (BATS) site. These colle ctions were made as oblique tows to 100 m on (i) one summer night in e ach of 1986, 1989 and 1991, (ii) one night per month on 13 cruises bet ween April 1991 and August 1992, and (iii) three consecutive summer ni ghts of 1992. The mean wet displacement volume (WDV) of siphonophores in the these collections was 1.4 +/- 0.6 ml per 100 m(3), or 20 +/- 10 % of the total catch biovolume. The most common species belonged to th e Calycophorae families Diphyidae and Abylidae: the combined numerical abundance of Chelophyes appendiculata, Eudoxoides mitra, Lensia subti lis, Eudoxoides spiralis, Bassia bassensis and Abylopsis eschscholtzi averaged 300-400 per 100 m(3). Combined numbers of these common specie s varied little when sampled hour by hour at the same location during consecutive summer nights or at different night-time sampling location s 10(1)-10(2) km apart when these were sampled within 3-5 h. In contra st, most of these six species exhibited 2- to 3-fold differences in ni ght-time abundance between summers of different years and between seas ons. Three seasonal groups were detected, with a winter group being th e most distinct from the others. Changes in the abundance of several s pecies were correlated with seasonal changes in the depth of the mixed layer.