Distribution, abundance and benthic-pelagic coupling of suspended hydroidson Georges Bank

Clytia spp. hydroids (Phylum Cnidaria), typically attached to a substrate d uring their asexual, polyp stage, have been found in significant numbers wi thin the mesozooplankton on Georges Bank, North Atlantic Ocean. We examined unpublished historical records of the 1939-1941 cruises of the R/V Atlanti s and obtained samples at Four-study sites on Georges Bank in June/July 199 5 in an attempt to (1) quantify the planktonic and benthic distributions of hydroids on Georges Bank, and (2) determine the coupling between benthic a nd pelagic habitats of this population. We found that planktonic hydroids h ave a patchy distribution, varying both spatially and temporally (most abun dant in summer months, absent in winter). In 1939-1941 the planktonic hydro ids were most broadly distributed following a spring(1940) with strong wind events; hydroids were absent from all samples in 1941. In 1995 we found th e highest abundance of planktonic Clytia spp. hydroids (6213.5 +/- 1343.6 h ydranths m(-3)) in the central crest of the bank, "downstream" in the Georg es Bank circulation pattern from sites along the northeast peak of the Bank where large populations of benthic Clytia spp. hydroids were found (up to 6465 hydranths m(-2)). Our plankton sampling did not show significant numbe rs of hydroids in the water column at the Northeast peak sites, indicating that large numbers of planktonic hydroids are not being introduced into the Bank's circulation patterns from off-Bank sites to the northeast (e.g. Sco tian shelf). The source population for planktonic hydroids found in the cen tral region of the Bank is most likely the benthic habitats on the northeas t peak of the Bank. We hypothesize, and our limited data suggest, that hydr oids are detached from the benthos by storm action or other disturbance, ad vected clockwise with the mean residual circulation, and concentrated and r etained in the central, low-advective region of the Bank. (C) 2000 Elsevier Science Ltd. All rights reserved.