Erich Horgan

Voracious planktonic hydroids: unexpected predatory impact on a coastal marine ecosystem

Abstract-Hydroids are typically attached, benthic cnidarians that feed on a variety of small prey. During sampling on Georges Bank in spring 1994, we found huge numbers of hydroids suspended in the plankton. They fed on young stages of copepods that are an important prey for fish, as well as on young fish themselves. Two independent methods were used to estimate feeding rates of the hydroids; both indicate that the hydroids are capable of consuming from 50% to over 100% of the daily production of young copepods. These results suggest that hydroids can have a profound effect on the population dynamics of zooplankton and young fish on Georges Bank. Copyright 0 1996 Elsevier Science Ltd INTRODUCTION The life history of marine hydrozoans (Phylum Cnidaria) is typically an alternation between a sessile hydroid phase and a motile medusa. The hydroids form colonies attached to seaweed, rocks or other organisms, and their feeding polyps (hydranths) prey on small organisms, eggs and detrital matter. The medusae are usually released to feed and disperse in the plankton (Hyman, 1940). We report observations from Georges Bank on the occurrence of suspended colony fragments of the hydroid

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

Abstract Clytia spp. hydroids (Phylum Cnidaria), typically attached to a substrate during their asexual, polyp stage, have been found in significant numbers within the mesozooplankton on Georges Bank, North Atlantic Ocean. We examined unpublished historical records of the 1939–1941 cruises of the R/V Atlantis and obtained samples at four-study sites on Georges Bank in June/July 1995 in an attempt to (1) quantify the planktonic and benthic distributions of hydroids on Georges Bank, and (2) determine the coupling between benthic and pelagic habitats of this population. We found that planktonic hydroids have a patchy distribution, varying both spatially and temporally (most abundant in summer months, absent in winter). In 1939–1941 the planktonic hydroids were most broadly distributed following a spring (1940) with strong wind events; hydroids were absent from all samples in 1941. In 1995 we found the highest abundance of planktonic Clytia spp. hydroids (6213.5±1343.6 hydranths m−3) in the central crest of the bank, “downstream” in the Georges 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 numbers of hydroids in the water column at the Northeast peak sites, indicating that large numbers of planktonic hydroids are not being introduced into the Banks circulation patterns from off-Bank sites to the northeast (e.g. Scotian shelf). The source population for planktonic hydroids found in the central region of the Bank is most likely the benthic habitats on the northeast peak of the Bank. We hypothesize, and our limited data suggest, that hydroids are detached from the benthos by storm action or other disturbance, advected clockwise with the mean residual circulation, and concentrated and retained in the central, low-advective region of the Bank.

LAPIS: A new imaging tool for macro-zooplankton

Accurate sampling of macrozooplankton and micronekton has always been challenging. As an alternative sampling approach, camera systems can obtain images of particles or plankton in a quantitative fashion. However, planktonimaging devices built to date operate at too small a spatial scale to be useful for macrozooplankton and micronekton, which range in size from 1 cm to 1 m, with varied shapes, transparency and reflectivity. We have designed and built the Large Area Plankton Imaging System (LAPIS) to address this gap in current imaging capabilities. LAPIS uses hardware and software similar to the Video Plankton Recorder but illuminates much larger volumes in order to image macroplanktonic organisms, including crustaceans, fish and gelatinous animals. This paper briefly describes the current instrument, which was first used on a research cruise in February and March, 2006, and summarizes it capabilities and potential, with examples of image data and their interpretation