Horgan Erich F.

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Horgan
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Erich F.
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  • Preprint
    Effects of warm water intrusions on populations of macrozooplankton on Georges Bank, Northwest Atlantic
    ( 2004-06-21) Brown, Harmon ; Bollens, Stephen M. ; Madin, Laurence P. ; Horgan, Erich F.
    As part of the Georges Bank/North West Atlantic GLOBEC (Global Ocean Ecosystems Dynamics) Program, macrozooplankton and micronekton were collected on 30 Broad Scale Survey Cruises between January – June, 1995 –1999, using a 10 m2 MOCNESS (3mm mesh). The objective of this study is to examine the effects of warm water intrusions on populations of macrozooplankton, namely Salpa spp., Phronima spp., Neomysis americana, and Crangon septemspinosa, on Georges Bank. Salpa spp. and Phronima spp. showed a large degree of horizontal co-occurrence, being found predominantly in Upper Slope/Gulf Stream Water and Georges Bank/Gulf of Maine Water. Abundances of these taxa showed striking interannual variability, and were only abundant on the southern flank and in the Northeast Channel in late spring/early summer of 1995 and 1999, periods during which AVHRR imagery and hydrographic data showed the presence of warm water intrusions. These intrusions seemed to have little effect on the distribution of other macrozooplankton (e.g., Neomysis americana and Crangon septemspinosa). Warm water intrusions can directly affect Salpa spp. and Phronima spp. populations by advecting them onto Georges Bank, although other, more resident populations, especially those inside the 100m isobath, seem to be little affected by such intrusions.
  • Preprint
    Effects of turbulence on the feeding rate of a pelagic predator : the planktonic hydroid Clytia gracilis
    ( 2005-12-14) Adamik, Peter ; Gallager, Scott M. ; Horgan, Erich F. ; Madin, Laurence P. ; McGillis, Wade R. ; Govindarajan, Annette F. ; Alatalo, Philip
    Relatively little is known about the role of turbulence in a predator - prey system where the predator is a passive, pelagic forager. The Campanulariid hydroid Clytia gracilis (Cnidaria, Hydrozoa) is unusual because it occurs as planktonic colonies and is reported to forage passively in the water column on Georges Bank, Massachusetts, USA. In this study we investigated the role of various turbulence conditions on the feeding rate of C. gracilis colonies in laboratory experiments. We found a positive relationship between turbulence velocities and feeding rates up to a turbulent energy dissipation rate of ca 1 cm2 s-3. Beyond this threshold feeding rate decreased slightly, indicating a dome-shaped relationship. Additionally, a negative relationship was found between feeding efficiency and hydroid colony size under lower turbulent velocities, but this trend was not significant under higher turbulence regimes.