Planktonic hydroids on Georges Bank: effects of mixing and food supply on feeding and growth

Huge numbers of hydroids (principally Clytia gracilis) were recently reported suspended in the plankton over the shallow, well-mixed region of Georges Bank, where preliminary feeding experiments suggested that these planktonic predators could have a potentially devastating effect on their zooplankton prey (Madin et al., 1996). Based on these initial findings we undertook a more extensive set of laboratory experiments examining the effects of particulate food concentration and mixing (turbulence) intensity on the feeding and growth of suspended hydroids. Not surprisingly, we found a clear effect of particulate food concentration on the growth of hydroid colonies. After 7 days at 15°C, both colony size (number of hydranths colony−1) and specific growth rate (hydranth hydranth−1 day−1) were significantly greater in well-fed (80–160 Artemia nauplii L−1) versus starved treatments. More interesting was the additional significant effect of turbulent mixing (ε=9×10−5 W kg−1) on hydroid growth. Consumption rates (4.5 Artemia nauplii hydranth−1 day−1) were not significantly different between mixing vs. non-mixing treatments, indicating that the enhanced growth rate in the mixing treatments could not have been due to turbulence-enhanced predator–prey contact rates. An alternative hypothesis for the apparent advantage that mixing seemed to confer on hydroid growth is that reduced boundary layer thickness around the hydroids served to replenish the local supply of DOM and oxygen and/or remove waste products. This study indicates that growth rate of planktonic hydroids is dependent on both food concentration and mixing intensity, a finding that helps explain why these organisms are vastly more abundant in the central, shallow, well-mixed region of Georges Bank compared to the stratified flanks of the Bank.