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JOURNAL OF PLANKTON RESEARCH | VOLUME 12 | NUMBER 4 | PAGES 831-850 | 1990
© Oxford University Press

research-article

Zooplankton biomass gradient off south-western Nova Scotia: nearshore ctenophore predation or hydrographic separation?

Iain M. Suthers1 and Kenneth T. Frank2

Department of Biology, Dalhousie University Halifax B3H 4J1 1Present address: School of Biological Sciences (A08), University of Sydney Sydney, NSW, Australia 2006 2Marine Fish Division, Bedford Institute of Oceanography Dartmouth, Nova Scotia, Canada B2Y 4A2

Received on October 5, 1989; accepted on March 19, 1990 A persistent large-scale cross-shelf gradient in zooplankton biomass >1050 µm was evident off south-western Nova Scotia during annual spring surveys between 1985 and 1987, with relatively low levels inshore and higher levels offshore. Conversely, the abundance of the tentaculate ctenophore Pleurobrachia pileus was the greatest inshore, and distributed reciprocally to zooplankton >1050 µm. The principle prey of both adult ctenophores and post-larval cod is zooplankton >1050 µm (primarily calanoid copepods), and cod growth rates are strongly influenced by prey biomass. Ctenophore predation appears to have been responsible for the low nearshore zooplankton biomass, whereas the influence of hydrographic factors on the zooplankton gradient was minimal. On a smaller scale, persistent, abrupt changes in zooplankton biomass >1050 µm and ctenophore density existed 3–30 km from shore, in contrast to linear gradients in water density ({sigma}1) during a 5 week sampling period in spring 1987. Ctenophores were confined to depths <55 m and zooplankton >1050 µm predominantly occurred at depths >55 m. High concentrations of chlorophyll and phaeopigment were evident at depths <55 m also suggesting intense predation by ctenophores on large herbivores. The relatively high proportion of smaller zooplankton (153–308 –m) in the nearshore is also consistent with the predation hypothesis. The reduced growth experienced by post-larval cod inshore appears generated by ctenophore predation of a common prey resource.


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