Mixing an enclosed, 1300m3 water column: effects on the planktonic food web

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Abstract
The nutrient, phytoplankton, and zooplankton dynamics in three enclosed water columns (1300 m3) are described. Two of the enclosures were mixed using a bubbling chamber at depth. Young chum salmon (Oncorhynchus keta) were added to one of the mixed enclosures and the unmixed enclosure. No other manipulations were imposed. Copepods appeared in large numbers (e.g. especially Pseudocalanus minutus s.l. and Paracalanus parvus) and population growth rates were estimated. Ctenophora did not appear in large numbers despite presumably ideal food environments; it is suggested that in one enclosure this is a consequence of fish predation on the ctenophores. The fish experienced high mortalities and low growth rates presumably due to unsuitable prey size. Weekly collections of sediment permitted isolation of two major sediment contributors, the first from phytoplankton sinking and the second from biogenk fallout associated with herbivore production. It was found that the more oligotrophic enclosure (unmixed) experienced proportionally higher utilization of organic carbon. Some of these results are explained by our data while others require more sophisticated experimentation, both in the design of the containers and in the types of observations.

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