Iron limitaion of natural phytoplankton assemblages associated with the Pacific Northwest ECOHAB Pseudo-nitzschia blooms

M.J. Wells¹, W. P. Cochlan² and C.G. Trick³

1 School of Marine Sciences, University of Maine, Orono, Maine
2 Romberg Tiburon Center for Environmental Studies, San Francisco State University, Tiburon, CA
3 Department of Biology, The University of Western Ontario, London Ontario

Community bioassay experiments were performed during the first two cruises of the Pacific Northwest ECOHAB project – a study designed to investigate environmental factors that influence the formation and domoic acid content of Pseudo-nitzschia spp. that periodically bloom in a local large scale eddy (Juan de Fuca eddy) and coastal upwelling regions along the Washington and Oregon coasts. Surface waters were collected from regions of the Juan de Fuca eddy and micronutrient availability regulated by iron and copper enrichment, or the addition of metal chelators (desferal) during on-deck incubation experiments. Growth responses were determined from the increase in whole community chlorophyll concentrations, and the concomitant drawdown of macronutrients. The resulting impact of the supplements on the community composition was recorded through microscopic observations, flow cytometry, the productivity of phytoplankton (14C-uptake) and heterotrophic bacteria (3H-leucine) and concentration of dissolved and particulate domoic acid.

The findings from the initial ECOHAB cruise (June 2003) showed that the planktonic community was iron-stressed despite being closely associated with water influenced by coastal discharge. The effect of iron additions was minimized with the addition of the iron chelator, desferal. These bioassay experiments will be repeated during the second cruise in September 2003 to determine if there are seasonal differences in the iron nutrition in of this region, and the resultant effect of micronutrient manipulations on domoic acid production.