New Zealand Journal of Marine and Freshwater Research abstracts

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Nitrogen uptake by the summer size-fractionated phytoplankton assemblages in the Westland, New Zealand, upwelling system

F. HOE CHANG
JANET M. BRADFORD-GRIEVE

National Institute of Water & Atmospheric
Research Ltd
P. O. Box 14-901, Kilbirnie
Wellington, New Zealand

WARWICK F. VINCENT*
PAUL H. WOODS**

National Institute of Water & Atmospheric
Research Ltd
P. O. Box 11-115
Hamilton, New Zealand

Present addresses:

*Département de biologie, et Centre d'études nordiques, Université Laval, Québec, QC G1K 7P4, Canada

**Institute of Environmental Health and Forensic Sciences, P. O. Box 29181, Christchurch, New Zealand

Abstract The assimilation of nitrate (), ammonium (), and urea by size-fractionated natural assemblages of phytoplankton, as a function of the individual ambient nitrogen (N) nutrient concentrations, was investigated off the west coast of South Island, New Zealand, in February 1989. New production ([[rho]]) contributed by all three size classes was higher inshore than offshore, and accounted for 40-61% and 19-29% of the total N production ([[Sigma]][[rho]]N), respectively. The reverse was true for regenerated production [[[rho]] + [[rho]]urea], a clear indication of the importance of and urea in offshore waters. Picoplankton (< 2 um fraction), both inshore and offshore, was the major user of the regenerated N, and accounted for 43-62% of total regenerated production. The assimilation rates of all three size components achieved saturation at the larger additions of , , and urea, and the half-saturation constants (Ks) of all three N nutrients for the three size classes in summer were higher than the respective ambient N concentrations. Low ambient dissolved nitrogenous nutrients (DNN), low dissolved reactive phosphate (DRP), and low Vamb/Vmax ratios recorded in summer off Westland imply that the natural assemblages of phytoplankton, especially the larger components in inshore waters, were under nutrient stress.

Keywords summer ¹⁵N assimilation; f-ratio; half-saturation constant (Ks); maximum uptake rate (Vmax); diel ¹⁵N uptake; new and regenerated production; microplankton; nanoplankton; picoplankton; upwelling

New Zealand Journal of Marine and Freshwater Research, 1995: Vol. 29: 147-161

0028-8330/95/2902-0147 $2.50/0 (c) The Royal Society of New Zealand 1995

PDF file of entire paper: medium quality (1568K); (scanned from paper original: notes about this process)

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