New Zealand Journal of Marine and Freshwater Research abstracts

---

Nitrogen utilisation by size-fractionated phytoplankton assemblages associated with an upwelling event off Westland, New Zealand

F. HOE CHANG

New Zealand Oceanographic Institute National Institute of Water and
Atmospheric Research Ltd P.O. Box 14901, Kilbimie Wellington, New Zealand

WARWICK F. VINCENT* PAUL H. WOODS

Taupo Research Laboratory
National Institute of Water and Atmospheric Research Ltd
P.O. Box 415
Taupo, New Zealand
*Present address: D6partement de biologie, University Laval, Quebec G1K 7P4, Canada

Abstract Uptake rates of ammonium (NH₄⁺), nitrate (NO₃""), and urea by three subgroups of phytoplankton (< 200, < 20, < 2 (jm) off Westland, were measured using ¹⁵N tracer techniques in midwinter 1988, after a recent upwelling. For all size fractions at surface irradiance (1_1Oo), nitrogen (N) was taken up primarily as NO₃~. This accounted for 67-85% of total N uptake (SpN), whereas at 40 and 7% of surface irradiance, the regenerated N (NH₄⁺) and urea) made up 31-72% of EpN. Depth profile experiments for all three size components showed that uptake was most light-sensitive, followed by that and urea. The irradiance and nutrient availability plot indicated that light was substantially more important than the nutrient concentrations in controlling the assimilation of N by microplankton (20-200 |jm). Nano- (2-20 pm) andpicoplankton (< 2 pm) however, were not as sensitive to either light or nutrient concentrations. High winds and the resulting deep mixing, combined with offshore and alongshore advection in the midwinter, were suggested to be the major cause of the low biomass and N productivity.

Keywords ¹⁵N assimilation; new and regenerated production; total nitrogen uptake;/ratio; upwelling; mixed layer depth; microplankton; nanoplankton; picoplankton; particulate nitrogen; chlorophyll a; Westland

New Zealand Journal of Marine and Freshwater Research, 1992, Vol. 26: 287-301The Royal Society of New Zealand 1992
Received 29 October 1991;

PDF file of entire paper: medium quality (1190K); (scanned from paper original: notes about this process)

---

This year's abstracts | Journal home page | All abstracts | Publishing home page