a Study of Plankton Dynamics and Nutrient Cycling in the Central Gyre of the North Pacific OCEAN1
Abstract
The dynamics of phytoplankton growth in relation to nutrient concentrations were studied in the subtropical central gyre of the North Pacific in November 1971. Rates of excretion of phosphate, ammonium, and urea‑N by zooplankton and rates of assimilation of carbon, nitrate, ammonium, and urea‑N by phytoplankton were measured. The growth rate of phytoplankton was estimated to be about 0.2–0.3 doublings day−1 in the 70–80‑m mixed layer, apparently limited by concentrations of both nitrogen and phosphate. Only nitrogen concentration was so limiting at a station near the western edge of the California Current. No diel changes in concentrations of ambient nutrients were observed. Urea‑nitrogen appears to be an important source of nitrogen for phytoplankton growth in these waters and to be an important excretory product of zooplankton.Concentrations of phosphate and ammonium were extremely low, but turnover times were estimated to be of the order 3–5 days for ammonium and >10 days for urea and phosphate. Bioinass of phytoplankton in the mixed layer was also very low, and corresponded approximately to that expected if a laboratory culture were operated as a nitrogen‑limited chemostat with a concentration of about 0.48 µg‑atom N liter−1 in the incoming culture medium and a dilution rate of about 0.13 per day.Physiological differences were noted between the phytoplankton in the mixed layer and that living below the thermocline, as were differences in chemical composition (ratio of C:Chl a and C:N).
- Publication:
-
Limnology and Oceanography
- Pub Date:
- July 1973
- DOI:
- 10.4319/lo.1973.18.4.0534
- Bibcode:
- 1973LimOc..18..534E