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-' 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 bc of the order 3-5 days for ammonium and >lO days for urea and phosphate. Biomass of phytoplankton in the mixed layer was also very low, and corresponded approximately to that expected if a laboratory culture were operated as a nitrogenlimited chemostat with a concentration of about 0.48 pg-atom N liter-l 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).The central gyre of the North Pacific Ocean is a trans-Pacific body of water extending approximately from 40"N to 15"N and maintained by the surrounding, anticyclonic pattern of surface circulation. Because of the gyre's size, the effects of land masses and of waters of different origins are buffered in its center, which is therefore an appealing area in which to study plankton-nutrient relationships. As a result of generalized downwelling and mild winters, a thermocline appears to persist within the euphotic zone over a time measured at least in months, if not in years, and to isolate from deeper waters an environment. which is relatively stable in comparison with equatorial, temperate, and polar seas, or with coastal waters. Hence it is not far-fetched to think of stability in the mixed layer on a time scale rather long in comparison with the expected generation times of phytoplankton ( days) and zooplankton (weeks), notwithstanding seasonal fluctuations in the thickness of the mixed layer2 and in its temperature (Robinson and Bauer 1971).

May 1989Volume 34Number 3Physical transport of nitrate across the nitracline is the main nitrogen source for new production by phytoplankton in the surface ocean. The intersection of the nitracline with the bottom of the photic zone makes this depth interval a region of rapid nitrogen cycling, involving assimilatory and regenerative transformations, as well as physical transport. We investigated the pathways of nitrogen transformations in surface, nitracline, and subnitracline samples by following changes in concentration and lSN : 14N ratio in dissolved and particulate pools in the same bottles for 24 h. Fluxes of nitrite and nitrate among pools were detected from tracer distributions but were not always reflected in changes in nutrient concentrations. For example, the production of nitrate by nitrification could be detected by tracer methods even when nitrate concentration decreased. Nitrification was detected within and below the nitracline by both oxidation of 15N02-and dilution of initially labeled nitrite (ammonium oxidation) or nitrate (nitrite oxidation) pools. Significant nitrate production by nitrification, relative to nitrate assimilation, implies that some of the nitrate assimilated by phytoplankton is functionally regenerated rather than new nitrogen. Our observations also suggest an important role for a labile organic nitrogen pool of unknown identity, possibly involving bacterial mediation, in the nitrogen cycling of surface waters.

The transport of nitrate into the euphotic zone appears to be a major factor regulating the standing stock and production of phytoplankton in southern California coastal waters. The evidence is as follows. 1) The rate of photosynthetic carbon assimilation is proportional to the rate of nitrate assimilation and to the ratio of nitrate:total N assimilated.2) The phytoplankton standing stock (g C-m-") and its production rate are related to the depth of the vertical nitrate concentration gradient.3) The chemical composition of the particulate organic matter, as the POC:PON ratio, is rclatcd to the carbon:nitrogen assimilation ratio of the phytoplankton.4) Regenerated production, measured as ammonium assimilation, is proportional to the nitrate assimilation rate, implying parallel and concurrent increases in the production of heterotrophic microplankton and phytoplankton attending new inputs of nitrate into the cuphotic zone. 5) The vertical diffusion of nitrate, when calculated from the vertical nitrate concentration gradients and nitrate assimilation rates, appears to give reasonable cstimates of the vertical eddy diffusivity coefficient for nitrate.

SUMMARY The oceanic diatom Thalassiosira pseudonana Hasle and Heimdal (formerly Cyclotella nana) was grown with 12L:12D illumination cycles in nitrogen‐limited continuous culture with a mixture of ammonium and nitrate as the N source. Measurements included, at 3 different growth rates (degrees of N limitation), cell concentration, cell carbon, nitrogen, and chlorophyll a contents, cell volume, photosynthetic carbon assimilation vs. irradiance, short‐term uptake of ammonium and nitrate vs. their ambient concentrations, and in vitro activities of the assimilatory enzymes nitrate reductase and glutamic dehydrogenase. The various parameters showed either an increase (pattern a) or a decrease (pattern b) with increasing N limitation. Those following pattern a were nitrate reductase activity and the capacity to assimilate nitrate and ammonium. Those following pattern b were glutamic dehydrogenase activity, photosynthetic rate, nitrogen:carbon and chlorophyll a:carbon composition ratios. Results are discussed in terms of the interpretation such measurement for natural phytoplankton and effects of circadian periodicity.