No 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-' 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).

The grazing rates of 4 species of Calanus were measured on several species of marinc phytoplankton, both singly and in mixtures.Grazing rates varied inversely with phytoplankton concentration, the duration of the expcrimcnt, and the age of the phytoplankton cultures used.Calanus spp. generally removed large cells at higher rates than small when feeding on a mixture of two species of phytoplankton.Bccausc of this selectivity and the difference in volume between large and small cells, the large cells contributed by far the greater fraction of the total volume of food ingested by the copepods, even when considerably less abundant than the smaller cells. Comparison of the grazing rates of the copepodite stage V and adult female animals of C. finmarchicus, C. glacialis, and C. hyperboreus on a mixture of 7 species of diatoms demonstrated further the importance of large cells. Marked quantitative differences in rate of food intake were found between the different developmental stages of each species and between the diffcrcnt species of copepods, and some qualitative differences in selective feeding were also indicated.

In the size range from lO-4 to 10' pg (carbon) body weight, the biomass of plankton in the euphotic layer of the North Pacific Central Gyre decreases as an allometric function of body weight. Even in a steady state ecosystem such as that analyzed here, there is variability in space and time; this suggests that one must be careful in extrapolating the relation to less predictable marine areas. In obtaining dynamic information from biomass spectra, one must distinguish changes due to the flow of energy within the spectrum (growth, predation, reproduction) from changes due to emigration from or immigration into the spectrum of the particular area sampled, such as those due to the diel vertical migration of macrozooplankton in the largest size classes.