A STUDY OF PLANKTON DYNAMICS AND NUTRIENT CYCLING IN THE CENTRAL GYRE OF THE NORTH PACIFIC OCEAN1

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 l Supported by U.S. Atomic Energy Commission Contract AT( 11-l )GEN 10, P.A. 20, National Science Foundation Grant No. GA-31167X, ind the Marine Life Research Program. Ship time supported by the National Science Foundation Alpha Helix program. Contribution from Scripps Institution of Oceanography. 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). 2 In February 1973 we observed a mixed layer extending to 150 m throughout much of the central gyre, suggesting some mixture of deeper water into the surface layers. LIMNOLOGY AND OCEANOGRAPHY 534 JULY 1973, V. 18(4) NORTH PACWIC PLANKTON DYNAMICS 535 Although concentrations of plant nutrients and organisms are low (Reid 1962; Univ. California 1967, 1970; Sagi 1969; Marumo 1970)) absolute fluctuations in concentrations so far recorded are small in comparison to those in other areas. These variations appear to be of the order twofold to threefold about the mean both areally and seasonally, Sampling problems, for purposes of broad areal representation, would be eased if the apparent uniformity in concentrations of nutrients and organisms persists under careful measurement. The patchiness of diatom species has been compared quantitatively to that found in the subarctic North Pacific (Venrick 1972). All this information suggests that a steady state might pertain with respect to nutrient concentrations, plankton standing stocks, specific growth rates, and grazing. Such a situation would simplify the interpretation of measurements and permit the calculation or prediction of certain stocks and rates not readily measured with existing methods. Dugdale and Goering (1967) and Dugdale (1967) have provided a conceptual framework for the measurement and interpretation of nutrient-phytoplankton-zooplankton relationships in oligotrophic ocean waters with a persistent surface mixed layer. They conceive a two-layered water column: Specific growth rate of phytoplankton would be nutrient-limited in the mixed layer and light-limited below the thermocline. Grazing in the mixed layer would be closely coupled with phytoplankton production, with nitrogenous excretory products of the zooplankton the primary source of the growth rate-limiting nutrient for phytoplankton. Losses of nutrient to deep water, via sinking and migration of zooplankton, would be balanced by vertical turbulent diffusion of nitrate nitrogen from deep water, input of ammonium in rain ( cf. Menzel and Spaeth 1962) or by fixation of molecular nitrogen. This paradigm has many historical sources and seems consistent with present information, including the extensive studies of the Woods Hole group in the Sargasso Sea (Menzel and Ryther 1960, lQ61a,b; Steele and Menzel 1962) and the results of the EASTROPAC program in the eastern tropical Pacific Ocean (Thomas 1970a,b; Thomas and Owen 1971). Usually lacking in such studies have been measures of the degree of coupling between growth of phytoplankton and grazing by herbivores and between rates of excretion by zooplankton and rates of assimilation of nutrients by phytoplankton. Some estimates are available for coastal waters (Harris 1959; Martin 1968; Hargrave and Geen 1968). Pomeroy et al. (1963) presented an estimate for excretion relative to assimilation in the Gulf Stream as well as waters over the continental shelf. There has been no quantitative assessment of the rate of loss of nutrients to deep water, or their rcplaccment via upward diffusion or nitrogen fixation. The deeper water, in which growth of phytoplankton would be light-limited, usually shows a zone of chlorophyll concentrations higher than in the mixed layer above, and this zone may contribute an important fraction of the primary production to the water column (Anderson 1969, 1972; Hobson and Lorenzen 1972; Venrick et al. 1973). The elevated concentration of chlorophyll is due in part to a higher chlorophyll content of each phytoplankton cell ( cf. Steele 1964)) but in some cases the concentration of cells may also be elevated (Anderson 1972). Nitrate as a source of nitrogen for phytoplankton in this layer is relatively more important than in the mixed layer (Goering et al. 1970). We examined the rate of primary production of this deep chlorophyll layer and the physiological adaptations of phytoplankton to the very low light intensities and restricted spectral distribution of energy in hope of achieving a more adequate understanding of production processes in oligo-