Surface waters of the eastern equatorial Pacific Ocean present the enigma of apparently high plant-nutrient concentrations but low phytoplankton biomass and productivity1. One explanation for this ‘high-nitrate, low-chlorophyll’ (HNLC) phenomenon has been that growth is limited by iron availability2,3. Here we use field data and a simple silicon-cycle model4 to investigate the HNLC condition for the upwelling zone of this ocean region. Measured silicate concentrations in surface waters are low and largely invariant with time, and set the upper limit on the total possible biological utilization of dissolved inorganic carbon. Chemical and biological data from surface waters indicate that diatoms—silica-shelled phytoplankton—carry out all the ‘new production’ (nitrate uptake)5. Smaller phytoplankton (picoplankton) accomplish most of the total primary production, largely fuelled by nitrogen regenerated in reduced forms as a result of grazing by zooplankton. The model predicts values of new and export production (the production exported to below the euphotic zone) that compare well with measured values6. New and export production are in balance for biogenic silica, whereas new production exceeds export for nitrogen. The HNLC condition in the upwelling zone can therefore be understood to be due to a chemostat-like regulation of nitrate uptake by upwelled silicate supply to diatoms: ‘low-silicate HNLC’. These results are not inconsistent with observations of iron-fertilized diatom growth during in situ experiments in ‘low-iron HNLC’ waters outside this upwelling zone2,3, but reflect the role of different supply rates of iron and silicate in determining the nature of the HNLC condition.
Access optionsAccess options
Subscribe to Journal
Get full journal access for 1 year
only $3.90 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
Barber, R. T. & Chavez, F. P. Regulation of primary productivity rate in the equatorial Pacific. Limnol. Oceanogr. 36, 1803–1815 (1991).
Martin, J. H. et al. Testing the iron hypothesis in ecosystems of the equatorial Pacific Ocean. Nature 371, 123–129 (1994).
Coale, K. H. et al. Amassive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean. Nature 383, 495–501 (1996).
Dugdale, R. C., Wilkerson, F. P. & Minas, H. J. The role of a silicate pump in driving new production. Deep-Sea Res. I 42, 697–719 (1995).
Dugdale, R. C. & Goering, J. J. Uptake of new and regenerated forms of nitrogen in primary productivity. Limnol. Oceanogr. 12, 196–207 (1967).
Murray, J. W., Johnson, E. & Garside, C. AU.S. JGOFS Process Study in the equatorial Pacific (EqPac): Introduction. Deep-Sea Res. II 42, 275–293 (1995).
Frost, B. W. & Franzen, N. C. Grazing and iron limitation in the control of phytoplankton stock and nutrient concentration: a chemostat analogue of the Pacific equatorial upwelling zone. Mar. Ecol. Prog. Ser. 83, 291–303 (1992).
Minas, H. J. & Minas, M. Net community production in “High Nutrient-Low Chlorophyll” waters of the tropical and Antarctic Oceans: grazing versus iron hypothesis. Oceanol. Acta 15, 145–162 (1992).
Wilkerson, F. P. & Dugdale, R. C. Silicate versus nitrate limitation in the equatorial Pacific estimated from satellite-derived sea-surface temperatures. Adv. Space Res. 18, 81–89 (1996).
Ku, T.-L., Luo, S., Kusakabe, M. & Bishop, J. K. B. 228Ra-derived nutrient budgets in the upper equatorial Pacific and the role of “new” silicate in limiting productivity. Deep-Sea Res. II 42, 479–497 (1995).
Broecker, W. S. & Peng, T.-H. Tracers in the Sea (Eldigio, New York, (1982)).
Brzezinkski, M. A. The Si:C:N ratio of marine diatoms: interspecific variability and the effect of some environmental variables. J. Phycol. 21, 347–357 (1985).
Dugdale, R. C., Wilkerson, F. P., Barber, R. T. & Chavez, F. P. Estimating new production in the equatorial Pacific Ocean at 150° W. J. Geophys. Res. 97, 681–686 (1992).
McCarthy, J. J., Garside, C., Nevins, J. L. & Barber, R. T. New production along 140° W in the equatorial Pacific during and following the 1992 El Niño event. Deep-Sea Res. II 43, 1065–1093 (1996).
Probyn, T.A. The inorganic nitrogen nutrition of phytoplankton in the southern Benguela: New production, phytoplankton size, and implications for pelagic food webs. S. Afr. J. Sci. 12, 411–420 (1992).
Nelson, D. M., Goering, J. J. & Boisseau, D. W. in Coastal Upwelling Vol. 1(ed. Richards, F. A.) (Am. Geophys. Un., Washington DC, (1981)).
Michaels, A. F. & Silver, M. W. Primary production, sinking fluxes and the microbial food web. Deep-Sea Res. 35, 473–490 (1988).
Wanninkhof, R. A. et al. Seasonal and lateral variations in carbon chemistry of surface water in the eastern equatorial Pacific during 1992. Deep-Sea Res. II 42, 387–409 (1995).
Bidigare, R. R. & Ondrusek, M. E. Spatial and temporal variability of phytoplankton pigment distributions in the central equatorial Pacific Ocean. Deep-Sea Res. II 43, 809–833 (1996).
Kaczmarska, I. & Fryxell, G. A. Microphytoplankton of the equatorial Pacific: 140° W meridional transect during the 1992 El Niño. Deep-Sea Res. II 42, 535–558 (1995).
Buesseler, K. O., Andrews, J. A., Hartman, M. C., Belastock, R. & Chai, F. Regional estimate of the export flux of particulate organic carbon derived from Thorium-234 during the JGOFS EqPac program. Deep-Sea Res. II 42, 777–804 (1995).
Barber, R. T. et al. Primary productivity and its regulation in the equatorial Pacific during and following the 1991–1992 El Niño. Deep-Sea Res. II 43, 933–969 (1996).
Sunda, W. G., Swift, D. G. & Huntsman, S. A. Low iron requirement for growth in oceanic phytoplankton. Nature 351, 55–57 (1991).
Fitzwater, S. E., Coale, K. H., Gordon, R. M., Johnson, K. S. & Ondrusek, M. E. Iron deficiency and phytoplankton growth in the equatorial Pacific. Deep-Sea Res. II 43, 995–1015 (1996).
Dugdale, R. C. & Wilkerson, F. P. in Primary Productivity and Biogeochemical Cycles in the Sea (eds Falkowski, P. G. & Woodhead, A. D.) 107–122 (Plenum, New York, (1992)).
Archer, D. E. et al. Daily, seasonal and interannual variability of sea surface carbon and nutrient concentrations in the equatorial Pacific Ocean. Deep-Sea Res. II 43, 779–809 (1996).
This work was supported by the US NSF and Region Provence Alpes Maritimes Cooperative Agreement.
About this article
Nature Climate Change (2019)
Marine Chemistry (2019)
Geophysical Research Letters (2019)
Journal of Plankton Research (2019)
Phytoplankton productivity and community structure variations over the last 160 years in the East China Sea coast in response to natural and human-induced environmental changes
The Holocene (2019)