Meridional variations in dissolved and particulate matter concentrations and stoichiometries in the tropical and subtropical Pacific Ocean

E. M. Grabowski, K. M. Björkman, M. J. Church, T. M. Clemente, S. E. Curless, L. A. Fujieki, T. K. Gregory, A. E. Harlan, C. W. Mahaffey, D. W. Sadler, B. V. W. Watkins, D. M. Karl

Department of Oceanography, University of Hawaii, 1000 Pope Road, Honolulu, HI 96822

Abstract

The partitioning of bioessential elements including carbon (C), nitrogen (N), and phosphorus (P) between organic and inorganic forms and among particulate and dissolved phases depends in part on the activities of planktonic microorganisms. In an effort to regionalize time series observations at Station ALOHA, the field site for the Hawaii Ocean Time-series (HOT) program, surface water concentrations of inorganic nutrients (nitrate+nitrlte (N+N) and soluble reactive phosphorus (SRP)) and organic nutrients (dissolved organic carbon, dissolved organic nitrogen and dissolved organic phosphorus) were sampled along a meridional transect from American Samoa to Honolulu, Hawaii in May 2005. This cruise provided an opportunity to evaluate spatial gradients in dissolved and particulate matter elemental composition between several open ocean biogeochemical provinces including both the North and South Pacific subtropical gyres and equatorial waters. The transition from the South Pacific Subtropical Gyre into equatorial waters was accompanied by sharp increases in the concentration of inorganic nutrients and corresponding changes in dissolved and particulate matter concentrations and stoichiometries. In particular, surface water concentrations of SRP increased approximately four-fold between 3 S and 3 N, while particulate C, N, and P concentrations increased approximately two-fold in the same region. In general, particulate matter C:N:P ratios varied widely along the transect, with particulate matter pools enriched in both C and N relative to P found in both the oligotrophic South and North Pacific subtropical gyres. Flow cytometric analyses of picoplankton community populations revealed relatively constant cell abundances of Prochlorococcus with latitude, while both Synechococcus and picoeukaryote abundances increased more than five-fold in the equatorial waters. Alterations in plankton community structure stemming from meridional variations in nutrient availability likely dictate the resulting particulate matter stoichiometries, and ultimately constrain the composition of organic matter exported to the deep sea.