Principal Investigator: Jia-Zhong Zhang
Collaborating scientist(s):
George Berberian
Calvin Mordy (PMEL)
Charles Fischer
Objective: Determination of dissolved nutrients (phosphate, silicate, nitrate and nitrite) in the ocean as a contribution to the OACES, WOCE, JGOFS and GLOBEC programs.
Rationale: The distribution of inorganic nutrients (phosphate, silicate, nitrate and nitrite) in the ocean is controlled by physical, biological and geochemical processes. Biological processes also link nutrient and CO2 cycles in the marine environment. In fact, the availability of inorganic nutrients in the euphotic zone is an important factor controlling primary productivity and the efficiency of the so-called biological carbon pump (and for that matter the energy available for secondary productivity). During regeneration, bacterial oxidation of dissolved and sinking organic matter yields CO2 and inorganic phosphate and nitrate. Nutrient concentrations corrected for bacterial remineralization can be used as conservative tracers in the study of ocean circulation as various water masses have distinctive distributions of "preformed" nutrients.

The major sampling effort in the Indian Ocean was focused along the I8N line. This is a repeat WOCE hydrographic line and also the sole mid-ocean meridional WOCE cruise during the southwest monsoon season. The depth and meridional extent of monsoonal upwelling can be be used to estimate the monsoonal upwelling nutrient flux for the central Indian Ocean. Nutrient data will aid in examining monsoonal effects on upper ocean circulation in the tropics, determining the seasonal variability of equatorial currents as well as the Southern Indian Subtropical Gyre, and in elucidating the deep water circulation in the central Indian Ocean.

Nutrient data collected along line P15S will be used to identify water masses within the thermocline as well as the intermediate and bottom waters flowing northward into the Southwest Pacific Basin and through the Samoa Passage. The nutrient data will also be used to estimate nutrient budgets for the Southwest Pacific Basin.

Method: An Alpkem Flow Solution System and a Technicon Autoanalyzer II were used as analytical instruments. Nutrient measurements on the oceanic cruises closely followed WOCE protocols for chemical analysis, standardization and data analysis (Gordon et al. 1991). Some modifications in the chemical analysis were required to optimize the Flow Solutions system.
Accomplishment: Nutrient analyses has been successfully completed to required specifications of precision and accuracy on the NOAA sponsored WOCE line I8N in the Indian Ocean along 80 degrees E (Sept.-Oct. 1995) and P15S in the Pascific Ocean along 170 degrees W (Jan.-Mar. 1996). Samples were analyzed for silicic acid, phosphate, nitrate and nitrite on an autoanalyzer in a temperature controlled van, usisng procedures similar to those specified in the WOCE nutrient protocol (Gordon et al., 1991). A total of 101 stations along I8N and 182 stations along P15S were occupied, resulting in about 10,000 and 24,000 nutrient data values along the I8N and P15S lines, respectively. Preliminary data was provided daily or weekly during the cruise, and a nearly complete preliminary data set was provided to the chief scientists before disembarking the ship. Some comparisons of our preliminary data with data from another recent WOCE cruise at a nearby site are shown in Figures1,2, 3. Assembly, processing and quality control of nutrient data collected along I8N and P15S is underway.
Key reference:
Gordon, L.I., Joe C. Jennings, Jr., Andrew A. Ross, and James M. Krest. 1992. A suggested protocol for continuous flow automated analysis of seawater nutrients (Phosphate, Nitrate, Nitrite and Silicic Acid) in the WOCE Hydrographic Program and the Joint Global Ocean Fluxes Study. OSU Coll. of Oc. Descr. Chem. Oc. Grp. Tech. Rpt. 92-1.
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