NITROGEN BIOGEOCHEMICAL CYCLING IN THE NORTHWESTERN INDIAN-OCEAN

The vertical distribution and fine scale structure of nitrate (NO3), n itrite (NO2), nitrous oxide (N2O), phosphate (PO4), oxygen (02) and ch lorophyll a (chl a) were determined in the North Western Indian Ocean (NWIO) along a meridional section (67-degrees-E) from the Equator to t he Gulf of Oman using an Autoanalyser for micromolar levels of nutrien ts, and chemiluminescence and gas chromatographic methods for nanomola r levels of NO3 and NO2 and N2O respectively. Three biogeochemically c ontrasting regimes were investigated: (1) the highly oligotrophic nutr ient-depleted subtropical gyre; (2) the monsoonal upwelling of nutrien t-rich intermediate waters off the southeastern Arabian Coast; and (3) the denitrifying 02-depleted zone (ODZ; ca 150-1200 m depth) in the A rabian Sea. Concentrations of NO3 and NO2 were severely depleted in su rface oligotrophic waters from the equator (average 43 and 3.6 nM resp ectively) to the subtropical gyre (12-15-degrees-N; average 13.3 and 2 .0 nM respectively) with similar levels in the more stratified Gulf of Oman. Upwelling waters off Southern Arabia had three orders of magnit ude higher NO3 levels, and throughout the NWIO, the calculated NO3-fue lled primary production appeared to be regulated by NO3 concentration. Existing Redfield DELTAO2/DELTANO3 regeneration ratios (=9. 1) previo usly derived for the deep Indian Ocean were confirmed (=9.35) within t he oxic upper layers of the NWIO. The ''NO''-potential temperature rel ationship (BROECKER, 1974, Earth and Planetary Science Letters, 23, 10 0-107) needed for the derivation of expected NO3 and NO3-deficits with in the denitrifying ODZ were refined using an isopycnal, binary mixing model along the sigma(theta) = 26.6%. density layer to take into acco unt the inflowing contribution of NO3-depleted Persian Gulf Water. Ver tically integrated NO3-deficits increased northwards from 0.8 mol NO3- N m-2 at Sta. 2 (04-degrees-N), up to 6.49 mol NO3-N m-2 at Sta. 9, at the mouth of the Gulf of Oman, then decreased to 4. 10 moles NO3 -N m -2 toward Sta. 11, near the Straits of Hormuz. When averaged for the d enitrification area of the Arabian Sea, this corresponds to a deficit of 118 Tg NO3-N. Adopting a recent Freon-11 based estimate of water re sidence time of 10 years (OLSON et al., 1993, Deep-Sea Research II, 40 , 673-685) for the O2-depleted layer, we calculate an annual net denit rification flux of 11.9 Tg N to the atmosphere or approximately 10% of the global water column denitrification rates. Supersaturated N2O con centrations were found in both surface oxic and upwelling waters (up t o 246%) and peaked at the base of the ODZ (up to 1264%) in the norther n Arabian Sea. Both nitrification in oxic waters and denitrification i n hypoxic layers can be invoked as sources of N2O. The inventory of ex cess N2O amounted to 2.55 +/- 1.3 Tg N2O-N, corresponding to annual pr oduction of 0.26 +/- 0.13 Tg from denitrification. This is comparable to earlier (LAW and OWENS, 1990, Nature, 346, 826-828) estimates of th e ventilation flux of N2O (0.22-0.39 Tg yr-1) from the upwelling regio n of the Arabian Sea. The decadal response times for circulation, deox ygenation, denitrification and ventilation of the ODZ-derived N2O and CO2 greenhouse gases and their monsoonal coupling implies the Arabian Sea is a sensitive oceanic recorder of global climate change.