Nitrogen biogeochemical cycling in the northwestern Indian Ocean

R. Fauzi, C. Mantoura, Clifford S. Law, Nick Owens, Peter H. Burkill, E. Malcolm S. Woodward, Robin J.m. Howland, Carole A. Llewellyn

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Abstract

The vertical distribution and fine scale structure of nitrate (NO3), nitrite (NO2), nitrous Oxide (N2O), phosphate (PO4), oxygen (O2) and chlorophyll α (chl α) were determined in the North Western Indian Ocean (NWIO) along a meridional section (67°E) from the Equator to the Gulf of Oman using an Autoanalyser for micromolar levels of nutrients, and chemiluminescence and gas chromatographic methods for nanomolar levels of NO3 and NO2 and N2O respectively. Three biogeochimically contrasting regimes were investigated: (1) the highly oligotrophic nutrient-depleted subtropical gyre; (2) the nonsoonal upwelling of nutrient-rich intermediate waters of the southeastern Arabian Coast; and (3) the denitrifying O2-depleted zone (ODZ; ca 150–1200 m depth) in the Arabian Sea.

Concentrations of NO3 and NO2 were severely depleted in surface oligotrophic waters from the equator (average 43 and 3.6 nM respectively) to the subtropical gyre (12–15°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 magnitude higher NO3 levels, and throughout the NWIO, the calculated NO3-fuelled primary production appeared to be regulated by NO3 concentration.

Existing Redfield ΔO2/ΔNO3 regeneration ratios (=9.1) previously derived for the deep Indian Ocean were confirmed (= 9.35) within the oxic upper layers of the NWIO. The “NO”-potential temperature relationship (Broecker, 1974 Earth and Planetary Science Letters, 23, 100–107) needed for the derivation of expected NO3 and NO3-deficits within the denitrifying ODZ were refined using an isopycnal, binary mixing model along the σθ = 26.6%, density layer to take into account the inflowing contribution of NO3-depleted Persian Gulf Water. Vertically integrated NO3-deficits increased northwards from 0.8 mol NO3-N m−2 at Sta. 2 (04°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 denitrification area of the Arabian Sea, this corresponds to a deficit of 118 Tg NO3-N. Adopting a recent Freon-11 based estimate of water residence time of 10 years (Olsonet al., 1993, Deep-Sea Research II, 40, 673–685) for the O2-depleted layer, we calculate an annual net denitrification flux of 11.9 Tg N to the atmosphere or approximately 10% of the global water column denitrification rates.

Supersaturated N2O concentrations were found in both surface oxic and upwelling waters (up to 246%) and peaked at the base of the ODZ (up to 1264%) in the northern Arabian Sea. Both nitrification in oxic waters and denitrification in hypoxic layers can be invoked as sources of N2O. The inventory of excess N2O amounted to 2.55 ± 1.3 Tg N2O-N, corresponding to annual production of 0.26 ± 0.13 Tg from denitrification. This is comparable to earlier (Law and Owens, 1990, Nature, 346, 826–828) estimates of the ventilation flux of N2O (0.22–0.39 Tg yr−1) from the upwelling region of the Arabian Sea. The decadal response times for circulation, deoxygenation, 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.
Original languageEnglish
Pages (from-to)651-671
Number of pages20
JournalDeep-Sea Research Part II - Topical Studies in Oceanography
Volume40
Issue number3
DOIs
Publication statusPublished - 1 Jan 1993

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