Spatial variability in nitrogen uptake rates and f-ratio in the northern Indian Ocean

Inorganic nitrogen is the major limiting nutrient in the northern Indian Ocean. The published and unpublished data of nitrate (NO₃⁻) and ammonium (NH₄⁺) uptake rates and estimated f-ratios (new to total production) for the past 3 decades were compiled for the northern Indian Ocean. Though the data covered seasonality in the northern Indian Ocean, however, it is biased with reference to space. Both NO₃⁻ and NH₄⁺ uptake rates displayed significant spatial variability associated with hydrographic properties. The mean NO₃⁻ uptake rates at surface were not different in the Arabian Sea (0.07 ± 0.1 mmol m⁻³ d⁻¹) than Bay of Bengal (BoB; 0.04 ± 0.02 mmol m⁻³ d⁻¹), however, photic zone integrated rates were higher in the BoB (4.5 ± 2 mmol m⁻² d⁻¹) than Arabian Sea (2.7 ± 2 mmol m⁻² d⁻¹) due to higher uptakes rates at the subsurface chlorophyll-a (Chl-a) maxima region in the BoB. The basin average NH₄⁺ uptakes at the surface were almost same in the Arabian Sea (0.4 ± 0.3 mmol m⁻³ d⁻¹) and BoB (0.5 ± 0.5 mmol m⁻³ d⁻¹), the photic zone integrated NH₄⁺ uptake rates are 6 times higher in the Arabian Sea (15 ± 8 mmol m⁻² d⁻¹) than BoB (2.3 ± 1 mmol m⁻² d⁻¹). This difference is mainly caused by deeper photic depth in the Arabian Sea (75 ± 8 m) than BoB (62 ± 5 m) due to high suspended load from rivers in the latter basin. High f-ratios were observed in the BoB (>0.5) than that of Arabian Sea (<0.2) suggesting that about half of the primary production is potentially exported from the mixed layer in the BoB whereas less than one-fifth only exported in the case of the Arabian Sea. The photic-zone integrated NO₃⁻ uptake rates and f-ratios displayed a significant relationship with surface temperature, salinity, surface Chl-a and nutrients concentrations suggesting that nitrogen uptake rates were mainly controlled by hydrographic properties. The compiled nitrogen uptake rates and f-ratio may be useful to improve simulations of cycling of carbon, nitrogen, and oxygen in the numerical models.