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

Abstract

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 The published and unpublished data of nitrate (NO3?)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.