Nitrate uptake at photic zone depths is not important for export in the subtropical ocean

Observations of nitrate (NO₃^-) uptake across the photic zone of the tropical and subtropical Atlantic Ocean are described. High NO₃^- uptake rates were commonly found at depth (>75 m), coincident with the deep chlorophyll maximum and the nitracline, whereas highest rates of carbon fixation were found in near-surface waters (0-50 m). Thus NO₃^- based estimates of in situ new production (NO₃^- uptake × 6.6) at depth are in excess of in situ measurements of carbon fixation implying NO₃^- uptake exceeds production requirements. Such surplus NO₃^- uptake may ultimately result in the production of dissolved organic nitrogen with its measurement within the particulate phase representing a transient enrichment. When only upper euphotic zone (to 14% irradiance depth) rates of NO₃^- uptake and carbon fixation are considered, new production is less than total carbon fixation, suggesting, as expected, that additional nitrogen sources are required to support the observed production levels. Comparison of the deep NO₃^- uptake rates with literature estimates of seasonal NO₃^- drawdown within the nitracline suggests that the drawdown observed represents only 1% of the actual NO₃^- uptake occurring in this region of the water column, and hence most NO₃^- uptake must be recycled locally rather than exported.