An analysis of continuous vertical nutrient profiles taken during a cold-anomaly off Peru

Abstract Continuous vertical profiles of nutrient concentrations to depths of ∼250 m with a vertical resolution of ∼2 m help to describe and explain conditions during a cold-anomaly in the waters off Peru. The data suggest that primary production (∼2 g C m 2 d 1 ) in the euphotic zone above an extraordinary shallow oxygen deficient zone (maximum nitrite value of 23 μM) was supported largely by vertical transports from a thin layer between 10 and 20 m. Calculations based on the profiles suggest that the denitrification rate in the shallow oxygen deficient feature (2–5 × 10 3 μ mol m −2 d −1 ) was also supported by vertical diffusion. Both calculations are based on a vertical diffusion coefficient of 1 cm 2 s −1 . Deeper oxygen deficient zones with secondary nitrite maxima embedded within them were also sampled; an unusual feature found north of 11°S centered at about 300 m and the main secondary nitrite maximum (MSNM) centred at about 200 m south of 11°S. Our data and calculations suggest that vertical diffusion was not the dominant control on denitrification rates in the northern feature. A possibly atypical station within the MSNM, however, revealed a steep vertical gradient at the relatively shallow upper boundary of the nitrite maximum. In this case, the denitrification rate required to balance losses through the upper boundary may have been on the order of 40% of the total rate. Comparison of the vertical nutrient profiles with variations in static stability shows structures with similar depth scales. The data also suggest that these features may arise from the interleaving of water masses with different histories and that the rates of microbial nitrogen transformations may display similar fine structure. A stability maximum is common at the upper boundary of the oxygen deficient zones, but our data agree with previous results suggesting that the nitrite maxima embedded within the cores of these features are not associated with unusually strong stratification.