The supply of nutrients due to vertical turbulent mixing: A study at the Porcupine Abyssal Plain study site in the northeast Atlantic

As part of a multidisciplinary cruise to the Porcupine Abyssal Plain (PAP) study site (49 degrees 00'N 16 degrees 30'W), in June and July 2006, observations were made of the vertical nitrate flux due to turbulent mixing. Daily profiles of nitrate and turbulent mixing, at the central PAP site, give a mean nitrate flux into the euphotic zone of 0.09 (95% confidence intervals: 0.05-0.16) mmol N m(-2) d(-1). This is a factor of 50 lower than the mean observed rate of nitrate uptake within the euphotic zone (5.1 +/- 1.3 mmol N m(-2) d(-1)). By using our direct observations to 'validate' a previously published parameterisation for turbulent mixing, we further quantify the variability in the vertical turbulent flux across a roughly 100 x 100 km region centred on the PAP site, using hydrographic data. The flux is uniformly low (0.08 +/- 0.26 mmol N m(-2) d(-1), the large standard deviation being due to a strongly non-Gaussian distribution) and is consistent with direct measurements at the central site. It is demonstrated that on an annual basis convective mixing supplies at least 40-fold more nitrate to the euphotic zone than turbulent mixing at this location. Other processes, such as those related with mesoscale phenomena, may also contribute significantly. (C) 2010 Elsevier Ltd. All rights reserved.