Evaluation of the utility of xanthophyll cycle pigment dynamics for assessing upper ocean mixing processes at Station ALOHA

We had the opportunity to evaluate the utility of the chromophyte xanthophyll cycle pigments (XCP), diadinoxanthin (DDX) and diatoxanthin (DTX), for assessing upper ocean mixing processes at Station ALOHA (22.75 degrees N, 158 degrees W) during August-September 2012. De-epoxidation state (DES, [DTX]/[DDX + DTX]) kinetic rate experiments were performed that quantified the conversion of DDX to DTX in the light. DES values ranged from 0.13 to 0.51 (weight: weight), and yielded a first-order rate constant of 0.16+/-0.02 min(-1). In addition, a time-series of chromophyte pigments was collected from the sea-surface in order to quantify the time-dependent rate of change in the ratio of XCP to chlorophyll a (XCP/CHLA). In situ XCP/CHLA values were measured by rapid collection of cells in the mixed layer using a diaphragm pumping system and the xanthophyll cycle activity inhibitor dithiothreitol. Turbulent transfer velocity (TTV) was estimated as the quotient of the XCP/CHLA photoadaptation rate and the XCP/CHLA gradient in the mixed layer. TTVs ranged from 0.3 to 0.5 cm s(-1), and agreed to within a factor of similar to 2 of the Lagrangian float-derived estimate of vertical mixing at a comparable friction velocity. Coincident measurements of dissolved gases suggest that XCP dynamics may be useful for the interpretation of trace gases species distributions, such as hydrogen and carbon monoxide, in the upper ocean.