Production and turnover rates of C 37 alkenones in the eastern Bering Sea: implication for the mechanism of a long duration of Emiliania huxleyi bloom

Abstract

Production and turnover rates of C 37 alkenones were determined by a 13C-labeling technique during the Emiliania huxleyi blooms in the eastern Bering Sea. The production rates of C 37 alkenones are variable in the surface water of the Bering Sea (during Emiliania huxleyi blooms), ranging from 0.019–0.043 μg l −1 day −1 for C 37:2 and 0.057–0.16 for μg l −1 day −1 C 37:3 respectively. The temperature difference (dT*) between in situ temperature and U k’ 37-based temperature (estimated by C 37 alkenone unsaturation index: U k’ 37) for seston is generally larger than the temperature difference between in situ temperature and Pr-U k’ 37-based temperature (estimated by the unsaturation index of C 37 alkenone production rate). Therefore, the application of Pr-U k’ 37 determined by the 13C labeling can be suggested as a more accurate calibration of in situ temperature. On the other hand, the C 37 alkenones concentration (3.1 μg l −1) at BR-10 (middle shelf) is 4–16 fold larger than the concentrations (0.19–0.77 μg l −1) at other stations. In addition, the U k’ 37-based seawater temperature at BR-10 (4.0 °C) was much lower than the in situ temperature (7.5 °C), showing the largest seawater temperature difference (dT*). This low temperature and high abundance of C 37 alkenones may be the result of turbulent mixing, like that caused by eddies on the continental slope area, considering the existence of very low temperature and high ammonium concentration water in the water mass beneath the surface layer. The uncoupling between the in situ temperature and U k’ 37-based seawater temperature in the middle of shelf area (BR-10) may indicate a link between the nutrient supply mechanism and the sustained Emiliania huxleyi bloom from summer through autumn.