Production and turnover rates of C37 alkenones in the eastern Bering Sea: implication for the mechanism of a long duration of Emiliania huxleyi bloom

Production and turnover rates of C37 alkenones were determined by a 13C-labeling technique during the Emiliania huxleyi blooms in the eastern Bering Sea. The production rates of C37 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 C37:2 and 0.057–0.16 for μg l−1 day−1 C37:3 respectively. The temperature difference (dT*) between in situ temperature and Uk’37-based temperature (estimated by C37 alkenone unsaturation index: Uk’37) for seston is generally larger than the temperature difference between in situ temperature and Pr-Uk’37-based temperature (estimated by the unsaturation index of C37 alkenone production rate). Therefore, the application of Pr-Uk’37 determined by the 13C labeling can be suggested as a more accurate calibration of in situ temperature. On the other hand, the C37 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 Uk’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 C37 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 Uk’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.