Distribution of pelagic phytoplankton-derived lipid biomarkers in the Northwest Pacific region: insights into their suitability as open-water indicators

Abstract

The phytoplankton marker-IP25 (PIP25) index has been proposed to more quantitatively reconstruct the past sea ice conditions. To date, brassicasterol, dinosterol and HBI triene have been used as pelagic phytoplankton-derived lipid biomarkers when calculating the PIP25 index. This approach has been generally applied within sedimentary work, with fewer efforts observed on material collected within the water column. Moreover, it is not clear which planktonic biomarker is more suitable for the PIP25 index. In this study, we collected suspended particulate matter (SPM) along a transect from the East Sea to the Bering Sea from 18 to 28 July in 2015 and analyzed highly branched isoprenoid (HBIs) and sterols. IP25 was not detected in any of the samples, with HBI triene only detected in the five stations across the Northwest Pacific and Bering Sea. However, all sterols considered in this study were detected in all stations. Interestingly, brassicasterol concentration showed a strong, positive relationship with cholesterol concentration, but no relationship with chlorophyll a, suggesting that the former might have been associated with not only marine phytoplankton but other sources in the study area, such as zooplankton. Dinosterol and HBI triene concentrations also showed no clear relationship with chl. a or brassicasterol concentrations, indicating likely different and diverse sources of these lipids in addition to marine phytoplankton. Therefore, our study suggests that applying brassicasterol, dinosterol, and HBI triene to PIP25 under the same sea-ice conditions may lead to different trends. Further studies on the seasonal and spatial variations of the planktonic biomarkers are needed to better constrain the use of these lipids as ice-free, open ocean biomarkers when using the PIP25 index in the western Arctic region.