Distribution of pelagic phytoplankton-derived lipid biomarkers along a transect from the East Sea to the Bering Sea: insights into their suitability as open-water indicators

Abstract

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. We then analyzed the samples for the Arctic sea ice proxy IP25 together with various phytoplankton-derived lipids including a tri-unsaturated highly branched isoprenoid (HBI triene) and two sterols (brassicasterol and dinosterol) to assess the suitability of these compounds for the so called PIP25 index in the western Arctic region as a proxy for sea ice change in the past. The distributions of some other commonly reported sterols (cholesterol and 24-methylene cholesterol) were also investigated. IP25 could not be detected in any of the samples analyzed, consistent with the nature of the sampling location and season, while the HBI triene was only detected at five sampling sites in the Northwest Pacific and the Bering Sea. In contrast, each of the sterols were detected at each sampling site. 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 with brassicasterol, indicating likely different and diverse sources of these lipids in addition to marine phytoplankton. Our study suggests that the use of brassicasterol, dinosterol, or HBI triene, as strict phytoplankton markers for use with the PIP25 index, might result in misleading outcomes. Hence, it is clear that more work is 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.