Estimating stable carbon isotope values of microphytobenthos in the Arctic for application to food web studies

Most studies on Arctic food webs have neglected microphytobenthos as a potential food source because we currently lack robust measurements of delta C-13 values for microphytobenthos from this environment. As a result, the role of microphytobenthos in high latitude marine food webs is not well understood. We combined field measurements of the concentration of aqueous carbon dioxide and the stable carbon isotopic composition of dissolved inorganic carbon (delta C-13(DIC)) from bottom water in the Beaufort and Chukchi seas with a set of stable carbon isotopic fractionation factors reflecting differences in algal taxonomy and physiology to estimate the stable carbon isotope composition of microphytobenthos-derived total organic carbon (delta C-13(p)). The delta C-13(p) for Phaeodactylum tricornutum, a pennate diatom likely to be a dominant microphytobenthos taxon, was estimated to be -23.9 +/- A 0.4 aEuro degrees as compared to a centric diatom (Porosira glacialis, delta C-13(p) = -20.0 +/- A 1.6 aEuro degrees) and a marine haptophyte (Emiliana huxleyi, delta C-13(p) = -22.7 +/- A 0.5 aEuro degrees) at a growth rate (A mu) of 0.1 divisions per day (d(-1)). delta C-13(p) values increased by similar to 2.5 aEuro degrees when A mu increased from 0.1 to a maximum growth rate of 1.4 d(-1). We compared our estimates of delta C-13(p) values for microphytobenthos with published measurements for other carbon sources in the Arctic and sub-Arctic. We found that microphytobenthos values overlapped with pelagic sources, yet differed from riverine and ice-derived carbon sources. These model results provide valuable insight into the range of possible isotopic values for microphytobenthos from this region, but we remain cautious in regard to the conclusiveness of these findings given the paucity of field measurements currently available for model validation.