SHIFTS IN THE δ13C OF SERRIPES GROENLANDICUS SHELL ORGANIC MATTER: A PROXY FOR INTRA- AND INTERANNUAL CHANGES IN PRIMARY PRODUCTION IN THE ARCTIC?

Primary production in the Arctic Ocean is partitioned primarily between ice algae and phytoplankton. Ice algae live both within the ice column and attached to the bottom of sea ice and bloom during spring, while phytoplankton live in the water column and typically bloom after the ice melts in early summer. These primary producers have different carbon isotope compositions, which are passed on, with some modification, to the tissues of primary consumers. The circumpolar Arctic bivalve, Serripes groenlandicus (hereafter Serripes), has pronounced seasonal and annual growth bands with dark and light layers representing winter and summer growth seasons, respectively. We evaluated the potential for using the inter- and intra-annual variability of the carbon isotope composition of Serripes shell organic matter as a means for determining changes in primary production.

Samples of live Serripes were collected from Kongsfjord on the west coast of Svalbard during July of 2004 and June of 2006. Winter and summer shell material from duplicate animals were sampled for the years 1996, 1997, 2000, and 2001 and entire growth bands were sampled from multiple organisms for each year between 1991 and 2002, except 1999, for carbon isotope composition of the shell organic fraction. Isotopic values of amino acids in bivalves can vary by as much as 28‰; thus, the amino acid composition of these shell layers was also determined.

Preliminary results suggest that there were statistically significant seasonal isotopic differences in 1997, 2000, and 2001. Shell organic matter from the winter growth layers was enriched in ¹³C relative to the summer layers. These isotopic shifts can not be attributed to seasonal differences in amino acid composition alone, and may reflect seasonal changes in the available food supply. The Serripes may have consumed more ice algae during the winter months, and more phytoplankton during the summer months. Additional isotopic and amino acid analyses are needed to confirm this interpretation. Between 1991 and 2000, the δ¹³C values of the annual growth bands were relatively consistent (avg δ¹³C = -18.6 ± 0.2‰). The years 2001 and 2002 were enriched in ¹³C by 1‰ relative to the rest of the time series, indicating that an environmental shift occurred in Kongsfjord after 2000, the nature of which is currently under investigation.