Links between phytoplankton dynamics and shell growth of Arctica islandica on the Faroe Shelf

The phytoplankton dynamics on the Faroe Shelf are strongly connected to higher trophic levels, and their inter-annual variability has great importance for many organisms, including the principal fish stocks. Hence, information on the marked phytoplankton variability is scientifically and economically valuable. We show here that the shell growth variability in Arctica islandica shells has the potential to identify periods of increased and decreased phytoplankton concentrations on the Faroe Shelf and in the wider Faroese region in previous centuries. The growth of A. islandica has often been linked to changes in phytoplankton concentrations, i.e., food availability. By cross-matching life-collected and sub-fossil A. islandica shells from two separate locations on the Faroe Shelf, we have built a master chronology, which reaches back to the 17th century. This master chronology correlates well with a Primary Production index for the Faroe Shelf (r=0.65; p<0.01) and average April–June chlorophyll a concentrations in the central part of the shelf (r=0.74; p<0.01). A link is also identified between the shell growth and phytoplankton concentrations over the wider Faroese Channel Region, as represented in the Continuous Plankton Recorder surveys, especially for the months June–September (r=0.39; p<0.01). In addition, an inverse relationship is observed between the master chronology and on-shelf water temperatures from June–September (r=−0.29; p<0.01), which is likely associated with a previously reported inverse relationship between temperatures and the on-shelf primary production. An analysis of the δ18O in the shells shows that the main growing season of the shells presumably occurs during the spring and summer months, which concurs with the main spring bloom.