Abrupt interruptions in the spring bloom on the Faroe Shelf

Since 1997, phytoplankton abundance (Chlorophyll a) has been monitored weekly during spring and summer at a coastal station, „Skopun”, in the central parts of the Faroe Shelf. These measurements have demonstrated a large inter-annual variability both in the timing, and the peak magnitude of the spring bloom. In the most productive years, the bloom starts several weeks earlier than in the bad years and may reach high peak values. In some years, however, a rapidly evolving spring bloom may be interrupted by one or more periods of decreasing phytoplankton abundance, until the bloom recovers. This was the case both in 2008 and 2009. Concurrent temperature measurements at various locations indicate that this may have been due to changes in the circulation pattern on the Shelf. Here, we suggest that the shallow parts of the northern shelf area, including Skopun, in some periods may become relatively isolated from the deeper parts of the shelf. Since this region is shallow, a rapid and intensive spring bloom may develop as long as the isolation is maintained. In some years, such periods of relative isolation seem to alternate with periods of intensive mixing between the shallow and deep parts, in which the phytoplankton in the shallow part is flushed out to the whole shelf. We present a simple box model of the system and show that this can explain the main features of the 2008 and 2009 spring blooms.