The zooplankton community structure in relation to its biological and physical environment on the Faroe shelf, 1989-1997

Abstract

The Faroe shelf water is separated from the offshore water by a persistent tidal front, which surrounds the islands. This shelf water contains a neritic zooplankton community, which, regarding species composition, production, seasonal development and environmental conditions, is quite different from that in the surrounding ocean. While during spring and summer the zooplankton in the oceanic environment are dominated by the copepod Calanus finmarchicus, the zooplankton in the shelf water are largely dominated by neritic copepods, mainly Acartia longiremis and Temora longi- cornis. Calanus finmarchicus occurs in interannually highly variable abundance in the Faroe shelf ecosystem. Meroplanktonic larvae, mainly Balanus spp. and decapod larvae, are also common in the shelf water during spring and summer. During the period presented (1989-1997), the Faroe shelf ecosystem has undergone very large changes in abundance of different zooplankton species. The midsummer abundance of C.finmarchicus, which originally is advected into the shelf from the open ocean, fluctuated from ~400 copepods m-3 in 1989 to ~25 copepods m -3 in 1994, and at the same time the neritic zooplankton increased from ~120 m-3 in 1989 to 450 m-3 in 1994. Consequently, the midsummer biomass in the shelf fluctuated by a factor of 10 during the same period. It is presumed that this variability between oceanic- and neritic-dominated zooplankton, their sizes and their biomass has greatly affected the entire pelagic ecosystem.