Long-term changes of euphausiids in shelf and oceanic habitats southwest, south and southeast of Iceland

Generalized additive models (GAMs) were used to test the hypothesis that changes in physical and biological environmental conditions affected by current climatic warming would negatively impact the euphausiid populations in the North Atlantic. Two zooplankton time series were used, one collected by the Marine Research Institute (MRI) on a transect south of Iceland during spring (1990-2011) and the other by the Continuous Plankton Recorder (CPR) survey (1958-2007) in the oceanic waters south of Iceland covering all months. Due to limitations of the sampling gears used, the results mainly reflect the variations of the early stages of euphausiids. On a spatial scale, results reveal a general decline of euphausiid abundance from the east coast of Greenland to the Faroe Islands. On a temporal scale, euphausiid numbers decreased in most CPR areas from 1958 to 2007. Conversely, an increase was observed in numbers of larvae during spring 1990-2011 for the shelf south of Iceland. Single variable-based GAMs indicated that phytoplankton biomass was generally the main environmental factor regulating euphausiid abundance. Multiple variable-based GAMs showed that phytoplankton biomass was the strongest predictor of euphausiid abundance in the west, whereas in the east temperature appears to be most important. In addition, the onset of the spring bloom also affected the long-term changes in euphausiid abundance. For the oceanic areas, it is concluded that a weakened temporal synchrony between the development of young euphausiids and the phytoplankton bloom influenced by recent climate warming may have led to the observed decrease in euphausiid populations.