Interannual variability in abundance of Calanus finmarchicus south and north of Iceland in relation to environmental conditions in spring 1990-2012

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A 23-year time series of the abundance of Calanus finmarchicus from two standard sections differing in hydrographical conditions (Atlantic Water south of Iceland, Subarctic Water north of Iceland) was examined in relation to hydrography, phytoplankton dynamics and large scale climatic forcing (NAO). In the sea area around Iceland, C. finmarchicus is the most abundant mesozooplankter. Large fluctuations in abundance and stages structure of C. finmarchicus between years were observed. The variations are not in tune in south and north, indicating that they are influenced by different forcing factors. In the south the year to year variability is mainly dictated bottom-up by phytoplankton growth, whereas in the north advection of animals from the south appears more important. In neither region, a unidirectional temporal trend in abundance of C. finmarchicus was detected. Based on a comparison with time series from nearby areas, it is hypothesized that the growth and development of C. finmarchicus in Icelandic waters is more affected by local conditions than large scale climatic forcing.