Long-term changes in abundance of Calanus finmarchicus south and north of Iceland in relation to environmental conditions and regional diversity in spring 1990–2013

Abstract

Abstract A 24-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 (North Atlantic Oscillation, NAO). In the sea area around Iceland, C. finmarchicus is the most abundant mesozooplankter. The long-term variations were evaluated using multivariate statistics, principal component analyses, redundancy analyses (RDA), and principal response curves. Both south and north of Iceland, significant (RDA, p < 0.05, Monte-Carlo permutation tests) fluctuations in abundance and stages structure of C. finmarchicus between years were observed. The variations were, however, not in tune in south and north, indicating that the C. finmarchicus abundance may be influenced by different forcing factors in the two regions. South of Iceland, the analyses failed to find an effect of any of the environmental variables tested, whereas in the north the year-to-year variability was related to freshwater thickness west of Iceland and temperature north of the island (RDA, p < 0.05, Monte-Carlo permutation tests), indicating that advection of animals with currents from the south is important. In neither of the two regions was it possible to detect a unidirectional temporal trend in abundance of C. finmarchicus (RDA, p < 0.05, Monte-Carlo permutation tests). Based on a comparison with time series from nearby areas and the lack of a relationship with NAO, 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.