Abstract
Kaartvedt distinguished between drifting and resident plankton and hypothesized that the latter were distinguished by their ability to maintain their horizontal position in desired habitats (Kaartvedt, 1993). In this study, the population genetic consequences of these two life-styles for copepods in four fjords of westrn Norway (Lurefjorden, Masfjorden, Sognefjorden and Sorfjorden) and one fjord in eastern Norway (Oslofjorden) is examined. Based on DNA sequence variation of a region of mitochondrial 16S rRNA, population genetic diversity and structure in drifting populations of Calanus spp. with that of resident populations of Acartia clausi is contrasted. With the exception of Sorfjorden (where Calanus spp. were rare), two or three species of Calanus co-occurred in significantly different proportions in the fjords. Based on a 350 base-pair region of mitochondrial 16S rRNA, Calanus spp. varied in molecular genetic diversity, with the highest values for C. helgolandicus. There was no evidence of siginficant genetic structure of fjord populations for either C. finmarchicus or C. helgolandicus; the population structure of C. glacialis could not be evaluated as the species was only abundant in Lurefjorden. Acartia clausi was abundant in all five fjords sampled for this study. Molecular genetic diversity of A. clausi, based on a 220 bp region of mt 16S rRNA, was within the range of Calanus spp. values. Population of A. clausi showed significant structure (i.e. haplotype frequencies differed markedly) among the fjords. The results of this study indicated that little exchange (gene flow) occurs between populations of A. clausi in different fjords, and suggested that the populations are longterm residents of a fjord. In contrast, most Calanus spp. fjord populations may be replaced periodically, as they drift with currents flowing to and from coastal and fjord environments