Population genetics of drifting (Calanus spp.) and resident (Acartia clausi) plankton in Norwegian fjords

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, we examined the population genetic consequences of these two lifestyles for copepods in four fjords of western Norway (Lurefjorden, Masfjorden, Sognefjorden and Sorfjorden) and one fjord in eastern Norway (Oslofjorden). Based on DNA sequence variation of a region of mitochondrial 16S rRNA, we contrasted population genetic diversity and structure in drifting populations of Calanus spp. with that of resident populations of Acartia clausi. 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 significant 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. Populations of A.clausi showed significant genetic 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 long-term 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.