Correlative evidence for competition between Fucus serratus and the introduced chlorophyte Codium fragile subsp. fragile on the southwest coast of Norway

The distribution of Codium fragile subsp. fragile and the native canopy-forming alga Fucus serratus was recorded at 51 sites in a 20 km long, sheltered region on the southwest coast of Norway. The purpose of the study was to examine if these species are potentially competing and how their distributions are related to wave-exposure and substrate. Codium fragile subsp. fragile was patchily distributed, a pattern which appears to have been sustained over time since its introduction to this area. It was almost always observed growing below mean low water, in the low intertidal and shallow subtidal. Both substrate type and wave-exposure influenced the vertical distribution of C. fragile subsp. fragile; growth occurred higher on the shore at sheltered sites and deeper in the subtidal on stony substrate. Its vertical range of growth overlapped with that of F. serratus and, when C. fragile subsp. fragile was abundant, F. serratus tended to grow higher on the shore and at lower abundances. This suggests that C. fragile subsp. fragile is affecting F. serratus in this area through competition, but only in the lower portion of the fucoid's vertical range and only at sites favorable for its own growth with regard to shelter and substrate.