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

Abstract

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.

Can an old alien benefit from rising ocean temperatures? An experimental and field study on the growth and local distribution of Codium fragile subsp. fragile (Chlorophyta)

“…If C. fragile was present but not forming a zone, and there were also fewer than 50 individuals present, it was recorded as 'low' abundance. This is the same system as used in Armitage et al (2014) when the original survey (2011) was carried out, and was done because the estimate of number of thalli could be susceptible to errors. By combining the growth pattern and abundance, a coarser but more reliable picture of C. fragile at the site is formed.…”

“…It has been present in Norway since at least 1932 (Armitage and Sjøtun 2016a) and is categorised as a high impact species according to the Norwegian blacklist (Gederaas et al 2012). Although patchily distributed, it can form the dominant sublittoral canopy at some sites, and is mainly restricted to below mean low water in this region (Armitage et al 2014). Codium fragile subsp.…”

Codium fragile in Norway: subspecies identity and morphology

“…fragile has been classified as a high-impact nonnative species due to its widespread distribution, long expected population lifetime and moderate ecological impact (Gederaas et al 2012). In some regions, it can become locally abundant, growing in patches in the upper subtidal and infralittoral zones of sheltered and moderately wave-exposed locations, especially with boulder/ cobble substratum (Armitage et al 2014).…”

Epiphytic macroalgae mediate the impact of a non-native alga on associated fauna

“…Along the south-west coast (northern North Sea), it can form locally dominant patches of canopy vegetation, typically at sheltered and semi-exposed rocky sites (Jorde, 1966;Armitage et al, 2014). These patches can extend into the low-intertidal zone usually dominated by the native canopy-forming brown alga Fucus serratus L. (Fig.…”

“…These patches can extend into the low-intertidal zone usually dominated by the native canopy-forming brown alga Fucus serratus L. (Fig. 1), where they may compete for space (Armitage et al, 2014). An increase in abundance of C. fragile at the expense of F serratus could have an impact on infralittoral seaweed-associated communities.…”