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)

Codium fragile subsp. fragile (hereafter C. fragile ) is a widespread non-native chlorophyte. Experimental and observational approaches were used to examine the influence of temperature on its growth and local distribution in southwestern Norway, where it has been established for over 80 years. Growth was measured in laboratory experiments at current and predicted seasonal minimum and maximum seawater temperatures, and under natural conditions by recording length over 1 year in the field. The results indicated that if temperatures increase, C. fragile growth rate may increase in spring, but not in mid-autumn/winter due to low light. Thallus fragmentation was common in autumn/winter in the field, but very rare in the winter-temperature laboratory experiment, indicating that low temperature is not the only driver. Growth occurred at temperatures which are reported as too low for growth in other locations, suggesting that C. fragile can acclimatise/adapt to local temperature regimes. Local distribution was examined by surveying abundance using categories based on the number of thalli and their arrangement (scattered/patch/zone) at 46 stations from 2011 to 2016. Codium fragile colonised the stations relatively often, but was frequently lost from sites with only bedrock compared to sites with stony substratum present (cobbles/boulders). It was more likely to be consistently present at sites with stones and could form dominant patches of canopy in the upper sublittoral on this substratum. An increase in the number of stations with C. fragile present was seen after two mild winters, suggesting that higher minimum temperatures may increase site occupancy of C. fragile in this region.