Competitive interactions between the invasive green alga Codium fragile ssp. tomentosoides and native canopy-forming seaweeds in Nova Scotia (Canada)

Abstract

Two concurrent 2 yr experiments were con- ducted along the Atlantic coast of Nova Scotia (1) to exam- ine competitive interactions between the invasive green alga Codium fragile ssp. tomentosoides and canopy-form- ing native seaweeds, particularly the dominant kelps (Laminaria longicruris and L. digitata) and opportunistic algae (Desmarestia viridis and D. aculeata), and (2) to identify conditions enabling the establishment and persis- tence of C. fragile. In the first experiment, Laminaria spp. and/or Desmarestia spp. were either removed or left intact (in an orthogonal factorial design) to examine their effect on C. fragile. At the end of the experiment, cover, density, and biomass of C. fragile did not differ significantly among treatments, although mean density was up to 2 times greater in plots where kelp was removed than in control plots. While removal of kelp positively affected growth of C. fragile (up to 12 cm mo -1 ) during the first year, growth was similar among treatments in the second year, when kelps were smaller and Desmarestia spp. were virtually absent (following decimation of native seaweeds by the invasive bryozoan Membranipora membranacea). Survival of marked individuals of C. fragile was high in all treatments in the first year (85 to 100%), but dropped dur- ing winter in the second year (40 to 60%). In the second experiment, the effect of stands of C. fragile on re- colonization by Laminaria spp. and Desmarestia spp. was examined by removing C. fragile in both Pulse and Press removal treatments. At the end of the experiment, density of Laminaria spp. in the Press treatment was more than 4 times higher than in the Pulse treatment. Dense stands of C. fragile in control plots inhibited recruitment of kelps and other seaweeds, and survival of marked thalli of C. fragile over 9 mo was high (90%). We conclude C. fragile exhibits competitive advantages over native seaweeds through opportunistic exploitation of disturbance- generated gaps within kelp beds. Once established as dense meadows, C. fragile prevents re-colonization by kelp and persists as the dominant canopy-forming sea- weed for prolonged periods.