Detrital subsidy from subtidal kelp beds is altered by the invasive green alga Codium fragile ssp. fragile

Invasive species have the potential to alter the dynamics of detrital subsidy from high to low productivity areas through changes in quantity and nutritional quality of detrital material. We examined the effect of the invasive alga Codium fragile ssp. fragile on the nature of detrital export from subtidal algal beds off Nova Scotia, Canada, by comparing changes in mass, nutritional quality (%C, %N, C/N ratio), concentration of dimethylsulfoniopropionate (DMSP, a secondary metabolite that deters grazers), and isotopic composition δ¹³C and δ¹⁵N) between C. fragile and the native kelp Saccharina latissima at 4 wk intervals over 16 wk of degradation in cages on a sand bottom at 19 m depth. C. fragile degraded more slowly, and had a consistently lower C/N and higher DMSP concentration, than S. latissima. Isotope signatures did not differ between algal species: δ¹⁵N became slightly enriched (~1%o) after 16 wk of degradation, with no change in δ¹³C. We also compared macrofaunal communities associated with degrading thalli of each algal species and found significantly higher abundances of invertebrates (mainly capitellid poly-chaetes) on S. latissima after 8 wk of degradation, resulting in lower evenness JJ') and diversity (H') on S. latissima compared to C. fragile. Macrofaunal community composition became similar between algal species at 12 and 16 wk in concordance with decreases in C/N ratio in S. latissima and DMSP concentration in C. fragile. Our results indicate that differences in biochemical composition and the rate of degradation between C. fragile and native kelps result in community-level effects in areas linked to shallow algal beds via the transfer of detritus.