Variability in the fractionation of stable isotopes during degradation of two intertidal red algae

Abstract

Macroalgae contribute to intertidal food webs primarily as detritus, with unclear implications for food web studies using stable isotope analysis. We examined differences in the thallus parts of two South African rhodophytes ( Gelidium pristoides and Hypnea spicifera) and changes in overall δ 13C, δ 15N signatures and C:N ratios during degradation in both the field and laboratory. We hypothesized that both degrading macroalgal tissue and macroalgal-derived suspended particulate material (SPM) would show negligible changes in δ 13C, but enriched δ 15N signatures and lower C:N ratios relative to healthy plants. Only C:N laboratory ratios conformed to predictions, with both species of macroalgae showing decomposition related changes in δ 13C and significant depletions in δ 15N in both the field and laboratory. In the laboratory, algal tissue and SPM from each species behaved similarly (though some effects were non-significant) but with differing strengths. Gelidium pristoides δ 13C increased and C:N ratios decreased over time in tissue and SPM; δ 15N became depleted only in SPM. Hypnea spicifera, δ 13C, δ 15N and C:N ratios all decreased during degradation in both SPM and algae. Over 60 days in the field, δ 13C was depleted in both species (1–2‰) and in naturally senescent Gelidium pristoides fronds. δ 15N was depleted in Hypnea spicifera (approx. 1‰), while C:N ratios of both species were unaffected. The two species differed in δ 13C, δ 15N and C:N after degradation, but only in C:N beforehand. We suggest isotope changes in the laboratory mainly reflect microbial effects, while in the field these are combined with leaching due to constant water replenishment and agitation. Differences between these two species in the isotope responses to degradation highlight the difficulty of linking the signature of SPM to its multiple sources.