A stable isotope approach to assess the ecological role and fate of macroalgal decomposition in the Antarctic benthos

Around Antarctica, disturbance from iceberg scouring and natural macroalgal senescence result in the accumulation of macroalgal detrital material. This detritus in turn supports a significant abundance of detritivorous invertebrates and provides material for carbon burial and sequestration. We tested the hypotheses that decomposing macroalgal detritus makes an increasing contribution to both macroinvertebrate diet, and sub-surface sediment organic matter. We analysed data from two independent in situ decomposition experiments conducted at Rothera Research Station in two successive years. Mesh cages, containing macroalgal detritus isotopically enriched in 13C and 15N were deployed at an Antarctic soft sediment site. Numerical abundances of macroinvertebrate species as well as the isotopic contribution of macroalgae to macroinvertebrates and sediments were estimated across four-time intervals for each experiment. Thirty-six (2021) and eighty-nine (2022) macroinvertebrate species were associated with experimental cages of macroalgal detritus. Multivariate and univariate analyses revealed temporal shifts in the macroinvertebrate assemblages within each study year, switching from amphipod to mollusc species-dominated assemblages throughout the experiment. The 15N enriched macroalgal detritus were incorporated within benthic sediment and macroinvertebrate tissue. Only the diet of arthropods was influenced by temporal macroalgal decomposition. Isotope measurements of local macroalgal accumulation in sediments showed contrasting trends in both experiments, either declining or accumulating over time with the more less chemically defended macroalgae contributing most to local sediment enrichment. Assessing the fate of macroalgal carbon is important in the polar regions where macroalgal biomass and its detritus are increasing in coastal habitats due to climate change opening new ice-free suitable habitat.