Spatio-temporal variability in the deposition of beach-cast kelp (wrack) and inter-specific differences in degradation rates

Coastal habitats dominated by marine macroalgae typically exhibit high rates of primary productivity and play a key role in local and regional carbon cycles and stores. In temperate regions, large brown algae (i.e. kelps and fucoids) contribute significantly to macroalgal primary production, most of which is exported from source habitats as detritus. The ultimate fate of this detritus and the processes controlling detrital pathways into food webs and carbon cycles remain poorly understood. Based on field surveys, we quantified the biomass of kelp-derived detritus (wrack) at sandy and pebble-dominated shores in Ireland and conducted a manipulative field experiment to test for inter-specific differences in detritus degradation rates and the effect of macroinvertebrate detritivores. Overall, accumulated wrack biomass was similar on all shores but varied temporally depending on habitat type. Degradation rates and the nutritional (C:N) and chemical (polyphenol concentrations) properties differed among kelp species. Interestingly, exclusion of macroinvertebrate detritivores did not affect kelp degradation rates, C:N ratios or polyphenol content. Our findings show that rates of macroalgal breakdown differ among kelp species and that, in contrast to other aquatic systems, macroinvertebrates appear to play a very limited role in the breakdown of these marine detrital subsidies, suggesting a key role for meiofauna and microbes in detritus processing. Increasing recognition for the role of detritus in coastal food webs and carbon cycles warrants a better understanding of the mechanisms underpinning degradation rates.