Taxon-specific differences in C and N cycling and metabolic activity of intertidal organisms: Part B – Long-term processes

Tidal flats cover less than 1% of the surface of our planet, but they provide important ecosystem functions and services, like carbon sequestration and nutrient (re-)mineralization, primary, secondary, and food production, and habitats for a variety of species. Despite their importance, they are threatened by environmental stressors and anthropogenic impacts. Hence, a better understanding of the interaction between macrozoobenthos, their metabolic activity, and their contribution to carbon and nitrogen cycling is important. Therefore, we conducted in-situ pulse-chase tracer experiments at a Pacific oyster ( Magallana gigas ) reef in the Eastern Scheldt (Southwest Dutch Delta, Netherlands) in summer and autumn 2020. During these experiments, we provided the submerged benthic community with ¹³ C and ¹⁵ N-enriched bacterioplankton and incubated it in seawater enriched in 1% deuterium oxide ( ² H ₂ O) for one tidal cycle (∼9 h), before the benthic community was kept at in situ conditions for another 14 d. Our aim was (1) to assess seasonal variations in ¹³ C and ¹⁵ N processing by macrozoobenthos, (2) to evaluate potential life-stage dependent differences in ² H uptake – a proxy for metabolic activity, and (3) to identify the macrozoobenthos taxon which had the highest metabolic turnover rate ( MTR ). In summer, most bacterioplankton-derived ¹³ C- and ¹⁵ N was taken up by sponges ( Halichondria panicea ), limpets ( Crepidula fornicata ), and clams ( Ruditapes philippinarum ), whereas in autumn, H. panicea, C. fornicata and the sponge Hymeniacidon perlevis dominated ¹³ C and ¹⁵ N incorporation. Statistically significant differences in ² H uptake between life stages (juvenile/ small vs. adult) was detected in Malacostraca (i.e., Rhithropanopeus harrisii ), but not in Polychaeta or Bivalvia. Therefore, this difference between taxonomic classes might be linked to mold-intermolt periods. Seasonal differences in MTR of H. panicea and M. gigas (i.e., the species with the highest MTR s in summer and autumn) might be linked to seasonal environmental variability, such as temperature and food availability.