Shallow Arctic sediments contain large infaunal blue carbon stocks
Total abundance, total biomass, and average individual biomass showed gradual changes between stations and accordingly no distinct station groups emerged from the significance of pairwise comparisons (Fig. 1). Nonetheless, strong patterns were observed within the data. Total biomass was more than one order of magnitude higher for the shallow Arctic location (IsfS) than for all other stations. Furthermore, total biomass was much higher at shallow locations in Arctic and temperate fjords than at deep locations in the same fjords, a pattern that was not observed for sub-Arctic fjords. Across all fjords, the average individual biomass was higher in the shallow habitats. The magnitude of this difference between the depth zones differed strongly between the fjords and was particularly high in the Arctic and the more northern sub-Arctic fjord (Fig. 1). The average individual biomass was highest in the Arctic fjord and particularly high for the shallow location. These biomass and individual biomass patterns indicate the shallow Arctic community as the community with outstanding high faunal blue carbon storage (high biomass) that has been stored for a long time (large sized individuals that are most likely old).
Long-lived and large suspension feeders are important blue carbon stores
Modalities for three biological traits related to resource utilization (feeding mode) and carbon storage potential (maximum size, life span) were assigned to 72 mollusk taxa (see Supplementary Information). Two RLQ analyses13 were conducted with two different community data sets (abundance; biomass) to explore how functional differences in the communities are related to the different habitat characteristic (climate and depth zone).
The biomass based RLQ analysis, linked high contribution of long-lived (> 10y), large (> 100mm), and suspension feeding taxa to shallow (< 150 m) and Arctic habitats (habitat and trait scores in Fig. 2). This result is in line with above identified blue carbon potential of this habitat and the latitudinal and depth patterns in suspension feeding10. Therefore, my results demonstrate that the potential loss of the long-lived suspension feeders from shallow Arctic communities (Fig. 2) will likely reduce the infaunal biomass by up to one order of magnitude (Fig. 1). My results appear to contrast with an earlier study11 that concluded that the infaunal size structure may be resilient to major climatic changes, based on a high consistency in biomass size spectra in different fjords along the same latitudinal gradient. All samples in Mazurkiewicz et al.11, however, were collected deeper than 150 m and even below 200 m for Svalbard fjords. Accordingly, the patterns in communities shallower than 150 m could not have been discovered by them and I conclude that the latitudinal consistency in biomass size spectra11 must be limited to deep habitats, where the majority of the biomass is associated with deposit feeders and predators (Fig. 2)10.
Faster biomass turnover and lower blue carbon stocks in the future
Furthermore, RLQ analyses of abundance and biomass associated intermediate life spans (3-10y) with sub-Arctic fjords and short life spans (1-3y) with temperate fjords (Fig. 2). This suggests that biomass and blue carbon turnover will accelerate with the progressing climate change. Accordingly, the potential of benthic infauna as blue carbon solution in fjords appears to be limited to newly emerging habitats due to sea-ice loss. For all other habitats that already contain an infaunal community, my results suggest that under a future warmer climate the blue carbon storage in infauna will be reduced and accordingly that a positive climate feedback loop exists.