Benthic respiration in a seep habitat dominated by dense beds of ampharetid polychaetes at the Hikurangi Margin (New Zealand)

Many biological seep studies focused on the distribution, structure, nutrition and food web architecture of seep communities as well as on their interaction with the seep geochemistry. However, overall respiration at cold seeps received only little attention. We conducted in-situ oxygen flux measurements in combination with ex-situ oxygen micro-profiles, respiration measurements, as well as rate determinations of microbial methane and sulfate turnover to assess respiration pathways as well as carbon turnover at a seep habitat that was recently discovered alongside the Hikurangi Margin offshore northern New Zealand. This habitat is dominated by dense beds of tube-building, heterotrophic ampharetid polychaetes. Average total oxygen uptake (TOU) from this habitat was very high (83.7?mmol m-?2 day-?1). TOU at a non-seep reference site ranged between 2.7 and 5.8?mmol m-?2 day-?1. About 37% (30.8?mmol m-?2 day-?1) of the average TOU was consumed by ampharetids. Considering mean diffusive oxygen uptake (8.5?mmol m-?2 day-?1) the remaining fraction of ~?53% of the TOU (44.4?mmol m-?2 day-?1) might be explained by respiration of epibenthic organisms as well as aerobic methane and sulfide oxidation at the sediment-water interface. The strongly negative carbon isotopic signatures (-?52.9?±?5[per mille sign] VPDB) of the ampharetid tissues indicate a methane derived diet. However, carbon production via anaerobic oxidation of methane (AOM) was too low (0.1?mmol C m-?2 day-?1) to cover the mean carbon demand of the ampharetid communities (21?mmol C m-?2 day-?1). Likely, organic carbon generated via aerobic methane oxidation represents their major carbon source. This is in contrast to other seep habitats, where energy bound in methane is partly transferred to sulfide via AOM and finally consumed by sulfide-oxidizing chemoautotrophs providing carbon that subsequently enters the benthic food web.