Benthic primary production and respiration of shallow rocky habitats: a case study from South Bay (Doumer Island, Western Antarctic Peninsula).

Rocky benthic communities are common in Antarctic coastal habitats; yet little is known about their carbon turnover rates. Here, we performed a broad survey of shallow ( < 65 m depth) rocky ice-scoured habitats of South Bay (Doumer Island, Western Antarctic Peninsula), combining (i) biodiversity assessments from benthic imaging, and (ii) in situ benthic dissolved oxygen (O2) exchange rates quantified by the aquatic eddy covariance technique. The 18 study sites revealed a gradual transition from macroalgae and coralline-dominated communities at ice-impacted depths (15-25 m; zone I) to large suspension feeders (e.g., sponges, bivalves) at depth zone II (25-40 m) and extensive suspension feeders at the deepest study location (zone III; 40-65 m). Gross primary production (GPP) in zone I was up to 70 mmol O2 m-2 d-1 and dark ecosystem respiration (ER) ranged from 15 to 90 mmol m-2 d-1. Zone II exhibited reduced GPP (average 1.1 mmol m-2 d-1) and ER rates from 6 to 36 mmol m-2 d-1, whereas aphotic zone III exhibited ER between 1 and 6 mmol m-2 d-1. Benthic ER exceeded GPP at all study sites, with daily net ecosystem metabolism (NEM) ranging from - 22 mmol m-2 d-1 at the shallow sites to - 4 mmol m-2 d-1 at 60 m. Similar NEM dynamics have been observed for hard-substrate Arctic habitats at comparable depths. Despite relatively high GPP during summer, coastal rocky habitats appear net heterotrophic. This is likely due to active drawdown of organic material by suspension-feeding communities that are key for biogeochemical and ecological functioning of high-latitude coastal ecosystems.