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 (O-2) 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 O-2 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.