Temporal variation in infaunal community structure of chemosynthetic and non-chemosynthetic environments in Barkley Hydrates, British Columbia, Canada

Increasing evidence points to greater temporal variation in deep-sea ecosystems than previously thought. In cold seeps, most available evidence focuses on successional stages of megafauna, with few studies on temporal variability in infaunal communities. We present a temporal study of infaunal communities and sedimentary organic matter characteristics from Barkley Hydrates outcrop, a chemosynthetic environment that receives organic matter from chemosynthetic and photosynthetic (phytodetritus) origins. In order to help interpret results from the outcrop, we contrast temporal variability in outcrop communities during two sampling periods with corresponding variation in an adjacent background sedimentary habitat (both habitats at ~900 m depth, offshore of British Columbia, Canada), that receives food input only from a photosynthetic source (phytodetritus). Comparisons with the background community help in differentiating chemosynthetic from photosynthetic food effects at the outcrop. Our results show contrasting temporal patterns for each community. First, lower (though not significantly) total density at the outcrop site in May 2014 corresponded to a significant change in species characteristic of that location (i.e., oligochaetes, dorvilleid polychaetes, and bivalves) in the previous sampling period (i.e., September 2013). Background communities did not change significantly in total density or species composition. Changes at the outcrop coincided with higher values of δ13C in sediment layers in May 2014, indicating reduced contributions of chemosynthetic organic matter to the total organic carbon pool relative to September 2013, and low bottom water methane concentrations. Second, although not significantly different, we detected more recruits in September 2013 for multiple species (Bipalponephtys cornuta, Cossura longocirrata, ampharetid polychaetes, Leitoscoloplos sp., and Levinsenia sp.) at the background site, following increased surface primary productivity in summer (June-August 2013). The presence of recruits of ampharetid polychaetes at both sites in September 2013 suggests that, although most individuals from the hydrate site depend on chemosynthetic food sources and therefore track temporal variation of organic matter chemosynthesised from methane, some outcrop site species also track phytodetritus pulses, much as observed in the background community. Our results demonstrate the utility of temporal studies in identifying the primary food source(s) fueling seafloor communities, and the dynamic nature of deep-sea infaunal communities from contrasting sites in Barkley Hydrates.