Food quantity and quality in Barkley Canyon (NE Pacific) and its influence on macroinfaunal community structure

Abstract

The highly heterogeneous nature of submarine canyon physical landscapes can influence organic matter spatial distribution and thus benthic community and food web structure. We therefore studied patterns in quantity and quality (i.e., nutritional value for benthic organisms) of sediment and bottom-water particulate organic matter and their influence on macroinfaunal community structure in Barkley Canyon (NE Pacific) at multiple spatial scales (10 s to 100 s of meters). At large scales (100 s of meters), we hypothesised that canyon heterogeneity would drive organic matter patterns: topographic features (slope, aspect, curvature, rugosity, and benthic positioning index) and other environmental variables (depth, dissolved oxygen concentration, surface primary productivity, and sediment type). Our multivariate distLM analysis identified mean grain size, surface primary productivity and benthic positioning index (concave/convex seafloor topography) as the primary drivers of organic distribution in sediments. Noting different degradation rates among food variables, we inferred that the freshest organic matter reaches bottom waters of Barkley Canyon at 400 m, where ambient currents at the canyon head region likely concentrate primary productivity from surface waters. Evidence of deposition (increased amounts of fresh food) first appeared at 600 – 800 m, coincident with finer sediments, convex topography, and convergence of canyon branches. Degraded organic matter accumulated at 1500 and 2000 m where the comparatively fine sediments adsorb a greater proportion of available organic material than at shallower depths but limited delivery of surface primary productivity reduces overall food quality. Despite clear differences in food quantity and quality among sites (100 s of m apart), dissolved oxygen primarily drove macrofaunal distribution, along with hydrocarbons, indicative of a chemosynthetic ecosystem. At smaller spatial scales (10 s of meters) we found greater food patchiness associated with the topographically complex upper canyon (≤ 800 m). We also found distinct communities at smaller spatial scales (10 s of m apart) at 200 m, where fatty acid biomarkers distinguished a food patch rich in zooplankton. Overall, canyon heterogeneity rather than depth primarily determines patterns of organic matter (quantity and quality) in Barkley Canyon, with greater food patchiness at sites ≤ 800 m depth. Organic matter distribution appears to influence macroinfaunal community structure more strongly at smaller spatial scales in contrast to major stressors (i.e., oxygen) that act over larger scales.