Epibenthic community assessments indicate high spatial and temporal variability among continental shelf hard bottom sites in a marine transition zone

•Onslow Bay, NC epibenthic communities varied significantly both by site and year.•Depth was the most important factor in structuring Onslow Bay epibenthic communities.•Macroalgae dominate the community at shallower sites.•Macroalgae and sessile invertebrates share dominance at deeper sites.•Depth in Onslow Bay reflects varying winter bottom water temperatures and light.

We studied 19 hard bottom sites within Onslow Bay, North Carolina during 2007–2010 using photoquadrat analyses to investigate the role of temperature, depth, and fish community variables in the structure of the epibenthic (macroalgae and sessile invertebrates) community. Although significant variation in community structure was found both by site and year, depth was the most important factor in structuring these epibenthic communities with significant differences found among five depth categories: 18–20 m; 24–29.2 m; 31 m; 32.5–37 m; 38.5–42 m. The largest community difference was found between depths ⩽⩽31 m and ⩾⩾32.5 m, resulting in a shift from macroalgae dominance at the shallower sites to shared macroalgae and sessile invertebrate dominance at deeper sites. Depth is a complicated variable as it relates to structuring the epibenthic community in this region because it reflects varying winter bottom water temperatures, light levels, and periodicity of nutrient influxes. The location of North Carolina marine habitats at the transition from cold-temperate to warm-temperate/tropical zones, and the spatial compression of this transition zone along inshore to offshore transects make this an ideal area for tracking climate change related shifts in marine communities. However, a better understanding of the relationship among variables such as depth, light, temperature and nutrients and the epibenthic community, as well as seasonal and short annual community variation, is needed before climate related shifts can be determined.