Processes Determining the Spatial Distribution of Optically-Imaged Particles in the Gulf of Alaska

The Gulf of Alaska is a seasonally productive ecosystem surrounded by glaciated coastal mountains with high precipitation. We measured the concentrations and size distributions of large marine particles (0.06-27 mm) using the Underwater Vision Profiler (UVP) during 4 cruises in 2014 and 2015. We analyzed the spatial distribution of particles of different size classes to determine the probable drivers. Total concentration of particles increased with proximity to glacial and fluvial inputs. We found a major contrast between shelf and offshore particle concentrations. Over the shelf, concentrations on the order of 1000-10,000/L were 1-2 orders of magnitude greater than offshore concentrations on the order of 100/L. Driving processes over the shelf included terrigenous inputs from land, resuspension of bottom sediments, and advective transport of those inputs along the shelf break. Offshore, biological processes were drivers of variability in particle concentration and size with depth. The dominance of inorganic material in shelf processes will inform the location of future studies of the biological pump in the coastal Gulf of Alaska, and could have implications for increased aggregate flux via lithogenic ballast.