Composition and functional diversity of macrofaunal assemblages on vertical walls of a deep northeast Pacific fjord

ABSTRACT: Fjords are temperate zone coastal features with strong horizontal and vertical environmental gradients, but the composition and function of biota living on the confining walls are poorly documented due to relative inaccessibility. We present results from remotely operated vehicle imagery of the subphotic (50 to 680 m depth) bedrock walls from 3 sites in Douglas Channel, a northeast Pacific fjord complex. We assessed the composition and abundance of the wall fauna and correlated these data with the water mass flux character of the fjord. Using a suite of morphological traits, we also identified areas of high function through habitat formation. This baseline record of hard substratum benthos in Douglas Channel revealed diverse assemblages marked by vertical zonation, dense animal cover (≥80% areal cover in some areas), and some variation from fjord head to mouth. The deepest portions of the fjord at our most seaward site (≥400 m) harbored the most taxonomically and functionally rich assemblages, with multiple species exclusive to this zone; there was a sharp increase in animal cover in shallow (≤150 m) areas caused by the appearance of dictyonine glass sponges and increases in articulate brachiopod, zoanthid, and encrusting sponge cover. Animal cover was positively correlated with winter kinetic energy density fluxes, indicating that a consistent oceanic influx augments biomass above 150 m most likely by increasing particle delivery rates. Our findings demonstrate that fjord walls support high biomass, high functioning, diverse, and expansive biosystems that warrant further study and consideration when developing coastal ocean management plans.