Nekton Community Responses to Seagrass Differ with Shoreline Slope

Abstract

Seagrass beds vary in patch size and shoot density, which can influence the distribution and abundance of nekton responding to habitat structure. In Washington State, USA, eelgrass (Zostera marina) occurs under two distinct shoreline slope conditions: shallow tidal flats where eelgrass forms extensive meadows, and steep slopes near channels where it forms narrow, often patchy, fringes adjacent to unvegetated sediment. We sampled nekton in these naturally occurring habitat mosaics with a crossed design: unvegetated, edge, and interior eelgrass (habitat) in flats and fringes. Multivariate community structure showed additive effects of habitat and slope, while aggregate body size did not vary across habitat, shoot density, or slopes. Total nekton abundance responded to structure on fringes (interior > unvegetated; edge = unvegetated; and interior > edge) but not on flats, while half of the most common taxa showed a significant habitat-by-slope interaction in abundance. Diversity and species richness were greater in fringes than flats, irrespective of habitat type. Since canopy height and shoot density did not differ significantly between fringes and flats, it is unlikely that fine-scale aspects of vegetation structure explain why fauna responded differently to eelgrass fringes and flats. We instead attribute these different responses to underlying differences between fringes and flats in within-habitat heterogeneity and connectivity to deeper habitats, as well as species-specific responses to greater edge/interior ratios in fringing beds. Our study shows that topographic and seascape heterogeneity as well as habitat connectivity may play key roles in the value of nearshore estuarine habitats for nekton in the Northeast Pacific.