Facultative feeding and consistency of trophic structure in marine soft-bottom macrobenthic communities

Abstract

We investigated the roles of facultative versus strict niche feeding in the maintenance of trophic consistency in soft-bottom macrobenthic communities in the Strait of Georgia, British Columbia, Canada. Changes in trophic structure across gradients in depth and percent fine sediments were examined over a broad regional scale by identifying trophic compartment(s) re sponsible for the resulting trophic changes. The use of proportional organic biomass data allows direct comparison of community trophic structure across diverse hydrographic regime(s), large ranges in overall biomass and productivity, and highly variable community composition. Cluster analyses revealed low overall dissimilarity in trophic structure across all substrate and depth ranges (24 and 28% divergence, respectively), suggesting an overall economy of trophic function. Similarity percentage (SIMPER) analyses revealed that low trophic dissimilarity is driven primarily by the remarkably even distribution of the 2 dominant facultative feeding groups in all habitat types. These facultative groups contained the most ubiquitous and abundant taxa found throughout the Strait, and likely confer strong resilience in these communities to habitat change. In contrast, the small but significant divergences in trophic structure over depth and percent fine sediment gradients was explained by the distributions of strict niche feeders: (1) macro-omnivores and herbivores dependent on non-detrital food sources were important in shallow areas (<25 m) with coarse sediments (<10% fine sediment), contributing to a unique trophic composition in these areas; and (2) subsurface deposit feeders were the only trophic group to vary significantly in proportional biomass explained by depth and percent fine sediment combined (22%; positive linear response to both factors).