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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) responsible 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).KEY WORDS: Deposit feeding · Suspension feeding · Strait of Georgia · Salish Sea · British Columbia · Facultative feeding · Trophic structure Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 445: [129][130][131][132][133][134][135][136][137][138][139][140] 2012 2001). Most studies of macrobenthic trophic structure tend to classify benthic fauna in terms of strictly defined trophic groups only (e.g. Chardy & Clavier 1988, Gaston & Nasci 1988, Ricciardi & Bourget 1999, García-Arberas & Rallo 2002, Coyle et al. 2007, Albano & Obenat 2009, Cacabelos et al. 2009, Dolbeth et al. 2009). Most commonly these trophic groups are: suspension feeders, deposit feeders (surface and subsurface), carnivores, herbivores and omnivores. However, many macrobenthic taxa exhibit more than one feeding mode (e.g. Okamura 1990, Brown et al. 2000, Gaudencio & Cabral 2007, Gray & Elliott 2009). This trophic flexibility has been proposed as an important feature of benthic marine communities (Cadée 1984). For instance, many species of facultatively carnivorous polychaetes are also known to deposit feed (e.g. Glyceridae, members of Lumbrineridae; Fauchald & Jumars 1979). Other taxa ...

Detailed knowledge of environmental conditions is required to understand faunal production in coastal seas with topographic and hydrographic complexity. We test the hypothesis that organic biomass and production of subtidal sediment invertebrates throughout the Strait of Georgia, west coast of Canada, can be predicted by depth, substrate type and organic flux modified to reflect lability and age of material. A basin-wide database of biological, geochemical and flux data was analysed using an empirical production/biomass (P/B) model to test this hypothesis. This analysis is unique in the spatial extent and detail of P/B and concurrent environmental measurements over a temperate coastal region. Modified organic flux was the most important predictor of organic biomass and production. Depth and substrate type were secondary modifiers. Between 69–74% of variability in biomass and production could be explained by the combined environmental factors. Organisms <1 mm were important contributors to biomass and production primarily in shallow, sandy sediments, where high P/B values were found despite low organic flux. Low biomass, production, and P/B values were found in the deep, northern basin and mainland fjords, which had silty sediments, low organic flux, low biomass of organisms <1 mm, and dominance by large, slow-growing macrofauna. In the highest organic flux and biomass areas near the Fraser River discharge, production did not increase beyond moderate flux levels. Although highly productive, this area had low P/B. Clearly, food input is insufficient to explain the complex patterns in faunal production revealed here. Additional environmental factors (depth, substrate type and unmeasured factors) are important modifiers of these patterns. Potential reasons for the above patterns are explored, along with a discussion of unmeasured factors possibly responsible for unexplained (30%) variance in biomass and production. We now have the tools for basin-wide first-order estimates of sediment invertebrate production.