On the trophic role of filter feeders in rocky intertidal shores
Mussels and barnacles are key structural components of rocky intertidal communities, but also important prey items for a number of consumers. By feeding mainly on plankton and particulate matter, and being capable to use detritus from subtidal kelp forests, filter feeders might channel a variety of autochthonous and allochthonous energy sources into the community, potentially affecting the food web in different ways. In particular, nutrient subsidies sourced from kelp forests and intertidal macroalgal detritus could both be effectively propagated by filter feeders through the trophic chains linked to them, having positive effects on the abundance of filter feeders and their consumers. We tested this hypothesis using four intertidal assemblages in south-central Chile, two of them adjacent to subtidal forests of Macrocystis pyrifera, assessing the local abundance of two key filter feeders (the mussel Perumytilus purpuratus and the barnacle Jehlius cirratus), their main predators, and the most common intertidal macroalgae, and collecting samples of filter-feeder species and their potential nutritional sources for stable isotope analysis. We then developed different structural equation models to explain the variation in filter-feeder abundance, and determined the proportional contribution of each food source to their diet using a Bayesian mixing model.
We found no significant association between the presence of kelp forests and the abundance of filter feeders or their consumers. None of the proposed models for barnacles agreed with the collected data, but intertidal macroalgae accounted for more than 30% of the variation in mussel density. While both mussels and barnacles consumed kelp tissue at all localities, the contribution of kelps to their diets was extremely low, being similar or much lower than that of intertidal macroalgae. All food sources showed between-site variations, but particulate organic matter was overall the most important component of the diet of the two filter feeders, followed by intertidal macroalge which were particularly important for barnacles, making up over 60% of their diet in some sites. In addition, the enrichment levels of carbon vs. nitrogen suggest also that zooplankton and phytoplankton are significant nutritional sources for barnacles and mussels, respectively. These results stress the need to examine the relative importance of autochthonous vs. allochtonous energy sources (or inshore vs. offshore production) for the sustainability and stability of rocky intertidal food webs, and the role of filter feeders in channelizing such resources.