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We evaluated how links between direct and indirect interactions and physical disturbance shaped trophic dynamics in a soft-sediment benthic estuarine community. We crossed presence of a burrow-excavating crab (Pachygrapsus crassipes) and nutrient enrichment (nitrogen and phosphorus) in cages containing herbivorous snails (Cerithidea californica) and benthic microalgae in a tidal mudflat and a tidal sandflat in Mugu Lagoon, southern California, USA. P. crassipes consumed up to 85% of C. californica in enclosures, but there was no evidence of a trophic cascade, as crab reduction of snail density did not increase benthic microalgal biomass. Rather, P. crassipes decreased diatom and cyanobacterial biomass by up to 50% in the mudflat and 80% in the sandflat, probably via bioturbation. Subadult C. californica lengths increased 15-20% over five weeks in treatments with neither crabs nor nutrients. In the presence of P. crassipes, C. californica lengths increased less than 5%, probably an indirect result of crab reduction of microalgal food availability or increased burial of surface-feeding snails. C. californica may have actively burrowed as an escape response from the crabs, or have been passively buried during crab burrowing activities. Nutrient addition did not reduce snail growth but increased snail mortality in both sites, possibly a result of nutrient-induced shifts towards toxic or poor nutritive quality cyanobacteria. The top-down impacts of P. crassipes reduced the relative bottom-up effects of nutrients in this habitat, illustrating the importance of evaluating both biotic and abiotic interactions simultaneously. Numerous indirect and non-trophic interactions revealed a community structure that was much more complex than suggested by food web structure. |
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