Linkages between rocky reefs and soft-bottom habitats: Effects of predation and granulometry on sandy macrofaunal assemblages

• We examined the structure of soft-bottom assemblages associated with rocky reefs.
• We tested the role of predation and granulometry on assemblage structure.
• There was a significant effect of distance from of reefs on macrofaunal assemblages.
• Predation and granulometry did not fully explain variation in assemblages.
• The study shows a complex influence of reefs on soft-bottom communities.

The movement of animals and materials among habitats can have an important influence on the structure of assemblages. The nature of such linkages is, however, not always an obvious one. Here we examined the influence of rocky reefs on nearby sandy macrofaunal assemblages. A preliminary descriptive study confirmed the hypothesis that sediment granulometry differs in relation to distance to rocky reefs and so did the macrofaunal assemblage composition, and relative abundance of three of the most abundant taxa, and that this effect was generally consistent among locations. In addition, there was a significant effect of sediment structure on community structure of benthic assemblages. A subsequent manipulative experiment tested, in an interactive way, the effects of predation and sediment grain size in structuring macrofaunal assemblages adjacent to a rocky reef. At the scale of the assemblage (richness and composition), results showed that there was an effect of sand granulometry and distance to the reef, while predation had no effect. Predation affected the abundance of one of the most abundant taxa (un-identified amphipod sp.1) but this effect was consistent adjacent and away from the reef. Sand granulometry had no influence on the numbers of the most abundant taxa, while the abundance of two of these taxa (Catapaguroides timidus and Ervilia castanea) still varied according to distance to the reef. This study adds to the wider literature by suggesting that there can be important movements of materials and organisms from rocky reefs to adjacent soft-bottom habitats but that rock-associated predators and sand granulometry may have a limited influence in structuring the small-bodied sediment infauna close to reefs. This study also stresses the need for caution when ascribing the importance of ecological processes based on descriptive studies alone, even when results seem ecologically logical.