Disturbance effects of stranded kelp on populations of the sandy beach bivalve Donax serra (Röding)

Abstract

Twelve beaches on the coast west of Cape Agulhas, South Africa, were surveyed to examine the influence of several physical factors, including stranded kelp, on the zonation, density and biomass of populations of the wedge clam Donax serra. Biomass and density of adult clams were significantly higher on beaches with lower cover of stranded kelp; this pattern did not hold for juveniles. Discriminant analysis of beaches with and without Donax classified these beaches 100% correctly — the most important factor being kelp cover followed by average winter temperature, wave height, morphodynamic state, beach length, wave period and beach slope. Beaches with Donax had a significantly lower percentage of the drift line covered by stranded kelp (mean=30%) than beaches without Donax (mean=77%). On two beaches of the southwest coast with similar morphodynamics and temperature regimes but different kelp cover, adult populations were centered in different zones: in the low intertidal to subtidal where kelp cover was higher (mean=42%) and in the mid-intertidal where no kelp was found. A disturbance hypothesis is developed to explain the observed patterns: kelp stranding in varying quantities interferes with Donax feeding and burrowing activities, dislodging animals of increasing size gradually downshore to the saturation and inner surf zones, where adult populations are eventually established. In the presence of extremely high quantities of stranded kelp, both juveniles and adults would be dislodged from the intertidal to the subtidal. The disturbance process may interact with several other physical factors, being magnified when coupled with low temperatures and small waves on reflective pocket beaches with steep slopes and coarse sands. Additionally, kelp gulls predate dislodged animals, explaining the absence of clams in a stretch of 20 m above the swash zone. It is thus concluded that the zonation patterns here observed are the result of a disturbance effect triggered by stranded kelp and mediated by multiple interacting factors. The adaptations of Donax serra that enable it successfully to colonize intertidal and subtidal habitats of the beach ecosystem are discussed.