MEPS 472:129-140 (2013)  -  DOI: https://doi.org/10.3354/meps10120

ABSTRACT: Our observations reveal that species living on rocky intertidal reefs can be positively and negatively associated with increasing sediment load. We therefore tested the hypothesis that sediment disturbance, including increased sediment loads associated with trampling by humans, alters the abundance of macroinvertebrates on a sheltered rocky intertidal platform in southern Australia. First we trampled the reef in shallow water at several disturbance intensities, simulating different numbers of people walking on the platform on a rising tide, and determined that sediment load was directly related to trampling intensity. Trampling displaced sediment, and up to an order of magnitude more sediment accumulated on the reef near intensively trampled areas compared to those with natural sedimentation. We then manipulated sediment load on the reef to mimic increased sedimentation due to trampling and other potential human (e.g. terrestrial run-off) and natural (e.g. storms) disturbances, and monitored changes to dominant species. Sediment addition increased the cover and depth of sediment on the reef. Increased sediment load negatively impacted barnacles, but not mussels, which occur naturally bound in a sediment matrix in small depressions on the platform. The dominant grazing gastropods Nerita atramentosa and Bembicium nanum were negatively influenced by increased sediments, although not at levels associated with trampling, whereas other gastropods were not affected or responded positively to increased sediment load. Changing sediment loads, including the cumulative effects of small-scale disturbances such as trampling, can alter macroinvertebrate species assemblages on rocky reefs and favour species that tolerate a range of environmental conditions and habitat types.

KEY WORDS: Australia · Grazing gastropod · Hormosira · Human impacts · Intertidal macroalgae · Marine invertebrate · Mussel bed · Resilience