California Sea Grant College Program

Natural and anthropogenic disturbances are important structuring agents in rocky intertidal communities. Here, the impacts of disturbance in the form of human trampling and periodic sand inundation on the invertebrates and algae of coralline algal turf communities were evaluated. First, a preliminary observational study of the turf communities at eight rocky intertidal sites indicated that differences in invertebrate community composition, diversity, and abundances existed among sites and sampling times. Density and diversity were highest in an area that has been closed to public use and lowest at the most heavily visited sites.

In order to quantify the timing and extent of sand inundations to a rocky intertidal area, observations of sand depth were made along a large, permanent grid system for 21 months. Inundations of sediment were relatively rapid and lasted on the order of one to two months, while erosion was equally rapid. The presence of sand was negatively correlated to the maximum significant wave height for that time period.

The use of time-lapse video proved to be an efficient method for collecting vast amounts of data regarding patterns of human visitation to the rocky intertidal. It was observed that the amount of visitation was highly site-specific. Time of day and week were more important predictors of visitation than was the height of the tide.

The coralline turf invertebrate communities exhibited rapid responses to disturbance in manipulative studies, but also appeared to recover rapidly. An acute, or “pulse”, disturbance in the form of controlled sand addition caused an immediate exodus of highly mobile taxa and then a gradual increase in psammophilic (sand-loving) gastropods. A more continuous, or “press”, disturbance in the form of human trampling caused a decrease in density and diversity of coralline turf invertebrates. In both cases, a return to control abundances occurred within one to nine months, indicating a resilient system.

The results of these studies fill an important gap in the knowledge of the natural history of rocky intertidal communities and the processes which shape them. Additionally, the techniques and results presented here will be valuable for future management decisions in protected coastal areas.