LONG‐TERM CHANGES IN ROCKY INTERTIDAL SEAWEED POPULATIONS IN URBAN SOUTHERN CALIFORNIA

Abstract

Murray, S. N., Goodson, J., Gerrard, A., & Luas, T.Department of Biological Science, California State University, Fullerton, CA 92834‐6850Long‐term changes in rocky intertidal macrophyte populations were studied at nine southern California sites in Orange and Los Angeles Counties. These sites were first studied by E. Y. Dawson in the late 1950s and were subsequently resurveyed in the late 1960s and1970s, and many again in the early 1980s. The photographs and field notes of previous investigators were obtained and used to relocate each study site. Using either line transect or plot‐based sampling methods, frequency and cover data were obtained for all macrophyte populations. Collected data were then compared with records from previous studies to determine changes in species composition and abundances. During the past fifty years, the human population increased greatly throughout southern California, resulting in extensive modifications of the coastal landscape and increased inputs of non‐point and point‐source discharges, particularly in rapidly growing Orange County. In addition, shifts in oceanographic climate have taken place over the five decadal period of study. Our results reveal that significant changes in macrophyte abundances have occurred throughout the region during this period. The abundances of disturbance‐resistant seaweeds have increased and of large, fleshy species have decreased. Macrophyte diversity has declined and most sites are now dominated by low‐producing crustose and articulated coralline algae, and small, turf‐forming algae. Increases in these forms, together with observed decreases in larger, and generally more‐productive fleshy seaweeds have resulted in a significant decline in the availability of frondose canopy habitat, changes in primary productivity, and possible shifts in food web structures in southern California rocky intertidal communities.