125 Changing Environmental Conditions and Changes in the Abundances of Rocky Intertidal Seaweeds on Southern California Shores

Abstract

Environmental conditions have changed along the southern California coast during the past five decades. The human population has greatly expanded, and coastal habitats have become increasingly influenced by human activities. In addition, coastal waters have been subjected to shifts in ocean climate, including the regime shift of the 1970s that affected coastal plankton, fish, and sea bird populations. Changes in the abundances of intertidal seaweeds during this nearly fifty‐year period are less well known. The earliest available data for tracking seaweed abundances were obtained by E. Y. Dawson in the late 1950s. During the mid‐1970s, Mark Littler and colleagues performed a series of more intensive ecological studies at additional southern California sites. We re‐sampled fourteen of these rocky intertidal sites between 1998 and 2002 to determine changes in seaweed abundances since the mid‐1970s. All sites were sampled using line transect or plot‐based methods similar to those used by earlier investigators. Our abundance data were then compared with values reported from the late 1950s through the mid‐1970s for the same study sites. We found that macrophyte communities at many of our sites have become increasingly dominated by low‐producing, crustose and articulated coralline algae and turf‐forming seaweeds. During this same period, cover of several larger, and more productive, frondose algae declined. These shifts in seaweed abundances appear to have changed the structure and functioning of southern California intertidal communities through changes in the principal contributors to community primary productivity and reductions in the availability of habitat provided by algal canopies.