A floristic analysis of the marine algae and seagrasses between Cape Mendocino, California and Cape Blanco, Oregon

The biogeographic area between Cape Mendocino, California and Cape Blanco, Oregon spans over 320 km and is characterized by prominent coastal headlands that act as genetic and species barriers for marine organisms. Because of a lack of a current macroalgal species list for this area, this study aimed to (i) compare patterns of intertidal macroalgal species composition for the poorly described coastline at four sites, and to (ii) compile a macroalgal and seagrass flora based on current findings and historical records. Collections were made in the spring and summers of 2010 and basic ecological habitat attributes for each species were recorded. Similarities in the macroalgal composition across the four sites were investigated using hierarchical clustering based on a presence/absence matrix for each species. A total of 162 species of marine macroalgae (103 Rhodophyta, 33 Heterokontophyta, Phaeophyceae, and 26 Chlorophyta) and 2 species of seagrasses (Anthophyta) were identified. The sites formed a latitudinal gradient of similarity; the two northern sites clustered together as did the two southern sites. Across all four sites, more than half of the taxa were found in the low intertidal. The within site comparison of taxa based on zonation revealed that Crescent City differered from the other three sites. One near-endemic species, Cumathamnion sympodophyllum M.J. Wynne K. Daniels, and one invasive species, Sargassum muticum (Yendo) Fensholt were found. Combined with historical accounts, the macroalgal flora between the Capes consists of 322 macroalgal taxa (201 Rhodophyta, 70 Heterokontophyta, Phaeophyceae, and 51 Chlorophyta) and 4 species of seagrasses. The results indicate a relatively high area of biodiversity of 134 species per degree latitude. The Cheney ratio designation is a cold-temperate flora with closer affinities to Oregon and Washington than to southern California. In comparison to historical records by Dawson and Doty, seven new records were found. Furthermore, Dawson and Doty's floras list 87 species that were not found, the discrepancy laying either in sampling efficiency or recent range shifts. This study was a first attempt to characterize the marine flora between Cape Mendocino and Cape Blanco and suggests that the capes act as a biogeographic barrier and important transition zone for some species of macroalgae in the Northeast Pacific.