Marine algae, or seaweeds, are the oldest members of the plant kingdom, extending back many hundreds of millions of years. They have little tissue differentiation, no true vascular tissue, no roots, stems, or leaves, and no flowers. Algae range in size from microscopic individual cells to huge plants more than 100 feet long. Though the flora is continuous along the coast, an abrupt change in overall species composition occurs at Point Conception, where nutrient-rich northern currents meet warmer southern ones.
Zonation patterns within algal assemblages are dictated by tidal exposure and wave impact, as well as by species interactions such as grazing by invertebrates, and by competition for space and light. The sea palm, Postelsia palmaeformis, for example, finds refuge on wave-pounded rocks where predators, such as sea urchins, are unable to follow. The sea palm's tough, cartilaginous stipe and holdfast (analogous to a stem and root in vascular plants) absorb wave shock, and the thick slippery cell walls reduce desiccation. In contrast, calmer waters support more delicate membranous and leaflike seaweeds such as Botryoglossum, Porphyra, and Plocamium species.
Common algae of the high intertidal include Turkish towel, Mastocarpus papillatus, and rockweed, Fucus distichus. Common in the middle and lower intertidal are sea lettue, Ulva spp.; feather boa kelp, Egregia menziesii; dead man's fingers, Codium fragile; and the iridescent, rubbery Iridaea species.
Kelp forests, found in subtidal waters as deep as 100-200 feet, are a unique ecosystem largely restricted to the west coast of the Americas, and best represented in California's coastal waters. Giant Kelp, Macrocystis pyrifera, and bull kelp, Nereocystis luetkeana, are the largest non-vascular plants known. Their blades are harvested for industrially valuable gels, called alginates. The multi-layered canopy of kelp fronds provides a complex aquatic habitat for thousands of fish and invertebrates.
In addition to seaweeds, marine flowering plants called seagrasses are also present in some intertidal assemblages. These plants can dominiate mudflats and exposed rocks, in some cases virtually excluding the algae. Eelgrass, Zostera marina, is common in quiet waters with a sandy or muddy substrate, whereas surfgrass, Phyllospadix spp., is common along exposed rocky shores, often in high-energy surf zones.