In September, strong northwest winds and upwelling events are infrequent, and water along many parts of the coast has become warmer and relatively stable. Meanders in the California current bring clear, blue offshore water toward shore. Pennate diatoms, dinoflagellates, and especially picoplankton dominate the phytoplankton.
Tongues of water from the California Current flow toward the coast, bringing clear, salty oceanic water and oceanic animals such as albacore tuna, jellies, and leatherback sea turtles.
Krill are still abundant, but not quite as abundant as in August. The same is true for blue whales. However, blue whales, as well as humpback whales and other marine mammals and seabirds still congregate around the shelf break to feed on krill swarms.
A wide variety of seabirds can be seen over the open coastal waters, as well as closer to shore. Some of these are just passing through as they migrate southward after nesting. Others will spend the winter feeding on the Central Coast.
Juvenile rockfish begin to settle to the seafloor, and become less abundant in the water column (where they have provided food for many seabirds during the summer).
The growth of the kelp canopy slows considerably due to reduced nutrients and overgrowth by epiphytes, though the community of algae and animals within the kelp forest remains lush and diverse. Many types of young rockfish are living in the kelp, feeding on the animals on the kelp canopy and holdfasts.
The wave climate becomes relatively quiescent, as northwest wind-waves subside. Occasional large swells from the south can do damege to the kelp beds, especially where the kelp has been overgrown by epiphytes.
Young crabs, worms, sea stars, and other animals continue to settle in tidepools after drifting around as larvae following the early summer reproductive frenzies. Many seaweeds begin to form reproductive structures or release gametes.
Estuaries support large numbers of breeding leopard sharks, as well as the crabs on which they feed. Thousands of migrating seabirds and water birds congregate in coastal estuaries to rest and feed as they migrate southward after nesting in Canada and Alaska. Sandy beaches also see many beach birds passing through and returning after nesting in the far north.