Shipwreck

The Shipwreck biome is the habitat that grows on a sunken vessel. A wreck puts a large, hard structure with tall sides, open holds, and many small spaces onto a seafloor that is usually sand or mud, and life settles on it much as it settles on a rocky reef. An estimated 3 million shipwrecks exist worldwide, according to a 2024 review in the journal BioScience. On the West Coast, some ships were lost in collisions and storms, and others were cleaned and sunk on purpose as artificial reefs, such as the Canadian navy destroyer HMCS Yukon off San Diego.

From bare hull to reef

Colonization starts almost as soon as a wreck settles. Microscopic algae, bacteria, and fungi arrive first and form a slimy film within days, Nicholas Ray of Nottingham Trent University writes in The Conversation. Barnacles, tube worms, and sponges follow, then larger animals such as soft corals and crustaceans. Nooks in the wreck shelter small fish, and the small fish draw larger predators.

A study of HMS Scylla, a British frigate sunk as an artificial reef off Cornwall, England, in 2004, followed that process for 5 years. Most of the wreck's typical species had settled by 2006, and after 5 years researchers had recorded 263 taxa, or kinds of organisms, on it. Its community still differed from the community on nearby natural reefs, and the authors cautioned against treating wrecks as replacements for damaged natural habitat.

Life arranged by height and flow

A new wreck stands tall off the bottom, and the life on it sorts itself by height and by current. The BioScience review describes high densities of large suspension feeders, animals that catch food drifting past in the water, on the upper parts of wrecks, and links the pattern largely to water flow. Smaller reef fish use the nooks and crannies inside the hull.

In San Diego's Wreck Alley, strawberry anemones (Corynactis californica), small pink to red relatives of sea anemones, are the most common animal on the wrecks, and they cover the NOSC Tower, a collapsed Navy research platform, and the Coast Guard cutter Ruby E. Records from the Yukon on iNaturalist, a citizen science app, include gorgonians (branching sea fans), rock scallops, spiny lobsters, and nudibranchs (sea slugs), along with fish such as calico bass, vermilion rockfish, and cabezon. In Puget Sound, a dive guide to Edmonds Underwater Park describes a sunken dry dock, a floating platform once used to lift ships out of the water, covered with giant plumose anemones and home to big rockfish and lingcod, and a sunken tug, the Triumph, where lingcod and cabezon gather.

Steel, wood, and concrete

What a ship is made of shapes what lives on it and how long it lasts. Iron and steel wrecks suit microorganisms that depend on iron, and some bacteria speed up corrosion, the slow chemical breakdown of the metal. The BioScience review notes that bacteria often drive this corrosion, and that measured rates of corrosion do not hold steady over time.

A wooden wreck is a large fall of organic matter on the seafloor, and some animals eat the wood itself. The main ones are shipworms, clams with long, soft bodies that bore into wood. The best-known species, the naval shipworm (Teredo navalis), uses the 2 small shells at its front end to rasp tunnels up to 60 centimeters (2 feet) long, and bacteria in its gills make the enzyme that lets it digest wood. A shipworm spends its whole life inside the timber.

Some ships were built of concrete. The SS Palo Alto, a concrete tanker built in 1919, was towed to Seacliff State Beach near Santa Cruz in 1930 and sunk in shallow water to serve as an amusement venue. Storms cracked it in 1932, split it in 1963, and tore off its stern in 2017. The wreck now works as an artificial reef, where seabirds perch, fish feed on algae growing in its shelter, and sea lions come to eat the fish. Whatever the material, wrecks break down over centuries, and as a wreck breaks down its spaces change, and so does the community that uses them, according to the BioScience review.

Wrecks on the West Coast

Wreck Alley, off Mission Bay in San Diego, holds several vessels sunk on purpose. The Ruby E, a Coast Guard cutter built in 1934, was sunk there in 1989 and sits upright on the bottom in about 85 feet of water. The San Diego Oceans Foundation bought the Yukon, and volunteers cleaned and prepared it. It was to be sunk on July 15, 2000, but it flooded in rough weather and sank the day before, coming to rest on its port (left) side in about 100 feet of water. A 2004 survey by the foundation and Scripps Institution of Oceanography found few attached invertebrates on the wreck and fish just beginning to settle. By August 2021, the Yukon Marine Life Survey, a citizen science project in which divers photograph the wreck's animals for identification on iNaturalist, had identified 59 species.

Vessels sunk as reefs in the United States are expected to be cleaned first. Guidance issued in 2006 by the US Environmental Protection Agency and the Maritime Administration sets cleanup goals for materials such as fuel, oil, and asbestos, though it does not bind the states.

Diving a wreck

A wreck's depth and shape decide how hard it is to dive. Because the Yukon sank early and landed on its side, its masthead, the top of its mast, sits about 60 feet down instead of the planned 30, which made it harder for recreational divers, and 5 people had died diving the wreck by December 2012. Going inside a wreck puts a diver in an overhead environment, a space with no direct path to the surface, where poor buoyancy control can stir up silt and cloud the water.

Wrecks are also protected property. California holds title to abandoned shipwrecks on state-owned submerged lands under the federal Abandoned Shipwreck Act of 1987 and the state Public Resources Code, which places them in the custody of the State Lands Commission. A commission staff report states that removing shipwreck material from state lands is a criminal offense.

Threats

Old wrecks can hold fuel, and failing hulls let it out. The SS Jacob Luckenbach sank on July 14, 1953, after colliding with its sister ship about 17 miles west-southwest of San Francisco, carrying 457,000 gallons of bunker fuel, the heavy oil that powers large ships. It rests in 180 feet of water. The wreck leaked on and off for years, and in 2002 its oil was linked to several mystery spills, including tarballs at Point Reyes. That summer the US Coast Guard and its partners removed much of the oil and sealed the rest inside the ship. The California Department of Fish and Wildlife estimates the leaks killed 51,569 birds between 1990 and 2003, including 31,806 common murres, along with 8 sea otters.

The problem reaches well beyond California. NOAA's database of sunken material in US waters holds about 20,000 shipwrecks, and its 2013 assessment narrowed 573 possible polluters down to 87 for full risk assessments, 36 of which could cause a worst-case discharge. Shallow wrecks leak too. After the Palo Alto's cracked fuel tanks were traced to a spill that killed dozens of seabirds, a 2006 cleanup costing $1.8 million pumped out 500 gallons of oil and found 200 more bird carcasses and 2 harbor seals inside the wreck.