Wood submerged in water degrades differently than wood on land. While traditional decay is largely inhibited in aquatic environments, wood remains susceptible to other forms of deterioration. Underwater conditions determine how quickly and by what means wood breaks down, from rapid destruction to remarkable preservation over millennia.
The Science of Wood Decomposition
On land, wood “rot” primarily results from fungi and certain bacteria. These microorganisms consume wood’s structural components, cellulose and lignin, through enzymatic action. For these terrestrial decay organisms to thrive, they require adequate moisture, oxygen, suitable temperatures, and the wood itself as a nutrient source.
Wood-decaying fungi are aerobic, requiring oxygen for their metabolic processes. Research indicates that fungal growth is severely inhibited or completely stops when oxygen levels drop below approximately 0.2%. These organisms break down the complex polymers of wood, leading to the characteristic softening and weakening associated with rot.
Underwater Conditions and Wood’s Fate
Submersion dramatically alters wood degradation conditions, primarily by limiting oxygen availability. In many aquatic environments, especially at greater depths or when buried in sediment, water becomes anoxic or hypoxic (very low to no dissolved oxygen). This lack of oxygen largely prevents the growth of typical wood-decaying fungi.
Despite the absence of fungal rot, wood underwater is still subject to other forms of degradation. In oxygenated marine waters, specialized biological agents known as marine borers pose a significant threat. These include shipworms (worm-like bivalve mollusks) and gribbles (small crustaceans). Shipworms bore extensive tunnels into wood, digesting cellulose with symbiotic bacteria, rapidly honeycombing submerged timber. Gribbles also tunnel into wood, but their activity often leads to superficial erosion, with wave action exposing fresh surfaces for attack.
Physical and chemical processes also contribute to underwater wood deterioration. Water currents can cause physical erosion, and abrasion from suspended sediments can wear away wood surfaces. Chemical degradation, such as hydrolysis, occurs at a slower rate, altering the wood’s chemical composition over time. The specific aquatic environment—whether freshwater or saltwater, shallow or deep, and its oxygen content—determines which of these degradation processes will be most dominant.
Preserving Wood in Aquatic Environments
The longevity of some underwater wood, such as ancient shipwrecks, can be remarkable due to specific environmental conditions that promote preservation. Deep burial in oxygen-poor sediments creates an anoxic environment where most biological decay organisms cannot survive. This natural embalming effect has allowed vessels to remain intact for thousands of years, as seen in the Black Sea.
Low water temperatures also play a role, as colder conditions slow biological activity and chemical degradation. The cold, brackish waters of the Baltic Sea, with their low salt content, contributed to the exceptional preservation of the 17th-century warship Vasa, inhibiting marine borers. Certain wood types possess natural durability due to their chemical composition, such as high oil or tannin content, making them more resistant to water and decay. Teak, Ipe, Iroko, Cedar, and Redwood are examples of woods known for their natural resistance.
For wood intended for aquatic use, human preservation methods offer protection. Pressure treatment infuses wood with chemical preservatives, such as copper-based compounds, which deter fungi, insects, and marine borers. Different treatment levels and chemicals are used depending on exposure to freshwater or saltwater. In archaeological contexts, salvaged waterlogged wood, like the Vasa, undergoes extensive conservation treatments involving substances such as polyethylene glycol (PEG) to replace water in cells and prevent shrinkage upon drying.