ABOVE: Part of a softwood crail or lobster pot, continually exposed to seawater for 6 months, showing extensive galleries carved preferentially into the early wood by gribble (limnoria sp.). Some of the white patches are colonies of ectoprocts.

The most immediate impacts on a shipwreck are due to physical interactions with the marine environment. These include the sinking event (buoyancy), break up of the wreck due to impacts and stress, movement of all or part of the wreck due to currents, and abrasion.

The wreck also has a physical impact on the marine environment. Upstanding features will change the sedimentation pattern, resulting in characteristic scour pits.

Once submerged, or subject to periodic immersion, rot and other microbial decay processes destroy organic materials quickly. If these materials are buried in anoxic sediments, however, the destruction process is arrested. Anoxic sediments are common in shallow coastal waters, which receive large amounts of organic matter from productive surface waters. This matter rapidly exhausts the oxygen in the sediment as it rots.

Under ideal conditions an excellent state of preservation can persist indefinitely. Fragile organic objects, such as leaves, have entered into the sub-fossil record as a consequence of initial preservation in anoxic mud.

Many metals, such as iron and copper are subject to chemical corrosion, resulting in gradual dissolution. The extent to which corrosion processes are mediated by biological organisms is unknown, but probably significant.

The shipwreck will also influence the chemistry and biology of the marine environment. This influence is usually long term and localised - massive chemical spills and their immediate effect on chemistry and wildlife over large areas are largely a modern phenomenon.

Over time structures provide solid substrates that can be colonised by organisms such as anemones and dead men's fingers. This can result in an area that is biologically distinct from its surroundings, if the wreck lies on a sandy or muddy seabed, and has been argued to contribute to bio-diversity in these areas.

Some organisms such as gribble and teredo (ship worm) burrow into wood, which they use as a shelter, a process that hastens its disintegration. Iron and objects made from other materials may act as a source of trace nutrients or toxins of local biological importance.

In conclusion, the environment dictates the extent of preservation of a shipwreck site. Most sites in shallow water are rapidly destroyed, so well-preserved, easily accessed shipwreck sites are at a premium. They represent a finite, irreplaceable resource.

Over the last ten years, we have been developing simple techniques that help us monitor the marine environment, and evaluate how the shipwreck and its environment might be interacting.