There are interesting and abundant marine habitats that were actually built by people. In an effort to control the movement of sand along a beach, jetties and groins were constructed up and down the coastline. Unfortunately their impact has been to alter the natural flow of sand by acting as a barrier and causing increased erosion. Groins are smaller structures than jetties and can be found scattered along the frontal beach. A jetty is located at the mouth of an inlet and is a much larger and longer structure often made of large boulders.
In spite of the erosion problems caused by these structures, they are also places of attachment for many marine species. In some cases, such as the jetty at the Cape Cod Canal, small pools of water can get trapped at high tide and harbor many marine animals such as sponges, sea anemones, and sea stars. I have seen large populations of salps (Thalia democratica) get trapped in these pools.
Most of the large jetties get periodically bombarded by strong waves and currents. The result is an irregular distribution of plants and animals along the rocks, a phenomenon called patchiness. The movement of water can sweep away the spores of algae or the larvae of animals before they get attached. But in those places where a plant or animal successfully colonizes a rock surface a form of zonation can occur. Near the upper portions of the rock an area called the splash zone may be slippery from a blue-green alga called Calothrix. Below this could be a widespread zone of barnacles, but these crustaceans do have to be submerged in order to feed.
In general the seaweeds follow a familiar pattern in which green algae, most likely Ulva or Enterormorpha, occupy the upper zone followed by the brown seaweeds (Fucus and Ascophyllum) in the middle zone, and the red algae (Polysiphonia and Chondrus) in deeper water where their pigments may absorb blue light. If the large seaweeds dominate a rock substrate then their fronds can slide back and forth in the waves preventing the colonization of other species.
The survival of any plant or animal in this habitat can be influenced by storms, predators, and severe competition. Both barnacles and rockweed can dominate a rock jetty habitat because of their ability to strongly attach to the rock surface. Because jetties attract many baitfish and crabs they are also excellent locations of striped bass fishing. While jetties and groins may indeed alter the shape of a beach they do provide additional habitats for many marine organisms.
Copyright Gil Newton 2009 Thanks to Chris Dumas for logo image.