COASTAL CORNER DECEMBER, 2009 The Value of Rockweed
An interesting example of a community of interacting plants and animals occurs along the banks of a salt marsh. At low tide one can observe a distinct zone in which the cordgrass (Spartina alterniflora) occupies the upper portion. Its extensive root system can be seen in the mud, along with a large ribbed mussel (Modiolus demissus) population. These animals are attached to the peat and each other by strong byssus threads. The mussels are filter-feeders and use their siphons to strain water for small particles of food.
Present also is a healthy population of rockweed (Fucus vesiculosus) which grows in the muddy banks and often covers the clumps of mussels. This alga is also common on jetties, pilings, and other hard surfaces. The general characteristics of rockweed are easy to identify. The brown, forked branches have pairs of small air bladders along the midrib of the frond which help the alga float in the water. The alga is attached to a hard surface by a structure called a holdfast. Sometimes the tips are swollen with small bumps which contain the reproductive conceptacles. The male conceptacles release motile sperm cells in the water when the tide comes in. The female conceptacles release the egg cells that are carried by water current. A single sperm cell fertilizes an egg cell, and the resulting zygote settles down on a solid surface. It then grows into a new rockweed plant.
Ecologically, rockweed has many functions. It forms a kind of canopy protecting the mussels, barnacles, and other life forms during low tide. Some invertebrates and small fish graze on the rockweed, while other species, like the periwinkles, scrape the microscopic diatoms and bacteria from the fronds. Like the cordgrass, the rockweed becomes food for other animals in deeper water. When the plants die, they decompose and break up into small organic bits of food called detritus. This gets swept up by the tides and currents, and transported to other areas as a source of nutrition.
Though rockweed is brown due to the presence of the pigment fucoxanthin, it can photosynthesize, and is a primary producer in the salt marsh. Therefore, it also releases oxygen and absorbs carbon dioxide. Physically it helps prevent erosion along the banks by reducing the impact of wave energy. Examine a small piece of rockweed with a hand lens and you may see the young larval stages of many animals, as well as tiny tube worms and encrusting bryozoans.
Copyright Gil Newton 2009 Thanks to Chris Dumas for logo image.