Phaeophyta: Life History and Ecology

Though there are a few rare freshwater species, the brown algae dwell almost exclusively in marine (or coastal) environments. Members of the group dominate many benthic marine biotas, sometimes reaching from the ocean floor to its surface. In general, they are not free-floating organisms, but are attached to rock, coral, or other firm surfaces.

The group is found primarily in colder waters of the northern hemisphere, with the largest forms occurring in cooler waters, rather than in the tropics. Many familiar species, such as rockweed, are intertidal, and are exposed to the air at low tide.

Kelp forests

Pictured above is a kelp bed located near Monterey, California. We will have more on these important communities at a later time.

The Sargasso Sea

The Sargasso Sea is a free-floating (pelagic) kelp-dominated ecosystem in the western North Atlantic. It is bounded by the Gulf Stream, the Canaries Current, and other currents which together produce an ever circulating boundary. The local pattern of distribution appears to also be affected by both wind and thermal fronts, and more broadly may be affected by the action of storms. Attempts to quantify the amount and distribution of Sargassum in the Atlantic have met with many difficulties, including changes with seasonality and regional patchiness.

Two species constitute the majority of the algae here, primarily Sargassum natans, and most of the rest is Sargassum fluitans. These two species apparently evolved from other anchored species of Sargassum, providing the basis of this bizarre ecosystem. Sargassum stays afloat by producing gas-filled bladders which act like buoys. You can see these in the picture at left; the picture also shows the typical jagged-edged blades. Life here is precarious for animals who are poor swimmers -- they must maintain a firm grip on floating mats of kelp, or be lost to the ocean depths.

Such a floating ecosystem of course will have difficulties in acquiring nutrients, and will therefore be severely limited by access to such nutrients. Many of the organisms which live here survive by being generalists, not limiting themselves to a single food source but making use of whatever is available. The most common crab is a generalist carnivore, eating many different kinds of prey. It is also interesting that this ecosystem has no animals which are strict herbivores, but rather they are omnivores, switching between diets of eating algae and animals. This may in part be due to the rubbery, chemical-laden nature of Sargassum.

The accumulated mats of Sargassum support a wide variety of animal life, some of which depend on the kelp for only a part of their life. Other organisms spend their whole life among the algae, and this diversity of life has been called a "floating jungle". Some of the more unusual forms include fish and crabs which are camouflagued to look like Sargassum. Perhaps the best known of these is the pipefish Syngnathus pelagicus, a relative of the seahorse. This fish is brownish-green, and is covered by flaps of skin which resemble the kelp blades. There are more than 50 fish species whose lives are linked to Sargassum, and a myriad of invertebrates, including gastropods, polychaetes, bryozoans, anemones, and sea-spiders. The most numerous inhabitants are hydroids and copepods.

Fucoid communities

Fucus, or rockweed, a picture of which appears on the introduction to the Phaeophyta page, is the organism which gives these communities their name. Communities dominated by members of the order Fucales are mostly subtidal and intertidal. In these regions, the stress of wave action makes life difficult for free-floaters; most organisms who live here attach themselves to the rocks. And it is not just the peaceful waves that must be withstood, but the powerful force of waves generated by storms as well. Those organisms not securely fastened to the rocks will likely be torn free and washed ashore or carried into the open ocean.

Algae and plants which live here must also contend with many herbivores which live in these near-shore communities. Heavy grazing may damage and weaken the holdfasts which anchor the algae. Exposure at low tide also means that these organisms risk dessication, which is reduced by the presence of gelatinous compounds such as algin.

The phaeophyte life cycle

Most brown algae have a sexual alternation of generations between two different multicellular stages. The differences in life cycle define a number of orders, some with a dominant diploid phase, and some with isomorphic phases, that is, they are quite similar in appearance to each other. Some, such as the Fucales, have no free-living gametophyte stage at all.

The largest kelps are diploid, and release flagellated swimming sperm into the water to find egg cells. It has been shown that chemical signals called pheromones aid the sperm in their quest in at least some phaeophytes.

More information about kelp forests is available from the Monterey Bay Aquarium or view a picture from the Stephen Birch Aquarium-Museum at the Scripps Institute of Oceanography in San Diego.

The Bermuda Atlantic Time-Series Study is collecting data on biogeochemical cycling in the Sargasso Sea. Their site includes much data and some nice satellite photographs. Or read this Research Paper by William W. Bushing on kelp populations around Santa Catalina Island, CA.

J. N. Butler, et al. 1983. Studies of Sargassum and the Sargassum Community. Bermuda Biological Station Special Publication No. 22.

J. H. Ryther. 1956. The Sargasso Sea. Scientific American 194(1):98-104.