The Seaweeds of Barnstable


The shorelines of Barnstable are replete with a wide variety of marine algae, commonly referred to as the seaweeds. Though these organisms resemble plants, they lack the transporting tissues xylem and phloem found in true plants. Algae do make their own food through photosynthesis, but they do not have true leaves, stems, roots, flowers, fruits, or seeds.

The seaweeds are divided into three main phyla or divisions based on the different color pigments in their cells. The three groups are the green algae (Chlorophyta), the brown algae (Phaeophyta), and the red algae (Rhodophyta). All these groups are represented at most Barnstable beaches such as Dowses Beach in Osterville, which has a high biodiversity of seaweeds throughout the year.

Yet as you walk and explore along these shores, you start to notice that several species tend to dominate an area. And some species tell us a great deal about the local environment, acting as indicators of change and possibly of water quality problems.

One very common and highly recognizable seaweed is the green sea lettuce (Ulva lactuca). This alga consists of large sheets that fold along the edges, much like salad lettuce. Sea lettuce is two cell layers thick and can grow up to three feet in length. There is a small inconspicuous root-like holdfast, though this alga can also be found floating in large sheets.

Sea Lettuce – photo by Chris Dumas

Sea lettuce can grow abundantly in areas of high nitrogen runoff. Nitrogen in excess can accelerate the process of eutrophication which can result in a drop of dissolved oxygen (DO). This decline in DO will lead to losses of crab, shrimp, and mollusk populations. There are several sources of nitrogen runoff including septic systems, lawn fertilizers, and atmospheric deposition.

The good news is that sea lettuce is an edible seaweed, though it should not be consumed if taken from areas of high septic runoff. This alga is rich in iron, iodine, and vitamin C. It’s about 15% protein and 1% fat. It is often used in soups and salads. Many seaweeds are edible, but for some people eating seaweed is an acquired taste.

Another common species seen is rockweed (Fucus vesiculosus). This brown alga grows primarily on hard surfaces such as jetties and pilings, but is also common along the muddy banks of salt marshes. Rockweed is easy to identify. The brown forked branches have pairs of small air bladders along the middle (midrib) of the branch (frond). Sometimes the tips (receptacles) are swollen with small bumps which contain the reproductive structures (conceptacles). These structures release male and female cells (gametes) into the water column where fertilization takes place.

Rockweed is an ecologically important seaweed. Its attachment on a substrate forms a protective canopy that shelters ribbed mussels, marsh crabs, and barnacles. Some invertebrates and small fish graze on the rockweed by scraping the microscopic diatoms and bacteria from the fronds. When the rockweed dies, it breaks up into small organic bits of food called detritus. This gets swept up by the tides and currents, and is transported to other areas of the ocean as a source of nutrition for many small animals.

Unfortunately, many beaches in Barnstable have also been invaded by the green alga Codium fragile, sometimes called the oyster thief. Introduced into these waters in the 1950’s from Asia, Codium has become highly invasive. The young alga can attach to practically any hard surface, and can grow over the filter feeding siphons of clams and block them. It can weigh down the motile scallops, preventing them also from feeding. And it can grow so thick that it blocks sunlight to eel grass beds, thus reducing their ability to photosynthesize and support many animal populations.

The branches of Codium are bright green and spongy in their texture. A thick, flat holdfast attaches to different objects such as clams, snails, and rocks. The alga can grow vegetatively by fragmentation of small pieces from the adult, but also through a process called parthenogenesis, in which a female gamete grows into an adult without fertilization from a male cell. Use this seaweed in your compost pile as you find it washed up on the beaches.

The last one to consider is the economically important red alga Irish moss (Chondrus crispus). This beautiful seaweed is attached to rocks, contains blades up to four inches long, and is dichotomously branched. It usually grows in the lower portions of the intertidal zone. Irish moss is a source of the substance carrageenan which has many commercial and industrial uses and is found as a thickening agent in puddings, beer, and ice cream. It is also used in some medicines, cosmetics, and paints. So the next time you brush your teeth or enjoy a refreshing ice cream on a hot day, keep in mind that you might also be ingesting this product from one of the red seaweeds!

Photos by Chris Dumas

Beachcombing for seaweeds can be a very entertaining and interesting activity at any time of the year. The Barnstable Land Trust sponsors walks to many of these local habitats led by naturalists and experts in a variety of fields. And the BLT remains committed to protecting these vital opens spaces and their resources for their biodiversity and wildlife.

We are always excited to publish blogs by Gil Newton! This one is special because it coincides with his new book, Mysteries of Seaweed: Questions and Answers. We have copies at the office, along with copies of Gil’s previous books. So come by, send us an email or give us a call to get a copy today! (It makes a great gift for the sushi lover in your life!)