A Large Red Seaweed Invades Narragansett Bay
Marlyn M. Harlin, Professor
Department of Biological Sciences, URI
Marilyn Harlin earned her AB and MS degrees from Stanford University and her PhD in botany from the University of Washington in Seattle. She was acting Chair in the Departments of Botany and Biological Sciences, where she has been on the faculty since 1971. Harlin has long been interested in how species become established and how they relate to one another. One of her avocations is travel; she recently visited the Galapagos Islands.
Martine Villalard-Bohnsack, Professor
Department of Biology, Roger Williams University
Martine Villalard-Bohnsack earned her BS and MS degrees at the University
of Montreal and a PhD in botany from URI. She has been on the faculty at Roger
Williams University since 1972 and is the author of the Illustrated Key to
the Seaweeds of New England (1995). Villalard-Bohnsack also enjoys traveling.
Alaska, Belize, and the Pacific Coast were among her recent trips. Harlin and
Villalard-Bohnsack have collaborated for almost 30 years on macroalgal research
and have co-authored a number of articles on the distribution and ecology of
marine algae.
A large red seaweed, Grateloupia doryphora, has invaded Narragansett Bay in the last five years. When we first reported this species in 1997, it had been unknown on the Atlantic seaboard of the United States despite a century of collections by phycologists (people who study algae) and our own intensive surveys during the last three decades. The first sighting of Grateloupia was on the lower rocky intertidal zone of the southernmost point of Beavertail State Park in 1994. By 1997, nine separate locations supported this species. In 1998 and 1999, the alga spread south, north, and west of these original stations. Because the direction of the spread coincides with the route of ships into Providence Harbor and patterns of Bay water circulation, we believe that spores were introduced from the ballast water of ships. Further support for this theory comes from the fact that the alga has not been found in the Sakonnet River which has the same water quality as the Bay but no ship traffic. Because the organisms in Rhode Island share more DNA similarities with each other than they do with the same species in other countries, we believe that the introduction was a one-time inoculum. We are continuing to study the behavior of this alien alga to learn what can happen when a species arrives in a new and favorable environment.
G. doryphora is highly variable in its morphology. Its thin, flat blade may be simple or divided and may have short marginal proliferations (narrow pointed offshoots). It is pinkish-red to maroon in color, its surface is slippery, and its definitive characteristic is the filamentous interior of the thallus (algal body). Near the entrance to Rhode Island Sound, on the west side, the alga grows to 30 cm long, and near Newport it grows to twice that length. In the subtidal zone north of Prudence Island, its length can exceed one meter and the shapes are mostly entire (undivided), a characteristic usually associated with this alga in deeper waters. In contrast to the undivided forms of Grateloupia in subtidal zones, Grateloupia in tide pools tend to be divided and have proliferations. G. doryphora does not survive very cold temperatures and thus in Rhode Island is restricted to subtidal zones or tide pools. This alga has a life cycle in which the gamete-bearing haploid phase (female or male) is the same shape and size as one of two spore-bearing diploid phases. A second spore-bearing diploid phase is characterized by a small bump-like parasite that is present on the surface of the female gametophyte.
When we set out to describe the invasion of this species, we did not know whether it would survive in waters that are near freezing in winter because original populations are located in warmer waters. Our populations did indeed survive, and we went on to investigate the direction and speed of the spread; the shape and size of the thallus with respect to position in the Bay; the timing, location, and amount of alga present; and recruitment onto artificial substrata. Percent cover is the unit of measurement that we find to be most useful for our research. It is defined as the percent of the surface that is covered by algae.
In Narragansett Bay, we measured a distinct seasonality with the highest percent cover in October and the lowest percent cover in May. The periods of highest cover and lowest cover are consistent at different sites throughout the Bay, but the amount of cover varies within each period. This seasonality is far more pronounced than that reported elsewhere, for example, in France. Within five years, not only has G. doryphora spread to new locations in Rhode Island, but it has also spread laterally at the specific locations that we are monitoring for percent cover. In October 1999 at Fort Wetherill, the average cover at 0.5 meters below mean low water (MLW) was 60 percent, a ten percent increase over the year before. Deeper, at 2 meters below MLW, G. doryphora averaged about 30 percent cover, a striking contrast to the spotty sightings at the same depth the year before.
To understand how and when new recruits arrive, we have introduced artificial substrata into the low intertidal zones where G. doryphora grows naturally. These substrata are plastic discs (13 cm diameter, 134 cm2) onto which sand grains have been glued with contact adhesive. The discs are then attached to bedrock with expansion bolts after making a hole with a Bosch drill and a 30 centimeter bit (long enough to operate in low intertidal zones). By monitoring the discs and putting them out at different times and depths, we have learned that recruits can arrive at any time of year and that the alga renews itself from a perennating (having survived the winter) red base that resembles other algae at the same site.
We have a unique opportunity to follow the recruitment, spread, and population dynamics of an invasive species from its introduction to virgin territory. Environmental conditions (hard substrata on which to settle, salinities, nutrient levels, and water dynamics) in Rhode Island are conducive to algal spread. The unusually large size of specimens in the northern portions of the Bay has been remarkable. This is one of the largest seaweeds in its subclass and one of the largest specimens of red algae in the North Atlantic, along with Porphyra amplissima (nori) in Maine.
Our research over the years has provided us with an extensive database on the distribution, abundance, and seasonality of macroalgae throughout Narragansett Bay and its contiguous waters. Without it, we could not have pinpointed the precise time of the introduction of this non-native species nor documented changes in flora at habitats in which it appeared.
Does the invasion of G. doryphora have a positive, negative, or neutral impact on Narragansett Bay? The answer depends in part upon one's point of view. We do not know what impact this species has had upon native organisms. If, on one hand, the alga is replacing native species, such an impact might be considered negative. We have noted that Palmeria palmata (dulse), a red alga superficially resembling G. doryphora, is unusually sparse this year and that Chondrus crispus (Irish moss) is less dense where it grows with G. doryphora. We do not yet have solid evidence that these changes are related to the Grateloupia invasion or that any native alga or sessile animal has been displaced. On the other hand, one could consider this species to be a potentially harvestable crop and therefore a positive occurrence. Elsewhere, in the Philippines, for example, Grateloupia has been raised as an aquaculture species because of its carrageenan content. That extract is used as a stabilizer and emulsifier for foodstuffs, paints, and toothpaste. In addition, Grateloupia has an antiviral compound that has been effective against viral diseases such as herpes and AIDS. The scientific significance of this work is that we are documenting what can happen when an alga settles in a new and favorable environment. Any measure of value, positive or negative, depends upon one's perspective and benefit or loss that may result from the invasion.
In 1986, the filamentous red alga, Antithamnion pectinatum, was the last seaweed to become established in Narragansett Bay prior to the arrival of Grateloupia. Antithamnion now ranges from Connecticut to Maine, where it is abundant as an epiphyte on other marine plants. In contrast to G. doryphora, A. pectinatum is small and inconspicuous. The first published reports of invasive macroalgae in the Northeastern U.S. are dated 1853 and contain the suggestion of algae being imported with ballast water. Ultimately, all species since the last ice age have arrived from somewhere else.