Ascophyllum nodosum is a brown seaweed and a ubiquitous member of intertidal communities throughout the temperate North Atlantic. This cold- and calm-water loving species has long strappy branches and air bladders along its axis. It grows in dense stands that are up to a meter tall – forming beautiful floating meadows at high tide and thick, floppy mats of seaweed at low tide. Ascophyllum’s high abundance, canopy formation, and place at the bottom of the food-chain makes it an important foundation species for intertidal rocky shore communities. So what would happen if Ascophyllum was suddenly gone tomorrow?

An aerial view of an Ascophyllum dominated intertidal community at low tide. Eager phycology students head down to survey the diversity at Quoddy Head State Park, Maine, USA. (C) Kylla Benes

Fortunately, there have been a few classic and long-term studies* that have addressed this question. Clearly, the removal of any foundation species would result in an immediate decline in associated and dependent species. But what about the long-term changes in intertidal communities?

First a little lesson on intertidal community organization. For sessile (attached and non-mobile) organisms, competition for space is a key driver of diversity on rocky substrate. Recruitment rates, growth rates, longevity, and position in the food-chain can determine which species are most abundant. Ascophyllum is not very good at recruiting. But its density, height, long life-span (up to 100 years), and low palatability mean that this species can out-compete many other sessile seaweeds and invertebrates. Large disturbances—such as strong waves or ice scour — that can remove Ascophyllum and/or high recruitment of other sessile species can lead to other seaweed and invertebrate species becoming dominant. Importantly, the identity of the dominant species can have major impacts on the associated community.

At locations where there is little recruitment, such as the northern-most reaches of the Gulf of Maine, the removal and absence of Ascophyllum could result in a mostly barren landscape or a sparse and low diversity community at best. At locations where there is high recruitment of invertebrates, barnacles and mussels can form near monoculture beds that provide little habitat for the intertidal community that is characteristically associated with Ascophyllum. Although, micro-invertebrates that can live in the interstices between mussels and predators of barnacles and mussels would be happy. In particular, mussels can become so dominant and persistent, that they can form stable and alternative intertidal communities to the Ascophyllum-based versions communities. At locations where predatory pressure from whelks and crabs is high enough to keep these invertebrates in-check, other brown seaweeds in the genus Fucus can be dominant space holders. As close relative to Ascophyllum, these species can play a similar role in intertidal communities, but they are not as long-lived and are more susceptible to damage from waves and herbivores. In the long-run, Fucus-dominated locations may be less stable compared to Ascophyllum-dominated intertidal communities.

The rocky intertidal at Mill Pond, Swans Island, Maine USA. From the 1980’s to 1990’s mussels and barnacles dominated this site (left). Beginning in 2000, disturbances from ice scour opened up space, allowing for a shift to a seaweed dominated community (right). Arrows indicate reference points at the site. Photos: (C) Steve Dudgeon and (C) Peter Petraitis – To see other photographs and learn more about the long-term studies of alternative stable states in the Gulf of Maine, visit the LTERB website.

The above, are all direct effects of the loss of Ascophyllum on the structure intertidal communities but there would also be some indirect effects. Many birds, fish, and invertebrates use Ascophyllum meadows as temporary habitat during migration or as juveniles. Further, seasonal release of gametes and senesce of Ascophyllum may be a large source of nutrients and carbon for intertidal animals or may even be exported to other ecosystems such as the deep subtidal. With Ascophyllum gone, these animals and ecosystems may suffer. These indirect, but potentially important, effects of Ascophyllum are much less well known and studied than direct effects.

Example seaweeds (A-C) and invertebrates (D-I) commonly associated with Asocphyllum. A) Palmaria palmata, B) Corallina officinallis, C) Ulva lactuca, D) Littorina littorea, E) Testudinium testudinalia, F) Littorina obtusata, G) Botryllus sp., H) Mytilus edulis, I) Semibalanus balanoides.
Photos: (C) Kylla Benes

Lastly, the disappearance of Ascophyllum would result in a loss of many ecosystem services that we humans are reliant upon. Lobster, cod, and several other fisheries species use seaweed meadows as nursery habitat for their young. The sudden disappearance of Ascophyllum would result in a reduction of these species, and ultimately income from these fisheries. And this would result in less of the surf, in your surf and turf dinners. Ascophyllum itself is harvested for use in fertilizer, nutritional and beauty products, and even as packaging to ship lobsters to distant restaurants (leading to a small temporary population in the San Francisco bay area). Last year, over 15 million pounds of seaweed were harvested from the coast of Maine, USA and the products of the seaweed industry are valued at about $20 million per year. In addition to its importance in fisheries, Ascophyllum is a foundation species of biodiversity hot spots (e.g., Cobscook Bay) and may be an important carbon sink, which could help mitigate the effects of ocean acidification.

Whether you’re an ecologist interested in community dynamics, think protecting the Earth’s biodiversity is important, or simply love a lobster dinner, we all have a reason to care about Ascophyllum.

*References on Intertidal Community Organization in the Gulf of Maine

Menge, B. A. 1976. Organization of the New England Rocky Intertidal Community: Role of Predation, Competition, and Environmental Heterogeneity. Ecological Monographs 46:355-393.

Petraitis, P. S. & S. R. Dudgeon. 1999. Experimental evidence for the origin of alternative communities on rocky intertidal shores. Oikos:239-245.

Bryson, E. S., G. C. Trussell, & P. J. Ewanchuk. 2014. Broad-scale geographic variation in the organization of rocky intertidal communities in the Gulf of Maine. Ecological Monographs 84:579-597.

May 31, 2016