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Rockweed (Ascophyllum nodosum) is a large, slow-growing, and long-lived seaweed of the rocky intertidal zone that is vulnerable to over-harvesting. Its ubiquitous, olive green and brown fronds are iconic to the Maine coast. The plants, which are technically algae, have a fan-like structure with forked branches that divide further as they grow, producing a single air bladder per year on each branch. Air bladders lift the fronds towards the sun as the tide rises and falls. Established plants resemble tall, underwater trees and can live for decades if the sturdy holdfast at the base of each plant is undamaged. To reproduce sexually, gametes from separate male and female plants fuse to form a fragile embryo that must find a foothold on bare rock. Rockweed is not a true kelp and it is not used in aquaculture.

Rockweed thrives in Maine’s sheltered coves and rocky ledges where it provides an essential habitat for many marine species. At high tide, the floating fronds create an underwater forest where small fish find food and shelter. At low tide, exposed rockweed plants are draped over each other, protecting other marine species from harsh conditions. The decomposing biomass of older rockweed plants stimulates productivity in coastal food webs and forms drift lines on beaches and mudflats where shorebirds feed.

Rockweed has been valued as a fertilizer for centuries, but industrial-scale use of rockweed for agriculture and animal food additives presents new challenges. Harvesting by commercial companies has damaged rockweed beds in parts of Nova Scotia. In Maine, companies are allowed to cut the plants down to 16 inches and vast amounts of extraction are permitted. Consequently, intensive harvesting has the potential to convert a foundational rockweed “forest” into a motley collection of dense “shrubs.” For more information, see Ecology and Management

An underwater view of the natural history of rockweed forests (2.5 min).