maërl

Maërl, also spelled maerl, is a marine sediment composed mainly of the calcium carbonate skeletons of calcareous red algae, particularly crustose coralline algae such as Lithothamnion and Phymatolithon. It occurs as living beds in shallow coastal waters and as loose, non-living carbonate deposits on the seabed. The term is used for both the live habitat and the surrounding calcified material.

Formation and structure: Maërl forms as fragments of algae accumulate and bind together through biofilms and other organisms, creating hard, reef-like sheets and nodules. Living maërl consists of active crustose coralline algae that contribute to calcification; dead maërl comprises the inert carbonate framework left after organisms decay. Beds can range in color from pink to reddish-brown and present as granular sediments or as consolidated patches on softer substrates.

Ecology and habitat: Maërl beds harbor diverse invertebrate communities, including sponges, bryozoans, echinoderms, crustaceans, and juvenile fish. They are considered biodiversity hotspots in some regions and provide ecological functions such as habitat structure and coastal protection. Growth is slow, and beds are highly sensitive to disturbance. They require clean, well-oxygenated water with relatively low turbidity; shading, smothering, or physical disruption can impair recruitment and persistence.

Distribution: Maërl is found along western and northern European coasts, including Brittany, Britain, Ireland, and parts of Portugal and Spain, in shallow sublittoral to intertidal zones, usually within depths of a few to a few tens of meters.

Conservation and use: Because of ecological importance and fragility, many maërl beds are protected or managed to limit disturbance, and extraction is restricted in several areas. Historically, maërl has been harvested as a soil conditioner or fertilizer due to its high calcium carbonate content, though environmental concerns limit such use.