Eco Marine consultants observe numerous different types of habitats during ecological surveys, one such habitat that we occasionally observe is maerl beds. Maerl is the generic name for a coralline alga which forms unattached and individual pink nodules or ‘rhodoliths’ which grow incredibly slowly (up to 0.5mm per year) by depositing calcium carbonate into its cell walls. This forms hard, brittle structures that can then live for over 5000 years.
Maerl beds form highly complex three-dimensional structures that create a wide range of ecological niches. As a result, they support high biodiversity and host large communities of both benthic fauna and macroalgae, which utilise the interstitial spaces between the branches. Similar to seagrass habitats, maerl beds serve as important nursery grounds for several commercially valuable species, including fish such as cod (Gadus morhua) and shellfish like scallops (Aequipecten opercularis).
There are thought to be three main species of maerl across Europe: Phymatolithon calcareum, Lithothamnion glaciale, and Lithothamnion coralloides. Beds of these species are distributed across the UK and primarily observed across the west coast of Scotland, the North-west coast of Ireland, and the South of coast of Cornwall.
As a photosynthesising organism, maerl depends on light, making its distribution closely linked to water clarity and depth. It typically occurs from the lower shore down to depths of around 40 metres. Seabed currents also play a crucial role, as they help prevent the accumulation of silt that could smother the maerl and as a result, more extensive maerl beds are generally found in areas with moderate to strong currents. Under favourable conditions, maerl can accumulate to form dense beds, often covering 30% or more of the seabed, which are most commonly found on coarse substrates such as gravel, clean sand along open coasts, or within tide-swept inlets.
Recent research has shown that maerl is more resilient to fluctuations in temperature and salinity than previously thought. However, it remains highly vulnerable to damage from smothering by fine sediments. Maerl has traditionally been extracted from the seabed for use as a soil conditioner—crushed into a fine powder—or is more commonly damaged as a bycatch during scallop dredging operations. Due to these threats, maerl is currently listed as a threatened species under Annex V of the OSPAR Convention. Growing awareness of its fragile nature, extremely slow growth rate, and ecological importance in supporting diverse marine communities has also led to increased conservation efforts. Bans on maerl extraction are becoming more widespread, including in areas such as the Celtic Sea and Greater North Sea. Additionally, ongoing monitoring, surveillance, and habitat mapping programmes are now in place.
Eco marine has a range of capabilities to survey this habitat, with the photo below showing a maerl bed Eco Marine observed during a drop-down video (DDV) survey. To discuss your project’s survey needs, please get in touch.
References:
- Bernard, G., Romero-Ramirez, A., Tauran, A., Pantalos, M., Deflandre, B., Grall, J., and Grémare, A. (2019). Declining maerl vitality and habitat complexity across a dredging gradient: Insights from in situ sediment profile imagery (SPI). Scientific Reports, 9:16463.
- Birkett, D.A., Maggs, C.A., and Dring. M.J. (1998). Maerl (Volume V). An overview of dynamic and sensitivity characteristics for conservation management of marine SACs. Scottish Association for Marine Science. (UK Marine SACs Project). 116pp.
- Hall-Spencer, J., Kelly, J., and Maggs, C. (2007). Assessment of Maerl Beds in the OSPAR Area and the Development of a Monitoring Program. 34pp.
Written by Amy Fisher.