Catchment News

The marl crusts of Lough Carra

The Lough Carra Catchment Association has supported the publication of ‘The Marl Crusts of Lough Cara’ by Philip Doddy. This booklet highlights some of the features that make this lake so unique…

Take a wander along the shore of Lough Carra and you will surely see many things of interest – the beauty of the landscape, a bird dabbling in the water, bright patches of wildflowers, maybe a few sheep quietly grazing in a nearby field. Another thing you might notice is that the rocks along the lakeshore are covered with a curious, whitish, quite slippery layer. This is a marl crust.

Krustenstein Boulders

What are marl crusts?

If you cut a slice of marl crust and look at it closely, you’ll see a green layer just beneath the surface. Below this will be a soft, whitish, crumbly layer, made up of quite fine grains. A magnified view of the green material shows that it contains many living things – a community of tiny life forms. The majority of these are cyanobacteria – ancient organisms which can grow as filaments, clusters of cells, or interwoven masses, depending on the species. Cyanobacteria have been around for billions of years; indeed some of the earliest traces of life on Earth are micro-fossils of cyanobacteria.

Various algae can also be seen in the living layer of marl crusts, along with microscopic animals such as nematode worms. Bigger animals live in these crusts too. Because thick crusts often have cavities or hollows, animals such as beetles, flatworms, and caddis fly larvae can be found living in them. A water spider will often emerge from a piece of crust if you examine it closely. A particularly rare beetle, called Ochthebius nilssoni, has recently been found living in Lough Carra crusts.

As well as these living creatures, marl crusts contain a lot of small grains of calcium carbonate (the main mineral that makes up limestone). In the upper layers of crust, these grains are bound together by many strands of filamentous cyanobacteria. This is why crusts have a firm texture. These grains also form the whitish material towards the base of the crusts. Because cyanobacteria need light, they cannot grow very deep within crusts, and so the lower parts of crusts are more loose and crumbly in texture.

Philip Doddy

Learn more:

The marl crusts of Lough Cara is available to download online:

Doddy, P., Roden, C.M. & Gammell, M.P. (2019) Microbialite crusts in Irish limestone lakes reflect lake nutrient status. Biology and Environment: Proceedings of the Royal Irish Academy Vol. 119, No. 1, 1–11.

Who is involved?

Quite simply, everyone in Ireland has a role to play. This can be from something as simple as making sure you don’t pollute your local stream, or a local community working together to establish a Rivers Trust to enhance the rivers and lakes in their area, to a Government Department or Agency helping a Minister implement a new policy to help protect and enhance all our water bodies.

This website has been developed and is maintained by the Environmental Protection Agency, and is a collaboration between the Department of Housing, Planning and Local Government, the Environmental Protection Agency, and the Local Authority Waters Programme.


Local Authority Waters Programme

The Local Authority Waters Programme coordinates the efforts of local authorities and other public bodies in the implementation of the River Basin Management Plan, and supports local community and stakeholder involvement in managing our natural waters, for everyone’s benefit.


Environmental Protection Agency

The EPA is responsible for coordinating the monitoring, assessment and reporting on the status of our 4,842 water bodies, looking at trends and changes, determining which waterbodies are at risk and what could be causing this, and drafting environmental objectives for each.


Department of Housing, Local Government and Heritage

The Department is responsible for making sure that the right policies, regulations and resources are in place to implement the Water Framework Directive, and developing a River Basin Management Plan and Programme of Measures to protect and restore our waters.