Ireland is rich in lakes, and for that we are fortunate. They bring beauty to our countryside, support a fascinating variety of wildlife, and are closely intertwined with our own history and heritage. Ireland is unusual in having many shallow limestone lakes. This story is about these, and some of the things that live in them.
Fifteen thousand years ago, Ireland was covered by deep ice, held in the grip of a long, long winter. There were no lakes. But over the next few thousand years, as the climate warmed, the ice slowly thawed and retreated, finally losing its hold on the land. Massive glaciers slumped down along mountain valleys; vast ice-sheets were in motion, scraping and grinding their way over the rock beneath; heaps of soil and stones were shifted around, finally coming to rest as the ice melted around them.
As this devastation subsided, streams flowed from high ground, and new rivers formed. Water gathered in the hollows and low places left behind after the glaciation – Ireland finally had its lakes. Many of these formed over limestone, especially in the west and the midlands. Today, these shallow limestone lakes are an Irish speciality.
As life crept over the new land, the lakes also developed their own living communities. What sort of vegetation grows in a shallow limestone lake? To answer this, we can look at some of the best examples remaining today. Muckanagh Lough and Lough Bunny, in Clare, are good examples. The most noticeable living things we see are the charophytes. These are green and look like plants, although they are not considered true plants – charophytes are so ancient that they evolved before any plants existed on Earth. They still grow in our limestone lakes, often forming dense beds and covering large areas. Tiny water animals hide among the charophytes to avoid being eaten by fish. Small fish can also hide here to escape big predatory fish, like trout or pike.
But there are stranger things here too. The margins of these lakes are covered by an odd, pale, spongy layer. This is a marl crust, and certainly does not look alive. And yet these marl layers are full of living things.
Pick up a piece of marl and look at it closely. It may well have holes or cavities, and these will surely contain some life. A water spider might suddenly emerge, carrying its bubble of air in a silken pouch. Flatworms make their way over the surface, rippling and
twisting as they go. You will see caddis larvae, midge larvae, and beetle larvae. Adult water beetles may also appear.
Now put a tiny fragment of the marl itself on a microscope slide and peer down the microscope. What a garden of curiosities appears! Clumps of cells, purple, green or amber, like tiny bunches of grapes. Strands of tiny cells, interweaving the grains of calcite, binding the marl together. Strings of bead-like cells, shining like tiny pearls. Long tapering filaments, bright orange in colour, twisting their way through the tiny garden. All of these are cyanobacteria, life forms from the deep past, some of the earliest living things to evolve on Earth. As we watch, a tiny animal, snake-like and almost transparent, glides by: a nematode worm.
How wonderful it is that a little fragment of marl contains such a variety of life! These little organisms live out their lives, all completely oblivious to humanity, with all our cares, conflicts, achievements and glories. And is it not rather a comfort that, all around us, even in the tiniest grain, life in its beautiful diversity endures?