Another Life: Land and sea locked into a timeless exchange

For every grain of mineral sand that whispers across the dunes as a fragment of a ground-down Irish mountain, another wind-blown particle began in shattered sea shells at the tideline – “the toe and fingernail parings of the sea” in Michael Longley’s indelible image.

Arriving ashore, the calcium of myriad marine molluscs – mussel, periwinkle, cockle, topshell, tellin, razor and an infinite remainder – becomes molecular material for the shells of land molluscs, the multicoloured snails that throng the dunes and machair. As a local piece of planetary recycling, I find this striking and delightful.

There are plenty of other Gaian examples on a grander, global scale. Where do the oceans get their new calcium from if not rivers rushing out dissolved limestone from the land? This is itself the rocky product of crushed molluscs, crustaceans, coccoliths, corals and so on in ancient, elevated seabeds. If it were not for the land the life of the sea would be much reduced – and, of course, vice versa.

Thanks to Triona McGrath at NUI Galway we now know that the generous outflow of Ireland’s alkalinity should help to sustain the coastal and shellfish-rich waters of Ireland. Like the rest of the world’s oceans they are threatened by excessive, man-made carbon dioxide absorbed into the water from the atmosphere and becoming corrosive carbonic acid.

For her PhD on chemical oceanography of Irish waters, with particular emphasis on ocean acidification, Dr McGrath sailed with scientists of the Marine Institute to analyse the biogeochemistry of water masses across our continental shelf and in the Rockall Trough.

She charted levels of acidification as far down as 2,200m (well past the depth of our susceptible cold-water coral reefs) and its impact on the water’s chemical nutrients, then turned to the distribution of a buffering alkalinity along our coasts.

Acidic and siliceous as the rocks in our rim of mountains may be, Ireland’s central raft of limestone is a source of calcium carbonate exceptional in the eastern north Atlantic. Its impact on alkalinity is, predictably, strongest at the island’s coasts where its distribution is complicated by upwellings of deep water and seasonal changes in marine life and productivity.

The Liffey is a high source of calcium to the Irish Sea; Munster’s Barrow, Nore and Suir help lime the Celtic Sea (but not the Lee, which scours a different bedrock); and the Shannon pours buffering alkalinity into coastal currents flowing north to Connacht. The island’s northern rivers, low in calcium, do little to raise the sea’s pH.

This could all be important to the success of shellfish farming, as the marine life most vulnerable to acidification includes many of the main human food species, such as oysters, mussels, scallops and clams.

A common prediction, tested in laboratory experiments, is that as the oceans become warmer and more acidic the shells of these will grow thinner. On the other hand, maintaining them as a defence against predators and rough seas could take more energy, perhaps resulting in smaller sizes with less marketable flesh.

How shellfish actually produce their shells from the calcium carbonate in seawater is still not fully known. A study of this, and how they will react as acidic water attacks their shells and skeletons, is the focus of a new €3.6 million EU research programme called Cache – Calcium in a Changing Environment.

Laboratory research in the US has shown some unexpected responses. Scientists at Woods Hole Oceanographic Institution tested the effects on 18 marine species of progressively raising the levels of carbon dioxide in the air above their tanks. Along with dissolving into oceans to produce carbonic acid, the carbon of carbon dioxide combines with calcium in the water to offer calcium carbonate used for shell-building. This is a balance that climate change could upset.

As the carbon-dioxide levels were increased at Woods Hole, clams, oysters and scallops built less and less shell – but cool-water urchins and limpets actually grew best at more than double the current level of carbon dioxide. And a crab, lobster and large prawn contrived heavier shells as the greenhouse gas increased.

The column’s theme invites a poetic conclusion:

Shells and dust of shells.
Evolution got it right:
no us without them.

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