Another Life: Refuge and nursery in our undersea meadows

As June envelops the acre the view from my desk disappears. A rising wave of fuchsia has left me a hazy glimpse of Inishturk, a few last green fields, and enough of the ocean to take my thoughts seaward, where other meadows, too, are reaching the peak of summer.

In sheltered bays and inlets with shores of sandy mud or muddy sand, one small group of flowering grasses can live only in the sea. The tallest, Zostera marina, takes the plunge at the lower shore and marches on, its thin leaves floating in the water like a mermaid's emerald tresses, perhaps a metre long or more. In clear water with strong light, it can form dense beds to about five metres deep, well below the tides.

A second species of seagrass, Zostera noltii – usually called dwarf eelgrass – lives between the tides and draws migrant flocks of brent geese to feed at Strangford Lough, in Co Down, in early autumn, and later at Merrion strand, on Dublin Bay.

In times long past Zostera fringed every sheltered coast on both sides of the north Atlantic, offering rich spawning grounds for fish and protective nurseries for myriad marine species. Building and dredging harbours and pouring out urban waste erased a good many of the most productive meadows.

Then, in the 1920s and 1930s, a natural wasting disease produced a catastrophic dieback of seagrass on both Atlantic shores. Its recovery has been slow and fragile, in beds still precious to the coastal ecosystem. Europe's Ospar Commission, which protects the northeast Atlantic, finds them endangered and declining throughout the region.

Ireland has seagrass beds in more than 20 bays and estuaries, almost all of them protected under EU nature directives. In 2007 scientists at University College Cork studied sample beds at two sites, in the entry creek to the Lough Hyne nature reserve, in west Cork, and at Ventry Bay, in Co Kerry. Their focus was on natural dynamic trends in the growth of Zostera and the diversity of sea life living among the grass (npws.ie, Irish Wildlife Manual 28) They listed 124 species in 81 families, including a multitude of amphipods, crustaceans and bivalve molluscs, along with sea anemones and brittlestars. Even this, they felt, was an underestimate, missing many grazing sea snails and sediment worms. Each site had about 40 species not found in the other, showing the value of a wide spread of seagrass beds to coastal biodiversity.

Their importance to any maritime nation is shown in a descriptive atlas just published by Spain's institute of oceanography (www.ieo.es/web/ieo/atlas-praderas- marinas).

Even without adequate Spanish, I find the marine photography in its 680-odd pages estupendo and its essay in English most enlightening. It's not just the man in the street who needs to know what seagrass is, says Prof Cornelis den Hartog, but civil servants making environmental decisions.

Ireland may not have given "eelgrass" – as all our agencies call all Zostera – such handsome treatment, but we know where most of it is (142 hectares of beds, for example, in Clew Bay, where so much of our aquaculture is based). Recent discoveries, in at least three bays, have been made by volunteers for Coastwatch, the NGO led by the marine ecologist Karin Dubsky, of Trinity College Dublin. They drew local knowledge from fishermen and divers – and often from their memories of where eelgrass used to be.

One discovery is in Lough Foyle, where Z marina grows in wide beds and patches along 8km of the Donegal shore, ending just short of Greencastle. On a survey in the summer of 2012 the water at their seaward edge was full of small fish, and more flushed from the beds when they were approached from the shore.

One of them crossed the path of a proposed sewage-outfall pipe, raising issues of enrichment and erosion. Declining water quality, shellfish farming, fishery trawling and coastal development such as new marinas led the threats to Ireland's Zostera reported to Ospar from within the wildlife service.

These are a familiar litany of risks to our coastal marine life. New ones have arisen in the arrival of smothering aliens, such as the Japanese seaweed Sargassum muticum, now widely present around our shores. Its floating mats shut out the sunlight that Z marina needs.

Climate change threatens more and worse storms to tear up the shallow-rooted seagrass, and sea-level rise could dim out the sun faster than the plants could travel back uphill. But a recent sequencing of the Z marina genome has yielded insights into the plant's unique evolutionary persistence. It has adapted its life, three separate times, from the land into the sea. It seems to be one of Earth's survivors – given, of course, enough time.

Michael Viney's Reflections on Another Life, a selection of columns from the past four decades, is available from irishtimes.com/irishtimesbooks