At the end of a summer free of fierce winds or peaty floods, the waters of the Shannon’s great Lough Derg are at their clearest, though still tawny enough in the depths. Much of its clarity comes from better management of waste-water, but some is owed to the greedy appetites for plankton of masses of new shellfish, the zebra mussel and Asian clam. Almost 20 such alien species are now changing the lake’s ecology.
Not every new discovery is an alien invader, obviously. At the lake's southern end, for example, some 3m to 5m down, wave tall meadows of a charophyte called Nitellopsis obtusa, or starry stonewort.
Broken away and drifting in currents, they can spread one plant to grow a colony
With whorls of branchlets and translucent green stems, starry stonewort is one of the family of brittle, skeletal plants, rich in calcium, that grow in Ireland’s limestone lakes since the last ice age. Some scientists group the charophytes as algae, watery forerunners of plants on land.
What makes the stonewort “starry” are white clusters of tiny bulbils studding its stems like star-shaped earrings. Broken away and drifting in currents, they can spread one plant to grow a colony.
From the meadows in Lough Derg they have also seeded several hectares of the Parteen Reservoir downstream. Here, plants dragged up with a grapnel were well over 1m long – twice as long as usual.
Is the starry stonewort a native Irish species that knows its place or an introduction that could spread to choke lakebed life?
Wielding the grapnel was biologist Dr Dan Minchin, joint founder with Dr Rick Boelens of the independent Lough Derg Science Group. With charophyte expert Dr Cillian Roden, he describes the discovery of the starry stonewort and the big question it poses in this month's Irish Naturalists' Journal (INJ 35: 105-109), edited by Dr Nigel Monaghan, keeper of the Natural History Museum in Dublin.
Cryptogenic
An expert on invasive aquatic and marine aliens, Minchin needs to decide is the starry stonewort a native Irish species that knows its place or an introduction that could spread to choke lakebed life with its dense and shadowy stands.
Lough Derg’s charophytes have been without scrutiny for years and, for the moment, the starry stonewort is described as “cryptogenic”, its origins uncertain.
It was first reported from a lake at Dooaghtry, in the machair below me, in a survey undertaken from Britain some 30 years ago. But Dr Roden, snorkelling the lake much more recently, could find no trace.
Starry stonewort is so rare as a native in these islands that the UK seeks to conserve its main stands in Norfolk and Suffolk, a region that includes the popular boating waters of the Broads. This suggests at once that, like the highly invasive zebra mussel, the stonewort arrived in Ireland with UK leisure craft coming to cruise the Shannon.
Genetic fingerprinting may be needed to trace the stonewort to the deep, reedy channels of East Anglia
That does sound more likely than other possibilities scrupulously listed by the science group, such as loose bulbils lodged in the feathers of migrant diving ducks that come to Derg in winter. Like DNA analysis that confirmed the transfer of zebra mussels from the UK, genetic fingerprinting may be needed to trace the stonewort to the deep, reedy channels of East Anglia.
Ballast water
Its invasive potential may be judged from the Great Lakes region of the US, where fragments were thought to have been discharged in ballast water from European vessels arriving up the St Lawrence River.
Since 1978 it has spread to a score of inland lakes (perhaps, indeed, in the feathers of diving ducks as well as on anglers' boats and fishing gear). Clearing its underwater thickets from fishing and swimming areas is costing local communities dear. (More on the US Geological Survey website at iti.ms/2wtGt0m.)
Its invasions have followed those of the Great Lakes by the alien zebra mussel, originally from eastern Europe. This now coats lake beds in the billions, along with piers and slipways, and chokes industrial underwater intakes.
Spreading as densely as the zebra mussel, they threaten tributary spawning grounds of salmon and trout
In Ireland, since 1994 the mussel has spread from Lough Derg through the Shannon system and many separate lakes as far north as Lough Erne. It has smothered native aquatic life beneath carpets of its shells and fertilised lakebeds to promote dense growth of underwater plants.
Minchin’s assessment of an “invasional meltdown” of Lough Derg’s natural ecosystem was hastened by his discovery of the first Asian clams in 2011. Spreading as densely as the zebra mussel, they threaten tributary spawning grounds of salmon and trout, and make demands on plankton and dissolved oxygen that compete with fish and other water life.
Swarm at night
Smallest of the Lough Derg aliens is the "bloody red shrimp", Hemimysis anomala, a tiny, stalk-eyed crustacean that can swarm at night at up to 6,000 per cubic metre. Another migrant from eastern Europe, its first swarm in Derg was recorded in in 2008, and has spread through the Shannon system to the Erne.
Voracious in appetite, will the bloody red shrimp out-compete fish for lake insects or offer them a rich new food? An ongoing “citizen science” study of Derg’s fish species and numbers, undertaken by the lake’s angling clubs, may eventually offer some news.
Michael Viney's Reflections on Another Life, a selection of columns from the past four decades, is available from irishtimes.com/irishtimesbooks; viney@anu.ie