Environmental disasters caused by our carelessness are tough on nature, but nature can also be tough on itself. There is a brackish sea lagoon in Co Galway that periodically self-destructs, wiping out its fish and crustaceans and killing all but the strongest organisms.
Ardbear "Salt Lake" is along the coast south of Clifden, and a NUI Galway team has been studying why the lake does this. Ardbear has a surface area of 4.5 square kilometres and for the most part is between four and six metres deep. There is a much deeper depression in one part which reaches a depth of 27 metres. It is filled with sea water, but only small amounts of fresh sea water reach it at high tide, no more than 10 per cent of the total volume. "It is a tidal basin with limited flushing," explained Prof Brendan Keegan of NUI Galway's Zoology Department.
The lake periodically experienced extreme and rapid oxygen depletion, or hypoxia, which in turn led to a dramatic collapse of its ecosystem, he said. The key to these events was the formation of a thermocline, a narrow layer separating warmer, shallower water and deeper, colder water. Measurements last summer showed the thermocline formed in the depression 10 to 15 metres down and when formed it can have powerful effects.
It created a barrier that prevented oxygen from reaching the deeper water, Prof Keegan explained. "The longer this persists, the less oxygen gets through." Oxygen levels plummet and toxins build, making life under the thermocline impossible for most species. Any fixed organisms die and decompose, causing the lake to "sour", he added.
Species that can move do so, with the lake's limited fish life, lobsters and crabs endeavouring to keep above the layer to avoid suffocation below.
They migrate into shallower waters, crowding the edges of the lake.
"It is a summer-time phenomenon," he said. Sometimes there is sufficient decomposition to sour the entire lake and steal all the available oxygen, causing a total collapse of lake life. This happened during an earlier study and virtually all life was killed off. The process could happen very rapidly taking from six to eight weeks, he said.
The lake is also home to a spectacular population of marine worms, Serpula vermicularis, which form reefs along the rocky lake-bed. These could be up to two metres high and 20 metres across, he said. They flourish above the thermocline and can tolerate poor conditions better than most other species.
"If they experience a few years when oxygen depletion is less than extreme, these reefs can cover up to 25 per cent of the floor of the lake."
The main researcher on the three-year project, now in its second year, is Ms Louise Henry, a graduate student. She is carrying out extensive studies of the water column at all depths, including detailed chemical analysis.
The thermocline forms in calm warm weather and is broken up by storms, and its barrier effect is less when the temperature gradient is low. This allows reoxygenation and a recolonisation of the salt lake until the next time that Ardbear decides to turn on itself.