Tuesday, October 18, 2022

Characterising survey for designation of a Habitats Directive priority lagoon habitat at Tawin

AQUAFACT was contracted by the National Parks and Wildlife Service, part of the Department of Culture, Heritage and the Gaeltacht, to determine if Tawin falls within the definition of a lagoon within the Habitats Directive.

Tawin (as it is referred to locally) is located at the eastern end of Galway Bay on the western end of the Tawin peninsula. Despite its informal name, it was yet to be assessed under the Habitats Directive to see if it met the criteria to be formally classed as a lagoon.

An aerial photograph of Tawin

Image of Tawin taken by AQUAFACT’s Shay Fennelly

Under the Habitats Directive, a coastal lagoon is a lake or pond that is fully or partially separated from the sea by a permeable barrier. This barrier can be entirely natural such as shingle or can be an artificially constructed embankment. As coastal lagoons (habitat code 1150) are a priority habitat in Annex I of the Habitats Directive, they are included in the inventory of Irish coastal lagoons and are therefore under increased site protection measures to ensure their continued survival, as so much of the habitat in Europe had, for a variety of reasons, disappeared or been degraded.

Lagoons typically contain brackish water (seawater that has been diluted to varying degrees), so they can be a habitat for marine and freshwater flora and fauna. There will also be the presence of lagoonal “specialists”, those plants and animals that thrive in these conditions. As their survival depends on lagoonal conditions, it is these species that would be threatened with extinction without site protection measures.

The seas within Tawin are a series of interconnecting water bodies that have five silled openings to the sea (with a rock barrier, or ‘sill’). Locally, these waters are referred to as “malloor”, a word that derives from the Irish term “mall mhuir” which literally translates as “slow sea”. Mall mhuir is the Irish word for Neap Tide, and the use of the term to describe the bodies of water within Tawin indicates that local people were aware that, compared to tidal conditions in the open sea, tides inside Tawin were slack.

Surveys were carried out to characterise the hydrological regime within Tawin and compare it to the open water regime and to determine water quality and depth. AQUAFACT also assessed the character and quality of the barriers, and described typical animal and plant species including lagoonal “specialists” as well as the presence or absence of negative indicator species. The surveys also determined the area of the aquatic habitat within Tawin.

AQUAFACT gathered data on current velocity, direction, and tidal elevation data, which were compared with tidal data from Galway Port. Bathymetric surveys using an echo sounder provided accurate information about the depth of the lagoon, which was used to produce depth contour maps.

There were terrestrial coastal habitats to survey, including extensive areas of salt marsh on stony soils and mid-upper marsh, as well as vegetated shingle, some of which formed the barrier to the sea. On this shingle substrate, AQUAFACT noted the presence of Yellow-horned poppy Glaucium flavum, a species that is very rare along the Atlantic coast of Ireland, and the shingle beaches of Inner Galway Bay are the most northerly sites known along the west coast.

A previously unrecorded bed of Eel Grass, Zostera marina, was recorded in one part of the lagoon.

A photograph of a rare yellow horned poppy growing on shingle vegetation at Mweeloon

The rare Yellow horned poppy (Glaucium flavum) growing on shingle vegetation at Mweeloon

Intertidal and subtidal species of flora and fauna were recorded using methodologies including water and sediment sampling, granulometry and photos from land, boat and SCUBA dives. The presence of lagoonal specialists and flora that were tolerant of lower salinities were recorded, as well as a rarely recorded bivalve and an eyeless species of marine worm that had not previously been recorded, which AQUAFACT recommended be investigated further.

An underwater photograph showing a bed of Zostera marina on sandy ground.

Zostera marina bed (listed in the Habitats Directive as a lagoonal specialist) and sandy ground.

AQUAFACT concluded that Tawin fits the definition of a coastal lagoon as defined by the Habitats Directive: it is an expanse of “shallow coastal salt water, of varying salinity and water volume, wholly or partially separated from the sea by sand banks or shingle”. Its physical and salinity characteristics classify it as a lagoon as it varies in both salinity and volume, and AQUAFACT recorded the presence of two lagoonal faunal “specialists” listed in the EU Habitats Directive, although its physical oceanography was the most convincing reason for its classification.

Tawin will now be included on the National list of lagoons.