Loch Obisary

Also within

Western Isles County

Condition (NatureScot)

Geology & landscape

Rock Type: Gneiss

Description

Loch Obisary Loch Obisary is an unusually deep brackish loch in North Uist with particular attributes which are unknown elsewhere in the British Isles. Loch Obisary, underlain by lewisian gneiss, is divided into two main basins connected by shallow channels amongst a chain of islands. The mean loch depth is 7.8m, with ten deeper parts exceeding 15.2m and one in the northern basin of 45m. The northern basin is connected to Loch Euphoirt by a shallow inlet channel, along which sea water ingressions are estimated to occur on over half of high tides. The northern basin is stratified into two distinct water layers. There is no apparent winter overturn of the layers, as happens in most stratified systems in the British Isles. By contrast, the water in the southern basin is completely mixed and it is rare to find this adjacent to a stratified basin. The salinity in the northern basin upper water layer (epilimnion), and the entire southern basin, ranges from 12 to 14 parts per thousand (ppt), whereas the northern basin lower water layer (hypolimnion) has salinity in the range of 24 to 29 ppt. Throughout the northern basin there is a stable and pronounced saline gradient (halocline), with associated steep oxygen and temperature gradients. This is located below the base of the upper wind-mixed layer, at a depth of around 4 to 5 metres. The water below 10 to15m in the northern basin does not contain oxygen (anoxic). There are evident depth zonations of plant and animal communities which are related to water quality (salinity, oxygen and temperature) in the northern basin; and related to light in the southern basin. A brackish marginal community is found in the upper water layer of the northern basin and throughout the southern basin. It consists of freshwater, salt-tolerant (euryhaline) species, such as fennel-leaved pondweed Potamogeton pectinatus and the spire shell snail Potamopyrgus antipodarum; and brackish water species, including the brown alga Fucus ceranoides and the mussel Mytilus edulis. A marine community occurs at and below the saline transition zone (halocline) in the northern basin. It is composed of marine, salt-tolerant species, numerically dominated by the red alga Phyllophora pseudoceranoides, the sea squirts Ciona intestinalis and Ascidiella aspersa, and the sea slug Akera bullata. The stability of the saline gradient has resulted in the development of an unusual community, found only within it, which is dominated by blue-green algae. Communities within and adjacent to the inlet channel show an increased species diversity and similarity to the marine communities found in Loch Euphoirt.

Species mentioned