Discovering Freshwater Algae in the Outer Hebrides

There is a long tradition of amateur naturalists who have been recognised as experts in the identification, taxonomy and ecology of one or more groups of animals and/or plants. This tradition is continued by a number of biological recorders with a recognised knowledge of certain taxa.

Microscopic freshwater algae are widely under-recorded, but in the Outer Hebrides a dedicated recorder has made a significant contribution to the knowledge and understanding of freshwater habitats and the algae they support.

Freshwater biodiversity, especially of small habitats like ephemeral ponds, bog pools and small lochans, is vulnerable to threats from climate change, habitat loss, recreation, pollution and water abstraction. Records of the algae and microorganisms in these ecosystems contribute to reliable datasets on species diversity and distribution, which can be used to monitor conservation status and inform protection and management of freshwater habitats.

Desmids are a group of freshwater algae that are particularly diverse in nutrient poor habitats like moorland pools and shallow lochs, and could be used as indicators of water quality.

The Outer Hebrides is an archipelago of 15 inhabited islands and over 100 smaller islands and skerries, situated 30-50 km off the west coast of Scotland.

There are over 6,000 freshwater and brackish lochs in the archipelago and, although the islands comprise only 1.3% of the UK’s landmass, they contain 15% of its freshwater surface area. Their water chemistry is influenced by their proximity to the sea and the presence of calcareous deposits in the machair areas.

These habitats vary from the highly acidic dystrophic pools and lochs of the blanket bogs and mires to the natural eutrophic lochs of the coastal plains of the machair. The freshwater habitats are predominantly nutrient poor and acidic; they are present in a variety of forms from deep water lochs and shallow lochans to drainage channels and flooded abandoned peat cuttings. Sphagnum is common in the peatlands and around shallow lochs.

Relatively mild and wet winters are characteristic of the Hebridean climate and during the winter large ephemeral pools form on the machair. These, together with ephemeral pools found in granite rock depressions and wet rock faces with mosses and liverworts, are fascinating atmophytic habitats and have a small, but interesting desmid flora.

The importantly rich and diverse desmid flora of areas such as north-west Scotland, western Ireland and the Lake District (Cumbria) has long been recognised, and although there were some small surveys in 1903, 1905 and the latter part of the 20th century the records were either scattered through various publications or remained unpublished.

The current survey began in 2018 with the intention to compile a biodiversity baseline for freshwater algae, concentrating on the single celled, microscopic algae known as desmids. Over the past 6 years we have collected water samples from over 200 sites from a wide range of habitats. So far, 3 new species have been described, and a considerable number of new taxa (species and varieties) have been added to the UK species index.

The  Outer Hebrides Algae website  illustrates over 650 taxa and the Outer Hebrides Biological recording database holds over 6,700 biological records, which are available on the  NBN Atlas .

The Outer Hebrides Desmid Survey has led to new discoveries, listed below, demonstrating the important role of recorders in enriching our knowledge of these poorly understood habitats.

1. Cosmarium pankakoskiiforme is currently only known from Loch na Gearrachan, Claddach, North Uist.
2. Cosmarium tasiussaqense was rediscovered in South Uist.
3. Cosmarium charnainense is currently only known from a shallow moorland pool, near Loch a’ Charnain, South Uist.
4. Cosmarium askernishense is currently only known from Loch a’ Chafain, South Uist.

Machair is a Gaelic word meaning fertile, low-lying grassy plain, formed over centuries by wind-blown, calcareous, shell-sand forming stable fixed dune systems. A rare and unique habitat occurring on the exposed west-facing shores of Scotland and Ireland. Being low-lying, it readily floods in the late autumn through to early spring, with the ample Hebridean rainfall.

During the repeated sampling of an area of flooded machair (South Uist, Outer Hebrides) an unfamiliar Cosmarium alga species was discovered which further research eventually revealed it to be Cosmarium tasiussaqense. This species was originally described from Tasiusaq Island within Upernavik Archipelago, Greenland from a site with a similar ecology to the South Uist site.

This original find occurred in the realm of the Nearctic. The South Uist find is the second record worldwide and the first for the western Palearctic.