The site contains the largest area of open water on St Mary's, which supports a wide range of aquatic plant and animal species. The area is associated with topogenous mire habitats which are developed on peat and alluvium. A vegetated sand and shingle bar separates this site from the open sea at Porth Hellick.
The shingle and sand bar at the back of Porth Hellick has strandline vegetation, dominated by Sea sandwort, Honkenya peploides, with a small population of Sea kale, Crambe maritima. There are also records of the rare Boccone’s sand-spurrey, Spergularia bocconii, occurring here. The narrow band of maritime grassland behind has abundant Red fescue, Festuca rubra, and Thrift, Armeria maritima, merging into the wetland habitats around the lake.
The pool is freshwater but occasional salt intrusion occurs which allows some salt-enduring species to survive at the seaward end. Examples of these species include Sea club-rush, Scirpus maritimus, Saltmarsh rush, Juncus gerardii, Brackish water-crowfoot, Ranunculus baudotii and Sea-milkwort, Glaux maritima. Common reed, Phragmites australis, fringe most of the pool backed by a border of Grey willow, Salix cinerea; Bulrush, Typha latifolia, is also present.
The wetland habitats include reedbeds and marsh areas, Soft rush, Juncus effusus, Yellow iris, Iris pseudacorus, Lesser spearwort, Ranunculus flamula, Gypsywort, Lycopus europaeus, Water mint, Mentha aquatica, Hemlock water-dropwort, Oenanthe crocata and Ragged robin, Lychnis flos-cuculi, are abundant together with populations of Royal fern, Osmunda regalis, Greater tussock-sedge, Carex paniculata and Southern marsh orchid, Dactylorhiza praetermissa, a species rare in Scilly.
More acidic bog conditions are indicated by small populations of Bog pimpernel, Anagallis tenella, Star sedge, Carex echinata, Marsh St John’s-wort, Hypericum elodes, Marsh willowherb, Epilobium palustre, Bog stitchwort, Stellaria alsine and Bog pondweed, Potamogeton polygonifolius.
The stream flowing from Holy Vale into the pool is the only running water habitat of any size on Scilly. There is a dense growth of Hemlock water-dropwort in the lower reaches whilst further upstream there is a narrow band of fringing woodland with English elm, Ulmus procera and Grey willow.
The wetter marsh to the west has Marsh willowherb, Yellow Iris, Creeping forget-me-not, Myosotis secunda, Purple loosestrife ,Lythrum salicaria and Wild angelica, Angelica sylvestris. The slightly higher and drier adjacent areas are dominated by Bracken, Pteridium aquilinum, and Bramble, Rubus fruticosus with several populations of the nationally rare Balm-leaved figwort, Scrophularia scorodonia.
The pool and the surrounding reed beds are important for dragonflies and damselfies, like the Blue-tailed damselfly, Isnura elegans. Breeding bird species include Mallard ,Anas platyrhynchos, Gadwall, A. strepera, Teal, A. crecca, Coot, Fulica atra, Moorhen, Gallinula chloropus and Sedge warbler, Acrocephalus schoenobaenus.
The site also provides valuable food and shelter for wintering birds, especially Wigeon, Anas penelope, Pochard, Aythya farina, Snipe, Gallinago gallinago, and Water rail, Rallus aquaticus. This freshwater habitat on the Isles of Scilly is nationally famous for rare vagrant birds, and also attracts less common passage migrants such as Jack snipe, Lymnocryptes minimus.
Higher Moors and Porth Hellick Pool are SSSI sites which lie within the Isles of Scilly Area of Outstanding Natural Beauty (ANOB) and Heritage Coast.
Natural England surveyed the area and found the site to be in a favourable condition. To uphold this status, careful management of the land is crucial.
To maintain the redbeeds, the other wetland habitats and shoreline, cutting and clearing vegetation from areas in and around the edges of the pool is important to help control the spread of more dominant species of plants and to maintain species diversity.
There is a good mixture of grass, wetland species and bracken and management is achieved by rolling bracken, grazing and exposing damp hollows.