Wednesday, 24 August 2011

Salty or sweet Sir?

Redshank, Black-tailed Godwits, Ruff and Dunlin feeding on the lagoon at Conwy as the water drops to a suitable feeding depth; Ragworm or bloodworm, which looks tastiest?

At what salinity should coastal lagoons be kept to maximise bird use? Coastal lagoons have a salinity that will vary between seawater (at 35 parts per thousand (ppt)) and fresh (1 ppt). The salinity will influence the invertebrates that dominate the lagoon and therefore the birds that are found there. This week I visited Conwy in north Wales where a proposal to change the lagoons from fresh to saline is being considered. How might this affect the birds that feed on the lagoons?

Low salinities (less than 8 ppt) will favour non-biting midges (chironomids) in the mud, with water boatmen (corixids) and opossum shrimps in the water. At higher salinities (8-40 ppt) ragworms Nereis and mud shrimps Corophium will join the chironomids and the shrimp Palaemonetes varians may be abundant in the water. The maximum biomass of the chironomid/corixid and ragworm/shrimp fauna will occur at about 6 and 24 ppt respectively.  However, the other key factor for attracting birds is the water level.  Good numbers of birds will only occur if the prey items are available at suitable water depths so good control of the water levels is desirable.

At Minsmere, various parts of the scrape are kept at different salinities; North Girder (in front of South Hide) at 15-35 ppt, whilst East Scrape is usually less than 15 ppt. The freshest scrapes (eg West) can become dominated by aquatic vegetation such as Mare’s-tail; an occasional dosing with salt water will sort this out. Switching between high and low salinities will kill off invertebrates. However, most of the less specialised invertebrates can re-colonise rapidly. Such switching may be useful as an occasional management tool but not on a regular basis.

The excellent scrapes at Cley are fed by fresh water draining from higher land, and are usually less than 1 ppt. However, high tide surges can bring an incursion of saltier water. Vange Marsh in south Essex, a seriously good addition to the scrape world, is also kept at a low salinity, usually less than 5 ppt, attracting sandpipers, shanks and godwits.

Last Friday I was at Havergate Island, where the lagoons support scarce lagoon invertebrates such as the Starlet Sea-anemone. Here the invertebrates rather than the birds determine the water regime as the salinity needs to remain in the favoured range of 25-40 ppt for these lagoon specialists. The lagoons did however, hold hundreds of birds, including 14 Spoonbill, 350 Avocets as well as numerous Knot, Black-tailed Godwits, Dunlin, Grey Plover and Redshank.

Below - How many Starlet Sea-anemones can you see?

No comments: