Friday, 28 June 2013

Lac de Grand-lieu

The Lac de Grand-lieu has been the origin of many of the colour-ringed Great White Egrets seen in the UK; a power-house for herons and another launching pad for UK colonisation.  A day on a boat with the warden Sebastien Reeber brought home what an amazing wetland this is.   The Lac de Grand-lieu has open water (the lake), reed, marsh extensive willow scrub, wet woodland and surrounding wet grassland.   The water is around 0.8-1.5 m deep in summer but rises by 2 m in winter to cover an amazing 6,000 ha. The levels fluctuate annually to maintain a huge dynamic wetland.

The summer extent of the lake is around 2,000 ha and of this 1,000 ha is covered in water lilies.  It took half an hour or so to pass through the lily zone, mobbed all the time by Whiskered Terns.  Sebastien listed the breeding birds: 2,000 pairs of Whiskered Tern, 1,000 pairs of Cattle Egret, 160 pairs of Spoonbill, 300 pairs of Little Egret, 160 pairs of Purple Heron, 250 pairs of Night Herons, 700 pairs of Grey Heron…….  In winter, there are 13,000 Shoveler, 5,000 Pochard...  The Great White Egrets have been established as a breeding bird since 1994 and have increased to a current total of 160 pairs over the last 20 years.

The wet grassland beyond the lake will typically be submerged by 1m or more of water during the winter before the levels drop in April/May.  The breeding birds include Black Tern, Black-necked Grebe and occasional Ruff amongst the more familiar waders.  The ‘yellow’ wagtails on site are an intergrade between flava and iberiae with a corresponding range of head patterns.  An endangered plant now restricted to just a handful of sites in the UK, Starfruit was abundant along the tracks, along with Lesser Water-plantain.

The wetland is however, far from being in a ‘natural’ state.  Invasive non-native species are a major problem, with some of the most obvious being Red Swamp Crayfish, Sacred Ibis, Black Swan, Coypu and Muskrat.  We don’t want any of these in the UK!   And yes, there are plenty of Ruddy Ducks.  The Sacred Ibis are being controlled.  Formerly 800 pairs, they are now reduced to around 300.  The Ibis are a major predator of the eggs of birds on the wet grassland but perhaps more of this later.

Red Swamp Crayfish is now abundant in the wetland – abundant to the tune of 2 tonnes of crayfish per hectare. Perhaps their most obvious effect is the devastating loss of submerged aquatic weeds.  RSC have previously spread throughout the Camargue, where they have been well studied since they were first found in the 1990s.  The RSC have a varied diet (insects, fish, amphibians, plants and seeds) essentially made up (80%) of organic matter and detritus.  Introduction of crayfish into ponds showed that after 10 weeks a density of 3 RSC m2 leads to an 80% decrease in the biomass of macrophytes and a 33% decrease in the diversity of macro-invertebrates.  On the plus side, they now form the major prey item for some wetland birds; 67% of the diet of Bitterns and up to 80% of the diet of Glossy Ibis, Spoonbill and Cattle Egret.  The Red Swamp Crayfish create a direct link in the food web between organic detritus and the top predators, providing the latter with a large invertebrate prey (primary consumer) in high numbers, increasing the overall amount of food available to them.   Even I can’t eat enough crayfish to get us out of this mess.
Above - lilies, Whiskered Tern, and nest.  Below: not-so-sacred Ibis, Starfruit, Flava/iberiae wag.

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