Wednesday, 20 November 2013

The effect of an invasive Crayfish on European wetlands.

The North American Crayfish Procambarus clarkii, widely known as the Louisiana Crayfish or Red Swamp Crayfish, is a highly invasive species which originates in the southeastern United States but which is now known to have colonized waterways across North and South America, as well as in Europe, Asia, Africa and many island nations. It is generally held to have a negative impact on environments it invades, but its exact role can be hard to determine, as it is often found in waterways that have otherwise been affected by human behavior, i.e. drained, dammed, polluted or subjected to multiple species introductions.

A Red Swamp Crayfish, Procambarus clarkii, in a creek near Lake Hodges, California. Cindy Bingham Keiser/Project Noah.

In a paper published in the journal PLoS One on 24 October 2013, a team of scientists led by Jessica van der Wal of the Department of Aquatic Ecology at the Netherlands Institute of Ecology, examined the role of Procambarus clarkii on the development of submerged macrophytes (rooted plants living beneath the water surface) in Lake Terra Nova, a peat lake between Amsterdam and Utrecht in The Netherlands.

The lake had until the mid-1970s boasted clear, well oxygenated water about 1.4 m deep, with extensive growth of benthic macrophytes. However it is now largely lacking in macrophytes, with extensive algal growth at the surface and turbid, poorly oxygenated water beneath. The waters are known to have a high phosphorous content, believed to be from organic content in the sediment; phosphorous promotes turbidity and algal growth. Previous experiments in which Cyprinid Fish (Loach, Carp etc.), which are voracious eaters of macrophytes, were excluded, and in which a variety of treatments to reduce the phosphorous content of the water were tried, had met with little success. Procambarus clarkii was discovered in the lake in 2006, and it has been suggested that the Crayfish may increase the level of phosphorous in the water column through bioturbation (stirring up sediments).

The approximate location of Lake Terra Nova. Google Maps.

Van der Wal et al. set up a series of enclosures in which Fish and Crayfish were present, only Fish were present, only Crayfish were present and where both Fish and Crayfish were excluded. In addition enclosures were divided between two ponds isolated from the main lake, one of which was treated with iron chloride to reduce the phosphorous load, and the other untreated.

After six weeks it was found that the macrophytes had grown more successfully in the enclosures where Crayfish were absent, and that Crayfish were able to suppress plant growth every bit as efficiently as herbivorous Fish. Some (but not all) plants did better with the iron chloride treatment, though the population of Crayfish was significantly lower with the treatment. Examination of the stomach contents of Crayfish suggested that they were consuming significant quantities of macrophyte material.

Van der Wal et al. concluded that the Crayfish did present a significant constrain on the re-establishment of macrophyte biomass in the lake, even where all other factors had been accounted for; though they were by no means the only problem facing the lake. Attempts to remove or manage Procambarus clarkii where it has become established have proved highly problematic, leading to the recommendation that measures should be taken to prevent the species from reaching other European wetlands wherever possible.


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