NUMBER TWENTY NINE - OCTOBER 1998
NUMBER TWENTY NINE - OCTOBER 1998
| < Previous | Next > |
5. General evidence for the alternative states model
Evidence for existence of alternative states in shallow lakes comes from several sources. First, surveys of shallow lakes in Denmark [87] (Fig 10) have shown that at similar phosphorus concentrations, there exist lakes with very high transparency and lakes with extremely low transparency. Transparency is measured as the depth at which a black and white sectored disc, about 20cm in diameter (a Secchi disc) disappears from view when lowered into the water and is taken as a surrogate for the gamut of characteristics of each state. Secondly, both states can be found in separate basins of the same lake, where free interchange of water and movement of organisms can occur through connecting channels (e.g. Hoveton Great Broad and Hudsons Bay [145], Box1)
Fig 10 Measures of transparency in Danish lakes show that either clear water or turbid water can exist at the same phosphorus concentration over a very wide range of phosphorus concentrations. From Jeppesen et al [86]
Thirdly, either state can be maintained under similar nutrient loading in experimental ponds [5,82], as discussed above, and fourthly, there is a range of instances where the plant-dominated state has been switched to the algal-dominated state through mechanisms operating independently of nutrient changes. In Lake Apopka, in Florida [127], initial destruction of plants by hurricane storm damage, and in Lake Zwemlust, the Netherlands, by application of herbicides 148, led to such a switch. Fifthly, there are well-documented examples where nutrient control has failed to restore a clearwater state but subsequent modification of the fish community has succeeded (e.g. Cockshoot Broad, UK [114]). Lastly, some lakes (e.g. Little Mere, UK, Box 3) [34] have retained clear water and submerged plant communities despite the presence of even abnormally high concentrations of nitrogen and phosphorus resulting from direct ingress of sewage effluent.
| < Previous | Next > |