Factors controlling hydrochemical and trophic state variables in 86 shallow lakes in Europe

In order to disentangle the causes of variations in water chemistry among European shallow lakes, we performed standardised sampling programs in 86 lakes along a latitudinal gradient from southern Spain to northern Sweden. Lakes with an area of 0.1 to 27 000 ha and mean depth of 0.4–5.6 m located in low to high altitudes were investigated within the EC project ECOFRAME 1–4 times during June–October 2000–2001. Several variables like conductivity, alkalinity, abundance of submerged plants, concentrations of suspended solids, total nitrogen and phosphorus were latitude-dependent decreasing from south to north. Secchi depth, concentrations of total nitrogen, total phosphorus, suspended solids, and chlorophyll a correlated strongly with the presumed quality classes of the lakes. We came to the conclusion that the variability of shallow lakes in Europe is still mostly dependent on natural differences. Variables connected to lake morphometry, seasonality, basin geology and climate explained altogether nearly half of the total variability of lakes. The trophic state factor, describing mostly the human influence on lakes, was the strongest single factor responsible for nearly a quarter of the total variability of the studied European lakes [KEYWORDS: ecological status, European shallow lakes, hydrochemistry, latitude]

Factors controlling hydrochemical and trophic state variables in 86 shallow lakes in Europe

In order to disentangle the causes of variations in water chemistry among European shallow lakes, we performed standardised sampling programs in 86 lakes along a latitudinal gradient from southern Spain to northern Sweden. Lakes with an area of 0.1 to 27 000 ha and mean depth of 0.4–5.6 m located in low to high altitudes were investigated within the EC project ECOFRAME 1–4 times during June–October 2000–2001. Several variables like conductivity, alkalinity, abundance of submerged plants, concentrations of suspended solids, total nitrogen and phosphorus were latitude-dependent decreasing from south to north. Secchi depth, concentrations of total nitrogen, total phosphorus, suspended solids, and chlorophyll a correlated strongly with the presumed quality classes of the lakes. We came to the conclusion that the variability of shallow lakes in Europe is still mostly dependent on natural differences. Variables connected to lake morphometry, seasonality, basin geology and climate explained altogether nearly half of the total variability of lakes. The trophic state factor, describing mostly the human influence on lakes, was the strongest single factor responsible for nearly a quarter of the total variability of the studied European lakes [KEYWORDS: ecological status, European shallow lakes, hydrochemistry, latitude

Statistical quantification of the effect of thermal stratification on patterns of dispersion in a freshwater zooplankton community

The vertical distribution of crustacean zooplankton species was examined during 2000 in Windermere, Cumbria. Patterns of dispersion were evaluated quantitatively using two different approaches. Firstly, Morisita’s index was used to test whether patterns of dispersion differed significantly from a state of randomness and, secondly, the relative distribution of zooplankton individuals between the epilimnion and hypolimnion was investigated, for a series of standardised vertical profiles of organism density. All six of the dominant species of planktonic crustaceans showed aggregated patterns of dispersion throughout the year. For most species, patterns of dispersion were affected by the onset and breakdown of thermal stratification in the lake. The degree of aggregation in the vertical plane, measured using Morisita’s index, increased when the lake became thermally stratified. Furthermore, for most species, there was a positive association between the degree of vertical differentiation in abundance across the thermocline, and the degree of temperature differentiation in the stratified water column. The results of the present analysis provide quantitative evidence for the phenomenon known as ‘zooplankton stratification’ and for temporal variation in patterns of zooplankton dispersion