Cadmium Release in Soil Solutions at Low Moisture Content

Abstract

Speciation of cadmium (Cd) was studied in four spiked agricultural soils at moisture content corresponding to 1.2 times field moisture capacity (FMC) and in the range from 1.2 FMC to soil–water 1∶10. Cadmium desorption isotherms were nonlinear in all soils, resulting in the decrease in Cd partition coefficient with loading. The Windermere Humic Aqueous Model (WHAM VI) was applied to predict Cd concentration in the solutions, and predicted values were compared with the measured ones. Based on total Cd content in soils, with concentrations of dissolved organic carbon (DOC), calcium (Ca), magnesium (Mg), and sodium (Na) and soil solution pH as the input variables, WHAM VI predicted Cd concentration in soil solutions with the root‐mean‐square error (RMSE) of log[Cd] RMSE_log[Cd]=0.54 (n=37). Using total Cd content in soils, average concentrations of Ca and DOC in soil solutions, and soil pH instead of soil solution pH enabled prediction of Cd concentration in soil solution with RMSE_log[Cd]=0.56. Calculation of Cd concentration as a function of moisture content resulted in RMSE_log[Cd]=0.25 (n=20).     

Changes in the population density of pelagic salmonids in relation to changes in lake enrichment in Windermere (northwest England)

From July 1989 to December 1994, an echo sounder provided monthly estimates, usually for both day and night, of pelagic salmonid densities in the North and South Basins of Windermere, the largest natural lake in England. Sampling was along contiguous transects, three in the North Basin and five in the South Basin. Records for Arctic charr (Salvelinus alpinus) could not be separated from those for brown trout (Salmo trutta), but previous sampling by gill-nets and anglers showed that charr formed over 90% of this mixed population in the North Basin and about 60–75% in the South Basin. Associated with the increasing eutrophication of the lake, there has been a decline in anglers' catches of charr and, since 1984, an increase in brown trout taken in the pelagic zone of the South Basin. The echo-sounder data showed that pelagic salmonid density in the North Basin was about two to five times that in the more eutrophic South Basin in 1989, 1990 and 1991. Since the start, in April 1992, of the reduction of phosphorus discharged from sewage works, this ratio has decreased, especially at night when the highest densities were recorded. This improvement was chiefly due to a significant (P<0.001) increase in the density of small fish (length <20 cm), in both the upper (depth <20 m) and deeper (depth >20 m) water layers. Although a similar improvement has still to be shown in the upper water layer by larger fish above the size limit for removal by angling (20 cm), there has been a significant increase (P<0.01) in the density of these fish in the deeper water layer of the South Basin. The increased density of small fish suggests that the stock available to charr anglers (fish >20 cm at water depths <20 m) should increase in the next few years, especially in the South Basin. It is therefore important to continue the monitoring program and thus ensure that there is advance warning of any marked changes in charr stocks.     

The effect of large-scale spatial variation of pelagic fish on hydroacoustic estimates of their population density in Windermere (northwest England)

From July 1989 to December 1992, an echo sounder provided monthly estimates, usually for both day and night, of pelagic fish densities in the north and south basins of Windermere, the largest natural lake in England. Sampling was along contiguous transects, 3 in the north basin and 5 in the south basin. It was impossible to separate records for Arctic charr (Salvelinus alpinus) from those for brown trout (Salmo trutta), but previous sampling by gill-nets and anglers showed that charr formed about 90% of this mixed population in the north basin and 60–75% in the south basin. In each basin, estimates of population density for the combined transects did not agree with estimates obtained by treating the transects as a contiguous sample of 9 sampling units; only the latter, of course, provided a measure of the precision of each estimate. Analyses of the contiguous samples showed that the variance was significantly greater than the arithmetic mean in most samples, indicating that the fish were distributed nonrandomly in a patchy or clumped pattern. The relationship between the large-scale spatial variance and mean for these samples was well described by a power function; the parameter estimates did not vary significantly between basins, day and night samples or years. As the power in this equation did not differ greatly from 2 (value for pooled data was 1.70 ± 0.11, n= 136), the variance was stabilized by a log transformation of the data, and the geometric mean, rather than the arithmetic mean, provided the best estimate of population density when some measure of precision was required. These conclusions may be applicable to other echo-sounding estimates of population abundance, and similar comparisons should be made for pelagic fish in other lakes.