Method of evaluating nutrient loads through the atmosphere onto lakes

The amounts of atmospheric depositions were measured by two types of bulk deposit-samplers (BDS). One of them was a commonly used dry-type BDS mounted with a conical polyethylene (PE) funnel on a PE-reservoir (d-BDS). The second BDS was a wet-type BDS which was devised to evaluate the nutrient loads deposited directly from the atmosphere onto lakes, and mounted with a cylindrical funnel filled with 0.005-M H2SO4 at 5 cm in depth (w-BDS). The water sample in the reservoirs was collected after an appropriate time interval. As the funnel sizes of d-BDSs decreased from 20 to 9 cm in diameter, the trapping efficiencies of TN and TP increased, as measurement accuracies decreased. The efficiencies measured by w-BDSs were not affected by the funnel size in the tested range from 20 to 43 cm. Based on the data continuously measured by d-BDS20 and w-BDS20 mounted each with a 20 cm funnel for one year, the atmospheric deposition of TN and TP measured by d-BDS20 were 18.1 and 0.745 kg ha−1 y−1, which were apparently smaller than 20.7 and 0.921 kg ha−1 y−1 measured by w-BDS20. It is recommended from the results that the direct nutrient-loads from atmosphere into lakes should be evaluated on the base of the more reliable unit loads measured by a w-BDS mounted with a funnel larger than around 30 cm in diameter.

Dry-type bulk deposit samplers (d-BDS) mounted with conical-shaped funnels with orifice radius from 10 to 20 cm have been widely used to measure atmospheric deposition for studies of material balance in forests and agricultural lands. However, the size of the funnels has never been standardized. It was shown in this study that the trapping-efficiencies of d-BDSs with funnel smaller than 20 cm were characterized by an inverse relationship with the orifice radius and by decreasing accuracy of measurement, regardless of the tested matter. Whereas the trappingefficiencies of d-BDSs with funnels larger than 30 cm in diameter became asymptotically stable. It is suggested that the observed phenomena resulted from a fringe effect which can be defined by the ratios of the fringe lengths to the orifice areas for conical-shaped funnels. It is recommended from the results that d-BDSs with funnels larger than 30 cm should be used for measurements of atmospheric depositions on terrestrial systems to avoid possible overestimation and to maintain the accuracy of measurements. d-BDSs have been uncritically used to measure the amounts of materials supplied directly from atmosphere onto lake ecosystems. Wet-type bulk deposit samplers (w-BDS) mounted with the cylindrical funnels were prepared in this study and filled with 0.005-M H2SO4. From the results obtained from the experiments using the funnels larger than 20 cm in orifice diameter, it was shown that the trapping-efficiencies were not affected apparently for the funnel-sizes from 20 to 43 cm. However, the accuracy of the w-BDS with a 20-cm funnel was inferior to the larger funnels. Then, the trapping-efficiencies were compared between w-BDS20 (orifice diameter: 20 cm) and d-BDS21 (21 cm) for one year. The depositions of TN and TP measured by w-BDS20 were 20.7 and 0.921 kg ha−1 y−1, respectively, and 18.1 and 0.745 kg ha−1 y−1 by d-BDS21. The results showed that the evaluation of nutrient loads based on the d-BDS method might underestimate the effect of the atmospheric depositions on the eutrophication of lakes. Consequently it is recommended that the direct nutrient-loads from atmosphere onto lakes should be measured by wet-type bulk deposit samplers mounted with funnels larger than around 30 cm in diameter.