Comparison of "Constant Rate" and "Slug" Addition Methods in the Study of Nutrient Retention in Rivers.

Within the spiralling length framework, the nutrient retention efficiency in rivers has become object of numerous works of investigation in fluvial biogeochemistry. Although the most popular method used to estimate the nutrient retention efficiency is the “constant rate” addition method, several studies are based on the “slug” addition method which is simpler and less expensive in time, instruments and reagents. However, the extent to which the results using both methods are comparable it is unknown. In order to reduce this gap we performed 10 double slug and constant rate addition experiments in two low- order Mediterranean streams: Fuirosos (Barcelona, Catalunya), draining granitic watershed, and Simbrivio (Rome, Italy), draining calcareous watershed. During each addition we estimate the phosphorus uptake rate (K, s-1) by means of a mass balance equation. K is a first-order uptake rate coefficient and can be used as an estimate of the stream nutrient retention efficiency. The experiments were carried out with stream discharges ranging from 3.5 to 25 ls-1 in Fuirosos and from 46 to 203 ls-1 in Simbrivio. In 70 % of the cases, the uptake coefficient determined by constant rate addition (Kc) resulted greater than the uptake coefficient determined by slug addition (Ks). The relative differences among the two estimates ranged between -54% and 86% in Fuirosos and -18% and 64% in Simbrivio. In more detail, in Simbrivio these differences increased significantly at high discharges (r=0.76, p<0.05). These preliminary results suggest that the uptake rates estimated with the slug method are not easily comparable to that estimated with the constant method.