Space–time configurations of solute input and biological uptake in river systems traversing limestone uplands (Yorkshire Dales, northern England)

Abstract

Solute concentrations, including major plant nutrients, are surveyed from sampling during 1985–89 in river systems variably influenced by upland Dinantian (Carboniferous) limestone. Concentrations of most ions generally increase downstream, but there are widespread dilution effects after rainy periods for major ions and local depletions of mostly biological origin for major plant nutrients. In the upper reaches, these depletions develop mainly under low flow from the influence of benthic algae and macrophytes; they affect inorganic C as HCO₃ ^- plus dissolved gaseous CO₂ and soluble reactive forms of Si, N and P. Inflow – outflow differences of an upland lake result from biological influence on these constituents and also Ca⁺ and K⁺. Experimental injections of NH₄-N, PO₄-P and K⁺were made to an acid headwater of the River Swale, together with Na⁺ and Cl^- as supposed conservative reference ions. Uptake is insignificant for K⁺ but marked for NH₄-N and PO₄-P; the residual percentage decreases exponentially with time. Relative rates of net uptake per unit time are greater for P than for NH₄-N and larger under conditions of higher flow velocity and benthic algal growth. Although limestone is lacking in some headwater regions, where atmospheric inputs appear to predominate, overall its chemical denudation as Ca²⁺ and HCO₃ ^-provides most of the solute flux downstream, as well as in some karstic headwaters where the highest values are limited by CaCO₃ saturation.