Response of surface microlayers to artificial acid precipitation in a meso-humic lake in Norway

Abstract

A meso-humic lake was divided into two basins. One basin and its catchment area were treated with an acid solution (pH 3.0–3.5), while the other served as control. To study the influence of the treatment on the physical and chemical properties of surface microlayers (SM), samples were taken with a remotely controlled slick collector, fitted with a hydrophilic Teflon-coated rotating drum. SM of acid-treated (A) and non-treated (B) basins and respective subsurface water (SW) were compared to identify possible effects of the acid treatment. The average enrichment factor ( E f) of particulate organic carbon (POC), i.e. the quotient of SM and SW concentrations, was 16 in SM A and 18 in SM B during the 1994 investigation. The particulate fraction ( > 0.45 μm) of organic carbon constituted 25–30% of total organic carbon in SM A and 10–15% in SM B during the study. E f values of dissolved organic carbon (DOC) rarely exceeded 3 and varied little during the day-night cycle. UV 254 absorbance capacity of SM was 10–15% lower in the treated basin than in the control during the whole study. In 1992, SM B contained relatively high amounts of UV-absorbing high-molecular-weight (HMW) material, while SM A contained two distinct fractions, one of higher and one of lower molecular weight. This coincided with a higher biomass of phytoplankton in SM B than in SM A. Acetic acid was the most abundant simple organic acid detected in SM and is a common product of organic acid degradation. The volatile carboxylic acids (C 2–C 6) represented a lower fraction of the DOC than the less volatile fatty acids (C 8–C 18). Yet, these compounds contributed less than 1% of total DOC. Protein material, measured as particulate organic nitrogen, was greater in SM A than in SM B. Nitrogen fractions, base cations and trace metals concentrations in SM A were qualitatively different from SM B. This could have been an effect of the treatment, but it was probably also related to differences in hydrology and geomorphology in the catchment area. The acid treatment of the catchment and lake influenced the microlayer properties by increasing particle formation and by acid degradation of HMW organic material. The acid degradation resulted in reduced UV extinction in the SM of the treated basin.