RESPONSE OF SURFACE MICROLAYERS TO ARTIFICIAL ACID PRECIPITATION IN AMESO-HUMIC LAKE IN NORWAY

A meso-humic lake was divided into two basins. One basin and its catch ment area were treated with an acid solution (pH 3.0-3.5), while the o ther 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 no n-treated (B) basins and respective subsurface water (SW) were compare d to identify possible effects of the acid treatment. The average enri chment factor (Er) of particulate organic carbon (POC), i.e. the quoti ent of SM and SW concentrations, was 16 in SM A and 18 in SM B during the 1994 investigation. The particulate fraction ( > 0.45 mu m) of org anic carbon constituted 25-30% of total organic carbon in SM A and 10- 15% in SM B during the study. E-r values of dissolved organic carbon ( DOG) rarely exceeded 3 and varied little during the day-night cycle. U V254 absorbance capacity of SM was 10-15% lower in the treated basin t han in the control during the whole study. In 1992, SM B contained rel atively high amounts of UV-absorbing high-molecular-weight (HMW) mater ial, while SM A contained two distinct fractions, one of higher and on e of lower molecular weight. This coincided with a higher biomass of p hytoplankton in SM B than in SM A. Acetic acid was the most abundant s imple organic acid detected in SM and is a common product of organic a cid degradation. The volatile carboxylic acids (C-2-C-6) represented a lower fraction of the DOC than the less vola tile fatty acids (C-8-C- 18). Yet, these compounds contributed less than 1% of total DOG. Prote in material, measured as particulate organic nitrogen, was greater in SM A than in SM B. Nitrogen fractions, base cations and trace metals c oncentrations in SM A were qualitatively different from SM B. This cou ld have been an effect of the treatment, but it was probably also rela ted to differences in hydrology and geomorphology in the catchment are a. The acid treatment of the catchment and lake influenced the microla yer properties by increasing particle formation and by acid degradatio n of HMW organic material. The acid degradation resulted in reduced UV extinction in the SM of the treated basin. (C) 1997 Elsevier Science Ltd.