Gases (CH4, CO2 and N2) and pore water chemistry in the surface sediments of Lake Orta, Italy: acidification effects on C and N gas cycling

Lake Orta, a subalpine, warm monomictic lake in northwestern Italy was heavily polluted from rayon factory discharges of ammonium and copper since 1926. In the 1950s accumulations of contaminants resulted in whole lake pHs of 3.8-4.0 from ammonium oxidation. Partial remediation started in the 1950s, but by 1985-89 the water remained acidified at pHs of 4.0. Artificial liming (14,500 t) in 1989-90 resulted in improved water quality and substantial recovery of the biological community. Sediment gases, sampled in 1989 before liming, from the lake's four basins showed severe inhibition of methanogenesis (CH4 = 0.0-0.15 mM) in the surface sediments (0.5-5 cm) of the southern basin, location of the plant effluent, as compared to the deep central and northern basins (0.9-1.4 mM). Four years after liming, cores collected in 1994 near the 1989 southern basin sites showed a slight change in surface sediment methane (0.07-0.82 mM), yet suggested continual sediment toxicity, at least to carbon cycling through methanogenesis. Calculations of diffuse flux of CH4 at the sediment-water interface (SWI) in 1989 were 6.6-7.4 mM m-2 day-1 for the central and northern basins and 0.13 for the southern basin. CH4 fluxes increased 16x to 2 mM m-2 day-1 in 1994 in the southern basin, possibly from remediation of near surface sediments. The impact of pollution on denitrification (formation of sediment N2 gas) was not so obvious since two processes could counteract each other (high NO3 - stimulating denitrification versus possible negative effects from acidity and metals). The calculated flux of N2 from the southern basin sediments increased 5x four years after liming compared to the period of acidification, suggesting possible toxicity towards denitrifiers during the earlier period. Core overlying water (0.68 mM) exhibited N2 concentrations close to saturation, while most surface sediments were twice as much (1.5 mM). Surface (0-6 cm) sediment N2 was similar at most sites, with the exception of 0.82 mM in the low porosity sediments of the Omegna basin at the lake’s outlet. The calculated diffuse flux of N2 at the SWI averaged 4 mM m-2 day-1 for the entire lake and varied between 1.3 at Omegna to 7.3 at the Central basin. It is suspected that surface sediment N2 production from denitrification and subsequent flux from sediments to overlying waters should resolve much of the deficit in the mass budget for water column nitrogen during the acidification period where only 35 percent of the ammonium oxidation could be accounted for as nitrate.