Concentration data of NO3-, NH4+, PO4, and Si from filtered sediment porewaters. Basis for the flux data “F” in Table 1

Porewater samples were conserved with 0.2% chloroform and analyzed at EAWAG, Switzerland, for NO3-, NH4+, SiO2, and o-PO4 using standard photometric methods (DEW, 1996). In 2002, NH4+ in porewater was measured on board of the research vessel with the indophenol method (DEW, 1996) and a portable photometer (Merck Spectroquant). In March and July 2001, porewater measurements of O2, NO3−, and NH4+ were performed with ion-selective electrodes on retrieved sediment cores from the South Basin, Vydrino, and Posolskoe High on the ice and in the hydrological institute on shore at Listvijanka to give vertical concentration profiles with high spatial resolution. Measurements are described in detail by Maerki et al. (submitted for publication). Diffusive fluxes across the sediment–water interface were calculated from the chemical gradients of their porewater concentration profiles assuming steady-state conditions and using Ficks first law of diffusion (Berner, 1980):