Seasonal variation of water chemical gradients in three boreal fens

Surface water chemistry was monitored in three fens through one growing season by weekly sampling in the southern aapa mire zone, middle boreal Finland. The fens included a soligenous moderately rich fen, a soligenous extremely poor fen, and a topogenous extremely poor fen. The chemical variables most clearly related to the poor—rich gradient were pH, alkalinity, Ca, Mg, and Na concentrations. The soligenous and topogenous extremely poor fens differed in DOC, Al, Fe, Si, and S concentrations. The main chemical gradients were related to the season. In early summer (June to July) the chemical gradients were variable and weakly definable. In late summer (August to September) the gradients were more stable and separated the three fens clearly. Late summer is suggested optimal for comparable sampling of surface water in boreal fens.