Element-specific downward fluxes impact the metabolism and vegetation of kettle holes

Abstract

Given that water-filled kettle holes are mostly undergo a wet–dry cycle, and are directly fuelled by terrigenous material, it was hypothesized that the downward flux of matter, including P and its binding partners, varies between and within kettle holes, and is closely coupled to the prevalent water regime. Sedimentation was studied in two kettle holes close to Rittgarten (RG) and Kraatz (KR), Uckermark, NE Germany. Pairs of cylindrical traps at three sites in each kettle hole were sampled biweekly (June 2013–July 2014). Mean fluxes decreased with decreasing water level. KR was Fe-dominated binding P, and had submersed macrophytes. RG was Ca dominated and had low Fe concentrations suggesting that both apatite and oxidized Fe compounds equilibrated P release, with finally a surplus in P. Thus, RG was covered by duck weed. The higher C flux fuelled the sulphate reduction at higher rates than in KR, as also favoured by oxygen deficits due to duck weed coverage. Thus, internal eutrophication, i.e. where sulphate reduction and Fe sulphide formation lead to a lower Fe availability for P binding, is an issue for kettle holes increasingly degrading their ecosystem services.