Hydro-Period Influence on Kettle Hole Biogeochemistry in NE Germany

Abstract:

Hydro-Period Influence on Kettle Hole Biogeochemistry in NE Germany

Kettle holes are glacially created ponds (<0.01 km²) that form within landscape depressions and, while small, they are numerous across young moraine landscapes in Europe and North America. Kettle holes are only seasonally connected to streams or groundwater and therefore they undergo pronounced short-term changes in the hydro-periods, i.e. water level fluctuations that include complete desiccation and rewetting. Little is known about kettle hole biogeochemistry in NE Germany, especially with regards to the hydro-period. We hypothesized that a connection exists between kettle hole hydro-period and sediment biogeochemistry. We surveyed kettle hole water in NE Germany over several years to capture the seasonal isotopic composition (δD, δ¹⁸O). Within a subset of the surveyed kettle holes we measured the δ¹³C and δ¹⁵N composition of sediments at two different depths from one season. Our objective was to link the abiotic influences demarked by the evaporative isotopic signal from kettle hole water, to biotic processes, such as microbial turnover and contributions of vegetation, imprinted in the δ¹³C and δ¹⁵N signals in sediment organic matter. Based on the upper sediment isotopic signal, we were able to classify two categories: permanently and temporarily filled kettle holes. Other kettle holes, for example those found in forests, were not as easily classified. Within the deeper sediment layers we found a distinct curve linear response between δ¹⁵N and C/N ratios, where temporarily filled kettle holes were consistently enriched, indicating a higher level of microbial transformation. We evaluated our evaporation estimates against the sediment-based classification to test evaporation as a major mechanism behind kettle hole biogeochemistry. While the temporarily filled kettle holes are the most biogeochemically dynamic, due in large part to their hydro-period, the mechanisms underlying the hydro-period and the concurrent effects on biogeochemical cycles are diverse.