Biogeochemistry of natural ponds in agricultural landscape: Lessons learned from modeling a kettle hole in Northeast Germany

Abstract Kettle holes, small shallow ponds of glacial origin, represent hotspots for biodiversity and biogeochemical cycling. They abound in the young morainelandscape of Northeast Germany, potentially modulating element fluxes in a region where intensive agriculture prevails. The Rittgarten kettle hole, with semi-permanent hydroperiod and a surrounding reed belt, can be considered as a representative case study for such systems. Aiming to provide insights into the biogeochemical processes driving nutrient and primary producerdynamics in the Rittgarten kettle hole, we developed a mechanistic model that simulates the carbon, nitrogen, phosphorus and oxygen, phytoplankton, and free-floating macrophytebiomass dynamics. After model calibration and sensitivity analysis, our modeling exercise quantified the simulated nutrient fluxes associated with all the major biogeochemical processes considered by the model. Seasonality of nutrient concentrations, magnitude of primary productivity rates, and biogeochemical process characterization in the pond were reasonably reproduced by the model from July 2013 to July 2014. Our results suggest that the establishment of a phytoplankton community well-adapted to low light availability, together with the differential use of N and P from free-floating macrophytesand phytoplankton can explain their coexistence in kettle holes. Sediment nutrient release along with decomposition of decaying submerged macrophyteare essential drivers of internal nutrient cyclingin kettle holes. Our results also suggest that the Rittgarten kettle hole act as a net source of CO 2 to the atmosphere on an annual scale, which offers a testable hypothesis for kettle holes with structural and functional similarities. We conclude by discussing the need to shed light on the effects of water level fluctuations on nutrient dynamics and biological succession patterns, as well as the relative importance of external sources and internal nutrient recycling mechanisms.