Field evidence for buoyancy-driven water flow in a Sphagnum dominated peat bog

SummaryNocturnal buoyancy-driven water flow in bogs is proposed as a mechanism to replenish the nutrient availability in the top of the acrotelm. In an earlier paper, we provided evidence for buoyancy-driven water flow on theoretical and experimental grounds. In this paper, field evidence is given for the occurrence of nocturnal buoyancy cells. Measurements in a small isolated bog in the Netherlands showed that temperature differences between day and night and hydraulic conductivity resulted in a Rayleigh number that is sufficiently high to induce buoyancy flow. Irregularities between diurnal heating and nocturnal cooling gave evidence that indeed convective heat exchange occurred. Moreover, we observed a temperature rise at 5–10 cm depth around midnight, which would be expected on theoretical grounds if buoyancy-driven water flow occurs. Furthermore, the oxygen cycles at 5 cm depth showed an irregularity, which exactly matched the temperature irregularity. This also strongly suggests that buoyancy-driven water flow occurred. Nutrient samples taken from the same ombrotrophic bog showed a significant increase in the concentrations of NH4+, Ca2+, Mg2+ and CO2 with depth, whereas K+ showed a significant decrease with depth indicating that buoyancy-driven water flow may potentially redistribute these nutrients. The size of the buoyancy cells was assessed under controlled conditions in a peat core taken from the bog. We observed two spots of upward flow of warm water and two spots of downward flow of cold water. The cell sizes were approximately 25–100 cm2. We conclude that nocturnal buoyancy-driven water flow occurs in the field were it may be an important mechanism for relocation of oxygen and nutrients.