Oscillating peat surface levels in a restiad peatland, New Zealand-magnitude and spatiotemporal variability

Abstract

Hydrology, particularly the water table position below the surface (relative water level, RWL), is an important control on biogeochemical and ecological processes in peatlands. The surface elevation (SE) in a peatland oscillates in response to changes in effective stress on the peat matrix mainly caused by water level fluctuations. This phenomenon is called peatland surface oscillation (PSO). To investigate the spatiotemporal variability of PSO, surface elevation and the water level above sea level (AWL) were measured monthly (23 sites) over one year in a warm-temperate restiad peatland, New Zealand. At one site peat surface elevation was measured indirectly by monitoring AWL and RWL continuously with pressure transducers. Annual PSO (the difference between maximum and minimum surface elevation) ranged from 3•2 to 28 cm (mean = 14•9 cm). Surface elevation changes were caused by AWL fluctuations. Spatially homogenous AWL fluctuations (mean 40 cm among sites) translated into RWL fluctuations reduced 27–56% by PSO except for three sites with shallow and dense peat at the peatland margin (7–17%). The SE-AWL relationship was linear for 15 sites. However, eight sites showed significantly higher rates of surface elevation changes during the wet season and thus a non-linear behaviour. We suggest flotation of upper peat layers during the wet season causing this non-linear behaviour. Surprisingly, PSO was subjected to hysteresis: the positive SE-AWL relationship reversed after rainfall when the surface slowly rose despite rapidly receding AWL. Hysteresis was more prominent during the dry season than during the wet season. Total peat thickness and bulk density together could only explain 50% of the spatial variability of PSO based on manual measurements. However, we found three broad types of SE-AWL relationships differing in shape and slope of SE-AWL curves. These oscillation types reflected patterns in vegetation and flooding.