Pore-water Feedbacks and Resilience to Decay in Peat-filled Bedrock Depressions of the Canadian Shield

Abstract

Northern peatlands are able to persist on the landscape and continue to accumulate carbon in the long-term thanks to a suite of ecohydrological feedbacks that confer resilience to disturbance such as the drier and warmer conditions associated with climate change. One feedback of particular interest operates between peat pore-water residence time and chemistry, whereby changes in hydraulic structure with depth restrict turnover in deeper layers, allowing decay end-products to accumulate and thermodynamically suppress decomposition. In this way, the burial of peat facilitates its continued recalcitrance. While this feedback has been observed in more extensive northern peatlands, at least on the side of carbon dynamics and geochemistry, there has been no observational study of profiles of pore-water residence time nor has it been assessed in smaller peat-forming systems. The peat-filled bedrock depressions of the Canadian Shield offered a unique opportunity to study this feedback in systems where primary peat formation occurs under geological constraints on growth in the form of the largely impermeable bedrock. These systems play important hydrological, biogeochemical and ecological roles on the landscape. Understanding their resilience on the landscape may reveal key insights into their evolution and their response to disturbance, which is increasing in the eastern Georgian Bay region. These systems have previously exhibited a hydrological feedback between water table depth and specific yield that varies with depression size. To assess the hydraulic structure that constrains pore-water transport to support continued recalcitrance, profiles of hydrophysical properties and pore-water residence time in four deep (>0.4 m mean depth) and five intermediate (<0.4 m) depressions. Hydraulic structure varied by depression size and depth in the profile, with very low hydraulic conductivities measured in the catotelms of deep sites. The two classes of depressions exhibited distinct hydrology, in the form of dampened water table fluctuations and hydraulic gradients in the deeper sites. Stable isotope analysis of δ2H and δ18O was used to estimate relative pore-water residence times using the simplified inverse transit time proxy (ITTP) for samples collected from May-August 2017. These estimates were observed to have similar controls to hydraulic structure and a close relationship with depth-averaged conductivity on a whole-site basis. While it was hypothesized that the catotelms of deeper depressions would have less pore-water turnover than that of shallower depressions, the ITTP was only able to differentiate between catotelm-acrotelm and deep-intermediate individually. The relative residence time of pore-water in deep catotelms based on δ2H was longer than in intermediate catotelms, but not significantly. These results broadly supported previous pore-water residence time work despite the likely ubiquitous promotion of turnover in the wetter-than-average study period. Carbon accumulation was quantified from extracted peat cores and pore-water chemistry was assessed as dissolved organic matter (DOM) quality using fluorescence spectrometry of monthly pore-water samples. Fluorescence and absorption indices varied by the same depression characteristics as hydraulic structure of site size and depth, but only the humification index exhibited significant temporal variation. Characterization of pore-water DOM was somewhat unclear across the seven indices calculated, although the DOM of intermediate sites appeared to be less humified, more recently produced and autochthonous in nature compared to deep sites Carbon accumulation was predominantly driven by the waterlogged, relatively stable carbon stored deep in the catotelm. Total carbon accumulated in the profile, and even more so the amount stored in the catotelm, were strongly related to depression depth. The thickness and carbon storage of the acrotelm was insensitive to depression morphology, with some intermediate sites being considered all acrotelm based on their water table behaviour. Overall, deeper peat-filled depressions showed stronger signs of the pore-water residence time-chemistry feedback, suggesting the carbon stored in their deep peat layers is more resilient to decay, by way of less conductive deep peat, longer relative pore-water residence times and more humified, less biologically active DOM. In order to comprehensively assess this feedback, longer stable isotope records are essential to ensure robust residence time estimates through differing moisture conditions, and a greater variety of depression sizes may allow for elucidation of threshold depression sizes where hydrological behaviours diverge. This study, at least on a categorical basis, can be used to inform conservation strategies of the relative vulnerability of these important reptile habitats and carbon stores, as well as guide restoration efforts to construct sufficiently deep, resilient systems.