Capturing temporal and spatial variability in the chemistry of shallow permafrost ponds

Abstract

Abstract. Across the circumpolar north, the fate of small freshwater ponds and lakes ( < 1 km 2 ) has been the subject of scientific interest due to their ubiquity in the landscape, capacity to exchange carbon and energy with the atmosphere, and their potential to inform researchers about past climates through sediment records. A changing climate has implications for the capacity of ponds and lakes to support organisms and store carbon, which in turn has important feedbacks to climate change. Thus, an improved understanding of pond biogeochemistry is needed. To characterize spatial and temporal patterns in water column chemistry, a suite of tundra ponds were examined to answer the following research questions: (1) does temporal variability exceed spatial variability? (2) If temporal variability exists, do all ponds (or groups of ponds) behave in a similar temporal pattern, linked to seasonal hydrologic drivers or precipitation events? Six shallow ponds located in the Hudson Bay Lowlands region were monitored between May and October 2015 (inclusive, spanning the entire open-water period). The ponds span a range of biophysical conditions including pond area, perimeter, depth, and shoreline development. Water samples were collected regularly, both bimonthly over the ice-free season and intensively during and following a large summer storm event. Samples were analysed for nitrogen speciation (NO − 3 , NH + 4 , dissolved organic nitrogen) and major ions (Cl − , SO 2− 4 , K + , Ca 2+ , Mg 2+ , Na + ). Across all ponds, temporal variability (across the season and within a single rain event) exceeded spatial variability (variation among ponds) in concentrations of several major species (Cl − , SO 2− 4 , K + , Ca 2+ , Na + ). Evapoconcentration and dilution of pond water with precipitation and runoff inputs were the dominant processes influencing a set of chemical species which are hydrologically driven (Cl − , Na + , K + , Mg 2+ , dissolved organic nitrogen), whereas the dissolved inorganic nitrogen species were likely mediated by processes within ponds. This work demonstrates the importance of understanding hydrologically driven chemodynamics in permafrost ponds on multiple scales (seasonal and event scale).

Using radon to quantify groundwater discharge and methane fluxes to a shallow, tundra lake on the Yukon-Kuskokwim Delta, Alaska

“…There is a significant amount of variability in the groundwater samples when considering both field samples and incubations. 222 Rn activities in groundwater at Landing Lake are likely controlled by the mineral content of soils, which is known to increase with depth in peatlands (Morison et al 2017a). Using the average endmember (Table 2, 24,000 ± 5,000 dpm m -3 ), the 222 Rn flux via groundwater (300 ± 100 to 1000 ± 400 dpm m -2 d -1 ) and Equation 3, we calculated groundwater fluxes of 0.012 ± 0.006 and 0.043 ± 0.020 m d -1 (1.2 ± 0.6, 4.3 ± 2.0 cm d -1 , Table 5), respectively, for conservative and upper limit estimates.…”

Snow, ponds, trees, and frogs: how environmental processes mediate climate change impacts on four subarctic terrestrial and freshwater ecosystems

“…Additionally, ponds in the HBL are extremely sensitive to evaporative desiccation, especially in years of low snowmelt (Bouchard et al 2013;MacDonald et al 2017). Increases in open-water evaporation (projected to increase by nearly 25% from the recent past to the end of this century) could increase evapoconcentration (Morison et al 2017b) and potentially lead to a further increase in productivity within HBL freshwater ecosystems. Changes to inpond primary production could have implications for higher trophic levels within the food web, including wood frog and boreal chorus frog food availability (Stephens et al 2015), although these increases could be curbed by pond temperatures approaching algal thermal maximum tolerance values (Chen 2015).…”

“…The magnitude of rainstorms is expected to increase, from an annual single-day maximum precipitation of 27-34 mm from the recent past to the end of the century (Table 2), although projected changes to summer precipitation are extremely varied across space, and generalizations are therefore more difficult (Shook and Pomeroy 2012;Harder and Pomeroy 2013). Across multiple years, short-term (i.e., hour-to-hour) variations in pond chemistry exceeded seasonal-scale variation in pond chemistry (Morison et al 2017a(Morison et al , 2017b, illustrating the potential of individual precipitation events to dictate factors mediating pond hydrochemical responses to climatic change. The relative vulnerability of these shallow pond and wetland systems to short-term, eventbased hydrochemical changes may be controlled hydrologically, particularly for conservative chemical species, in which the relative proportion/contribution of new and old water will dictate the extent of these short-term responses.…”

“…When large precipitation events occur during the ice-free season, they can have outsized consequences for pond ecosystems. For instance, pond hydrochemistry has been shown to be more variable within and following a 24 h period surrounding a large precipitation event than an entire ice-free season of variation (Morison et al 2017b). In fragile and complex ecosystems such as the HBL, extreme events can result in state changes and reset the impacts of other climate-driven changes.…”