Linking testate amoeba assemblages to paleohydrology and ecosystem function in Holocene peat records from the Hudson Bay Lowlands, Ontario, Canada

Abstract

Peat cores from boreal bog and fen sites in the Hudson Bay Lowlands of Northern Ontario, Canada, were analysed to calculate Holocene carbon accumulation rates, and to show how testate amoeba taxonomic assemblages, inferred depths to water table, and four morpho-traits that may be linked to function (mixotrophy, aperture size, aperture position, and biovolume) changed since peatland initiation. Carbon accumulation rates were on average higher for the Holocene in the fen record (19.4 g C m−2 yr−1) in comparison with the bog record (15.7 g C m−2 yr−1), which underwent a fen-to-bog transition around 6900 cal yr BP. Changes in rates of carbon accumulation were most strongly driven by changes in rates of peat vertical accretion, with more rapid rates in the fen record. Carbon accumulation rates were highest following peatland initiation when reconstructed water tables were highest, and in the late Holocene, when water table positions were variable. Taxa with larger biovolumes and apertures were generally more abundant when reconstructed water tables were higher, most notably following peatland initiation. Mixotrophic taxa were more prevalent in drier conditions and in the bog record. Changing frequencies of morpho-traits suggest that testate amoebae may occupy a higher trophic position in the microbial food web during wetter periods, signaling the possibility of internal feedbacks between peatland ecohydrology and critical ecosystem functions including long-term carbon accumulation.

Ecohydrological controls on apparent rates of peat carbon accumulation in a boreal bog record from the Hudson Bay Lowlands, northern Ontario, Canada

“…At HRST 13-01, there is a notable absence of Centropyxis taxa (Supplementary Fig. S2) in the lower portion of the core, despite being common in rich fen stages in peatland records (Bunbury et al, 2012; Lamentowicz et al, 2013; Bysouth and Finkelstein, 2020). Therefore, the poor preservation of testate amoebae supports the interpretation of drier and alkaline conditions in the lower portion of the core.…”

“…Paleoecological records from HBL peat cores confirm that autogenic succession, climate, hydrology, and regional uplift have all played a role in vegetation dynamics and peat carbon accumulation throughout the Holocene (Bunbury et al, 2012; O'Reilly et al, 2014; Bysouth and Finkelstein, 2020). Because of the proximity of the decaying Laurentide Ice Sheet, orbitally controlled temperature maxima were delayed in the HBL region into the mid Holocene (7—5 ka; Glaser et al, 2004; Viau and Gajewski, 2009) compared with other boreal regions (~11–9 ka; Renssen et al, 2009; Yu et al, 2010).…”

“…Because of the proximity of the decaying Laurentide Ice Sheet, orbitally controlled temperature maxima were delayed in the HBL region into the mid Holocene (7—5 ka; Glaser et al, 2004; Viau and Gajewski, 2009) compared with other boreal regions (~11–9 ka; Renssen et al, 2009; Yu et al, 2010). Warmer temperatures during the regional Holocene thermal maximum (HTM) are associated with higher apparent peat CARs at some sites in the HBL and adjacent regions (van Bellen et al, 2011a; Bunbury et al, 2012; Holmquist and MacDonald, 2014; O'Reilly et al, 2014; Packalen and Finkelstein, 2014; Packalen et al, 2014, 2016; Bysouth and Finkelstein, 2020). However, these high CARs are often associated with early successional stages of peat (typically fens), and there is not always a clear increase in CARs with warmer temperatures, suggesting that peatland vegetation dynamics and local hydrology also play a role in long-term carbon dynamics in these areas (van Bellen et al, 2011a; Bunbury et al, 2012; Packalen and Finkelstein, 2014; Bysouth and Finkelstein, 2020).…”

“…Neoglacial cooling over the last four millennia (Viau and Gajewski, 2009; Hargan et al, 2020) is generally associated with lower rates of peatland initiation and lower apparent CARs in some HBL peat records (Kettles et al, 2000; Bunbury et al, 2012; O'Reilly et al, 2014; Packalen and Finkelstein, 2014; Bysouth and Finkelstein, 2020). In several records, the slowdown in peat CARs is associated with a shift from fen to bog conditions after 2500 cal yr BP.…”

“…In several records, the slowdown in peat CARs is associated with a shift from fen to bog conditions after 2500 cal yr BP. However, there is significant variability in the timing of this transition (Bunbury et al, 2012; Holmquist and MacDonald, 2014; Bysouth and Finkelstein, 2020). Thus, ecohydrological processes must be considered in addition to regional climate to determine the drivers of this change (Loisel and Garneau, 2010; Bunbury et al, 2012).…”