Wetlands in the Prairie Pothole Region of the North American Northern Great Plains perform multiple ecosystem services and are biodiversity hotspots. However, climatological changes can result in sudden shifts in these important ecosystems. For example, marked increases in precipitation in the last few decades have resulted in a widespread shift in wetlands across the Prairie Pothole Region to a new ecohydrological state. We used multi-proxy analyses (diatom community composition and invertebrate stable isotopes) of 210Pb-dated sediment cores from two adjacent, but morphologically and hydrologically different, prairie-pothole wetlands to assess the effects of hydroclimatic variability on these wetland ecosystems. Our results provide evidence that the recent ecohydrological shift in the region’s wetlands is unprecedented over the past ca.178 years. Oxygen stable isotopes in chironomid head capsules provide a record of paleo-hydrology changes. The most recent sediments (i.e., those deposited after the state shift) from both wetlands revealed novel changes in diatom communities that differed greatly from earlier community compositions. Additionally, a depleted signal in deuterium and 13C carbon stable isotopes observed in chironomid head capsules and Daphnia ephippia, respectively after 1993 is likely related to an increase in methane production in these wetlands. Our study highlights the importance of considering basin morphometry including whether a wetland has an overflow point, and multiple biological indicators to study climate-change influences on freshwater ecosystems. Research using these techniques can lead to an improved understanding of recent ecosystem shifts, an understanding that will be essential for future climate-change adaptation and mitigation in this ecologically important region.

Diatom community analyses

Diatom frustules were prepared for enumeration following the methods of Battarbee et al. (2001). Briefly, hydrochloric acid (10%) was used to remove carbonates before processing. Hydrogen peroxide (30% concentration) was added at ~80 °C to remove organic material. A portion of the digested slurry was dried on cover slips and mounted with Naphrax®. Diatom identification and enumeration were performed under oil immersion at 1,000X magnification using an Olympus model BX60 microscope with phase contrast. A minimum of 400 diatom frustules were identified and counted from each sample. Diatom taxonomy followed Krammer and Lange-Bertalot (1986–1991). Updated naming conventions were obtained from Diatoms of North America (Spaulding et al., 2022). Community data were expressed as percent abundance of the total diatom sum in each sample. Diatom species were categorized as either planktonic or benthic following their habitat preferences based on Spaulding et al. (2022).

Invertebrate stable-isotope analyses

Stable isotope analyses of chironomid head capsules and Daphnia ephippia were conducted following the methods of Wang et al. (2008). Briefly, 2 to 4 g of freeze-dried sediment were treated with 10% KOH in a warm (60 – 70 °C) water bath for 15 to 20 min. After the sample cooled, sediments were rinsed with distilled water through a 100-μm sieve (Wang et al. 2008). Residual material was refrigerated at 4 °C in a 50-ml centrifuge tube until sorted. For sorting, small aliquots of aqueous residual material were transferred into a Bogorov counting tray using a pipette and examined under a dissecting microscope at 25X magnification. Chironomid heads capsules (0.05 – 2.5 mg) and Daphnia ephippia (0.07 – 0.6 mg) were separately transferred using fine forceps into pre-weighed, tin capsules filled with distilled water. After all invertebrate remains were transferred into tin capsules, the open tin capsules were allowed to air dry overnight. The tin capsules were then crimped to leave a small opening and freeze dried for > six days. The tin capsules were then reweighed, folded into a ball, and stored in a 96-well culture plate in a desiccator. Empty tin capsules were included every five samples in sample preparation (total eight empty tin capsules for 43 samples) for the oxygen and hydrogen isotopes blank correction (Wang et al. 2008). The mass of each sample was calculated by subtracting the tare weight (i.e., weight of the empty tin capsule). The chironomid samples were then loaded into a zero-blank auto sampler attached to an on-line pyrolysis thermochemical reactor (ThermoElectron TC/EA) coupled via a Conflo Ⅲ with a thermoFinnigan Deltaplus XP IRMS at the Alaska Stable Isotope Facility (University of Alaska Fairbanks, Fairbanks, Alaska, USA). Chironomid head-capsule results are reported in units of δ18O and δD per mil (‰) relative to Vienna Standard Mean Ocean Water (V-SMOW). Daphnia ephippia stable isotopes were quantified using a Costech ECS4010 elemental analyzer (EA) attached via a conflo Ⅲ to an IRMS (thermoFinnigan Deltaplus XL). δ13C values are reported relative to Vienna Pee Dee Belemnite (V-PDB) and δ15N values are expressed relative to atmospheric (atm.) nitrogen. Analytical precision for δ18O, δD, δ13C and δ15N were 0.6‰, 4‰, 0.2‰ and 0.1‰, respectively. Invertebrate stable isotopes were quantified for wetland P1 and P8 to a core depth of 21 cm and 22 cm, respectively.