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Subfossil chironomids (insecta: diptera) and climatic change at high elevation lakes in the Engelmann spruce-subalpine fir zone in southwestern British Columbia Palmer, Samantha Louise


Because very little is known about past climate change in the Ashnola region of southwestern British Columbia , a stratigraphic analysis of fossil head capsules of chironomids (Diptera: Chironomidae) was performed for two lakes located at treeline (approx. 2250m) in southernmost B.C.: North Crater Lake and Lake-of-the-Woods. Distinct changes in the chironomid (non-biting midge) communities implied changes in climate throughout the Holocene (10 000 yr. B. P. to present). Prior to 10 000 yr. B.P., cold conditions are indicated by the lack of temperate taxa and the presence of cold-stenotherms such as Sergentia and Heterotrissocladius. The early Holocene (after 9500 yr. B.P.) in both lakes shows rapid increases in the abundance and diversity of warm-adapted taxa (e.g., Dicrotendipes, Microtendipes, Polypedilum and Cladopelma), while cold-indicators disappear. This is indicative of warm, dry (i.e., xerothermic) conditions. Just prior to Mazama ash deposition, and later in the mid-Holocene, several warm-adapted taxa decrease in both lakes. This coincides with the warm, yet moist conditions of the mesothermic. The late-Holocene assemblages (~ 4500 yr. B. P. to present) indicate continued cooling with a continued reduction in diversity and abundance of warm-adapted taxa. Furthermore, cold-stenothermsreappear in Lake-of-the-Woods. To quantitatively assess these inferred climate changes, temperature reconstructions were performed using a newly-developed chironomid-paleotemperature inference model. Reconstructed temperatures are generally in agreement with those changes inferred qualitatively from the stratigraphic diagrams. For both lakes, mean summer temperatures were cold in the late-glacial (8-10 °C). Inferred temperatures increase rapidly in the early- Holocene, with the highest inferred temperatures (12-16 °C) occurring during the xerothermic interval (9500 to 7000 yr. B.P.). Both lakes show a slight drop in temperature just prior to the Mazama eruption. This cooling trend continues thereafter at North Crater Lake, to present day. The cooling trend begins slightly later at Lake-of-the- Woods, after 5400 yr. B. P. For both lakes, this inferred cooling trend is consistent with the time of neoglaciation in B.C., as inferred from glacial and paleobotanical evidence. For comparison, paleotemperature reconstructions were done for Cabin Lake and 3M Pond, located slightly northwest of my sites. The reconstructed temperatures for these lakes show similar trends as those inferred for North Crater Lake and Lake-of-the-Woods. As diversity often changes with changing community assemblages, diversity was also assessed quantitatively using the Shannon-Wiener Diversity Index. The inferred diversity changes within cores, parallel the inferred climate and temperature changes. Diversity is low during cold intervals, such as in the late-glacial. Diversity increases in the early-Holocene, corresponding with the warm reconstructed temperatures. Diversity drops in the late-Holocene (after 5400 yr. B.P.), and this is consistent with the cooler conditions of the neoglacial. Community trajectory analysis for both lakes showed that while species assemblages of the late-glacial did not reassemble in the neoglacial, distinct late-glacial, early Holocene and late-Holocene stages of development were indicated throughout the history of each lake.

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