| Summary: | Physical, chemical and biological variables were measured in 35 lakes from Wood Buffalo National Park, northern Alberta and the Northwest Territories, Canada. Of these lakes, 22 were sinkholes, situated on limestone and gypsum, five were situated on the Canadian Shield and eight were shallow, "muskeg" lakes located on calcareous shales. For each of the 35 lakes, 37 environmental variables were measured, and diatoms from the surface 1-cm of deep water sediments (i.e. a surface sediment calibration or training set) were identified and enumerated. Two hundred and seventy-six diatom taxa were identified in the 35 lakes; 199 of these taxa were in relative abundances of ≥ 1%. Principal component analyses (PCA) and canonical correspondence analysis (CCA) revealed that underlying geology strongly influenced limnic properties, and therefore diatom community composition. Shield lakes were characterized by higher concentrations of AI and Fe, and lower pH values, specific conductivities and concentrations of ions associated with calcareous rocks (e.g. Ca, SO₄, Li, Mg, Na, etc.) than either the sinkhole or the muskeg lakes. The muskeg lakes were differentiated from the sinkhole lakes by higher concentrations of nitrogen and organic carbon (i.e. NH₃, particulate nitrogen and dissolved organic carbon), which was reflected in proxies of higher productivity (particulate organic carbon and chlorophyll a) and lower secchi depths. Diatom distributions were strongly influenced by the environmental variables distinguishing the three lake types. The 22 sinkhole lakes were further classified by the type of surrounding vegetation. Six vegetation groups were recognized: 1) spruce; 2) pine; 3) mixed; 4) shrubs/poplar; 5) recently burned; and 6) rocky. These vegetation groups, in part, reflect fire history. Unlike geology, surrounding vegetation, and therefore fire, generally had little influence on limnic properties or diatom community composition. PCA and CCA showed that, based on water chemistry and diatom assemblages, of the six vegetation groups, only the spruce group was distinct. Spruce sites were characterized by high levels of dissolved inorganic carbon, and therefore unique diatom assemblages. The results from the analyses of modern limnic properties and surface sediment diatoms are corroborated by high-resolution analyses of fossil diatom and chrysophyte assemblages from a Rainbow Lake A (RLA) sediment core. RLA contains annually-laminated sediments which were sampled at 5-year continuous intervals in order to study the impacts of known fire events, long-term vegetation change and recent climatic change. Park records and dendrochronological techniques were used to determine that at least three fires (circa 1860, 1880 and 1894), and to a limited extent a fourth fire (circa 1943), burned the catchment area of RLA. Paleolimnological evidence, including biogenic silica, diatom and chrysophyte analyses, suggests that RLA did not respond markedly to fire occurrence, and therefore short-term (10's of years) vegetation change. Similarly, high-resolution diatom analyses (one sample for every 5 years) from RLA for the last 800 years revealed no change in diatom community composition coincident with a long-term (100's of years) vegetation change circa 1536. The lack of Iimnic response to this vegetation change may be related to the lake's relatively small catchment area or the calcareous bedrock that underlies the drainage area. Although there was no change in diatom community composition coincident with the circa 1536 vegetation change, there was a dramatic change in diatom community composition circa 1836, that persisted to the present time. This change most likely reflects an increase in the internal loading of phosphorus, possibly as a result of a breakdown in meromixis, due to variations in ice out and spring circulation, or an increase in the duration of summer stratification, as a result of warmer summer conditions. The timing of this event is coincident with the marked changes in diatom community composition noted in sediment cores from ponds on Ellesmere Island The results of this thesis, coupled with the work of others, suggest that recent environmental changes, most likely climate related, have been having widespread impacts on remote lacustrine environments since the early part of the nineteenth century. Unlike past, long-term vegetation shifts, neither underlying geology or catchment area, are mitigating limnic response to this recent change. Doctor of Philosophy (PhD)
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