The use of stable carbon and oxygen isotopes to examine the fate of dissolved organic matter in two small, oligotrophic Canadian Shield lakes.
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Allochthonous carbon can be a large proportion of the carbon budget in northern temperate and boreal lakes. This thesis uses stable carbon and oxygen isotopes to examine the fate of allochthonous dissolved organic matter (DOM) in northern temperate lakes, and to determine the importance of dissolved organic carbon (DOC) in lake carbon mass balances and in the δ¹³C of lake sediments. To use stable isotopes as a tool for studying DOC loss and sedimentation within lakes requires an understanding of processes that affect the δ¹³C and δ¹⁸O in aquatic systems. Photolysis is one mechanism that can account for the large allochthonous DOC loss within northern temperate lakes. There is, however, little research examining the effects of photolysis on stable isotopes (e.g. δ¹³C and δ¹⁸O) in aquatic systems, or how photodegradation of DOM affects the δ¹³C of lake sediments. To study the effects of DOM photodegradation on carbon and oxygen isotopes, stream waters from catchments with varying peatland coverage were incubated in Tedlar bags placed in water baths under natural sunlight. Results from three streams flowing into two oligotrophic headwater lakes (Harp and Dickie Lakes) indicate that O₂ consumption rates and dissolved inorganic carbon (DIC) production rates were an order of magnitude greater in light exposed treatments than in dark treatments, suggesting that light mediated processes control O₂ consumption and DIC production in incubations. The similarity between filtered, inoculated, and sterile treatments, indicate that photolysis was the dominant O₂ consuming and DIC producing process in the incubations, while the contribution of respiration to these processes was not detectable. Differences in both O₂ consumption rates and DIC production rates (normalized to DOC loss) among streams suggest that DOM photolability was an important factor in both O₂ loss and DIC production on a volumetric basis. A concomitant increase in δ¹⁸O-O₂ was observed with O₂ loss indicating that during the photo-oxidation of DOM, the lighter ¹⁶O isotopomer was preferentially consumed in the oxidation of DOC to CO₂. Fractionation factors for respiration, photolysis and other abiotic reactions were not a function of O₂ consumption rates and ranged between 0.988 and 0.995, which lies outside the range published for respiration (0.975-0.982). These are the first published photolytic fractionation factors. The δ¹³C-DIC produced collectively by photolysis, respiration, and other abiotic reactions in incubations exposed to natural sunlight ranged between –23‰ and –31‰, and were similar in the light incubations for each treatment, but different among streams. Together, the light and dark incubation data suggest that photolysis and other abiotic reactions were largely responsible for the DIC concentration and δ¹³C-DIC changes observed, while respiration is a relatively minor contributor. During the incubations, as DOC photodegraded to CO₂, the lighter ¹²C isotope was preferentially mineralized (or the moieties cleaved were depleted in ¹³C) leaving the residual δ¹³C-DOC 1‰ to 4‰ enriched, creating enrichment (ε) values up to ~–3‰. The change in final δ¹³C-DOC after DOM photodegradation was different for each inflow, ranging from ~1 ‰ to 8.0 ‰, and provides an average enrichment of –2.1‰ (Harp Inflows ε: –1.2‰; Dickie Inflows ε: –3.4‰). These ε values are in agreement with the average ε from previous incubations on 3 of the inflows and 3 published studies based on UV exposed bog water (Osburn et al., 2001), riverine waters (Opsahl and Zepp, 2001), and lyophilized Juncus leachate dissolved in lake water (Vähätalo and Wetzel, 2008) (average ε = –2.9‰). The structure of DOM changed during photolysis. Absorbance data indicated that the aromaticity, colour, UV absorption and the average molecular size of the DOC decreased. Additionally, after exposure to sunlight, C/N ratios of the DOC changed from high values (24-55), indicative of terrestrial inputs, to lower values (4-13) traditionally thought to be representative of algal or microbial inputs. This contradicts the conventional view that terrestrial DOC has C/N ratios >20, and shows that abiotic processes can alter allochthonous carbon structure and the residual allochthonous carbon can have C/N values similar to, or overlapping with, C/N ratios expected from algal or microbial carbon. With the loss of 61-90% of the DOC, the particulate organic carbon (POC) created accounted for 20-90% of the DOC lost. Values of δ¹³C-POC ranged from –25.7‰ to –27.7‰, with 80% of the samples within 1‰ of the initial δ¹³C-DOC indicating that the particulate carbon created from the photodegradation of DOM that settles to the lake sediments could be isotopically similar to the source DOC. Overall, these incubations indicate that the photodegradation of DOM can affect both concentrations and isotopes of O₂, DIC, DOC, and POC of the stream waters flowing into Harp and Dickie Lakes and are important to consider in lake dynamics of high DOC retention lakes. Two independent methods were used to examine the importance of allochthonous DOC to lake sediments. The first method used a two end-member mixing model to estimate the proportion of allochthonous and autochthonous carbon within the lake sediments. Inflow δ¹³C-POC data, δ¹³C-leaf litter measurements, and DOC photodegradation experiments were used to calculate average annual δ¹³C-POC values for the allochthonous end member. The average annual δ¹³C-POC values for the autochthonous end member were calculated using estimates of productivity, surface δ¹³C-CO₂ values and estimated average annual fractionation factors. Average annual δ¹³C-POC values from allochthonous and autochthonous sources for these lakes were distinct. Using the end members to calculate the relative contributions of allochthonous and autochthonous carbon to lake sediments revealed that the δ¹³C of the lake sediment can be significantly affected by the ratio of autochthonous and allochthonous contributions. Furthermore, peaks in the allochthonous contributions of carbon accompany the δ¹³C peaks in the sediment records to the lake sediments. This suggests that climate change and/or anthropogenic changes to the landscape, and the concomitant changes in DOC inputs to lakes, can be recorded in the sediment record indicating that sediment records are not just productivity signals, but also mass balance signals in high DOC retention lakes. In the second method carbon isotope budgets were completed to accompany the carbon mass budgets for Harp and Dickie Lakes. Mass-weighted average annual δ¹³C-DOC values from the inflows and outflows and δ¹³C-DIC values from the inflows varied by 0.2‰ to 1.3‰, suggesting the values are well constrained. Conversely, the range of weighted δ¹³C-DIC values from the outflows were larger (2.2‰) than those of the inflows. Calculated δ¹³C values of the lake sediment were not equal to the measured δ13C values of the lake sediments for either Harp or Dickie Lakes suggesting a problem lies within the mass balances, or the weighted average annual δ¹³C values used in the isotope budgets. To examine the sensitivity of the average annual weighted δ¹³C values for the carbon entering and exiting the lakes, and the mass of carbon entering the lakes δ¹³C of the lake sediments, a mass and isotope budget model was created. The model indicated that the δ¹³C of the lake sediments is sensitive to a number of parameters including the amount of DOC entering the lake, the δ13C-CO2 evaded from the lake, the areal water discharge rate (qs), the gas exchange coefficient (k), and pH. Many of these parameters required adjustments for the masses of carbon to match those presented in the mass balances suggesting that the mass balances averaged over 8 years have errors associated with them. However, changing the DOC load to the lakes in the model by the variability observed over all the years of the mass balances) indicates that the isotopic signature of the lake sediment could change by up to 2.5‰. This isotope change is large enough to account for the historical δ¹³C changes observed in the δ¹³C sediment record, suggesting that allochthonous DOC can drive the sediment record.