project IV
project I

project II

project III

project IV

project V

project VI

project VII

project VIII

project IX

project X

current vacancies


project IV: DOC as an integrator of the effects of multiple stresses on aquatic ecosystems

The importance of DOC as a constituent of natural waters because of the many roles it plays in controlling the chemical and physical environment was emphasized in Project III. The role of DOC as a common factor linking the effects of multiple stresses, including acid deposition and climate change, was also described. Project IV focuses on the interactions and effects of two very specific stresses, the increase in sulphur and nitrogen deposition expected in the next few years, and the El Nino of 1997-98, on the DOC cycle in the two lakes and their watersheds that are studied in Project III.

It is well established that the formation of DOC is primarily a wetland phenomenon (Bayley et al. 1986, Dillon and Molot 1997). Consequently, an important factor controlling DOC levels in streams and lakes is the extent and nature of wetlands in their watersheds. The nature and extent of wetlands in the watersheds is, in turn, affected by temperature and precipitation regimes,which are, of course, altered by climate change; for example, climate change in north-western Ontario led to changes in the DOC levels in lakes (Schindler et al. 1990, 1997). The magnitude and rate of water table response within wetlands to summer drought, such as caused by El Nino events, has been shown to be a function of the hydrogeologic setting (Devito and Hill 1997). Predicted water table draw downs in the watersheds of the study lakes may influence DOC production within and hydrologic flow paths to and through wetland areas. The export of DOC from watersheds, and thus the concentrations in lakes, is also affected by acid deposition (Dillon et al. 1987). Although the mechanism is uncertain, it is probably the result of reduced solubility of DOC at lower pH, enhanced precipitation of DOC by higher levels of soluble metals (Al, Fe),enhanced adsorption onto hydrous oxides, or a combination of these mechanisms.

The objectives of this study are to determine how two stresses (acid deposition, drought resulting from El Nino) interact to effect the quantity and quality of DOC exported from the study watersheds. The magnitude of the fluxes of DOC are controlled by position of the water table,vegetation community (labile carbon, vegetation type and litter quality) and nutrient supply (e.g.,the addition of nitrogen can stimulate both peat decomposition and net primary productivity). Each of these biological and environmental controls are directly and indirectly affected by both external and internal hydrology (Schiff et al. 1990, Waddington and Roulet 1997, Devito and Hill 1997). Anthropogenic stresses (e.g., climate and landuse change) can significantly alter the hydrologic regime and thus, the quality and quantity of DOC exported.

The specific hypotheses that will be tested include: (a) the amount and quality of DOC exported will differ between several wetland systems in the two watersheds and over the season with differences in water table position and hydrologic flow path; (b) the impact of drought on thermal regime, water table fluctuations and the amount and quality of DOC export is a predictable function of the hydrogeologic setting of the wetland - wetlands isolated from terrestrial runoff (e.g. bogs) will demonstrate the greatest change in DOC export following drought, and wetlands connected to continuous groundwater sources will demonstrate the weakest response; and (c) the effects of the increase in acid deposition and the drought-induced water table draw down in the next few years will be additive, and will result in significant decreases in DOC fluxes into the lakes. These hypotheses will be tested in several wetlands representing a range of hydrogeologic conditions. The study sites will include a nutrient poor conifer peatland receiving seasonal groundwater inputs and a minerotrophic conifer peatland with continuous groundwater connections.