Historic reconstruction of watershed land-use impacts on water quality in freshwater systems




Das, Biplob

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Watershed land use has a pervasive influence on a variety of properties within aquatic ecosystems including productivity and community composition. Long-term data, which are rare or absent for many lakes are crucial for taking or making management decisions. Paleolimnological approaches can provide long-term data. In this study, emphasis was placed on exploring how the traditional proxies can be applied in alternative ways as well as to evaluate novel tools to reconstruct water quality with contrasting watershed land uses. I examined Sooke Lake Reservoir (SOL) and Shawnigan Lake (SHL), located on Vancouver Island, British Columbia. Canada. SOL is an impounded lake that is used as a drinking water reservoir, and has a watershed with restricted public access and development. SHL has a similar limnological regime, but the surrounding watershed has been developed extensively for residential uses. In recent years elevated primary production was observed in SHL as compared to SOL. Results also suggest increased variability for most phytoplankton groups in SHL during post-disturbance periods compared to SOL. I also investigated how the history of local disturbance in a watershed can influence the regional coherence of ecosystem properties in lakes, by measuring sedimentary d13C, d15N, C:N and %BSiO2 (SOL and SHL). Data suggest that local disturbances can influence the %BSiO2 (proxy for lake productivity) and C:N ratio of lake sediments, but is less Iikely to alter the regional coherence of %C, %N and d13C between lakes. Fossil pigment records along with other geochemical indicators (d13c, d15N, C:N and %BSiO2) in lake sediments can provide information on changes in aquatic productivity, temporal coherence and variability due to either natural or anthropogenic disturbances and thus can be used to guide and manage lake water quality. The relationship between phytoplankton community composition (as inferred from diatoms and fossil pigments) and trophic status was different with contrasting watershed land uses. My results suggest that the hump-shaped (or unimodal) relationship between diatom species richness and production may be limited to high productive systems with maximum richness and diversity in intermediate states. In addition, fossil pigment records as proxies for algal functional groups reveal that in a mid to high productive system with intense watershed scale disturbances, community composition of algal functional groups declined favoring certain diatoms. These findings have broader implications for aquatic ecosystem management. Given that continuing pressure on land and subsequent land-use changes is a threat to freshwater resources, it is critical to trace watershed disturbances and subsequent alterations in accumulation of organic matter and impacts on aquatic ecosystems. An alternative approach to reconstruct organic matter accumulation using stable isotopes from lake sediments was explored. Stable isotope signatures from defined organic matter sources from the catchment and surface water of Shawnigan Lake (SHL) and Elk Lake (ELL), were compared with sedimentary proxy records. Results from this study reveal that terrestrial inputs and catchment soil coinciding with the watershed disturbance history probably contributed to the recent trophic enrichment in SELL. In contrast, cultural eutrophication in ELL was partially the result of input from catchment soil (agricultural activities) with significant input from lake primary production as well. The organic matter source detection technique that I have developed in this study can be applied to limit the effects of land use change in the vicinity. Bacterial production in the water column is positively correlated with algal primary production and therefore, may be responsive to watershed land-use changes. Bacteria release extracellular enzymes to hydrolyze high molecular weight organic compounds and are sensitive to the amount and type of organic matter in the aquatic environment. Aminopeptidase activity (a protease enzyme) in sediment core was strongly related to the watershed land-use history and subsequent changes in organic matter in the aquatic environment. The enzymatic activity changed with the degree of watershed disturbances, and therefore, suggests that enzymatic activity in lake sediments could be a proxy to infer historical productivity.



Water quality, Sooke Reservoir, Shawnigan Lake, British Columbia