LE3 .A278 2009

2009

O'Driscoll, Nelson

Acadia University

Bachelor of Science

Honours

Environmental Science

Earth & Environmental Sciences

Dissolved organic carbon (DOC) absorbs ultra-violet radiation and is photo-oxidized, producing a variety of dissolved inorganic carbon (DIC) compounds of which CO2 is the greatest. We lack quantitative models to describe the kinetics of this photo-oxidation. The objective of this study was to quantify the rate of abiotic conversion of DOC to DIC in five lakes in Kejimkujik National Park, Nova Scotia, with DOC concentrations ranging from 3.27 to 12.3 mg L-1. The rates of DOC photo-oxidation and DIC production were determined by irradiating lake water samples with UV-B and UV-A radiation under controlled conditions. The quantity of DOC and DIC remaining was then measured using a Shimadzu high sensitivity Total Organic Carbon (TOC) analyzer. These measurements were combined with attenuation spectra from the lakes to produce a predictive model for DOC photo-oxidation in freshwater. The following conclusions were drawn: i) attenuation coefficients for UV-A were positively correlated with DOC concentrations, ii) UV-B radiation is 2.36 times more efficient at photo-oxidizing DOC than UV-A radiation, iii) rates of DIC production ranged from 6.00  10-5 to 3.05 10-4 mg L-1J-1 (UV-B) and from 3.73 10-5 to 1.13 10-4 mg L-1J-1 (UV-A); higher rates of DIC production were observed in higher DOC lakes, iv) CO2 was produced at greater depths in lakes with smaller DOC concentrations for UV-A exposures, and v) low DOC lakes can produce similar amounts of CO2 per m2 as high DOC lakes under UV-A radiation; production is simply distributed over a greater depth. Although CO2 contributions from the photo-oxidation of DOC in lakes are on their own small, ranging from 4.24 kg year-1 (UV-B) to 1704 kg year-1 (UV-A), the impacts on individual lake functioning and ecology may be significant.

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