Photosynthesis, respiration, gas-exchange, diffusion, and stable isotopes of oxygen and carbon in a small mesotrophic lake (Castle Lake, CA)
AuthorBrown, Julie Mitchell
AdvisorPoulson, Simon R
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The purpose of this study was to investigate the application and usefulness of dissolved oxygen (DO) concentration, dissolved oxygen stable isotope (d<super>18</super>O–DO), dissolved inorganic carbon (DIC) concentration, and dissolved carbon stable isotope (d<super>13</super>C–DIC) measurements for quantifying biological activity in a lake over the course of a summer season. The hypothesis was that collection of this suite of measurements would potentially allow for quantification of the rates of various processes (such as photosynthesis, respiration, and gas–exchange) that control the behavior of DO and DIC in lakes. These measurements are applied to calculate the ratio of the rate of photosynthesis to the rate of respiration (P:R) using DO and d<super>18</super>O–DO. This study presents DO concentration, d<super>18</super>O–DO, DIC concentration, and d<super>13</super>C–DIC data collected every 2m from 2–30m in Castle Lake, CA, during events sampling events about two weeks apart from 12 July 2010 to 6 November 2010. Large ranges of d<super>18</super>O–DO (9.8 to 33.8 per mil) and d<super>13</super>C–DIC (–18.7 to –10.0 per mil) values were observed. These parameters demonstrate strong and systematic variation vs. depth as a result of the predominance of photosynthesis operating at mid–range depths (below the surface mixed layer, SML), where d<super>18</super>O–DO = 9.8 to 11 per mil, and d<super>13</super>C–DIC = –13.5 to –10.0 per mil. d<super>18</super>O–DO and d<super>13</super>C–DIC values indicate that respiration is the dominant process at ∼20–30m in the lake; however it is possible that other reactions that consume DO and produce DIC (and isotopically fractionate both species) are operating at depth. Short–term (diel) fluctuations of DO concentration and pH were areas investigated, but were not observed. Two series of rate of photosynthesis to rate of respiration ratio (P:R) calculations were performed: (1) DO–derived P:R calculations using formulae available in the literature, and assuming that gas–exchange with the atmosphere is taking place; (2) DIC–derived P:R calculations using formulae derived in this study, and assuming that gas–exchange with the atmosphere is taking place, and (3) calculations that assumed diffusion not gas–exchange was taking place. All three series of P:R ratio calculations require that DO and DIC concentrations and stable isotope compositions are at (or near) steady state, which appears to be a reasonable assumption for Castle Lake during peak summer conditions. The DO–derived calculations yield P:R ratios that varied from 0.18 to 2.19 that vary systematically with depth, and indicate that photosynthesis dominates above, and respiration dominates below about 10 – 18 m. A sensitivity analysis reveals that DO–derived P:R calculations are most sensitive to the fractionation factors associated with photosynthesis and respiration. Dissolved inorganic carbon–based calculations yield P:R ratios from 0.79 to 1.20 that also vary systematically with depth, and indicate that photosynthesis dominates above, and respiration dominates below ∼17 m. A sensitivity analysis reveals that DIC–derived P:R calculations are most sensitive to the photosynthesis and respiration fractionation factors, and the d<super>13</super>C value of the organic matter consumed during respiration. This study demonstrates that measurement of DO concentration, d<super>18</super>O–DO, DIC concentration, and d d<super>13</super>C–DIC, with subsequent calculation of P:R ratios, is a useful technique for quantifying biological activity in a lake. However, future work is needed to successfully incorporate diffusion into P:R calculations for use below the surface mixed layer (SML), and to better quantify fractionation factors for P and R in order to increase the accuracy of P:R ratio calculations. Further work is also needed to assess what additional processes, if any, are operating in Castle Lake that also may affect DO and DIC concentrations and stable isotope compositions, e.g. anaerobic respiration, precipitation and/or dissolution of calcium carbonate, anaerobic decomposition of organic matter, or input of DIC–rich surface and groundwaters.