Patterns Of Dissolved Organic Carbon And Particulate Organic Carbon In The Guadalupe River And Canyon Lake Reservoir

Addresses reservoirs as unique ecological systems and presents research indicating that reservoirs fall into two or three highly concatenated, interactive ecological systems ranging from riverine to lacustrine or hybrid systems. Includes some controversial concepts about the limnology of reservoirs, which make for interesting reading.

Instantaneous amounts of organic carbon, both particulate and dissolved, in a number of freshwaters ranging from unproductive tarns to moderately productive reservoirs were measured by acid-persulfate digestion and infrared absorption. Organic carbon was designated as net seston carbon, filter seston carbon, or dissolved carbon based on filtering techniques. Repeated sampling showed amounts of organic carbon, either dissolved or particulate, fluctuated considerably over short periods of time; the dissolved fraction probably varying less than particulate forms. Almost daily measurements in samples incubated over three week periods also revealed erratic changes, with no clear reduction in total organic carbon. The data have descriptive value, although they cannot, at this time, be related satisfactorily to the general limnological knowledge.

I analyzed the 21 year dynamics of dissolved organic carbon (DOC) in 55 lakes in five sites across Eastern Canada in relation to regional and global variables. Regional variables included total solar radiation (TSR), precipitation (PPTN), air temperature (T) and sulfate deposition (SO4). Global variables included the Southern Oscillation Index (SOI), North Atlantic Oscillation (NAO) and Pacific Decadal Oscillation (PDO). A synchronous pattern in DOC was found among lakes within each region; however, a synchronous pattern in DOC was not found between sites, except for Kejimkujik and Yarmouth which were only 80 km apart from each other. This suggested that the variation of the long-term DOC pattern was in response to the temporal pattern of regional variables, and it supports the recent understanding that regional factors have a strong influence on many lake properties. Significant long-term trends in DOC were not evident except at the Experimental Lakes Area (ELA), where an increase in DOC was observed together with a decrease in summer TSR and an increase in summer precipitation. Annual mean air temperature has increased at the Nova Scotia and Turkey lakes sites over the study period. The relationship between the long-term pattern in DOC with the regional and global variables was analyzed for each study site to determine the key variables that could best explain the variation in the long-term pattern in DOC. TSR and PPTN were important independent variables across all sites, except for the Turkey Lakes Watershed site (TLW). Summer TSR (annual TSR for Kejimkujik and Yarmouth) had a negative relationship, while summer precipitation had a positive relationship with the long-term DOC pattern for all sites except TLW. TSR and PPTN explained 78%, 49% and 84% of the variation in the long-term DOC pattern at Dorset, ELA, and Nova Scotia (NS) sites, respectively. In contrast, the long-term pattern in DOC at TLW only had a weak relationship with the regional and global variab.

Rivers transform, mineralize, and export organic carbon and are a substantial portion of regional and global organic carbon (OC) budgets. Quantifying the drivers of OC transformation and mineralization are needed to understand how rivers export OC to downstream ecosystems. A substantial flux of OC in rivers is dissolved organic carbon (DOC). DOC sources and fates in large rivers, which drive riverine fluxes, are poorly understood. Furthermore, reservoirs decrease OC export by trapping particulate organic carbon (POC), but the effect of reservoirs on DOC export is less clear. Tailwater ecosystems directly below dams are fundamentally different than reaches upstream of reservoirs, but how these ecosystems alter OC export is less understood. I conducted three studies with the overall objective to quantify source specific fluxes of organic carbon in the Colorado River Basin. I quantified autochthonous DOC in a tailwater ecosystem as a focus of my first chapter. Few studies have focused on source specific fluxes of DOC, especially autochthonous DOC. I found that although the flux of autochthonous DOC was 7% of the total DOC flux, it was a 43% of gross primary production. Therefore, autochthonous DOC can contribute to daily DOC budgets and a large fraction of primary production in tailwater ecosystems. In chapter 2, I quantified sources and fates of DOC in the Grand Canyon reach of the Colorado River. Because sources and fates of DOC are empirically difficult to measure in large rivers, I developed a model to estimate source specific fluxes of DOC, including influx and uptake rates of DOC. Using a transport model along with a Bayesian approach, I was able to quantify fluxes of algal and terrestrial-derived DOC. The influx of algal-derived DOC was 2X greater than the influx of terrestrial-derived DOC. Terrestrial-derived DOC was piped and exported downstream. Algal-derived DOC uptake rates were similar to DOC uptake rates empirically measured in streams 1000-fold smaller than the Colorado River, Grand Canyon. The modeling approach I developed can be used to quantify sources and fates of DOC in rivers where otherwise empirical measurements are not possible. In chapter 3, I estimated OC fluxes along with DOC composition and quality to quantify how OC export changed from free-flowing rivers to reservoirs to tailwater ecosystems. I found that reservoir-tailwater ecosystems alter the sources of OC from more terrestrial to more algal-like. The OC transformation from terrestrial to more algal-like may lead to underestimation of the quantity of terrestrial OC that was buried in reservoirs because total fluxes of OC did not represent transformation processes of OC in river-reservoir-tailwater ecosystems.