In order to obtain more information on the isotopic signature of biological oxygen, laboratory experiments have been conducted to determine the isotopic composition of oxygen produced by different phytoplankton cultures. Recently, a controversy has emerged in the literature over these parameters (Kaiser, 2011) and one of the goals of this research is to provide additional data to resolve this controversy. Two parameters are key for this calculation: the isotopic composition of dissolved O2 in equilibrium with air and the isotopic composition of photosynthetic oxygen. This approach exploits the relative 17O/16O and 18O/16O isotope ratio differences of dissolved O2 compared to atmospheric O2 to work out the rate of biological production. A biogeochemical approach based on triple oxygen isotope measurements in dissolved oxygen (O2) has been developed over the last few years, which allows the derivation of gross productivity integrated over the depth of the mixed layer and the time-scale of O2 gas exchange (Luz and Barkan, 2000). Traditional techniques to measure marine production are laborious and subject to systematic errors. The measurement of biological production rates is essential for our understanding how marine ecosystems are sustained and how much CO2 is taken up through aquatic photosynthesis. These findings suggest strong effects of freshwater-saltwater mixing on dissolved carbon dynamics, which should be taken into account in carbon processing and budgeting in the world's estuarine systems.Triple oxygen isotope composition of photosynthetic oxygen DIC concentration increased six times from freshwater (0.24 mM) to saltwater (1.64 mM), while DOC showed an opposing trend, but to a lesser degree (from 1.13 to 0.56 mM). For the entire study period, the river's freshwater discharged 0.25 x 10⁹ mol dissolved inorganic carbon (DIC) and 1.77 x 10⁹ mol dissolved organic carbon (DOC) into the mixing zone. In the freshwater-saltwater mixing zone with wide channels and river lakes, however, a much larger amount of carbon (3.04 x 10⁸ kg) was emitted to the atmosphere during the same period. In the short freshwater river reach before a saltwater barrier, 0.079 × 10⁸ kg carbon was emitted to the atmosphere during the study year. The average CO₂ outgassing fluxes at site 1 through site 6 were 162, 177, 165, 218, 126, and 15 mol m⁻² year⁻¹, respectively, with a mean o f 140 mol m⁻² year⁻¹ for the entire river reach. We found that throughout the sampling period, all six sites exhibited CO₂ supersaturation with respect to the atmospheric CO₂ pressure during most of the sampling trips. From November 2013 to December 2014, we investigated freshwater-saltwater mixing effects on dissolved carbon concentrations and CO₂ outgassing at six locations along an 88-km-long estuarine river entering the Northern Gulf of Mexico with salinity increasing from 0.02 at site 1 to 29.50 at site 6 near the river's mouth. The delivery of dissolved carbon from rivers to coastal oceans is an important component of the global carbon budget.
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