6:15 PM - 7:30 PM
[MIS26-P01] In situ estimation of new and regenerated production in lakes using triple oxygen isotopes as tracers
Keywords:new production, regenerated production, gross primary production, lakes, triple oxygen isotopes, hypolimnion
The gross primary production rate is an essential parameter to study biogeochemical processes in hydrosphere, having strong relations with environmental changes in lakes and oceans, such as eutrophication and global warming. Supplying rates of fixed nitrogen, especially dissolved nitrate (NO3-) and ammonium (NH4+), to each hydrospheric environment often control each gross primary production rate. As a result, the primary production is often divided into the following two categories: “new production” that uses NO3- supplied either from atmosphere or from aphotic layer, and “regenerated production” that uses a recycled nitrogen in the form of NH4+ or dissolved organic nitrogen excreted or produced during biogeochemical processes within photic layer.
All the above-mentioned parameters had been traditionally estimated based on incubations of sampled water in bottles by adding isotope-labeled compounds such as 13CO2 or 14CO2 for the primary production rates and/or 15NO3- or 15NH4+ for nitrogen uptake rates. In these approaches, however, sampled water in bottles is incubated under artificial conditions that must be somewhat different from actual in-situ conditions and the results could represent different rates from the original in aquatic environments. Moreover, the estimated values based on the incubation corresponds to instantaneous uptake rates when sampling was done so that large errors could be expected for the hydrospheric environments with significant temporal variations, otherwise we must increase a number of observations using time and costs.
In this study, we determined the parameters using natural isotopes in lake-dissolved materials instead of using incubations. Most of the oxygen-containing molecules on earth show mass-dependent relative variation between 17O/16O ratios and 18O/16O ratios. On the other hand, atmospheric O3 photochemically produced from O2 shows an anomalous enrichment in 17O, so that residual atmospheric O2 is slightly depleted in 17O in comparison with the mass-dependent relative relation. Besides, at least one of the O atoms in atmospheric NO3- is derived from atmospheric O3 owing to the contribution of O atoms from O3 during the photochemical oxidation processes of NOx in atmosphere, so that the triple oxygen isotope ratios (∆17O values) of NO3- also deviate from the mass-dependent relative relation. Since ∆17O value does not vary during the general mass-dependent reactions such as decompositions, we can estimate the mixing ratio between atmospheric O2 and photosynthetic O2 from ∆17O value of O2 and that between atmospheric NO3- and remineralized NO3- from ∆17O value of NO3-. If we determine the ∆17O values of both dissolved O2 and NO3- in a hydrospheric environment as well as supplying rates of atmospheric O2 and NO3-, we can determine both the primary production rate and NO3- uptake rate simultaneously. One of the priorities of this ∆17O method is that the estimated rate corresponds to the average value of each rate, so that the values can be a more reliable and accurate than the values estimated from the incubation methods.
In this study, we determined both gross primary production rate and new primary production (NO3- uptake) rate simultaneously based on the ∆17O value of dissolved O2 and NO3- in two oligotrophic lakes (Lake Shikotsu and Lake Kuttara) and one mesotrophic lake (Lake Biwa) in Japan. The regenerated production rate was then calculated by deducing the later from the former. Water samples were collected twice (spring and summer) in a year for each lake. Both primary production rates and NO3- uptake rates were determined from the vertical distribution of ∆17O values of O2 and NO3- and their difference between the seasons. We found that the f-ratios (relative use of NO3- among the total use of nitrogen) were lower in oligotrophic lakes than in the mesotrophic lake.
All the above-mentioned parameters had been traditionally estimated based on incubations of sampled water in bottles by adding isotope-labeled compounds such as 13CO2 or 14CO2 for the primary production rates and/or 15NO3- or 15NH4+ for nitrogen uptake rates. In these approaches, however, sampled water in bottles is incubated under artificial conditions that must be somewhat different from actual in-situ conditions and the results could represent different rates from the original in aquatic environments. Moreover, the estimated values based on the incubation corresponds to instantaneous uptake rates when sampling was done so that large errors could be expected for the hydrospheric environments with significant temporal variations, otherwise we must increase a number of observations using time and costs.
In this study, we determined the parameters using natural isotopes in lake-dissolved materials instead of using incubations. Most of the oxygen-containing molecules on earth show mass-dependent relative variation between 17O/16O ratios and 18O/16O ratios. On the other hand, atmospheric O3 photochemically produced from O2 shows an anomalous enrichment in 17O, so that residual atmospheric O2 is slightly depleted in 17O in comparison with the mass-dependent relative relation. Besides, at least one of the O atoms in atmospheric NO3- is derived from atmospheric O3 owing to the contribution of O atoms from O3 during the photochemical oxidation processes of NOx in atmosphere, so that the triple oxygen isotope ratios (∆17O values) of NO3- also deviate from the mass-dependent relative relation. Since ∆17O value does not vary during the general mass-dependent reactions such as decompositions, we can estimate the mixing ratio between atmospheric O2 and photosynthetic O2 from ∆17O value of O2 and that between atmospheric NO3- and remineralized NO3- from ∆17O value of NO3-. If we determine the ∆17O values of both dissolved O2 and NO3- in a hydrospheric environment as well as supplying rates of atmospheric O2 and NO3-, we can determine both the primary production rate and NO3- uptake rate simultaneously. One of the priorities of this ∆17O method is that the estimated rate corresponds to the average value of each rate, so that the values can be a more reliable and accurate than the values estimated from the incubation methods.
In this study, we determined both gross primary production rate and new primary production (NO3- uptake) rate simultaneously based on the ∆17O value of dissolved O2 and NO3- in two oligotrophic lakes (Lake Shikotsu and Lake Kuttara) and one mesotrophic lake (Lake Biwa) in Japan. The regenerated production rate was then calculated by deducing the later from the former. Water samples were collected twice (spring and summer) in a year for each lake. Both primary production rates and NO3- uptake rates were determined from the vertical distribution of ∆17O values of O2 and NO3- and their difference between the seasons. We found that the f-ratios (relative use of NO3- among the total use of nitrogen) were lower in oligotrophic lakes than in the mesotrophic lake.