Abstract
Many methods are available for the measurement of dissolved carbon dioxide in an aqueous environment. Standard titration is the typical field method for measuring dissolved CO2 in aquaculture systems. However, titrimetric determination of dissolved CO2 in marine water aquaculture systems is unsuitable because of the high dissolved solids, silicates, and other dissolved minerals that interfere with the determination. Other methods used to measure dissolved carbon dioxide in an aquaculture water included use of a wetted CO2 probe analyzer, standard nomographic methods, and calculation by direct measurements of the water's pH, temperature, and alkalinity. The determination of dissolved CO2 in saltwater based on partial pressure measurements and non-dispersive infra-red (NDIR) techniques with a CO2 gas analyzer are widely employed for oceanic surveys of surface ocean CO2 flux and are similar to the techniques employed with the head space unit (HSU) in this study. Dissolved carbon dioxide (DC) determination with the HSU using a infra-red gas analyzer (IRGA) was compared with titrimetric, nomographic, calculated, and probe measurements of CO2 in freshwater and in saltwater with a salinity ranging from 5.0 to 30ppt, and a CO2 range from 8 to 50mg/L. Differences in CO2 measurements between duplicate HSUs (0.1–0.2mg/L) were not statistically significant different. The coefficient of variation for the HSU readings averaged 1.85% which was better than the CO2 probe (4.09%) and that for the titrimetric method (5.84%). In all low, medium and high salinity level trials HSU precision was good, averaging 3.39%. Differences existed between comparison testing of the CO2 probe and HSU measurements with the CO2 probe readings, on average, providing DC estimates that were higher than HSU estimates. Differences between HSU and titration based estimates of DC increased with salinity and reached a maximum at 32.2ppt. These differences were statistically significant (P<0.05) at all salinity levels greater than 0.3ppt. Results indicated reliable replicated results from the head space unit with varying salinity and dissolved carbon dioxide concentrations.
Highlights
In intensive recirculating aquaculture systems (RAS) metabolicderived carbon dioxide can accumulate if there is a lack of degassing processes or limited system water exchange
Barometric pressure (BP) was stable at 749 mm Hg, the total dissolved solids concentration was approximately 0.6 g/L, water temperature and dissolved oxygen ranged between 15.2–15.6 ◦C, and 10.1–10.7 mg/L, respectively
head space unit (HSU) based DC results were lower than those measured by the CO2 membrane probe or the titration method
Summary
In intensive recirculating aquaculture systems (RAS) metabolicderived carbon dioxide can accumulate if there is a lack of degassing processes or limited system water exchange. Development of non-dispersive infra-red (NDIR) techniques have yielded total dissolved carbon dioxide measurements for flowing seawater stream with 0.1% accuracy and precision with a e-folding time (travel time through sampling tube) of 7–10 s (Bandstra et al, 2006). In aquaculture applications the pH/alkalinity and the titrimetric Standard 4500-CO2 methods by American Public Health Association (APHA, 1998) are often the most utilized and cited The shortcoming of these APHA standard methods is that the titration of CO2 is not recommended for waters of high total dissolved solids, such as those encountered in seawater and the pH/alkalinity method is subject to the limitations of accurate pH measurement. The alternative methods included standard methods of titration, nomographic, and calculation (APHA, 1998), use of the portable CO2 meter by OxyGuard, and an indirect method using a pH and pCO2 correlation curve
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