| Abstrakt: |
The stable isotope ratio of dissolved inorganic carbon (δ13CDIC) in rivers reflects the dominant vegetation type in the surrounding watershed, rates of chemical weathering, atmospheric CO2 fluxes, and the relative rates of photosynthesis and respiration. Reconstructing past δ13CDIC values may reveal changes in these characteristics before watersheds experience land‐use change and/or climate change. This study uses freshwater mussels, Elliptio complanata, and coeval environmental data to assess how high‐resolution changes in the oxygen and carbon isotope ratios of shell carbonate (δ18Oshell and δ13Cshell, respectively) can be used as proxies of paleotemperature and paleo‐DIC in rivers. To test our hypotheses, we analyzed time‐series δ18Oshell and δ13Cshell values, δ13C values in bivalve tissues, and environmental data collected fortnightly from the Neuse River, North Carolina. Shell microsamples milled every 150 μm along the maximum growth axis represent an average of 12 ± 5 days (n = 524; excluding periods of growth cessation of >30 days), which is similar to the environmental data resolution (∼14 days). Serially sampled δ18Oshell and δ13Cshell values did not capture the full range of environmental conditions due to growth cessation during winter shutdown and extreme weather events. Low water temperature and elevated turbidity appear to be significant drivers of growth cessation. Spatial and temporal differences in the amount of metabolic carbon incorporated in the shell (∼0%–44%) likely occur due to variable mussel metabolic rates within and among study sites. Though high‐resolution δ13Cshell values did not reflect variations in δ13CDIC values, average δ13Cshell values were indicative of average δ13CDIC values in the Neuse River. Plain Language Summary: Recently, extreme weather and land‐use change, such as urbanization and agriculture, have transported carbon from land to rivers. Many of these watersheds are affected by pollution, making them unsuitable for recreational activities, including swimming, boating, and fishing. Therefore, understanding carbon transfer from land to rivers is essential for maintaining healthy ecosystems. This study investigates how to reconstruct the carbon sources to rivers before human modification of watersheds. If live freshwater mussel shells record the carbon sources in modern rivers, then historical and fossil shells can be used to reconstruct carbon sources in the past. We compare chemical variations in freshwater mussel shells to chemical variations in the water in which they grew. We found that mussels shells cease growth in winter when the water temperature is cold or during extreme weather events when waters are turbid. Because the mussels do not grow for much of the year, the shells were not reliable recorders of seasonal carbon sources. However, there were similarities the carbon sources measured by the mussel shells and the waters in which they grew during summer months. Therefore, we can use historical and fossil mussel shells to observe broad trends in carbon fluxes in past rivers. Key Points: This study tests the utility of stable isotope ratios in freshwater mussel shells as proxies of paleotemperature and paleo‐dissolved inorganic carbon in riversDespite long periods of shell growth cessation, δ18O values in mussel shells can be used to reconstruct summer water temperaturesThe average δ13C values in mussel shells approximate the average δ13C values of dissolved inorganic carbon in the Neuse River, North Carolina [ABSTRACT FROM AUTHOR] |
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