Identifying the sources of Dissolved Inorganic Carbon (DIC) and Carbon isotopes (δ13C) in the Roosevelt Inlet (Lewes, DE, USA)
Date
2025
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Publisher
University of Delaware
Abstract
Over the past decade, researchers have studied how climate change alters the oceans, which has made oceans sinks of carbon dioxide (CO2). The release of hydrogen ions (H+) that results from the oceanic absorption of anthropogenic CO2 causes long-term decreases in pH; otherwise known as ocean acidification (OA). Despite its local importance, few studies examine the impacts of tidal variability on the biogeochemistry, particularly CO2 signals, of various endmember waters mixing. Here, we investigate the Roosevelt Inlet (Lewes, DE, USA), where endmembers namely Broadkill River (freshwater), Canary Creek (marsh water), Lewes and Rehoboth Canal (brackish) and Delaware Bay (seawater) mix. Two in situ autonomous biogeochemical sensors (SeapHOx V2 and SeaFET V2, Sea-Bird Scientific) were deployed to collect high resolution timeseries of coastal ocean biogeochemical data coupled with dissolved oxygen to identify biological influences during tidal events along with timeseries of light measured as Photosynthetically Active Radiation (EPAR). On five separate occasions, discrete samplings coinciding with the high resolution time series were also conducted to determine dissolved inorganic carbon (DIC), δ13C(DIC), total alkalinity (TA) and pH. ☐ Results revealed during the August and October sampling periods that low tide DIC was elevated by biological organic matter decomposition from the freshwater and marsh endmembers, producing high values of PCO2, low pH and O2 values while high tide produced lower values of DIC, PCO2, and higher pH and O2 values. In contrast, the sampling periods of December and February showed low values in DIC, PCO2, and high pH and O2 values, indicating the major processes were carbonate dissolution/precipitation, dilution/evaporation and summation of all aerobic processes. The DIC production in the April sampling period was impacted by an influence from photosynthesis that caused a source of higher pH, O2, and lower DIC and PCO2 values that originated from the freshwater endmember. Once endmembers were identified, the Fry (2002) mixing model for the conservative mixing of DIC and δ13C-DIC was applied to indicate the organic carbon source (decomposition) for the DIC. The modeled data resulted in a seasonal effect on δ13C and DIC, where the high-resolution and high and low tide discrete sampling in August and October had values of δ13C driven by S. alterniflora (-13 ‰) decomposition through marshland aerobic respiration rather than sulfate reduction. The December high resolution sampling period was also driven by values of δ 13C from S. alterniflora (-13 ‰) decomposition, while the high and low tide discrete sampling period had δ13C isotopic values driven by the freshwater endmember of Delaware Bay (-10 ‰, Deng et al., 2022) as noted in Fry (2002). The February discrete and high-resolution sampling pointed towards a dampened effect from marshlands with δ13C isotopic values that were driven by the freshwater endmember of Delaware Bay (-10 ‰, Deng et al., 2022). As such, the Fry (2002) conservative mixing model provides the science community another way to predict the source of organic carbon for DIC to other estuaries with similar characteristics. Overall, the present work utilized sensor and discrete sampling of carbon parameters to facilitate an assessment of the performance of the autonomous biogeochemical sensors and to elucidate the source of carbon parameters (DIC, TA, PCO2, pH, δ13C) to the Roosevelt inlet, driven by both tidal and seasonality effects. The high and low tide discrete sampling helped identify endmembers and high-resolution sampling identified daily and seasonal drivers of biogeochemical variability at the Roosevelt inlet, DE using continuous data logging with emphasis on the carbon dioxide system.
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Keywords
Dissolved inorganic carbon, Carbon dioxide, Freshwater, Photosynthesis, Biogeochemical variability