SOURCES OF DIC AND δ13C-DIC IN THE ROOSEVELT INLET

Abstract

This study investigates biogeochemical variability in Roosevelt Inlet (Lewes, DE), where mixing between four water sources—Broadkill River (freshwater), Canary Creek (marsh), Lewes-Rehoboth Canal (brackish), and Delaware Bay (seawater)—shapes marine carbonate chemistry. The goal of this work is to use discrete endmember tide cycle sampling with continuous data logging to identify contributions of dissolved inorganic carbon (DIC) and δ¹³C-DIC from marshes and Delaware Bay to the Broadkill Estuary. Previous studies of DIC in coastal systems have rarely incorporated δ¹³C data, thereby limiting insight into carbon source attribution. To address this, we conducted five seasonal samplings at each water source where we combined discrete endmember high/low tide and high-resolution tide cycle sampling surveys on separate days and weeks within the same months. Measured pH, pCO₂, total alkalinity (TA), DIC, and δ¹³C-DIC identified physical and biological controls on marine carbonate chemistry. Isotopic signatures revealed the influence of organic matter decomposition, with δ¹³C values consistent with inputs from Spartina alterniflora (–13‰) and freshwater endmembers (–10‰). A two-endmember mixing model demonstrated seasonal effects on DIC and δ¹³C, where August and October values were driven by marsh-derived aerobic respiration rather than sulfate reduction. August and October low tides were dominated by marshland, producing high pCO₂ and low pH and dissolved oxygen (DO) values. December and February were influenced by the Delaware Bay endmember (–10‰), and reflected carbonate precipitation and cumulative aerobic processes, whereas April conditions indicated enhanced photosynthesis. Here, the mixing model effectively traced organic carbon sources and may be applicable to similar estuarine systems.