Atmospheric carbon dioxide concentrations have been rising during the past century, leading to ocean acidification (OA). Coastal and estuarine habitats experience annual pH variability that vastly exceeds the magnitude of long-term projections in open ocean regions. Eastern oyster (Crassostrea virginica) reproduction season coincides with periods of low pH occurrence in estuaries, thus we investigated effects of moderate (pH 7.5, pCO2 2260 µatm) and severe OA (pH 7.1, pCO2 5584 µatm; and 6.7, pCO2 18480 µatm) on oyster gametogenesis, fertilization, and early larval development successes. Exposure at severe OA during gametogenesis caused disruption in oyster reproduction. Oogenesis appeared to be more sensitive compared to spermatogenesis. However, Eastern oyster reproduction was resilient to moderate OA projected for the near-future. In the context of projected climate change exacerbating seasonal acidification, OA of coastal habitats could represent a significant bottleneck for oyster reproduction which may have profound negative implications for coastal ecosystems reliant on this keystone species.

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Effects of land use on hydrology, organic matter sources, and processing may be proportionately greater in tidal creeks than large estuaries, yet tidal creek systems have been undervalued in assessments of emerging effects of anthropogenic land use. Through sampling of dated sediment cores, we identified indicators of historical land use change (past 150 yr) in a microtidal northern Gulf of Mexico tidal creek system in the early stages of urbanization. We found that tidal creeks differed from open water sites, and urbanized sites differed from less altered sites, primarily due to changes in carbon sources indicated by differences in sediment stable carbon isotope (δ13C) values, and concentrations of fecal indicator bacterium Clostridium perfringens. Total organic carbon (%TOC) and carbon : nitrogen (C : N) increased twofold in tidal creeks during upstream urbanization in the early 20th century, which led to elevated mid‐century sediment TOC accumulation rates (5–10 mg C cm−2 yr−1), followed by decreases in TOC accumulation in tidal creeks and open waters since the 1960s (0.4–1.8 mg C cm−2 yr−1). C. perfringens and nitrogen stable isotope values (δ15N) were, respectively, 1.8 and 1.5 times higher at wastewater‐influenced sites than at other sites, increasing through time or remaining high at wastewater‐influenced sites from approximately the 1950s‐present, when human populations quadrupled. Hence, urbanization altered estuarine inputs from upland C sources and increased inputs from human‐derived N and microbes. These findings suggest that tidal creeks are more sensitive archives of land‐use change than open water systems due to their proximity and greater connectivity to the watershed.