LAUSR

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.