Three centuries of biogeochemical change in a temperate embayment as revealed by sediment core stable isotopes, radiometric dating, and historical ecology.

Efforts to improve water quality in urbanized embayments may be complicated by changes that predate contemporary ecological monitoring efforts. Such is the case in Wickford H arbor, Rhode Island, one of the oldest continuous settlements in the northeastern USA, that is exhibiting degraded water quality after centuries of land use change, physical modifications, and nutrient loading. Here, we used historical ecology and sediment geochemical records to discern the biogeochemical impacts of these anthropogenic forcings over time. Segmented linear regressions fitted to the radiometrically dated sediment cores found break points in the geochemical record that align with physical modifications in the 1800s and nutrient enrichment in the 1930s. Reductions in grain size and sorting over time suggest that railway construction in the late 1800s constrained the hydrodynamic flushing of the study system and is an important driver of current water quality. Ratios of bulk carbon, nitrogen, and phosphorus content are indicative of a system that has been persistently eutrophic. Indeed, bulk N isotope composition reflects a 5‰ increase in δ¹⁵N since the colonial era, representing a shift to anthropogenic N sources that accompanied regional land use change. Subsequent increases in bulk C stable isotope composition and biogenic silica concentration suggest that primary production increased during the 18^th and late 20^th centuries. This work illustrates how ecological changes contributing to poor water quality can occur prior to con temporary nutrient loading, and efforts to restore systems in the absence of a historical ecological baseline are unlikely to produce a predictable ecosystem recovery.