Biogeochemical Stressors and Ecological Response in a Nitrogen‐Impaired New England Estuary

Abstract Estuaries are threatened by eutrophication due to anthropogenic nutrient loading. The Great Bay Estuary of New Hampshire and Maine has been designated as nitrogen impaired primarily due to a 65% loss in seagrass coverage between 1996 and 2023. This region has also experienced multiple consecutive years of low annual precipitation and more recent record precipitation years. The loss of seagrass, increased precipitation variability, and continued anthropogenic influence have biogeochemical consequences for the estuarine filter. Water quality budgets for a subregion of Great Bay Estuary were developed at annual timescales for nitrogen, orthophosphate (PO 4 3− ), dissolved organic carbon (DOC), and total suspended solids (TSS). Results show annual total nitrogen (TN) input loads, including tidal flux into Great Bay, (3,900 kg ha −1 year −1 ) are less than output loads whereas dissolved inorganic nitrogen (DIN) inputs (1,410 kg ha −1 year −1 ) exceed outputs, indicating net TN export and net DIN retention. PO 4 3− loads were nearly balanced, with a mean input and output of 257 kg ha −1 year −1 and 238 kg ha −1 year −1 , respectively. Seagrass acreage negatively correlated with DOC inputs (33,300 kg ha −1 year −1 ). High TSS output loads (227,000 kg ha −1 year −1 ) resulted in net TSS export. Despite critical seagrass loss and recent point source nitrogen load reductions, the system continues to retain bioavailable forms of nutrients, suggesting the importance of other primary producers to estuarine biogeochemical cycles. Water quality budgets for Great Bay provide useful insight into the biogeochemical capacity of an estuary during a time of habitat degradation and subsequent coastal management efforts.