Divergent responses of nitrogen-species loadings to future climate change in the Chesapeake Bay watershed

Study region: Chesapeake Bay Watershed Study focus: Climate plays a critical role in regulating N loading from terrestrial ecosystems to coastal waters and further affecting the health and functioning of coastal ecosystems. However, the sensitivities of riverine exports of different N species to climate change have rarely been investigated. This study examines the response of riverine exports of ammonia (NH4+), nitrate (NO3-), dissolved organic nitrogen (DON), and particulate organic nitrogen (PON) to future changes in precipitation and temperature. A suite of climate forcings was used to drive a processbased terrestrial-aquatic model, DLEM (Dynamic Land Ecosystem Model), to project changes in N loading to the Chesapeake Bay in the mid-21st century, relative to the 1990s. New hydrological insights for the region: Our simulations show that, despite a relatively small average change in freshwater discharge driven by future climate change, annual average NH4+, NO3-, DON, PON, and total nitrogen exports are likely to change by -12 %, +13 %, +2 %, -9 %, and +9 %, respectively. Driven by rising temperature, NH4+ decreases as a result of enhanced volatilization and nitrification, but NO3 - export may increase due to high mineralization and nitrification. The change in DON export is mainly regulated by discharge, and the PON change is highly uncertain due to its high sensitivity to extreme precipitation events. This study highlights the importance of considering different responses of N species to climate change when designing nutrient reduction strategies to mitigate estuarine hypoxia.