Science Inventory

Simulating Hypoxia in a New England Estuary: WASP8 Advanced Eutrophication Module (Narragansett Bay, RI, USA)

Citation:

Knightes, C. Simulating Hypoxia in a New England Estuary: WASP8 Advanced Eutrophication Module (Narragansett Bay, RI, USA). WATER. MDPI, Basel, Switzerland, 15(6):1204, (2023). https://doi.org/10.3390/w15061204

Impact/Purpose:

The release of nutrients from man-made sources have increased over the past decades. As nutrients have been added to estuaries and bays, oxygen concentrations in the water have decreased and algae concentrations have increased. These low concentrations of oxygen result in damaging the ecosystem and affecting fish and other wildlife in the estuary. Narragansett Bay ("the Bay") is a water body that has had large areas of low dissolved oxygen over the years. The State of Rhode Island has been implementing strategies to improve these conditions in the Bay. There is interest in understanding what governs dissolved oxygen concentrations in the Bay so that we may better understand how to manage and restore the Bay as well as look at long term scenarios due to land-use change, climate change, and management strategies. This study used computer software to develop mathematical models to simulate the concentrations of dissolved oxygen and phytoplankton throughout the Bay over the course of a single year (2009). Comparing simulated concentrations to observed concentrations, this modeling effort was able to to generally predict trends and patters of dissolved oxygen and phytoplankton with depth and space, though simulations were unable to approximate the large rises and falls of both oxygen and phytoplankton that occurred on a given day. By performing a series of modeling experiments, this work suggests that the surface waters are impacted by incoming nutrient loads from tributaries, while the mid-depth and deep layers of water were most connected to the sediments. Our simulations suggest the importance of the decades of nutrient loading into the Bay and the sediments on how the Bay will respond over the years and how long it will take for certain zones of the Bay to recover to pre-industrial levels.

Description:

Anthropogenic sources of nutrients cause eutrophication in coastal waters. Narraganset Bay (USA), the largest estuary in New England, has large seasonal zones of hypoxia. In response, management strategies have been implemented to reduce nutrient loadings. In this study, a mechanistic, mass balance fate and transport modeling framework was developed and applied to Narragansett Bay to improve our understanding of the processes governing hypoxia. Discrete and continuous observations were used for model comparison and evaluation. Simulations captured the general trends and patterns in dissolved oxygen (DO) with depth and space. Simulations were unable to capture the wide diurnal range of observed continuous DO and phytoplankton concentrations, potentially suggesting the need for improved understanding of processes at this time scale. Mechanistic modelling scenarios were performed to investigate how different sources of nutrients affect DO. Results suggest tributary sources of nitrogen affected upper layers of DO, while sediment oxygen demand and nutrient fluxes affected deeper waters. This work highlights the importance of understanding and simulating the legacy effects of historic nutrient loading to estuary systems to understand the magnitude and timing of long-term recovery due to reductions in nutrient loadings.