Citation:

Marois, D., Ted DeWitt, AND J. Stecher. A dynamic nitrogen budget model of a Pacific Northwest salt marsh. 2016 American Water Resources Association Summer Specialty Conference, Sacramento, CA, July 11 - 13, 2016.

Impact/Purpose:

Nitrogen pollution in coastal ecosystems is an issue of extensive concern across the globe. Anthropogenic activities such as agriculture and urbanization can lead to increased flows of nitrogen into natural systems. Higher levels of nitrogen in marine systems can increase the occurrence of eutrophication, harmful algal blooms, and hypoxic conditions. Salt marshes are intertidal wetland ecosystems that may have the capability to reduce nitrogen flows coming from upland systems before they reach marine waters. The question of whether salt marshes are ultimately a sink or source of nitrogen in relation to their adjacent estuaries has been a topic of ecosystem service research in recent decades. Thus far, the only solid consensus made in this regard is that the nitrogen sink or source status of salt marshes can vary depending on variables including tides, season, climate, and landscape. This research will improve the understanding of the importance of these variations on salt marsh nitrogen cycling using dynamic modeling that considers differing tidal, seasonal, and climatic conditions. This in turn can provide predictions on how the nitrogen budget of a salt marsh may change in scenarios of changing climate.

Description:

The role of salt marshes as either nitrogen sinks or sources in relation to their adjacent estuaries has been a focus of ecosystem service research for many decades. The complex hydrology of these systems is driven by tides, upland surface runoff, precipitation, evapotranspiration, and groundwater inputs, all of which can vary significantly on timescales ranging from sub-daily to seasonal. Additionally, many of these hydrologic drivers may vary with a changing climate. Due to this temporal variation in hydrology, it is difficult to represent salt marsh nitrogen budgets as steady-state models. A dynamic nitrogen budget model that varies based on hydrologic conditions may more accurately describe the role of salt marshes in nitrogen cycling. In this study we aim to develop a hydrologic model that is coupled with a process-based nitrogen model to simulate nitrogen dynamics at multiple temporal scales. To construct and validate our model we will use hydrologic and nitrogen species data collected from 2010 to present, from a 1.8 hectare salt marsh in the Yaquina Estuary, OR, USA. Hydrologic data include water table levels at two transects, upland tributary flow, tidal channel stage and flow, and vertical hydraulic head gradients. Nitrogen pool data include concentrations of nitrate and ammonium in porewater, tidal channel water, and extracted from soil cores. Nitrogen flux data include denitrification rates, nitrogen concentrations in upland runoff, and tidal nutrient flux rates. Results of this modeling study will provide insight into how the nitrogen budget of a Pacific Northwest salt marsh varies temporally across tidal cycles and seasons, and will be used to predict how the nitrogen budget could change with future variations in climate.

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