You are here:
Using Mud to Clear the Waters: Use of a Sediment Geochemical Marker to Derive Nitrogen Standards for Coastal Waters.
Citation:
Boothman, W. AND L. Coiro. Using Mud to Clear the Waters: Use of a Sediment Geochemical Marker to Derive Nitrogen Standards for Coastal Waters. SETAC North Atlantic Chapter, 28th Annual Meeting 2022, Groton, CT, June 27 - 28, 2022.
Impact/Purpose:
Interactions of factors influencing environmental effects of increased nitrogen in coastal waters (e.g., spring precipitation, wind speed and direction) make it difficult to obtain direct relationships between such effects and inputs of nitrogen. Molybdenum (Mo) in sediments may be used as a geochemical marker of periods of low oxygen (hypoxia) in bottom waters overlying those sediments, and because layers of sediment integrate process over periods of years to decades, the influence of factors confounding the relationship between nitrogen loads and effects may average out, enabling determination of more direct nitrogen load-effect relationships. Measuring Mo in sediments at multiple sampling sites in Narragansett Bay (RI, USA) demonstrated a strong correspondence between the frequency of hypoxia and nitrogen loading, adjusted for characteristics of the water bodies. This correspondence can be used to derive water quality standards for nitrogen that protect against hypoxia in coastal waters.
Description:
Despite significant study and improvements in modelling and measurement, direct quantitative relationships between increased inputs of nitrogen to coastal water bodies and environmental effects are difficult to obtain, making the task of setting standards for nitrogen loading to such water bodies problematic. Part of the problem in obtaining better relationships is the complex combination of factors that influence various effects of nitrogen at any one time / observation. Molybdenum (Mo) in sediments can be used as a geochemical marker of periods of hypoxia in bottom waters overlying those sediments. Reaction with sulfide in pore waters of sediments underlying hypoxic waters precipitates Mo present in seawater, a process that acts as a chronometer of the period the bottom waters are hypoxic. The longer time period encompassed by sampling sediments integrates the effects of nitrogen loading over long time periods (years) and may thereby reduce variability due to other shorter-term factors that affect the occurrence and severity of hypoxia (e.g., precipitation / river inflow, wind speed and direction). Mo was measured in surface sediments of coastal waters in Narragansett Bay (RI, USA). Multiple sampling sites were selected within and among the estuaries, so that the sites would be evenly distributed over the full range of expected normalized loading. The resulting estimates of frequency of hypoxic bottom waters correlated strongly with estimates of nitrogen inputs derived from a nitrogen land-use model, normalized for hydrodynamic characteristics of the water bodies. The strong relationship between normalized nitrogen loading and frequency / duration of hypoxia can then be used in a statistical / reference condition approach to set a standard for nitrogen loading to coastal water bodies to prevent / limit the occurrence of hypoxia.
