Tidal Flushing Rather Than Non-Point Source Nitrogen Pollution Drives Nutrient Dynamics in A Putatively Eutrophic...

Area of Science:

• Estuarine Ecology
• Biogeochemistry
• Stable Isotope Ecology

Background:

• Nonpoint source nutrient pollution impacts estuaries, often requiring multiple methods to assess. Barnegat Bay, New Jersey, is reportedly eutrophic, necessitating detailed investigation.
• Stable isotopes (carbon and nitrogen) are valuable tools for tracing nutrient sources and food web dynamics in aquatic ecosystems.

Purpose of the Study:

• To assess nitrogen sources and processing pathways in Barnegat Bay using stable isotope measurements.
• To identify geographic variations in nutrient dynamics and their relationship to water quality and estuarine health.

Main Methods:

• Stable isotope measurements (δ¹³C and δ¹⁵N) of sediment, water, primary producers, and consumers at 35 stations.
• Integration of water quality and hydrological data to create C and N isoscapes.
• Analysis of historical plant and shell samples (1880-2020) to track long-term isotopic changes.

Main Results:

• Four distinct geographic zones with unique isotopic baselines were identified, indicating variable nutrient sources and processing.
• Carbon isotopes reflected a terrestrial-marine gradient, while nitrogen isotopes were enriched near oceanic inlets.
• Lower δ¹⁵N values in urbanized areas suggested anthropogenic runoff is not a dominant N source; historical data showed decreasing δ¹⁵N values over time, especially in consumers.

Conclusions:

• Oceanic exchange plays a critical role in maintaining water quality and estuarine food webs in Barnegat Bay.
• Water quality issues are most severe in poorly flushed areas, highlighting the importance of water circulation.
• Estuarine nutrient dynamics are complex and influenced by both local conditions and broader oceanic influences.