Dissertations, Theses, and Capstone Projects

Date of Degree

2-2025

Document Type

Dissertation

Degree Name

Ph.D.

Program

Earth & Environmental Sciences

Advisor

Jennifer Cherrier

Advisor

Steve Morton

Committee Members

Jaye Cable

Brett Branco

Brian Lapointe

Pinar Balci

Subject Categories

Biogeochemistry | Environmental Chemistry | Environmental Indicators and Impact Assessment | Environmental Monitoring | Hydrology | Marine Biology

Keywords

Nitrogen, Septic System, Harmful Algal Bloom, Isotope, Submarine Groundwater Discharge

Abstract

The primary aim of this research was to elucidate the impact of submarine groundwater discharge (SGD) and septic effluent on coastal primary producers in the Great South Bay (GSB). The study focused on three main objectives: assessing the impact of precipitation patterns on SGD, tracing nitrogen sources into the GSB, and determining the impact of these nitrogen sources on coastal primary producers. The research revealed that current precipitation patterns significantly influence SGD across different seasons, highlighting the seasonal variability in groundwater discharge and its implications for nutrient loading in coastal waters. A suite of tracers was employed to identify and quantify nitrogen sources entering the GSB, including total dissolved nitrogen (TDN) and its constituents, with stable isotope analysis distinguishing between natural and anthropogenic nitrogen sources. The study evaluated which nitrogen source had the most significant effect on coastal primary production by assessing seasonal productivity through chlorophyll-a measurements and identifying dominant macroalgae and phytoplankton species. Isotopic analysis of biomass further elucidated the nitrogen sources driving productivity and potentially contributing to harmful algal blooms.

Key findings include a strong positive correlation (r = 0.982) between horizontal SGD and 10-day cumulative rainfall, following an exponential trend, with distinct spatial patterns in groundwater discharge, peaking at 17 meters offshore. Ammonium (NH₄⁺) was identified as the predominant form of nitrogen in sediment porewater, particularly at deeper depths, with multiple lines of evidence confirming the significant influence of septic systems on nitrogen loading, as indicated by elevated δ¹⁵N values and high sucralose concentrations. Ecological responses included distinct seasonal shifts in phytoplankton and macroalgae communities, transitioning from diatom-dominated to dinoflagellate-dominated systems during peak nitrogen loading periods. Stable isotope analysis revealed that human waste-derived nitrogen contributed up to 94% to macroalgae biomass during peak seasons. This study underscores the complex interplay between hydrological, chemical, and biological processes in coastal ecosystems and highlights significant implications for coastal management, particularly in addressing nutrient pollution and mitigating the impacts of climate change on coastal systems.

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