Abstract
Anthropogenic nitrogen (N) enrichment has degraded water quality and ecosystem function in southeastern Massachusetts (MA) estuaries and globally. In coastal communities like Cape Cod, nitrate from septic systems enters groundwater and discharges into estuaries, serving as a primary source of N pollution. This has prompted municipalities to explore innovative, cost-effective strategies to reduce N loading and restore impaired estuaries. This dissertation focused on in-transit and in-estuarine approaches for N reduction that rely on microbial conversion of nitrate to dinitrogen gas. Specifically, this dissertation evaluated: oyster-mediated denitrification, macrophyte-associated denitrification, and surface-water permeable reactive barriers (PRBs).Chapter One quantified denitrification mediated by Eastern oysters (Crassostrea virginica) using aquaculture oysters collected from three temperate estuaries. Significant denitrification rates were measured in association with both live oysters and empty shells. In Lonnie’s Pond (Orleans, MA), these rates contributed an estimated 12 kg N2-N annually to the N removal budget based on a standardized deployment of 225 kg dry tissue weight. N removal via oyster-mediated denitrification was comparable to oyster-enhanced sediment denitrification and made up almost half of the N removed through harvest. Thus, oyster-mediated denitrification represents a potentially significant pathway that should be included in future N removal budgets. Chapter Two evaluated macrophyte-associated denitrification through a case study of Mill Pond (Falmouth, MA), a temperate freshwater pond characterized by dense macrophyte growth and seasonal anoxia. The pond naturally attenuates N, reducing loading to the downstream Green Pond estuary. Previous research indicated that 50% of incoming N was attenuated within Mill Pond; however, known freshwater attenuation pathways (e.g. sediment burial and denitrification, and plant assimilation) failed to fully account for observed N losses. This study showed that macrophyte-associated denitrification accounted for an attenuation of 818 ± 80 kg N annually, resolving 67% of the previously unexplained N attenuation. This research suggests that macrophyte-associated denitrification is a dominant N removal process within this eutrophic freshwater pond, contributing to a 14% reduction in the potential watershed load entering Green Pond. Chapter Three examined surface-water PRBs as an innovative N reduction approach for agricultural freshwater flow-through systems. These carbon-based treatment barriers were evaluated in laboratory column and flume experiments and deployed in the channels of two cranberry bogs in southeastern MA. Results from column and flume experiments and field deployments were synthesized in a conceptual model to evaluate the underlying factors (e.g., PRB design parameters, and biological and physical timescales) which combined to produce the observed measurements. Results indicated that surface-water PRB success depends on the co-occurrence of labile carbon, sustained anoxic conditions and sufficient flushing to support measurable nitrate reduction. Collectively, this dissertation quantified N removal using three natural and engineered N reduction strategies currently being applied or considered in southeastern MA. Chapter One added empirical data necessary to determine the potential efficacy of oyster-mediated denitrification and for integrating it into an existing oyster N attenuation budget. Chapter Two refined the N budgets in a eutrophic freshwater pond, and established macrophyte-associated denitrification as an important N removal pathway. Chapter Three developed and evaluated a retrofitted approach to the traditional PRB to treat agricultural waters prior to their discharge into coastal ecosystems; however, effectiveness in the field was limited by hydraulic interactions with the PRB. Together this work offers municipalities new low-cost and innovative tools to manage their N loads to reach compliance with total maximum daily loads prior to discharge in coastal estuaries, particularly when combined with other attenuation methods.