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Couplings of watersheds to estuaries: Inputs, transport, and management of nitrogen from different sources.
Thesis information
Author:
Bowen, Jennifer LynnAdvisor(s):
Valiela, IvanDegree:
Ph.D.School:
Boston University.Year:
2005Full Abstract
Nitrogen (N) enrichment of coastal waters is a pervasive threat to estuaries because N limits growth of producers and eutrophies coastal waters. My dissertation focuses on how land use changes on coastal watersheds increase the supply of N to estuaries from atmospheric deposition, fertilizer use, and human-derived wastewater.;I first examined the coupling of watersheds and estuarine N cycling in temperate and tropical systems by means of freshwater transport by applying models and measuring N isotopic signatures of macrophytes. Eutrophication results from changing land uses on watersheds, but in arid regions flow of freshwater is insufficient to couple watersheds to estuaries. For management purposes it is necessary to focus on those systems in which land-derived loads may play an important role in stimulating eutrophication. In uncoupled systems, N fixation may be the dominant source of new N to estuaries.;I thus chose to focus the remainder of my dissertation on sources and management of land-derived N in coupled estuarine systems. First, to define changing N loads to a coupled estuarine system on Cape Cod, I reconstructed historical trends in atmospheric deposition of N from 1910--1995. N deposition increased through the century at a rate of 0.26 kg N ha-1 y-1. Increases in deposition expose coastal forests to N saturation and may increase N loads to estuaries.;I then defined how atmospheric deposition, fertilizers, and wastewater delivery changed since the 1930s. N loading more than doubled between 1938 and 1990. Wastewater N became the dominant N source during the 1980s even though the Waquoit Bay watershed retained 80% of N inputs, with only 20% entering the Bay. This increased N load altered the Bay, increasing biomass of phytoplankton and macroalgae, and decreasing the area of eelgrass habitat.;I also examined the effectiveness of potential management options to reduce N loads to Waquoit Bay. Among the most effective management options would be increasing the performance of on-site septic systems, preserving naturally vegetated land and wetlands, and implementing stringent zoning ordinances. The results of this research can be used to set priorities for decision-makers charged with the management of Waquoit Bay and similar environments elsewhere.
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