Nitrogen Inputs to Narragansett Bay: An Historical Perspective


6.1 Introduction

The overall alteration of the global nitrogen cycle has been of increasing concern to ecologists and resource managers for several decades (Vitousek et al., 1997), yet most impacts of excess anthropogenic nitrogen are placespecific and cannot be generalized. Thus, the implications of human impacts on the nitrogen cycle can often be best understood in the context of specific ecosystems. While anthropogenic inputs of nitrogen to the Narragansett Bay watershed are broadly representative of the patterns found in other eastern United States estuaries (Castro et al., 2001; Driscoll et al., 2001; 2003), the temporal and spatial patterns are specific to Narragansett Bay, and the detailed patterns need to be understood if effective management of nitrogen is to be undertaken. Understanding patterns of change in the nitrogen cycle requires knowing spatial and temporal dynamics; how many people and animals lived where, for how long, and with access to what type of technology. To construct temporally and spatially explicit models of nitrogen cycling, it is necessary to develop detailed databases; yet the amount of historical data available on nitrogen dynamics (see Nixon et al., Chapter 5) limits our ability to validate these models. It is possible, however, to use indirect evidence to test the robustness of the assumptions used to model historical N loading to the bay. A quantitative, spatially explicit understanding of changing human population and land use in the watershed over time is essential to the development of effective management strategies that recognize that the Narragansett Bay ecosystem is not in equilibrium with respect to nutrient cycling, climate, or anthropogenic use of the resource. Understanding how nitrogen inputs to Narragansett Bay have changed since the late 1800s, when detailed data on agriculture and the disposal of human waste became available, is a key to interpreting the more detailed data on nutrient dynamics that became available in the later part of the 20th century.

Several researchers have presented reconstructions of the history of anthropogenic nutrient loading to Narragansett Bay based on expected relationships to forcing variables such as population, land use, agricultural practices, and energy consumption (Nixon et al., 1995; Howarth et al., 1996; Jaworski et al., 1997; Roman et al., 2000; Boyer et al., 2002; Moore et al., 2004). To develop a temporal picture of changing nitrogen flux to Narragansett Bay from 1880 to 2000, this chapter expands upon these previous efforts by employing spatially explicit historical data on human populations, domesticated animals, and populations served by sewers. This period was selected as quantitative spatially explicit data are available, and it reaches back to a time prior to the construction of sewers within the watershed. To model changes in N loading over time, historical data on population and domestic animals at town and the subwatershed levels were compiled as shown in Fig. 6.1. A simple, spreadsheet-based numerical model estimating the N load from human and animal waste between 1880 and 2000 was constructed based on available archival records and a careful review of the narrative history of changing human use of the landscape over that time period. Using this model in conjunction with long-term datasets of river N concentrations, a more complete understanding of how human impacts have altered the N cycle in the Narragansett Bay watershed over the past 120 or so years is developed.

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A. Desbonnet, B.A. Costa-Pierce (eds.), Science for Ecosystem-based Management. c Springer 2008