The ecological effects of urbanization of coastal watersheds: historical increases in nitrogen loads and eutrophication of Waquoit Bay estuaries

Historical changes in land use on coastal watersheds have increased rates of land-derived nitrogen loading to estuaries and altered their biogeochemistry and food webs. We used information on human populations and land uses within the watershed of Waquoit Bay, Cape Cod, Massachusetts, U.S.A., to model how nitrogen loads derived from atmospheric deposition, fertilizer use, and wastewater disposal have changed since the 1930s. Nitrogen loading into Waquoit Bay more than doubled between 1938 and 1990. The predominant source of nitrogen added to the bay changed from atmospheric deposition to wastewater disposal during the 1980s, reflecting the increasing urbanization of Cape Cod. Larger nitrogen loads increased nitrogen concentrations in the water, altering the assemblage of primary producers and resulting in eutrophication of the estuary. Biomass of phytoplankton and macroalgae increased, and areal cover of eelgrass (Zostera marina) decreased, with increasing nitrogen load. An increase in nitrogen load from 15 to 30 kg N·ha–1·year–1 virtually eliminated eelgrass meadows. Land-use changes prompted by urban sprawl can therefore be linked to marked changes in water quality and eutrophication of receiving waters. Résumé : Les changements historiques d’utilisation des terres dans les bassins versants côtiers ont augmenté les taux de charge de l’azote d’origine terrestre dans les estuaires et y ont modifié la biogéochimie et les réseaux alimentaires. Des données sur les populations humaines et l’utilisation des terres dans le bassin versant de la baie de Waquoit du cap Cod au Massachussetts, États-Unis, ont permis de modéliser comment les charges d’azote reçues par précipitation atmosphérique, les engrais et les eaux de rejet ont changé depuis les années 1930. La charge d’azote à la baie de Waquoit a plus que doublé entre 1938 et 1990. Durant les années 1980, la source principale d’azote dans la baie est passée de la précipitation atmosphérique d’azote à l’azote des eaux de rejet, un reflet de l’urbanisation croissante du cap Cod. Les charges accrues d’azote ont augmenté les concentrations d’azote dans l’eau, ce qui a modifié la composition des producteurs primaires et résulté en l’eutrophisation de l’estuaire. Les biomasses du phytoplancton et des macroalgues ont augmenté avec l’apport plus important d’azote, alors que la surface couverte par la zostère (Zostera marina) a diminué. Les charges accrues d’azote de 15–30 kg N·ha–1·année–1 ont presque complètement éliminé les marais à zostères. Les changements dans l’utilisation des terres consécutifs à l’étalement urbain peuvent donc être reliés à des modifications marquées de la qualité de l’eau et à l’eutrophisation des eaux réceptrices. [Traduit par la Rédaction] 1500 Bowen and Valiela Introduction Enrichment of coastal waters from land-derived nitrogen is one of the most pervasive threats to aquatic ecosystems today (Carpenter et al. 1998). Human-induced alterations to coastal watersheds have invariably resulted in changes in the ecology of aquatic systems. Nitrogen typically limits the growth of estuarine producers (Ryther and Dunstan 1971; Howarth 1988) and increases in the supply of nitrogen induce eutrophication in coastal waters (Nixon 1995). In a large geographical context, all available nitrogen is derived from atmospheric sources through N2 fixation. Nitrogen in fertilizers is fixed from N2 gas and is then applied to major crops. The food we ingest transforms the nitrogen derived from fertilizer into nitrogen that is released as wastewater. On a watershed scale it is helpful to separate nitrogen delivery into three components: atmospheric deposition both within and outside the watershed, fertilizer application on lawns, turf, and agriculture, and disposal in humanderived wastewater (Lee and Olsen 1985; Cole et al. 1993). Some portion of the nitrogen that is deposited on watersheds from these sources is then delivered from land to coastal systems (Hinga et al. 1991; Correll et al. 1992). Numerous mechanistic and empirical models have been developed to predict both the amount of land-derived nitrogen entering surface water and groundwater and the ecological effects of that nitrogen on the receiving water body. These range from relatively simple models based on human population densities within watersheds (Cole et al. 1993) to more complex models that include terms for atmospheric inputs and land-use classifications (Valiela et al. 1997a). We used one such model to reconstruct changes in nitrogen loading to the estuarine complex in Waquoit Bay, Cape Cod, Massachusetts, U.S.A. (Fig. 1). Can. J. Fish. Aquat. Sci. 58: 1489–1500 (2001) © 2001 NRC Canada 1489 DOI: 10.1139/cjfas-58-8-1489 Received August 15, 2000. Accepted May 2, 2001. Published on the NRC Research Press Web site at http://cjfas.nrc.ca on July 6, 2001. J15920 J.L. Bowen1 and I. Valiela. Boston University Marine Program, Marine Biological Laboratory, Woods Hole, MA 02543, U.S.A. 1Corresponding author (e-mail: [email protected]). J:\cjfas\cjfas58\cjfas-08\F01-094.vp Tuesday, July 03, 2001 2:18:25 PM Color profile: Disabled Composite Default screen