Assessing the Impacts of Urbanization on Shellfish Growing Areas in Puget Sound
Description: This study explored the relationship between landscape patterns and nearshore water quality in shellfish growing areas of Puget Sound. We developed an empirical analysis of 32 basins selected to represent a gradient of urban land use/land cover patterns. Using bacterial contamination as an indicator of nearshore conditions, we developed a cross-sectional analysis across the 32 basins to assess what landscape factors best explain water quality conditions in Puget Sound’s shellfish growing areas. Our hypothesis was that variations in land cover composition, landscape configuration, land use intensity, and connectivity explain most of the variation in nearshore water quality conditions. The study is based on a landscape analysis approach. By combining remotely sensed data with land use and demographic data, we applied a set of landscape metrics developed in landscape ecology to quantify human settlement patterns, both in its composition and configuration of built elements and land uses on the landscape. Two scales of analysis were applied to assess influence of variables at the basin and local scale. A selection of variables was considered, including human population density, road density, percent land use, amount of impervious cover, aggregation of paved land, and amount and fragmentation of forest cover. Remote sensing and geographic information systems (GIS) have proven to be powerful landscape analysis tools, but the interpretation and analysis of the relationships between urban landscapes and nearshore environments present unique challenges, most notably gaps in important data sets and the complications inherent in sharing scientific data across disciplines and political boundaries.
The study shows that measurable differences in nearshore water quality can be detected across Puget Sound in watersheds with different amounts and fragmentation of forest cover at the basin scale (18 Puget Sound sites). Among the most urbanized basins (12 Puget Sound sites) the difference in water quality is associated with the amount and aggregation of impervious surface. While the amount of impervious area in the basins provides an effective measure of human impacts on nearshore water quality, variables measuring its spatial configuration (i.e., aggregation of paved land) and connectivity (i.e., total length of roads) show that the relationship between urbanization and water quality is not a linear one.
Land use and wastewater infrastructure are suspected to influence the impact of increasing human population on coastal environments. In order to explore this relationship further, additional data is required to research the interactions. The existing data that describe land use and infrastructure variables were limited and need to be improved to test hypotheses on the role that these factors play in mediating effects of urbanization on nearshore environments.