A new tool for quantifying urbanization gradients across seascapes predicts habitat-forming macroalgal distribution patterns in the Salish Sea

Nearshore environments on all continents are being transformed by rapid urbanization, which is projected to intensify in future decades. Landscape ecologists working in terrestrial and freshwater systems have developed several quantitative tools to help identify generalizable ecological patterns and responses to urbanization. These have yet to be fully utilized in the marine realm, though they require standardized measures of urbanization intensity that can be easily derived across cities and regions and at multiple spatial scales that reflect the varied nature of urban stressors and biotic response across functional groups. We developed an open-source resource in R that produces standardized measures of urbanization intensity based on a single spatial data layer of population density that is available at high resolution for all parts of the world. The tool was then employed to a case study evaluating urban-related gradients in the structural complexity of benthic macroalgal assemblages in the Salish Sea. Data from benthic photographic quadrat surveys at 22 sites distributed across ~90 km supported prior evidence from other regions that understory and canopy cover, kelp occurrence, kelp thallus density, and overall biogenic structural complexity decrease with increasing levels of land-based urbanization intensity. The standardized proxy of urbanization calculated for each site outperformed a suite of natural environmental variables, including temperature and water flow, in predicting abundances of kelp and other structurally complex macroalgae. These findings underscore the importance of urban stressors for habitat-forming macrophytes while providing a novel, open-source resource for performing urban-rural gradient analysis in urbanized seascapes.