Urbanization reduces understory kelp abundance, macroalgal function, and biogenic complexity in the Salish Sea

The Salish Sea, like coastal zones globally, is undergoing rapid urbanization, causing substantial changes to marine habitats and ecosystems. Urban gradient analysis is a common approach in terrestrial and freshwater systems for discerning ecological patterns relative to urban land-use. This study employed a similar approach to assess urban gradients in benthic macroalgal assemblages in Puget Sound. Using benthic photoquadrat data from 38 sites in Central, South, and Whidbey Basin, we characterized the relative importance of urban- and natural-environmental predictors on: (1) the presence/absence of understory kelp, (2) the composition of macroalgal functional groups, and (3) the structural complexity of macroalgal-formed biogenic habitat. Kelp occurrence was negatively correlated with land-based urbanization (as reflected by population density, impervious cover, high density land development, and the extent of shoreline armoring). Urbanization tended to have greater predictive power than natural drivers of kelp abundance, such as urchin density and temperature, in model comparisons. Macroalgal functional groups followed expectations from Steneck and Dethier (1994), with fewer leathery and corticated macrophytes in low flow, low salinity environments (which are characteristics of urban centers), and with filamentous and foliose macroalgae occurring throughout the study region, including in turbid, low salinity environments near city centers. These patterns coincided with decreasing structural complexity of macroalgal beds with increasing urbanization intensity. Our findings support recent reports of reduced structural complexity near cities on the coasts of Brazil, Australia, and elsewhere, underscoring the importance of urban development and human proximity in structuring benthic community composition and functioning.