How was urbanization linked to biodiversity in freshwater and coastal waters?

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A large proportion of the world's population lives in urban areas along coasts. As cities expand, the ability of coastal ecosystems to provide benefits such as fisheries food, coastal defense and maritime transportation is in question. Although coastal development is known to change ecosystems, quantitative evidence on how terrestrial urbanization alters the structure and function of adjacent freshwater and downstream coastal marine habitats remains rare; its impacts are generally expected to weaken from land to freshwater to coastal marine habitats. Empirical assessments are especially important for species that use both freshwater and coastal marine habitats at multiple life stages, including endangered and threatened Pacific salmon (Oncorhynchus spp.).

Researchers studied associations between landscape-scale urbanization and ecosystem structure, measured as biodiversity of epibenthic invertebrate taxa, and function, measured as benthic net primary productivity and decomposition. They compared freshwater and coastal marine habitats across six pairs of more and less urbanized coastal watersheds in Puget Sound, Washington, USA, using principal components analysis, analysis of covariance and Mantel tests.

Greater upland urbanization was associated with greater reductions in freshwater biodiversity, measured by the density and evenness of epibenthic invertebrate families. Surprisingly, coastal marine biodiversity, measured in the same way, tended to be higher at more urbanized sites, suggesting that low to moderate urbanization-related disturbance could play a role in coastal marine biodiversity patterns. There was no statistical association between urbanization and ecosystem functions in either habitat; these functions were estimated from changes in algal biomass accumulated on tiles for benthic net primary productivity and biomass loss from litter bags for decomposition. Nor was there evidence that changes in structure and function with urbanization were more severe in freshwater than in coastal marine habitats, as might be expected if the land-sea boundary diminished the effects of landscape-scale urbanization. The researchers concluded that urbanization's effects can be complex and that considering terrestrial, freshwater and coastal marine systems together will produce more effective ecosystem-based management.