Urban Sprawl Has Surprising Effect on Marine Life in Pacific Northwest

We started collecting invertebrates from the bottoms of streams and nearshore waters around Moorea, French Polynesia. Instead, they found something that challenged assumptions about how cities affect the sea. While freshwater sites near developed areas had fewer and less diverse communities of bottom-dwelling creatures, the marine sites told the opposite story—they were actually more biodiverse near urban areas.

We wanted to understand how landscape-scale urbanization fundamentally alters ecosystem structure and function across the boundary between land and sea. Working across six pairs of coastal watersheds around Puget Sound—each pair containing one more urbanized and one less urbanized watershed—We measured biodiversity by counting and identifying families of epibenthic invertebrates, the small creatures that live on the bottom. They also measured ecosystem functions by tracking how much algae grew on artificial tiles and how quickly organic matter decomposed in mesh bags.

The results split cleanly along the freshwater-marine divide. Greater upland urbanization was associated with greater reductions in freshwater biodiversity, measured as the density and evenness of epibenthic invertebrate families. But coastal marine biodiversity—measured using the same metrics—tended to be higher at more urbanized sites. We found no statistical association between urbanization and either freshwater or coastal marine ecosystem functions, estimated from changes in accumulated algal biomass on tiles and loss of biomass from litter bags. Surprisingly, there was no evidence that changes in ecosystem structure and function with urbanization were more severe in freshwater than coastal marine habitats.

"Surprisingly, there was no evidence that changes in ecosystem structure and function with urbanization were more severe in freshwater than coastal marine habitats."

What surprised We most was that marine biodiversity didn't just resist urban impacts—it seemed to benefit from them. They suggest this reflects the potential role of low to moderate levels of urbanization-related disturbance in determining coastal marine biodiversity patterns, though the mechanism remains unclear. We also expected the effects of terrestrial development to attenuate as they moved from land to freshwater to marine environments, but found no such gradient. The boundary between land and sea didn't seem to diminish the effects of landscape-scale urbanization the way anticipated.

These findings matter enormously for species like Pacific salmon that depend on both freshwater and marine habitats throughout their life cycles. The biodiversity of invertebrate prey items for juvenile salmon in both environments is particularly important to early marine growth and survival. If urbanization is creating different pressures in connected habitats that salmon use, restoration efforts need to account for these complex, sometimes counterintuitive patterns. Our results suggest that attention to terrestrial, freshwater, and coastal marine systems in concert will produce more effective, ecosystem-based management.

We still don't fully understand why marine systems seem to benefit from moderate urban disturbance while connected freshwater systems decline. The processes linking urban land development to changes in marine biodiversity remain largely hypothetical. Does urbanization introduce new species to marine environments? Does it alter nutrient flows in ways that benefit some marine communities? And how do these patterns play out over longer time scales? Until these questions can be answered, scientists are working with an incomplete picture of how expanding cities reshape the edge of the sea.

Citation

Samhouri, Jameal F.; Shelton, Andrew Olaf; Williams, Gregory D.; Feist, Blake E.; Hennessey, Shannon M.; Bartz, Krista; Kelly, Ryan P.; O’Donnell, James L.; Sheer, Mindi; Stier, Adrian C.; Levin, Phillip S. (2022). How much city is too much city? Biodiversity and ecosystem functioning along an urban gradient at the interface of land and sea. Frontiers in Marine Science.

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