Benthic subtidal assemblages and ecological processes in urbanized seascapes of Puget Sound, Washington, USA
Heery, Eliza C.
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Urbanization is a major process altering nearshore habitats in many parts of the world. One important aspect of urbanization in marine settings is the proliferation of artificial structures, such as seawalls, breakwaters, and jetties. Urban artificial structures can fundamentally shift marine communities and alter ecological processes at multiple spatial scales. Though they are common in both intertidal and subtidal habitats, their effect on subtidal ecosystems is particularly understudied. I examined the communities that form in association with subtidal artificial structures and their effects on surrounding sedimentary habitats in an urbanized estuary. In the first chapter, I evaluated detrital influx from artificial structures to surrounding sediments. Photoquadrat and sediment surveys indicated that red macroalgae and epilithic invertebrates were the major producers of detrital material on artificial structures in the Seattle area and that detritus from artificial structures was moving into adjacent sediments. Through a series of experiments, I then assessed the potential effects of these detrital inputs on macrofaunal assemblages. Sediments receiving one-time additions of red macroalgae and shell material were relatively resilient to detrital influx and exhibited little to no change in macrofaunal composition. However, rapid reductions in sediment chlorophyll and phaeopigment following detrital additions suggested that delivery of red macroalgae into sediments surrounding artificial structures may be frequent. In a follow up experiment, sediments were enriched with red macroalgae on a weekly basis to reflect more frequent delivery rates. Though I hypothesized that red macroalgae would serve as a subsidy for macrofaunal assemblages, I observed no positive opportunistic responses among macrofauna to weekly additions. Rather, frequent inputs of red macroalgal detritus led to decreases in abundance for the majority of macrofaunal taxa. Red macroalgae may therefore have negative impacts on macrofaunal assemblages, though this effect is likely minor compared with hydrodynamic alterations and other changes to sedimentary habitats that are associated with artificial structures. In the second chapter, I examined urban-related spatial distribution patterns and habitat-use the giant Pacific octopus (Enteroctopus dofleini). Urbanization is known to facilitate certain terrestrial mesopredators and I sought to evaluate whether similar patterns relative to urbanization were evident for this marine mesopredator. Modeling of citizen-contributed octopus presence/absence data suggested that urbanization impacts differed with depth. Octopus occurrence probability was positively correlated with urbanization intensity in deeper habitats only (> 24 m). In shallower environments (< 18 m), occurrence probability was higher in rural areas than in urban areas. In separate field surveys, I found that octopus diets were unrelated to urbanization, and that octopus abundance was positively correlated with the number of artificial structures on the seafloor. Though trophic mechanisms for urban-related distribution patterns of giant Pacific octopus are therefore unlikely, provisioning of shelter and denning habitat from artificial structures may be an important factor for octopus populations in urban areas. In the final chapter, I examined benthic composition on rocky artificial structures and natural reefs across an urban gradient. Photoquadrats were collected at 36 sites across Puget Sound. Consistent with studies in other regions, I found that artificial structures supported distinct and more variable benthic assemblages than natural reefs. In addition, rocky subtidal habitats in heavily urban areas had fewer kelps and more filamentous algal turf than those in less urban areas. Importantly, analyses from this study highlighted an important challenge in evaluating benthic composition relative to urbanization. Coastal cities tend to be located in protected bays and at the mouths of rivers, where benthic communities are subject to strong salinity gradients and low water flow. Strong collinearity in these naturally occurring environmental variables and urbanization intensity will be an important consideration for future studies that aim to characterize effects of urbanization on marine ecosystems.
- Biology