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Abstract

Marine food webs in coastal regions are among the most productive and culturally significant globally, yet their position at the land-sea interface makes them vulnerable to terrestrial disturbances. Terrestrial inputs of freshwater, organic matter, and nutrients heavily shape the structure and productivity of coastal marine food webs. Urbanization introduces novel organic materials to coastal ecosystems including stormwater runoff and wastewater discharge. Concurrently, altered nutrient dynamics can influence marine primary and microbial productivity. Through these changes, urbanization can shift the quantity and quality of the organic matter pool available for marine primary consumers. Such changes have implications for food web nutritional health and trophic interactions. To investigate the fate and implications of organic matter types in urban-based coastal food webs, I assessed the assimilation of organic matter and subsequent effects on nutritional health in suspension feeding mussels. Mussels were collected monthly for one year from eight sites around Burrard Inlet. Two approaches were applied to estimate uptake of marine (phytoplankton and macrophyte) and urban-derived (rivers, stormwater, and wastewater) organic matter types: 1) deriving fatty acid biomarkers for each organic matter type, and 2) applying a Bayesian mixing model combining fatty acid and stable isotope tracers. Both approaches yielded similar results (average difference = 4.14 %), giving confidence in the estimates. Phytoplankton were always dominant contributors to mussel diets (~ 50 - 70 %), underscoring the sustained high primary production despite urban pressures. Urban-derived contributions were mostly moderate (~ 15 - 20 %) and highest in an urban centre site (Shipyards) and during the wet season (October - February) when phytoplankton production was low and urban material flux was expected to be high. Nutritional variation in mussels was associated with shifts in organic matter contributions. Urban-derived inputs were primarily associated with saturated fatty acids, which are insufficient to support physiological demands for marine consumers. Coastal regions will face the continued rise of urbanization coupled with anthropogenically driven climate change, further disrupting terrestrial inputs and marine food webs. Ultimately, this study outlined a generalizable framework for examining the impact of these anthropogenic influences on coastal food webs, linking urban pressures to consumer health.