Molluscan live-dead mismatch as a gauge of urban footprints and unpolluted baselines in cold waters hostile to shell preservation

Assessing the health of benthic ecosystems is crucial for understanding anthropogenic impacts and guiding remediation efforts, but conventional methods relying upon a one-time sampling face many challenges in estimating the pre-impact state and identifying unaffected areas. Here, we used species-level abundance data for living and dead mollusks acquired from a single sampling (2014) of the monitoring grid for the Macaulay Point outfall (City of Victoria, British Columbia, Canada), which discharged untreated municipal wastewater into the cold-temperate Juan de Fuca Strait at 60 m water depth from the early 1970s until upgrading to tertiary treatment in 2020. The footprint of outfall-related nutrients was readily detected: despite corrosive seawater and scant sediment accumulation hostile to shell preservation, molluscan death assemblages retained a sufficient local inventory of shells produced under pre-pollution conditions, which favored suspension feeders, to create a gradient of live-dead mismatch with detritivore-dominated living assemblages. Concerns about postmortem bias against vulnerable shell types were also allayed, as were concerns that strong tidal currents would obscure or obliterate habitat-level differences. Live-dead mismatch was relatively and absolutely small in 2 reference areas, i.e. comparable to the offset expected from under-sampling natural temporal variability in the source living community. These results support using molluscan live-dead analysis to fill pernicious knowledge gaps outside the warm-water settings where the method was developed and to gauge (1) the spatial and ecological footprint of human stressors where data are scarce or suspect, (2) pre-stress baselines within that footprint, and (3) outlying areas that might serve as reference areas for monitoring systems going forward.