Divergent responses to copper and nutrients in genetically distinct kelp populations sourced across an urbanisation gradient

Urbanisation is reshaping land and seascapes, forcing species to adapt, migrate or become locally extinct. While the ecological impacts of urbanisation on coastal species are well understood, we have little knowledge of the adaptive capacity of these species to cope with multiple interacting stressors and whether local adaptation to one stressor constrains performance under another (a)biotic stressor, creating physiological trade‐offs that may compromise resilience.

We combined intraspecific variation with experimental ecotoxicology to investigate the independent and interactive effects of nutrient enrichment and copper exposure on two genetically and phenotypically distinct populations of the habitat‐forming kelp Ecklonia radiata from Sydney Harbour, Australia. We hypothesised that kelp from locations chronically exposed to urban stressors (inner harbour) would display reduced physiological responses to contamination compared to populations from less contaminated locations (outer harbour), but increased metal bioaccumulation in kelp tissues (and hence increased bioremediation potential). We expected that these population‐level differences would be greatest when exposed to both stressors.

Consistent with adaptation driven by anthropogenic selective pressures, kelp populations exhibited divergent responses. Inner harbour kelp demonstrated physiological metal tolerance through copper release from tissues, maintaining tissue integrity and showing minimal bioaccumulation, suggesting adaptation to multiple stressors. In contrast, outer harbour kelp accumulated more copper but showed greater physiological stress.

Nutrient enrichment buffered copper effects on some response variables (tissue nitrogen maintenance, photosynthetic performance) while exacerbating copper effects on others (tissue degradation, weight loss). Photosynthetic pigments increased under single‐stressor treatments in both populations, yet multi‐stressor conditions attenuated pigment fluctuations exclusively in outer harbour populations.

Synthesis and applications. Our findings suggest that urbanisation can generate genetically and physiologically distinct populations with divergent functional responses to stressors. This has important implications for marine restoration and management, where current provenance frameworks and single‐stressor risk assessments may overlook population‐level adaptation and multiple interacting urban stressors. Our findings highlight that urban adaptation may be incorporated into restoration practice by matching source populations with specific characteristics (e.g. tolerance to multiple urban stressors, such as nutrients and metals) to management objectives and extending existing climate‐informed provenance tools to include urban resilience.