Rapid Decreases and Performance Declines in Northeast Pacific Seamount Foundation Species Detected in an Oxygen Minimum Zone.

Seamount ecosystems are increasingly exposed to rapid oceanographic change, including warming waters, declining oxygen concentrations, and the upward migration of carbonate saturation horizons. Together, these processes are compressing the depth ranges of suitable habitat for many deep-sea organisms and altering the environmental conditions structuring benthic communities. While deep-sea environments have historically been considered relatively stable due to low environmental variability, empirical evidence documenting how populations respond to ongoing ocean change remains scarce. Here, we use high-resolution photogrammetric reconstructions of 12 monitoring sites (350-1111 m depth) across three Northeast Pacific seamounts to assess changes in the abundance and condition (i.e., health) of cold-water corals and sponges. Baseline reconstructions established in 2018 were compared with repeat surveys conducted 3-5 years later. Contrary to expectations for these slow-growing, long-lived species, significant declines in both abundance and condition were observed. Across the 12 sites, 163 of 844 individuals were lost between surveys, with abundance declining at five sites and condition declining at nine. The most severe losses occurred at a single site on Explorer Seamount, where 51% of individuals were lost, including approximately 80% of the dominant sponge species. Sponges experienced greater declines than corals across all metrics, and the most impacted sites were not consistently located within the lowest oxygen concentrations of the expanding oxygen minimum zone. Although abundance change did not differ significantly among oxygen zones, condition scores were lower at sites with the lowest oxygen levels. These findings suggest that early impacts of ocean change may already be occurring in deep-sea foundation species, highlighting the importance of repeat monitoring to detect rapid ecological change in environments traditionally assumed to be stable.