Fine‐Scale Variability of Cold‐Water Coral and Sponge Communities on Protected Northeast Pacific Seamounts

ABSTRACT

Cold‐water corals and sponges (CWCS) on Northeast Pacific seamounts form important biogenic habitat that is intersected by a broad oxygen minimum zone (OMZ) and exposed to ongoing deoxygenation. Many seamounts in this region lie within marine protected areas (MPAs), making an understanding of fine‐scale community structure particularly relevant for conservation and management. Using visual survey data from five seamounts, we modelled depth–occupancy patterns for CWCS functional groups and evaluated how these patterns vary among features, among transects and in relation to the core of the 800‐ to 1200‐m OMZ. Depth emerged as the dominant organizing gradient, with many groups concentrated in or near the OMZ core. However, the depth, shape, and magnitude of these occupancy peaks differed strongly across seamounts. At Union Seamount, depth–occupancy patterns also varied among sampled flank transects, revealing substantial horizontal heterogeneity over comparable depth ranges. Even when functional groups occupied similar vertical zones across seamounts, the dominant taxa within those peaks often displayed feature‐specific patterns. Community distinctiveness was highest at transitions around the OMZ core and in middepth zones above it, suggesting these boundary layers are key locations for detecting ecological reorganization. Functional groups also differed in their alignment with the OMZ core, indicating that shoaling or deepening of low‐oxygen waters would alter the estimated overlap between CWCS depth distributions and the OMZ core differently across seamounts. Together, these results reveal substantial habitat heterogeneity within and among seamounts, forming a depth‐structured mosaic that is consistent with regional oxygen gradients and local seamount context. This habitat heterogeneity highlights the need for multitransect, multidepth‐band monitoring and supports consideration of whole‐feature protection under continued global ocean deoxygenation.