UC San Diego

The deep ocean, exceeding 200 meters in depth, represents Earth's largest habitable space, yet it remains its least explored region. This study focuses on the Southern California Borderland (SCB), an area characterized by uneven and heterogeneous topography; and varying depths, temperatures, and oxygen concentrations. Due to its variability, this environment serves as an optimal setting for investigating the relationship between mineral-rich hardgrounds and benthic fauna. The deep ocean plays a crucial role in resource provisioning, but human activities, including deep-seabed mining, may threaten these oceanic functions. Two mineral-rich substrates, ferromanganese (FeMn) crusts and phosphorite rocks, are among the deep ocean mineral types being considered for their resource potential due to their enrichment in valuable metals in some regions. However, these geological features support deep-ocean biodiversity by acting as specialized substrates for macrofaunal communities and enabling key biogeochemical processes. This study aims to characterize macrofaunal (> 300 μm) density, diversity, and community composition on mineral-rich substrates in the SCB, focusing on FeMn crusts and phosphorite rocks. Macrofaunal samples were collected using remotely operated vehicles (ROVs) during expeditions in 2020 and 2021. Through quantitative analysis, I explore the faunal association with different substrate types, sites in the SCB, and various environmental variables, including oxygen, depth, temperature, and proximity to shore. Additionally, I assess the relationship between megafauna presence and macrofaunal density and diversity. A total of 3,555 macrofauna individuals were counted and 417 different taxa were identified from 82 rocks from depths between 231 m and 2,688 m. Average density for SCB macrofauna was 11.08 ± 0.87 individuals /200 cm2 and mean diversity per rock was H’(loge) = 2.22 ± 0.07. A relationship was found between site, substrate type, and macrofaunal communities. Phosphorite rocks had the highest diversity on a per-rock basis and when pooled, FeMn crusts had the highest number of species. Of all the environmental variables, depth explained the largest variance in macrofauna community composition. Macrofauna density and diversity had similar values at sites within and outside the oxygen minimum zone (OMZ). Understanding the intricate relationships between macrofaunal assemblages and mineral-rich substrates is essential, especially in the context of environmental disruptions associated with deep-seabed mining or climate change. This study is the first to analyze the macrofaunal communities of mineral-rich hard substrates in the SCB. The findings contribute crucial baseline information for effective conservation and management of the SCB and will support scientists in monitoring changes in these communities due to environmental disturbance or human impact in the future.