Rare glimpse at understudied ecosystem prompts caution on deep-sea mining

Some of the animals identified in the deep-sea that spend their life in the benthic boundary layer.
Photo Credit: Gabrielle Ellis

Scientific Frontline: Extended "At a Glance" Summary: The Abyssal Benthic Boundary Layer

The Core Concept: The abyssal benthic boundary layer is an enormous, poorly understood marine region located just a few meters above the global ocean seafloor. It hosts a dynamic community of tiny organisms, such as zooplankton, snails, bivalves, and barnacles, forming a crucial and interconnected component of the broader deep-sea ecosystem.

Key Distinction/Mechanism: Unlike the historical assumption that the deep abyss is a largely static environment, the benthic boundary layer is highly responsive to seasonal changes. The ecosystem is driven by the varying levels of organic material (food) sinking from the productive surface waters above, resulting in stark structural changes to the biological community between spring and fall.

Major Frameworks/Components:

• Organic Matter Flux: The dependence of the deep-sea benthic community on the downward transfer of organic material from the surface ocean.
• Temporal Dynamics: The significant seasonal variations in the community structure of deep-sea zooplankton based on surface productivity.
• Larval Dispersal Pathway: The boundary layer functions as a critical transit zone and habitat for the larvae of wide-ranging abyssal species before they settle on the seafloor.
• Anthropogenic Disruption: The mechanisms by which deep-sea mining harms the ecosystem, including ambient water removal, sediment plumes that interfere with filter-feeding, and the removal of polymetallic nodules that serve as essential settling habitats for larvae.

Branch of Science: Oceanography, Marine Biology, and Earth Science.

Future Application: Establishing critical ecological baselines to accurately assess normal biological variability, which will be essential for creating rigorous environmental frameworks, conservation strategies, and regulations regarding both deep-sea mining operations and the ongoing impacts of climate change.

Why It Matters: The research reveals that deep-sea mining will have severe, unavoidable impacts on biodiversity regardless of the time of year it occurs. Mining disruptions will not be isolated to specific nodule fields but will propagate through the deep ocean by removing dispersing larvae, threatening wide-ranging abyssal populations and their interconnected habitats.

Lander used to collect small deep-sea animals.
Photo Credit: Gabrielle Ellis

An enormous but poorly understood region of the global ocean–referred to as the abyssal benthic boundary layer–lies a few meters above the seafloor and has only been sampled a handful of times. A study by oceanographers at the University of Hawaiʻi at Mānoa provided the first in-depth look at this habitat, revealing a dynamic community that may be more sensitive to seasonal changes than previously understood. The research, published in Limnology and Oceanography, also concluded that deep-sea mining could have significant and unavoidable impacts on biodiversity, regardless of the time of year.

“Given the remoteness of this environment, we have extraordinarily limited knowledge of the animals that inhabit this zone,” said Gabrielle Ellis, lead author of the study and recent oceanography graduate from the UH Mānoa School of Ocean and Earth Science and Technology. “This study represents a significant contribution to our understanding of the benthic boundary layer community, and it starts to unravel temporal dynamics in the abyss.”

Sampling to assess seasonality

This community of organisms, like much of the deep-sea ecosystem, is reliant on organic material that falls from the surface ocean down to great depths. Using seawater pumps attached to a structure that descends to the seafloor, the research team collected tiny animals from about 10 feet above the seafloor during both spring and fall. Through both genetic and visual analysis, they discovered that the community of zooplankton, such as snails, bivalves and barnacles, changes dramatically between seasons, responding to varying levels of food sinking from the surface ocean.

“We didnʻt expect the results to be as stark as they were!” said Ellis. “These animals may be quite sensitive to changes in productivity in surface waters, which ultimately drive these patterns in the abyss.”

The benthic boundary layer is a crucial, interconnected part of the deep-sea ecosystem, serving as a home for unique animals and a pathway for the larvae of many species before they settle on the seafloor. The research highlights the complex behavior of animals across their life cycles, and the connectedness of the deep ocean more broadly.

“In the event of deep-sea mining, the organisms in this region will be impacted via ambient water removal and the generation of sediment plumes that interfere with feeding, in addition to the removal of nodules, which will effectively remove the settling habitat for larvae, likely leading to further declines in local recruitment,” said Erica Goetze, study co-author and oceanography professor. Goetze and Craig R. Smith, co-author and professor emeritus of oceanography, co-advised Ellis for her doctoral degree.

Larvae of deep-sea animals that grow and settle on the seafloor.
Photo Credit: Gabrielle Ellis

“These effects will not be isolated to nodule fields where the mining is occurring, but will also impact a variety of deep-sea habitats through the removal of their dispersing larvae that connect populations of wide-ranging abyssal species,” added Jeffrey Drazen, study co-author and oceanography professor. “Based on our findings, it appears that mining during any particular time period is likely to result in impacts to these organisms.”

Future research aims to conduct repeat sampling over several years to better understand what constitutes normal variability in this ecosystem.

“Our results highlight how much we have to learn about the dynamics of these abyssal ecosystems in order to provide a vital baseline for assessing the impact of both human activity and climate change,” said Smith.

Reference material: What Is: Abyssopelagic Zone

Published in journal: Limnology and Oceanography

Title: Zooplankton community structure in the abyssal benthic boundary layer varies over time due to nonuniform species response to seasonal organic-matter fluxes

Authors: Gabrielle N. Ellis, Jeffrey C. Drazen, Craig R. Smith, and Erica Goetze

Source/Credit: University of Hawaiʻi

Reference Number: mb100525_01

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