PhD DEFENSE | Victoria Assad | Ecology of mesopelagic micronekton of the Eastern Tropical Pacific: Insights into resiliency and OMZ specialization
Title: Ecology of mesopelagic micronekton of the Eastern Tropical Pacific: Insights into resiliency and OMZ specialization
Micronekton are a vital part of midwater food webs and have the potential to be impacted
by deep-sea mining via the release of sediment plumes, including dissolved metals. Micronekton communities in the Clarion-Clipperton Zone (CCZ) region of the eastern Pacific Ocean have been studied very little; baseline community descriptions are needed should mining commence to assess impact and resiliency. Micronekton samples were collected from 0–1500 m during the day and night in spring (March–April) and fall (October–November) of 2021, prior to mining activities, utilizing a 10 m 2 Multiple Opening Closing Net and Environmental Sensing System. Tows were conducted at both a Preservation Reference Zone (intended for monitoring against mining impacts) and a designated mining site (approximately 60 nautical miles or 111 km apart) in the NORI-D license area of the eastern CCZ, licensed to Nauru Ocean Resources Inc by the International Seabed Authority. I found higher springtime densities and biomasses of fishes, crustaceans, and cephalopods compared to fall samples, principally due to juvenile recruitment. Depth-integrated micronekton community composition and seasonal patterns in biodiversity levels were distinctly different between the sites. Seasonal differences were likely driven by primary productivity and the seasonal dynamics of the North Equatorial Current, North Equatorial Countercurrent, and eddies. I found high amounts of migrating biomass and described the vertical distributions and migration patterns of 52 taxa. Micronekton DVM patterns and vertical distributions corresponded to spatiotemporal changes in oxygen concentrations and many seasonal shifts in distributions at both the broad taxonomic level and species level were observed. Communities differed above, below, and within OMZ depths indicating the importance of oxygen on micronekton community structure. Over 40% of species identified were endemic to the eastern tropical Pacific or oxygen minimum zone- adapted. Biodiversity was found to be highest in the upper bathypelagic (1000-1500 m), a depth proposed for mining discharge at the time of this study. Due to the oceanographic variability found in NORI-D, and the lack of long-term time-series studies in the CCZ, additional sampling is needed to establish a sufficient baseline from which the potential impacts of deep-sea mining could be separated from natural variation. Finally, to evaluate potential resiliency to anthropogenic impacts I used otolith-increment analysis to determine the age and growth rates of four species of biomass dominant fishes in the CCZ, including two species in Family Melamphaidae and two in Family Myctophidae. I found these species to vary from annuals to ageing as long as 5 years, and all species to have reached reproductive maturity, suggesting these communities are able to reproduce at fast enough rates to replenish populations.
Location: POST 723
Date & Time: Tuesday, November 18, 2025 at 9:00am
