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

Abstract

The abyssal benthic boundary layer (BBL) community relies on pelagic organic matter (OM) export, yet community dynamics under changing surface productivity remain poorly understood. Here, we describe temporal variability within the BBL zooplankton community, focusing on benthopelagic adults and meroplanktonic larvae at a site with seasonal OM flux. We test the hypothesis that BBL community composition, richness, and structure are stable over time, but with higher absolute abundances occurring under greater OM flux. Zooplankton were collected from three meters above the seafloor in the Nauru Ocean Resources Inc. D (NORI‐D) mining contract area of the Clarion‐Clipperton Zone in spring and fall of 2021 using a benthic lander mounted with two large‐volume pumps (22 deployments) and analyzed by both morphotaxonomy and metabarcoding. Abundance of benthopelagic adults was elevated in springtime, as were abundances of bryozoan, bivalve, and gastropod larvae. Community structure was distinct between sampling periods ( p = 0.002, R 2 = 0.18), with high rates of species replacement between seasons. Springtime patterns co‐occurred with elevated surface productivity and OM flux, suggesting that distinct assemblages occur between periods of high and low OM flux. Biodiversity loss associated with polymetallic nodule mining is unlikely to be reduced if mining occurs only during select time periods, as high species richness and distinct community structure were observed in both sampling periods. Understanding the potential impacts of polymetallic nodule mining will require greater knowledge of abyssal BBL taxonomy and life history, and the nature of temporal variability in BBL communities, including climate‐related shifts in OM fluxes to the seafloor.

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