Fine Scale Assemblage Structure of Benthic Invertebrate Megafauna on the North Pacific Seamount Mokumanamana

Abstract

Changes in benthic megafaunal assemblage structure have been found across gradients of environmental variables for many deep-sea habitats, but patterns remain under-investigated on seamounts. To assess the extent of variability in benthic communities at the scale of within a single seamount, and to assess environmental drivers of assemblage changes, Mokumanamana, also known as Necker Island, a seamount in the Papahānaumokuākea Marine National Monument with no known history of human impacts, was surveyed. Replicate 1 km transects were conducted along depth contours at 50 m depth intervals from 200-700 m on three sides of Mokumanamana using the AUV Sentry. Megafaunal abundance and substrate parameters were obtained from 26,119 total images. The dominant megafaunal taxa were sponges, corallimorpharians, cup corals, and benthic ctenophores. Sea pens and alcyonacean octocorals were also abundant. Overall, abundance and diversity of megafauna increased with depth. Beta-diversity through species substitution with depth was very high. Beta-diversity was also high between the sides and likewise defined almost exclusively by species substitution. Crossed ANOSIM by depth and side showed community structure differed on Mokumanamana for both factors. NMDS and cluster analyses of Mokumanamana show nine assemblages that were defined by depth and reflect differences between sides of the seamount. Environmental modeling with DISTLM indicates sediment, oxygen, substrate variability and roughness, POC, and surface currents are correlated with these assemblage differences. These results suggest that microhabitats on seamounts can promote unique assemblages along depth gradients as well as on different sides of a feature, and this diversity may be easily overlooked without fine-scale sampling. These findings have implications for management and conservation of seamounts as well as future ecological studies of seamounts, as seamounts are generally sampled on much coarser spatial scales.

Benthic Assemblage Composition of South Atlantic Seamounts

“…Our observations suggest that the major large-scale drivers of seamount benthic assemblage structure in the South Atlantic include latitude, depth, longitude, FBPI, and slope (Table 2). This concurs with the current understanding that variables correlated with latitude and depth are strong drivers of epifaunal community composition on seamounts (McClain and Lundsten, 2015;Du Preez et al, 2016;De la Torriente et al, 2018Morgan et al, 2019), as well as in other deep-sea ecosystems (Rowe and Menzies, 1969;Rex, 1981;Billett et al, 2001;Ruhl and Smith, 2004). Latitude in itself is not ecologically relevant, but it acts as a proxy for other variables.…”

“…Contrastingly, the EEZs of Ascension and St. Helena are characterized by less complex water mass structures, with less variability in water mass characteristics, potentially driving less divergence in faunal communities (Peterson and Stramma, 1991). Topographic variables including slope and bathymetric position index (BPI) are, as our results support, often identified as key drivers of benthic assemblages in the deep sea, both at the inter-and intra-seamount level (Boschen et al, 2015;Morgan et al, 2019). Slope and BPI can be used to infer the geomorphology of a region (Greene et al, 1999), which in turn can provide insight into the hydrodynamics, and thus substrate types observed within an area (Stephens and Diesing, 2015).…”

“…There are many environmental factors that can influence both the composition and distribution of benthic assemblages on seamounts and oceanic islands, as well as their overall species richness. These include, but are not limited to, depth (Boschen et al, 2015), biogeographic region (McClain et al, 2009), local hydrodynamic regime (Levin and Thomas, 1989), surface productivity (Hernández-León et al, 2020;Bridges et al, in review), particulate organic carbon (POC) flux to depth (Morgan et al, 2019), temperature (O'Hara and Tittensor, 2010;Woolley et al, 2016), and topography and substrate type (Lundsten et al, 2009). In the South Atlantic, O'Hara et al (2010) identified distance from shore, temperature, dissolved oxygen and POC as important environmental predictors of assemblage composition along the VTC.…”

Megafaunal Ecology of the Western Clarion Clipperton Zone

“…Features providing positive elevation above the seabed, such as seamounts, are common on abyssal plains and specifically in the far western and eastern areas of the CCZ (Yesson et al, 2011;Wedding et al, 2013;Harris et al, 2014;Washburn et al, 2021a). Large variations in seabed environmental conditions found on seamounts, particularly those related to substrate and organic matter availability, influence the benthic communities present (Fock et al, 2002;Piepenburg and Muller, 2004;Samadi et al, 2007;Clark et al, 2009;McClain et al, 2009;Bo et al, 2011;Williams et al, 2011Williams et al, , 2015Boschen et al, 2015;Rogers, 2018;Morgan et al, 2019;Cuvelier et al, 2020;Jones et al, in review). Current speeds are enhanced and directions altered around seamounts (Genin et al, 1986), resulting in enhancements to suspended particle flux and locally deposited organic matter (Kiriakoulakis et al, 2009;Vilas et al, 2009).…”

First description of deep benthic habitats and communities of oceanic islands and seamounts of the Nazca Desventuradas Marine Park, Chile

“…These organisms are called habitat-forming species and are characterized by being small-scale engineers of autogenous ecosystems. These organisms, and the biogenic structures they produce, contribute to increasing spatial complexity in soft-sediment benthic systems 25 , promoting spatial heterogeneity on seamounts 26 , but they are often easily overlooked because seamounts are sampled at much more general spatial scales 26 . In recent years, the importance of sampling at smaller scales has been accentuated with the understanding that the characteristics of the bottom and the presence of habitat-forming species increases the heterogeneity of the habitat and fulfill the function of a structural habitat 27 .…”