Comparison between infaunal communities of the deep floor and edge of the Tonga Trench: Possible effects of differences in organic matter supply

Abstract

a b s t r a c tHadal trenches are characterised by environmental conditions not found in any other environment, thereby providing new opportunities to understand the processes that shape deep-sea benthic communities. Technological advances have led to an increase in the number of investigations in hadal trenches over the last two decades. However, more quantitative samples including the deepest parts of trenches is needed to better understand trends in benthic diversity, abundance, biomass and community structure in these extreme habitats, and how these may be shaped by environmental and/or evolutionary factors. In this study, we describe and compare the abundance, biomass, vertical distribution in the sediment, diversity, and community structure of nematodes and other infauna in sediments from the Horizon Deep ( $ 10 800 m) in the Tonga Trench and a site on the edge of the trench ( $ 6250 m). Mean nematode abundance was six times greater at the Horizon Deep site (387 ind. 10 cm À 2 ) than at the trench edge site (65 ind. 10 cm À 2 ). A similar pattern was observed for biomass (15 vs 2 μgDW 10 cm À 2 , respectively), which likely resulted from elevated organic matter supply at the Horizon Deep site. There was no significant difference in nematode species richness between the two sites, but diversity measured using rarefaction was significantly greater at the trench edge site than at the Horizon Deep site [ES(20); 13.8 vs 7.8]. Dominance was much more pronounced in the Horizon Deep, which may be due to competitive exclusion by a small number of opportunistic species. Nematode community structure differed significantly both between sites and among sediment depth layers. The presence of subsurface peaks in pigment concentrations, bacteria abundance, and nematode abundance at the Horizon Deep site is consistent with a recent turbidite event, and may also reflect high rates of bioturbation by larger fauna resulting from high food availability. Determining the relative influences of different environmental factors on hadal trench benthic communities will require further investigation based on quantitative samples encompassing the trench axis as well as the oceanic and continental slopes.

Characteristics of meiofauna in extreme marine ecosystems: a review

“…Similar patterns have been observed for nematodes (Tietjen 1989;Gambi et al 2003;Leduc et al 2016) and harpacticoid copepods (Kitahashi et al 2013). These trends are often ascribed to shifts in food availability, but the evidence is equivocal and other factors such as the physical characteristics of the sediment, disturbance and larger-scale evolutionary factors may also be important (Kitahashi et al 2013;Gambi et al 2003;Leduc et al 2016). The small size of trench habitats relative to the surrounding abyssal plains, as well as their geographical isolation, may limit both the size of species pools and connectivity among trenches, and may account for the limited diversity observed particularly in the deepest parts of trenches (Jamieson 2015).…”

“…A number of early and recent studies have shown a negative relationship between depth and the diversity of benthic organisms within trenches (Belyaev 1972;Fujii et al 2013). Similar patterns have been observed for nematodes (Tietjen 1989;Gambi et al 2003;Leduc et al 2016) and harpacticoid copepods (Kitahashi et al 2013). These trends are often ascribed to shifts in food availability, but the evidence is equivocal and other factors such as the physical characteristics of the sediment, disturbance and larger-scale evolutionary factors may also be important (Kitahashi et al 2013;Gambi et al 2003;Leduc et al 2016).…”

“…, whilst biomass is usually on the order of 10 to 100 μgDW 10 cm −2 (Leduc et al 2016); these values are broadly similar to values from abyssal environments (Schmidt and Martínez Arbizu 2015). A notable exception comes from a study of the PeruChile Trench, which yielded a mean meiofaunal abundance of about 6400 ind.…”

“…Food supply in hadal environments is not considered to be lower than in abyssal environments. Several studies have shown that food availability is higher in the deepest parts of hadal trenches than in nearby trench edge environments, suggesting that the topography of trenches act to concentrate food particles along the trench axis (Glud et al 2013;Leduc et al 2016). However, food availability can vary substantially, both among trenches as a result of differences in surface primary productivity and within trenches as a result of water depth gradients, local topography and lateral transport triggered by seismicinduced landslides and turbidity currents (Ichino et al 2015;Jamieson 2015).…”

“…Although turbidites do not occur often relative to the lifespan of meiofaunal organisms, their effects are felt over very large areas and are thought to be long lasting (Levin et al 2001;Lambshead et al 2001). The physical disturbance created by these events may be a major factor influencing the structure of meiofaunal communities , through transport and burial of both the meiofauna and their food resources (Nomaki et al 2016;Leduc et al 2016).…”