Abstract
Assemblages of megabenthos are structured in seven depth-related zones between ∼700 and 4000 m on the rocky and topographically complex continental margin south of Tasmania, southeastern Australia. These patterns emerge from analysis of imagery and specimen collections taken from a suite of surveys using photographic and in situ sampling by epibenthic sleds, towed video cameras, an autonomous underwater vehicle and a remotely operated vehicle (ROV). Seamount peaks in shallow zones had relatively low biomass and low diversity assemblages, which may be in part natural and in part due to effects of bottom trawl fishing. Species richness was highest at intermediate depths (1000–1300 m) as a result of an extensive coral reef community based on the bioherm-forming scleractinian Solenosmilia variabilis. However, megabenthos abundance peaked in a deeper, low diversity assemblage at 2000–2500 m. The S. variabilis reef and the deep biomass zone were separated by an extensive dead, sub-fossil S. variabilis reef and a relatively low biomass stratum on volcanic rock roughly coincident with the oxygen minimum layer. Below 2400 m, megabenthos was increasingly sparse, though punctuated by occasional small pockets of relatively high diversity and biomass. Nonetheless, megabenthic organisms were observed in the vast majority of photographs on all seabed habitats and to the maximum depths observed - a sandy plain below 3950 m. Taxonomic studies in progress suggest that the observed depth zonation is based in part on changing species mixes with depth, but also an underlying commonality to much of the seamount and rocky substrate biota across all depths. Although the mechanisms supporting the extraordinarily high biomass in 2000–2500 m depths remains obscure, plausible explanations include equatorwards lateral transport of polar production and/or a response to depth-stratified oxygen availability.
Highlights
Depth-related environmental factors structure marine benthic communities in the deep ocean, reflecting a well-documented general zonation along a shore to deep-ocean axis [1,2,3,4,5]
We describe and semi-quantitatively analyze seamount megabenthos assemblages on seamounts and associated rocky substrate along the continental margin of Tasmania, Australia to a depth of over 4 km (Fig. 1)
The overall decrease with depth correlates negatively with the proportion of images in each depth stratum in which sediment was the dominant substratum, confirming field observations that most of the observed megafauna was on hard substrata, either rock or of biogenic origin. The latter dominated the images only near and immediately below the peak of the Solenosmilia reef zone, between 1000–1300 m (Fig. 4), but it was conspicuous in two other depth zones, a patchy coverage of dead coral and coral rubble between roughly 1250–1600 m, and a zone between 2300–2700 m with extensive windrows of barnacle plates (Fig. 4b)
Summary
Depth-related environmental factors structure marine benthic communities in the deep ocean, reflecting a well-documented general zonation along a shore to deep-ocean axis [1,2,3,4,5]. The mechanisms that underlie this depth zonation are diverse and likely to differ between locations and between taxa [3,5]. They can be difficult to separate due to limited scope for experimental manipulation [7]. Biomass and density tend to decrease with depth, with the noteworthy exception of hydrothermal vent communities [8]. This is generally attributed to declining availability of surface-derived production with increasing depth [3,4]. Species richness tends to peak at intermediate depths [2,3,9] but fourth, declines overall with increasing depth [7,10,11]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
