Strong Depth-Related Zonation of Megabenthos on a Rocky Continental Margin (∼700–4000 m) off Southern Tasmania, Australia

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.

The Structure and Distribution of Benthic Communities on a Shallow Seamount (Cobb Seamount, Northeast Pacific Ocean)

“…Vertical zonation, reflecting ecologically significant environmental gradients correlated with depth, is common on seamounts [4, 7, 28, 62, 63] and has been previously reported on Cobb Seamount above 180 m [18] and 700 m depth [11]. In the present study, the nine communities were distributed within six significantly different depth ranges.…”

“…For example, vertical zonation of coral assemblages on a Hawaiian slope (<530 m depth) [68] and benthic communities on Tasmanian seamounts (<4 km depth) [63]. On Cobb Seamount, the two deepest communities (8 and 9) were differentiated by the distribution of cold-water corals and sponges.…”

High-Resolution Habitat Suitability Models for the Conservation and Management of Vulnerable Marine Ecosystems on the Louisville Seamount Chain, South Pacific Ocean

“…Chemical and other water property variables, including biological productivity variables available from various global or regional climatologies, were not considered as none were available with a native resolution similar to that of the MBES-derived variables. Several of these variables (e.g., temperature, salinity, surface-derived production, and aragonite saturation) are correlated with depth to varying degrees, and so a depth variable in this case can act as a surrogate for the changes in these parameters (Leathwick et al, 2006;Thresher et al, 2014). The categorical variable seamount, identifying the individual seamounts, was however added to MBESderived candidate variables.…”

A comparison of genetic connectivity in two deep sea corals to examine whether seamounts are isolated islands or stepping stones for dispersal

“…These two species of deep sea coral belong to the scleractinian family Caryophyllidae and are abundant and widely distributed on seamounts in the Southern Ocean. The solitary coral, Desmophyllum dianthus , has been recorded to depths of almost 2400 m42 and occurs throughout the world’s oceans. Genetic studies of ITS, 16S, and the mitochondrial control region have shown that populations of D. dianthus on widely separated ridge systems are isolated (1000s of km30) and there is evidence that oceanographic features such as interfaces between vertically stratified water masses within the Southern Ocean are barriers to vertical dispersal in this species15.…”