Macrobenthos relative to the oxygen minimum zone on the East Indian margin, Bay of Bengal

Abstract

The Bay of Bengal remains one of the least studied of the world's oxygen minimum zones (OMZs). Here we offer a detailed investigation of the macrobenthos relative to oxygen minimum zone [OMZ – DO (dissolved oxygen), concentration <0.5 ml·1−1] at 110 stations off the North East Indian margin (160 and 200 N) featuring coastal, shelf and slope settings (10–1004 m). Macrobenthos (>0.5 mm) composition, abundance and diversity were studied in relation to variations in depth, dissolved oxygen, sediment texture and organic carbon. Using multivariate procedures powered by SIMPROF analysis we identified distinct OMZ core sites (depth 150–280 m; DO 0.37 ml·1−1) that exhibited dense populations of surface‐feeding polychaetes (mean 2188 ind. m−2) represented by spionids and cossurids (96%). Molluscs and crustaceans were poorly represented except for ampeliscid amphipods. The lower OMZ sites (DO > 0.55 ml·l−1) supported a different assemblage of polychaetes (cirratulids, amphinomids, eunicids, orbinids, paraonids), crustaceans and molluscs, albeit with low population densities (mean 343 ind. m−2). Species richness [E(S100)], diversity (Margalef d; H’) and evenness (J’) were lower and dominance was higher within the OMZ core region. Multiple regression analysis showed that a combination of sand, clay, organic carbon, and dissolved oxygen explained 62–78% of the observed variance in macrobenthos species richness and diversity: E(S100) and H’. For polychaetes, clay and oxygen proved important. At low oxygen sites (DO <1 ml·l−1), depth accounted for most variance. Residual analysis (after removing depth effects) revealed that dissolved oxygen and sediment organic matter influenced 50–62% of residual variation in E(S100), H’ and d for total macrofauna. Of this, oxygen alone influenced up to ~50–62%. When only polychaetes were evaluated, oxygen and organic matter explained up to 58–63%. For low oxygen sites, organic matter alone had the explanatory power when dominance among polychaetes was considered. Overall, macrobenthic patterns in the Bay of Bengal were consistent with those reported for other upwelling margins. However, the compression of faunal gradients at shallower depths was most similar to the Chile/Peru margin, and different from the Arabian Sea, where the depth range of the OMZ is two times greater. The Bay of Bengal patterns may take on added significance as OMZs shoal globally.

Reviews and syntheses: Present, past, and future of the oxygen minimum zone in the northern Indian Ocean

“…Benthic communities and processes in the Bay of Bengal have thus far received less study than those of the Arabian Sea. It is however clear that oxygen exerts an important control on benthic communities across the margins of both basins (e.g, Ingole et al, 2010;Raman et al, 2015).…”

Impact of the 2004 tsunami on the macrofauna of the continental slope of the southeast coast of India

“…Weiss () also discussed the movement of sediment grains by passing tsunami waves to deeper waters. These variations cannot be taken as seasonal or yearly in the study area, because contemporary findings aboard FORV Sagar Sampada in the Bay of Bengal and Arabian Sea did not reveal seasonality in environmental parameters in the slope region (Jaleel, ; Raman et al., ) or between years in sedimentary characteristics (Kitahashi et al., ).…”

“…Sampling was done along fixed transects every year because studies done on the continental shelf did not suggest seasonal variations in species assemblage (Ganesh & Raman, ). Therefore, seasonal variations in the community structure of macrobenthos on the slope can be ruled out in the absence of a catastrophe (Jaleel, ; Raman et al., ). Also, yearly variations in the sedimentary characteristics were not found on the slope (Kitahashi et al., ), so species assemblages do not vary by year.…”

Distinctive Community Patterns With Exceptional Diversity of Polychaetes Around a Tectonically Active Archipelago in the Tropical Indian Ocean

“…These trends were more prominent in the Andaman Islands, where OMZ conditions (<0.5 ml.l −1 ) were encountered at 150-200 m depths. High densities of spionids, cirratulids, paraonids and ampharetids were found in this region (Ingole et al, 2010;Abdul Jaleel et al, 2014;Joydas and Damodaran, 2014;Raman et al, 2015) chiefly represented by Prionospio spp., Kirkegaardia dorsobranchialis, Aricidea lopezi, Levinsenia oculata etc. These are among the genera most commonly encountered in OMZ settings in other parts of the world (Lamont and Gage, 2000;Levin, 2003), which is attributed to their extensive branchiae, and deposit feeding life mode (Abdul Jaleel et al, 2014).…”

“…At the 200 m stratum, the relatively oxygenated (DO > 0.5 ml/l) sandy sediments of the Nicobar Islands harbored higher densities of crustaceans when compared to oxygen deficient (DO < 0.5 ml/l), sandy silt sediments of the Andaman Islands (Karakassis and Eleftheriou, 1997;Levin and Gage, 1998). The latter region was characterized by the dominance of polychaetes (91%), under the OMZ conditions (Abdul Joydas and Damodaran, 2014;Raman et al, 2015;Khan et al, 2017). Echinoderms, which are most vulnerable to oxygen depleted conditions (Diaz and Rosenberg, 1995;Parameswaran et al, 2018), were altogether absent in the oxygen deficient conditions prevailing at ∼200 m depth around the Andaman Islands.…”