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Raman, AV, Damodaran R, Levin LA, Ganesh T, Rao YKV, Nanduri S, Madhusoodhanan R.  2015.  Macrobenthos relative to the oxygen minimum zone on the East Indian margin, Bay of Bengal. Marine Ecology-an Evolutionary Perspective. 36:679-700.   10.1111/maec.12176   AbstractWebsite

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 <0.5ml1(-1)] at 110 stations off the North East Indian margin (16(0) and 20(0)N) featuring coastal, shelf and slope settings (10-1004m). Macrobenthos (>0.5mm) 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 150-280m; DO 0.37ml1(-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>0.55mll(-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(S-100)], 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(S-100) and H'. For polychaetes, clay and oxygen proved important. At low oxygen sites (DO <1mll(-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(S-100), H' and d for total macrofauna. Of this, oxygen alone influenced up to similar 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.

Gooday, AJ, Bett BJ, Escobar E, Ingole B, Levin LA, Neira C, Raman AV, Sellanes J.  2010.  Habitat heterogeneity and its influence on benthic biodiversity in oxygen minimum zones. Marine Ecology-an Evolutionary Perspective. 31:125-147.   10.1111/j.1439-0485.2009.00348.x   AbstractWebsite

Oxygen minimum zones (OMZs; midwater regions with O(2) concentrations <0.5 ml l(-1)) are mid-water features that intercept continental margins at bathyal depths (100-1000 m). They are particularly well developed in the Eastern Pacific Ocean, the Arabian Sea and the Bay of Bengal. Based on analyses of data from these regions, we consider (i) how benthic habitat heterogeneity is manifested within OMZs, (ii) which aspects of this heterogeneity exert the greatest influence on alpha and beta diversity within particular OMZs and (iii) how heterogeneity associated with OMZs influences regional (gamma) diversity on continental margins. Sources of sea-floor habitat heterogeneity within OMZs include bottom-water oxygen and sulphide gradients, substratum characteristics, bacterial mats, and variations in the organic matter content of the sediment and pH. On some margins, hard grounds, formed of phosphorites, carbonates or biotic substrata, represent distinct subhabitats colonized by encrusting faunas. Most of the heterogeneity associated with OMZs, however, is created by strong sea-floor oxygen gradients, reinforced by changes in sediment characteristics and organic matter content. For the Pakistan margin, combining these parameters revealed clear environmental and faunal differences between the OMZ core and the upper and lower boundary regions. In all Pacific and Arabian Sea OMZs examined, oxygen appears to be the master driver of alpha and beta diversity in all benthic faunal groups for which data exist, as well as macrofaunal assemblage composition, particularly in the OMZ core. However, other factors, notably organic matter quantity and quality and sediment characteristics, come into play as oxygen concentrations begin to rise. The influence of OMZs on meiofaunal, macrofaunal and megafaunal regional (gamma) diversity is difficult to assess. Hypoxia is associated with a reduction in species richness in all benthic faunal groups, but there is also evidence for endemism in OMZ settings. We conclude that, on balance, OMZs probably enhance regional diversity, particularly in taxa such as Foraminifera, which are more tolerant of hypoxia than others. Over evolutionary timescales, they may promote speciation by creating strong gradients in selective pressures and barriers to gene flow.

Levin, LA, Whitcraft CR, Mendoza GF, Gonzalez JP, Cowie G.  2009.  Oxygen and organic matter thresholds for benthic faunal activity on the Pakistan margin oxygen minimum zone (700-1100 m). Deep-Sea Research Part Ii-Topical Studies in Oceanography. 56:449-471.   10.1016/j.dsr2.2008.05.032   AbstractWebsite

A transition from fully laminated to highly bioturbated sediments on continental margins is thought to derive from increased animal activity associated with increasing bottom-water oxygen concentration. We examined faunal community responses to oxygen and organic matter gradients across the lower oxygen minimum zone (OMZ) on the bathyal Pakistan margin, where sediments grade from fully laminated sediment at 700m (0.12 mLL(-1) O(2) [5 mu M]) to highly bioturbated sediment at 1100 m (0.23 mLL(-1) O(2) [10 mu M]). High-resolution sampling of the seafloor (every 50 m water depth) was conducted along a single transect during inter- and post-monsoon periods in 2003 to address (a) the existence of oxygen thresholds regulating macrofaunal abundance, composition, diversity and lifestyles, (b) the interactive effects of organic matter quantity and quality, (c) associated community effects on sediment structure, and (d) potential seasonality in these processes. Macrofaunal biomass and bioturbation depth were positively correlated with organic matter availability, which peaked at 850-950 m (3.39-3.53% Org. Q. In contrast, macrofaunal diversity (HI), dominance (RID), and burrow number exhibited threshold responses at oxygen concentrations of 0.12-0.20 mLL(-1) [5-9 mu M]), with few animals and highly laminated sediments present below this concentration and most taxa present in fully bioturbated sediments above it. The highly mobile, burrowing amphinomid polychaete Linopherus sp. exhibited almost complete dominance and high density at 750-850 m (0.12-0.14 mLL(-1) O(2) [5-6 mu M]), but despite its activity, sediment laminae remained faintly visible. Formation of permanent burrows and detritivory were dominant macrofaunal lifestyles within the OMZ, allowing laminae to persist at surprisingly high animal density and biomass. Results reflect a shift from organic matter to oxygen regulation of body size and biogenic structures following the monsoon. This study suggests that for assemblages evolving under permanent severe hypoxia, food availability remains a significant determinant of animal abundance and biogenic structure depth. Oxygen influences patterns of diversity and dominance and interacts with organic matter to generate abrupt faunal transitions on the Pakistan margin. (C) 2008 Elsevier Ltd. All rights reserved.

Levin, LA, Rathburn AE, Gutierrez D, Munoz P, Shankle A.  2003.  Bioturbation by symbiont-bearing annelids in near-anoxic sediments: Implications for biofacies models and paleo-oxygen assessments. Palaeogeography Palaeoclimatology Palaeoecology. 199:129-140.   10.1016/s0031-0182(03)00500-5   AbstractWebsite

Anoxic or nearly anoxic conditions ( < 4 muM O(2)) have long been associated with the absence of bioturbation and animal traces. This premise has guided interpretation of paleoceanographic conditions from rocks and sediments. We recently observed a high-density, living assemblage of highly mobile, symbiont-bearing, burrowing, phallodrilinid oligochaetes within a nearly anoxic basin ( <1 muM O(2) [0.02-0.03 ml l(-1)]) on the Peru margin (305 m). These observations were made during the most intense part of the 1997-98 El Ni (n) over tildeo when there may have been slight oxygenation of an otherwise anoxic basin, but oligochaete presence prior to this event is likely. The occurrence of symbiont-bearing gutless oligochaetes mainly within the upper 5 cm of the sediment column coincided with a bioturbated zone overlying distinctly laminated sediments. Our observations redefine the lower oxygen limit of macrofaunal bioturbation to much less than2 muM, and indicate a need to modify currently accepted ideas about the relationship between bioturbation and paleo-oxygen concentration. These results also address an ongoing debate about the lifestyles of bioturbating organisms in oxygen-poor settings. (C) 2003 Elsevier B.V. All rights reserved.

Neira, C, Sellanes J, Levin LA, Arntz WE.  2001.  Meiofaunal distributions on the Peru margin: relationship to oxygen and organic matter availability. Deep-Sea Research Part I-Oceanographic Research Papers. 48:2453-2472.   10.1016/s0967-0637(01)00018-8   AbstractWebsite

A quantitative study of metazoan meiofauna was carried out on bathyal sediments (305, 562, 830 and 1210 m) along a transect within and beneath the oxygen minimum zone (OMZ) in the southeastern Pacific off Callao, Peru (12 degreesS). Meiobenthos densities ranged from 1517 (upper slope, middle of OMZ) to 440-548 ind. 10cm(-2) (lower slope stations, beneath the OMZ). Nematodes were the numerically dominant meiofaunal taxon at every station, followed by copepods and nauplii. Increasing bottom-water oxygen concentration and decreasing organic matter availability downslope were correlated with observed changes in meiofaunal abundance. The 300-m site, located in the middle of the OMZ, differed significantly in meiofaunal abundance, dominance, and in vertical distribution pattern from the deeper sites. At 305 m, nematodes amounted to over 99% of total meiofauna; about 70% of nematodes were found in the 2-5 cm. interval. At the deeper sites, about 50% were restricted to the top I cm. The importance of copepods and nauplii increased consistently with depth, reaching similar to 12% of the total meiofauna at the deepest site. The observation of high nematode abundances at oxygen concentrations <0.02mll(-1) supports the hypothesis that densities are enhanced by an indirect positive effect of low oxygen involving (a) reduction of predators and competitors and (b) preservation of organic matter leading to high food availability and quality. Food input and quality, represented here by chloroplastic pigment equivalents (CPE) and sedimentary labile organic compounds (protein, carbohydrates and lipids), were strongly, positively correlated with nematode abundance. By way of contrast, oxygen exhibited a strong negative correlation, overriding food availability, with abundance of other meiofauna such as copepods and nauplii. These taxa were absent at the 300-m site. The high correlation of labile organic matter (C-LOM, sum of carbon contents in lipids, proteins and carbohydrates) with CPE (Pearson's r = 0.99, p <0.01) suggests that most of the sedimentary organic material sampled was of phytodetrital origin. The fraction of sediment organic carbon potentially available to benthic. heterotrophs, measured as C-LOM/Total organic carbon, was on average 17% at all stations. Thus, a residual, refractory fraction, constitutes the major portion of organic matter at the studied bathyal sites. (C) 2001 Elsevier Science Ltd. All rights reserved.