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2009
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.

Gooday, AJ, Jorissen F, Levin LA, Middelburg JJ, Naqvi SWA, Rabalais NN, Scranton M, Zhang J.  2009.  Historical records of coastal eutrophication-induced hypoxia. Biogeosciences. 6:1707-1745.   10.5194/bg-6-1707-2009   AbstractWebsite

Under certain conditions, sediment cores from coastal settings subject to hypoxia can yield records of environmental changes over time scales ranging from decades to millennia, sometimes with a resolution of as little as a few years. A variety of biological and geochemical indicators (proxies) derived from such cores have been used to reconstruct the development of eutrophication and hypoxic conditions over time. Those based on (1) the preserved remains of benthic organisms (mainly foraminiferans and ostracods), (2) sedimentary features (e.g. laminations) and (3) sediment chemistry and mineralogy (e.g. presence of sulphides and redox-sensitive trace elements) reflect conditions at or close to the seafloor. Those based on (4) the preserved remains of planktonic organisms (mainly diatoms and dinoflagellates), (5) pigments and lipid biomarkers derived from prokaryotes and eukaryotes and (6) organic C, N and their stable isotope ratios reflect conditions in the water column. However, the interpretation of these indicators is not straightforward. A central difficulty concerns the fact that hypoxia is strongly correlated with, and often induced by, organic enrichment caused by eutrophication, making it difficult to separate the effects of these phenomena in sediment records. The problem is compounded by the enhanced preservation in anoxic and hypoxic sediments of organic microfossils and biomarkers indicating eutrophication. The use of hypoxia-specific proxies, such as the trace metals molybdenum and rhenium and the bacterial biomarker isorenieratene, together with multi-proxy approaches, may provide a way forward. All proxies of bottom-water hypoxia are basically qualitative; their quantification presents a major challenge to which there is currently no satisfactory solution. Finally, it is important to separate the effects of natural ecosystem variability from anthropogenic effects. Despite these problems, in the absence of historical data for dissolved oxygen concentrations, the analysis of sediment cores can provide plausible reconstructions of the temporal development of human-induced hypoxia, and associated eutrophication, in vulnerable coastal environments.

2003
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.