Faunal responses to oxygen gradients on the Pakistan margin: A comparison of foraminiferans, macrofauna and megafauna

Abstract

The Pakistan margin is characterized by a strong midwater oxygen minimum zone (OMZ) that intercepts the seabed at bathyal depths (150 to 1300 m). We investigated whether faunal abundance and diversity trends were similar among protists (foraminiferans and gromiids), metazoan macrofauna and megafauna along a transect (140-1850 m water depth) across the OMZ during the 2003 intermonsoon (March-May) and late/post monsoon (August-October) seasons. All groups exhibited some drop in abundance in the OMZ core (250-500 m water depth; O 2 : 0.10-0.13 ml l -1 = 4.46-5.80 μM l -1 ) but to differing degrees. Densities of foraminiferans >63 µm were slightly depressed at 300 m, peaked at 738 m, and were much lower at deeper stations. Foraminiferans >300 µm were the overwhelmingly dominant macrofaunal organisms in the OMZ core.Macrofaunal metazoans reached maximum densities at 140 m depth, with additional peaks at 850, 940 and 1850 m where foraminiferans were less abundant. The polychaete Linopherus sp. was responsible for a macrofaunal biomass peak at 950 m. Apart from large swimming animals (fish and natant decapods), metazoan megafauna were absent between 300 and 900 m (O 2 <0.14-0.15 ml l -1 = 6.25-6.69 μM l -1 ) but were represented by a huge, ophiuroid-dominated abundance peak at 1000 m (O 2 ~0.15-0.18 ml l -1 = 6.69-8.03μM l -1 ). Gromiid protists were confined largely to depths below 1150 m (O 2 >0.2 ml l -1 = 8.92 μM l -1 ). The progressively deeper abundance peaks for foraminiferans (>63 µm), Linopherus sp. and ophiuroids probably represent lower OMZ boundary edge effects and suggest a link between body size and tolerance of hypoxia. Macro-and megafaunal organisms collected between 800 and 1100 m were dominated by a succession of different taxa, indicating that the lower part of the OMZ is also a region of rapid faunal change. Species diversity was depressed in all groups in the OMZ core, but this was much more pronounced for macrofauna and megafauna than for foraminiferans. Oxygen levels strongly influenced the taxonomic composition of all faunal groups. Calcareous foraminiferans dominated the seasonally and permanently hypoxic sites (136-300 m); agglutinated foraminiferans were relatively more abundant at deeper stations where oxygen concentrations were >0.13 ml l -1 (= 5.80 μM l -1 ). Polychaetes were the main macrofaunal taxon within the OMZ; calcareous macrofauna and megafauna (decapods, 3 echinoderms) were rare or absent where oxygen levels were lowest. The rarity of larger animals between 300 and 700 m on the Pakistan margin, compared with the abundant macrofauna in the OMZ core off Oman, is the most notable contrast between the two sides of the Arabian Sea. This difference probably reflects the slightly higher oxygen levels and better food quality on the western side.

Macrofauna regulate heterotrophic bacterial carbon and nitrogen incorporation in low-oxygen sediments

“…Both these assemblages exhibit strong zonation across OMZs, with foraminiferal distributions controlled by sediment OM availability and high foraminiferal densities observed where metazoans are absent (for example, Gooday et al, 2009). By contrast, meiofaunal densities are positively correlated to macrofauna densities, potentially relying on macrofaunal bioturbation to provide suitable microhabitats (Cook et al, 2000).…”

“…In future climate projections, OMZs are predicted to expand with far-reaching implications for global biogeochemical cycles (Stramma et al, 2010). Changes in oxygen availability and sediment geochemistry across an OMZ influence sediment community composition, producing changes in the faunal and bacterial assemblages that control the rates and pathways for OM recycling (Woulds et al, 2007;Andersson et al, 2008;Gooday et al, 2009;Hunter et al, 2011Hunter et al, , 2012Moodley et al, 2011). Therefore, an understanding of the effects of oxygen limitation upon OM availability, sediment community structure and ecosystem processes is required in order to predict the effects of climate change upon global biogeochemical cycles.…”

Spring-to-summer changes and regional variability of benthic processes in the western Canadian Arctic

“…However, foraminiferan contribution to the variability in benthic processes was not statistically analysed, and their contribution to benthic carbon remineralisation was not measured in their study. As demonstrated by Gooday et al (2009) for the deep sea, the size and abundance of macrofaunal foraminifera in Arctic environments imply the need to consider this parameter in the examination of benthic processes.…”

Is the meiofauna a good indicator for climate change and anthropogenic impacts?

“…Meiofauna appear to better tolerate low concentrations of oxygen than macrofauna (Giere 1993). In the core OMZ regions, where oxygen concentrations typically drop below 0.1 mL/L, the fauna is primarily made up of small organisms, dominated by foraminifera and nematodes (Levin et al 1991;Levin 2003;Gooday et al 2009b), while macrofaunal organisms are rare or absent (Levin et al 1991;Gooday et al 2000Gooday et al , 2009aGooday et al , 2009bLevin 2003). Behavioural (escape, predation reduction) and physiological responses (growth rate decrease, dormancy, initiation of anaerobic metabolism) to hypoxic conditions have been recorded in meiofauna (Rabalais et al 2002), with a general decrease in meiofaunal densities due by the disappearance of many species (Diaz and Rosenberg 1995).…”

“…For an exhaustive review on this matter, please refer to paleoclimatology studies. Benthic foraminifera, particularly species with calcareous tests, have a superb fossil record and are widely used by micropaleontologists to reconstruct conditions in the historical and geological pasts (Gooday 2003;Jorissen et al 2007;Gooday et al 2009a;Schönfeld et al 2012). Their utility in palaeoceanography and palaeoecology is based on a considerable body of available knowledge regarding the adaptation of different species to particular environmental conditions (e.g., Murray 2006).…”