Uptake of phytodetritus by benthic foraminifera under oxygen depletion at the Indian margin (Arabian Sea)

Abstract

Abstract. Benthic foraminifera in sediments on the Indian margin of the Arabian Sea, where the oxygen minimum zone (OMZ) impinges on the continental slope, are exposed to particularly severe levels of oxygen depletion. Food supply for the benthic community is high but delivered in distinct pulses during upwelling and water mixing events associated with summer and winter monsoon periods. In order to investigate the response by benthic foraminifera to such pulsed food delivery under oxygen concentrations of less than 0.1 mL L −1 (4.5 µmol L −1 ), an in situ isotope labeling experiment ( 13 C, 15 N) was performed on the western continental slope of India at 540 m water depth (OMZ core region). The assemblage of living foraminifera (> 125 µm) in the uppermost centimeter at this depth is characterized by an unexpectedly high population density of 3982 individuals 10 cm −2 and a strong dominance by few calcareous species. For the experiment, we concentrated on the nine most abundant taxa, which constitute 93 % of the entire foraminiferal population at 0-1 cm sediment depth. Increased concentrations of 13 C and 15 N in the cytoplasm indicate that all investigated taxa took up labeled phytodetritus during the 4 day experimental phase. In total, these nine species had assimilated 113.8 mg C m −2 (17.5 % of the total added carbon). Uptake of nitrogen by the three most abundant taxa (Bolivina aff. B. dilatata, Cassidulina sp., Bulimina gibba) was 2.7 mg N m −2 (2 % of the total added nitrogen). The response to the offered phytodetritus varied largely among foraminiferal species with Uvigerina schwageri being by far the most important species in short-term processing, whereas the most abundant taxa Bolivina aff. B. dilatata and Cassidulina sp. showed comparably low uptake of the offered food. We suggest the observed species-specific differences are related to species biomass and specific feeding preferences. In summary, the experiment in the OMZ core region shows rapid processing of fresh phytodetritus by foraminifera under almost anoxic conditions. The uptake of large amounts of organic matter by few species within four days suggests that foraminifera may play an important role in short-term carbon cycling in the OMZ core region on the Indian margin.

Increased temperature causes different carbon and nitrogen processing patterns in two common intertidal foraminifera (<i>Ammonia tepida</i> and <i>Haynesina germanica</i>)

“…Research on the coupling of food-derived carbon and nitrogen in foraminifera is limited to a few in situ studies in the bathyal of the Arabian Sea, where foraminifera play an important role in benthic carbon fluxes (Enge et al, 2014(Enge et al, , 2016. Tidal flats function as both an important source and sink for nutrients (Joye et al, 2009) and represent essential ecological components of the earth's marine systems.…”

Living (Rose Bengal stained) benthic foraminiferal faunas along a strong bottom-water oxygen gradient on the Indian margin (Arabian Sea)

“…Nevertheless, we refer to it as H. cf. elegans because its occurrence in a severely hypoxic setting is unexpected, and for consistency with Enge et al (2014). Whether it is genetically coherent with H. elegans from oxic environments in other oceans remains to be determined.…”

“…Similar observations were made by Schumacher et al (2007) on the Pakistan margin, where C. oolina was found exclusively in the 0-0.5 cm layer across the OMZ (306-738 m depth). Species of the deep infaunal genus Globobulimina are also abundant in the first sediment centimetre on the Pakistan margin (at 576 m; Erbacher and Nelskamp, 2006). These species occur at several centimetres in depth of the sediment in better oxygenated environments (Corliss and Emerson, 1990;Kitazato, 1994;Rathburn et al, 2000).…”

“…1 and Praeglobobulimina sp. 1) demonstrated a large and rapid (∼ 4 days) carbon uptake in tracer experiments on the Indian margin using labelled phytodetritus (Enge et al, 2014). Uvigerina ex.…”

“…In the Arabian Sea, "live" (Rose-Bengal-stained) benthic foraminifera have been mainly studied on the Oman margin (e.g. Stubbings, 1939;Hermelin and Shimmield, 1990;Naidu and Malmgren, 1995;Gooday et al, 2000) and the Pakistan margin (Jannink et al, 1998;Maas, 2000;Erbacher and Nelskamp, 2006;Schumacher et al, 2007;Larkin and Gooday, 2009;Larkin et al, 2014;Enge et al, 2014). In addition, a few studies have focused on other areas, such as the Murray Ridge Caulle et al, 2014) and the central, eastern and western parts of the Arabian Sea (Kurbjeweit et al, 2000;Hemleben, 2003, 2006).…”