Carbon and Nitrogen Uptake of Calcareous Benthic Foraminifera along a Depth-Related Oxygen Gradient in the OMZ of the Arabian Sea

Abstract

Foraminifera are an important faunal element of the benthos in oxygen-depleted settings such as Oxygen Minimum Zones (OMZs) where they can play a relevant role in the processing of phytodetritus. We investigated the uptake of phytodetritus (labeled with 13C and 15N) by calcareous foraminifera in the 0–1 cm sediment horizon under different oxygen concentrations within the OMZ in the eastern Arabian Sea. The in situ tracer experiments were carried out along a depth transect on the Indian margin over a period of 4 to 10 days. The uptake of phytodetrital carbon within 4 days by all investigated species shows that phytodetritus is a relevant food source for foraminifera in OMZ sediments. The decrease of total carbon uptake from 540 to 1100 m suggests a higher demand for carbon by species in the low-oxygen core region of the OMZ or less food competition with macrofauna. Especially Uvigerinids showed high uptake of phytodetrital carbon at the lowest oxygenated site. Variation in the ratio of phytodetrital carbon to nitrogen between species and sites indicates that foraminiferal carbon and nitrogen use can be decoupled and different nutritional demands are found between species. Lower ratio of phytodetrital carbon and nitrogen at 540 m could hint for greater demand or storage of food-based nitrogen, ingestion, or hosting of bacteria under almost anoxic conditions. Shifts in the foraminiferal assemblage structure (controlled by oxygen or food availability) and in the presence of other benthic organisms are likely to account for observed changes in the processing of phytodetritus in the different OMZ habitats. Foraminifera dominate the short-term processing of phytodetritus in the OMZ core but are less important in the lower OMZ boundary region of the Indian margin as biological interactions and species distribution of foraminifera change with depth and oxygen levels.

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

“…To maintain an optimum energy budget, expenses of metabolic maintenance could therefore be compensated by a shift of H. germanica populations towards the subtidal zone. Foraminiferal C : N ratios are known from bathyal foraminifera of the Sagami Bay and from oxygen minimum zones of the Arabian Sea, where they range from 2.6 to 6.4 (Nomaki et al, 2008Enge et al, 2016). In our study, both species showed similar C : N ratios and natural 13 C and 15 N signatures (Tables 1 and 4), indicating equivalent trophic levels and similar nutritional demands.…”

“…Simultaneous detection of both stable isotopes allows us to determine the ratio in which pC relative to pN is retained in bulk foraminiferal cytoplasm over time. This helps to interpret phytodetrital uptake in relation to foraminiferal carbon and nitrogen coupling (Enge et al, 2016;Evrard et al, 2010;Hunter et al, 2012) and temperature influences might show imbalances of these ratios between treatments. Benthic foraminifera are used in the assessment of (palaeo-) environmental data.…”

“…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.…”

“…The laboratory feeding experiment was performed in incubation chambers, with the temperature being adjusted to 20, 25 and 30 • C. To obtain direct estimates of the uptake of carbon and nitrogen, phytodetritus was labelled with stable isotopes ( 13 C, 15 N). This method has provided important data on the in situ feeding behaviour of foraminifera in various environments (Moodley et al, 2002;Enge et al, 2016;Moodley et al, 2000;Middelburg et al, 2000;Witte et al, 2003;Nomaki et al, 2005Nomaki et al, , 2009Nomaki et al, , 2011Sweetman et al, 2009;Enge et al, 2011;Jeffreys et al, 2013). The temperatures chosen for this experiment correspond to experimentally determined values that cover optimum or tolerance ranges of physiological processes in intertidal foraminifera (Bradshaw, 1957(Bradshaw, , 1961Lee and Muller, 1973).…”

Effect of increased temperature on carbon and nitrogen uptake of two intertidal foraminifera (<i>Ammonia tepida</i> and <i>Haynesina germanica</i>)

“…The laboratory feeding experiment was performed in incubation chambers, adjusted to 20 • C, 25 • C and 30 • C. To obtain direct estimates of the uptake of carbon and nitrogen, phytodetritus was labelled with stable isotopes ( 13 C, 15 N). This method has provided important data on the in situ feeding behaviour of foraminifera in various environments (Moodley et al, 2002;Enge et al, 2016;Moodley et al, 2000;Middelburg et al, 2000;Witte et al, 2003;Nomaki et al, 2005Nomaki et al, , 2009Nomaki et al, , 2011Sweetman et al, 2009;Enge et al, 2011;Jeffreys et al, 2013). The temperatures chosen for this experiment correspond to experimentally determined values that cover optimum or tolerance ranges for growth and reproduction in laboratory cultures of intertidal foraminifera (Bradshaw, 1957(Bradshaw, , 1961Lee and Muller, 1973).…”