Community and trophic responses of benthic Foraminifera to oxygen gradients and organic enrichment
Global warming and eutrophication are driving an expansion of hypoxia in the World Ocean. This will favour organisms, such as Foraminifera (testate protists), that tolerate low-oxygen conditions and may lead to an overall decline in marine biodiversity. With this in mind, community and trophic responses of benthic Foraminifera were investigated at two contrasting sites in the upper boundary (140 m water depth; bottom-water oxygen concentrations = 2.05 mll-1 during the spring intermonsoon and 0.11 mll-1 during the SW monsoon) and the core (300 m water depth; bottom-water oxygen concentration consistently ~ 0.11 mll-1) of an intense, natural, mid-water oxygen minimum zone (OMZ) on the Pakistan Margin, NE Arabian Sea. Live macrofaunal (>300 µm fraction) Foraminifera (including softwalled species) and metazoans were examined at each site during the 2003 spring intermonsoon (April) and SW monsoon (October) seasons (4 replicate multicores/site/season, 25.5cm2 surface area, 0-5 cm depth). Wet-sorting revealed a low diversity assemblage dominated (> 60 %) by calcareous Foraminifera at both sites. A total of 36 species was recognised and diversity was not greatly affected by water depth or season. At both sites, >86 % of Foraminifera were restricted to the upper 0-1 cm layer of sediment and the Average Living Depth (ALD) decreased from the spring intermonsoon to the SW monsoon (140 m, ALD5 = 0.41 to 0.33; 300 m, ALD5 = 0.65 to 0.44). Foraminifera increased in mean abundance from 124 to 153 individuals per 10 cm2 from the spring intermonsoon to the SW monsoon at 140 m and from 86 to 122 individuals per 10 cm2 at 300 m. The calcareous species Uvigerina ex. gr. semiornata dominated communities and increased in mean abundance from 54 to 118 individuals (140 m) and from 41 to 69 individuals (300 m) per 10 cm2 following the SW monsoon. At 140 m, Foraminifera were 3.6 times more abundant than metazoans during the spring intermonsoon, rising to 13.9 times during the SW monsoon. The corresponding proportions at 300 m, where metazoans were rare, were 12.4 and 14.5. Fatty acid biomarkers suggest that foraminiferal diets vary between species. The calcareous species U. ex. gr. semiornata, Bolivina aff. dilatata and Globobulimina cf. G. pyrula selectively ingested phytodetrital material, whereas the agglutinated species, Ammodiscus aff. cretaceus, Bathysiphon sp. nov. 1, and Reophax dentaliniformis favoured bacteria. Moreover, U. ex. gr. semiornata, rapidly ingested (within two days) 13C-labelled diatoms in shipboard laboratory and in situ pulse-chase experiments at the 140-m site following the SW monsoon. This enabled the uptake and processing of organic matter (OM) to be tracked in the foraminiferal cell into individual fatty acids, using Gas Chromatography - Mass Spectrometry (selective ion scan). These results suggest that calcareous Foraminifera, in particular U. ex. gr. semiornata, play a central role in OM cycling on the sea-floor in the upper part of the Pakistan margin OMZ.