To test the pore density in benthic foraminifera as a potential proxy for bottom-water oxygenation, pore density analyses were carried out on tests of living (rose Bengal-stained) specimens of the deep-infaunal and anoxia-tolerant foraminiferal species Globobulimina turgida. Three stations within and two stations below the oxygen minimum zone (OMZ) off Namibia were investigated and compared to in situ-measured bottom-water oxygen content (BW-O2). Pore density was first conventionally assessed by rather time-consuming manual pore counting on SEM photographs and measurement of the analyzed test areas. To significantly shorten the measurement time we tested and evaluated an automation of the pore density measurement using the image analysis software package analySIS (version 5.0, Olympus Soft Imaging Solutions). Pore density data from automated analyses are compared to manually acquired data from G. turgida. Our study shows almost identical results for both manually and automatically acquired data. Consequently, we assume that the new technique provides an alternative and more rapid method to analyze the pore density of foraminifera.
For both methods, our results show a distinct negative linear correlation (automatically analyzed pore density: τ = −0.50, p < 0.001; manually analyzed pore density: τ = −0.49, p < 0.001) between pore density and BW-O2, suggesting that G. turgida increases its pore density in response to decreasing oxygen. Thus, we suggest that, similar to other recently described low-oxygen-tolerant benthic foraminiferal species, G. turgida may improve its O2 uptake by increasing pore density to survive in low-oxic environments. This morphological adaption might be useful for future studies to establish an independent proxy for BW-O2. In addition, pore density has been compared to in situ-measured bottom-water nitrate concentration (BW-NO3−). Our investigation of the pore density-to-BW-NO3− relationship for G. turgida suggests that nitrate seems to be a minor factor influencing pore density in this species compared to BW-O2.