The response of Nonionella iridea and other benthic foraminifera to “fresh” organic matter enrichment and physical disturbance

•Benthic foraminifera were exposed to sediment disturbance and organic matter (algae)•The highest foraminiferal abundance was in the unfed, undisturbed treatments•Nonionella iridea proliferated where there was no fresh organic material•We suggest that N. iridea may feed on microbes associated with hypoxic-suboxic sediment pore water conditions

Experimental and field evidence from sediments collected at an upper bathyal site in the outer Oslofjord, Norway, show that the benthic foraminifer Nonionella iridea lives closely associated with a steep oxygen concentration gradient in the surface sediment (0–2 cm). An experiment was designed to investigate the effect of organic matter enrichment, physical disturbance simulating bioturbation, and the resultant oxygen conditions on benthic foraminifera < 75 μm in size. At the beginning of the experiment, all individuals were < 75 μm, at the end of the eight week experiment the > 90 μm fraction was analysed. In the experiment, N. iridea was found most abundantly in oxic conditions but also grew under suboxic–hypoxic conditions. It has previously been reported to be an indicator of the arrival of phytodetritus to the seafloor. In this study N. iridea showed maximum abundance where no food was added, it probably fed on degrading organic matter and associated microbes. Weekly physical disturbance of the surface (0–2 cm) sediments caused a lower abundance of N. iridea by > 50% and created a significantly lower abundance in the overall foraminiferal assemblage compared to when the sediment was undisturbed. The disturbance probably has a negative impact because it disrupts biogeochemical gradients (e.g., rapid variation of oxygen concentration and gradients) and associated food within the sediment (e.g., patches of fresh food increasing microbial degradation and oxygen consumption). Both field and experimental data suggest that Brizalina skagerrakensis proliferates after the arrival of fresh phytodetritus in well oxygenated sediments. Based on microhabitat distributions, associated dissolved oxygen concentrations and previous reports, we hypothesise that B. skagerrakensis, N. iridea and Stainforthia fusiformis, in the order mentioned, show a successive ability to proliferate in habitats along a gradient from dominantly fresh to dominantly more refractory organic material and associated microbes related to organic matter degradation and the quality and quantity of organic matter.