The influence of a simulated diatom bloom on deep-sea benthic foraminifera and the activity of bacteria: A mesocosm study

Live benthic foraminifera were collected from the Portuguese continental margin from 900 m water depth. Under controlled laboratory conditions, Thalassiosira pseudonana, was used to create an artificial diatom bloom. Two different diatom loads, high (240 μg C/cm2) and low (80 μg C/cm2) load, were fed to benthos. The response of the benthic foraminifera, total standing stocks (63–150 μm, >150 μm size fractions) and vertical distribution in the sediment, was analysed at 28 and 56 days following the simulated food pulse. The treated cores, or mesocosms, were compared to controls that were harvested at the experimental setup, prior to the feeding, and at the end of the experiment. Bacterial activity was assessed in terms of benthic nitrate and ammonium fluxes across the sediment–water interface. Prior to the simulated diatom bloom the total standing stocks of foraminifera declined significantly. The population recovered following the feeding and, especially under the higher diatom load, increased in comparison to the control cores. In general, a ‘two-phased’ response was measured in different size classes of foraminifera. An increase in the smaller-sized (63–150 μm) subpopulation was followed by an increase in the larger-sized (>150 μm) subpopulation. Elevated effluxes of ammonium were measured following the simulated diatom bloom. The highest effluxes were seen in the high-load treatment, reflecting the highest remineralised carbon load. The ammonium effluxes normalised to control levels around 25 days after the bloom. In addition, high effluxes of nitrate were measured, indicating enhanced nitrification following the diatom deposition. The nitrate effluxes normalised to control levels around 40 days after the feeding event. Several foraminiferal taxa, e.g. Melonis barleeanum, Bigenerina cylindrica, Chilostomella oolina, responded to the diatom bloom (or increased bacterial activity) and increased in numbers. However, the vertical distribution of these infaunal species was not influenced by the added food. Only two infaunal species, Pullenia sp. and Trochammina sp., were observed to increase in numbers in the surficial sediments. These two taxa most likely migrated to the surface to consume the labile organic matter. In contrast, the other infaunal species perhaps benefited from the diatom bloom indirectly, e.g. from increased bacterial activity in the sediment.