Benthic microbial and whole-community responses to different amounts of 13C-enriched algae: In situ experiments in the deep Cretan Sea (Eastern Mediterranean)

The deep eastern Mediterranean Sea is one of the most oligotrophic regions of the world’s oceans and is therefore highly suitable for investigating the response of a resource-limited deep-sea benthic community to food pulses. Additionally, the benthic response to organic matter (OM) deposition can differ according to the amount of OM entering the sediment. Here, we report on 36-h pulse-chase experiments with 13C-labeled diatoms as a tracer of labile OM. Two loads of OM, equivalent to 25 mg C m^-2 (low-carbon [L] experiment) and 250 mg C m^-2 (highcarbon [H] experiment), were injected into chambers of a benthic lander. The fate of the added carbon was followed in total community respiration and bacterial assimilation (through 13C enrichment in bacteria-specific fatty acids). A benthic response was clearly evident in both experiments, but carbon turnover rates were higher with a higher OM addition. However, although a larger amount of carbon was assimilated by bacteria in the H experiment, bacterial assimilation accounted for a relatively lower proportion of total carbon processing compared with the L experiment. This result could indicate a modified bacterial response, a larger role of other biotic components in the high-load experiment, or both. This study demonstrates that an impoverished deep-sea benthic community rapidly responds to an input of fresh OM. Furthermore, the carbon flow pathways can differ according to the amount of OM entering the sediment. Consequently, comparison of benthic responses across systems can only be made in cases in which similar quantities and qualities of OM have been used.