Active colonisation of disturbed sediments by deep-sea nematodes: evidence for the patch mosaic model

An on-board experiment was performed during a research cruise to investigate the ability of deep-sea nematode species to actively colonise defaunated sediments. Small cylinders of 500 pm wire mesh filled with defaunated sediment were inserted into microcosms containing sediment with indigenous meiofauna collected from 1300 m depth in the Arctic Ocean. The defaunated sediments were either enriched with the diatom Thalassiosira weissflogii, or remained unenriched. Samples from the defaunated sediment were taken after 9 and 17 d. As controls, microcosms with sediment containing the indigenous meiofauna but without an internal cylinder were also sampled at each time interval. Nematodes colonised both enriched and unenriched sediments with abundances of up to 20% of the controls. Irrespective of the time of sampling, abundance and number of species were significantly higher in the enriched treatment, suggesting that the presence of food enhances colonisation and resilience. Nematode assemblages in the defaunated sediments were species-rich and differed from the controls. The majority of colonising species were rare or undetectable in the controls, suggesting that episodic disturbances may be necessary for their persistence in deep sea sediments. Colonisation was in part determined by species characteristics such as size and motility. At the same time, a large number of different species colonised the empty cylinders, resulting in poor similarity in community composition between replicates, particularly between samples enriched with diatoms. Our results indicate a poor predictability of community composition of recently disturbed sediments despite highly reproducible abundance and diversity patterns and lends experimental support to the existence of a spatio-temporal mosaic that emerges from highly localised colonisation patterns.