Living on the edge: single-species dominance at the Pakistan oxygen minimum zone boundary

Oxygen minimum zones (OMZs) are naturally occurring, low-oxygen water masses that create hypoxic conditions where they impinge on the seafloor. Their lower boundaries are characterised by elevated densities of hypoxia-tolerant fauna and an abundant food supply. The polychaete Linopherus sp. nov. (Amphinomidae) is the dominant taxon at the Pakistan margin (PM) lower OMZ, at near suboxic dissolved oxygen (DO) concentrations of 5 to 8 mu M. We explored the response of Linopherus sp. nov. to gradients in oxygen and organic matter (OM) availability from depths of 700 to 1100 m during the inter-and late monsoon periods. Linopherus sp. nov. was present from 800 to 1000 m, and highest densities were found at 850 m. Population size structuring was evident and smaller individuals were present at depths of lowest DO concentrations. Linopherus sp. nov. showed morphological adaptation to low DO, and respiratory surface areas were significantly larger in worms at sites of lowest DO concentrations. Stable carbon isotopes (delta C-13) revealed that Linopherus sp. nov. feeds mainly on sedimentary OM while enriched delta N-15 values suggest that Linopherus sp. nov. also utilises predation as a foraging strategy. Lipid biomarkers indicate an omnivorous lifestyle, in which Linopherus sp. nov. uses phytodetrital, bacterial and invertebrate/carrion food sources. Pulse-chase experiments demonstrated that Linopherus sp. nov. consumes phytodetritus and contributes significantly to OM processing, potentially altering OM quality and thus the availability of food resources to the benthic community. Severe oxygen stress leads to single-species dominance, which in turn simplifies macrofaunal ecosystems and thus reduces trophic complexity.