Oxygen deficiency and benthic communities in the peruvian upper continental margin
The oxygen minimum zone (OMZ) off the Peruvian coast, though shallower and more intense than other OMZs, exhibits a spatial distribution that varies with latitude, depth and distance to the coast. The spatial variability in oxygen content imposes latitudinal and bathymetric zonations of the benthic communities. Nevertheless the spatial patterns of the benthic communities are also modulated by the bottom topography and the sedimentary environments, which in turn are controlled to a large extent by the poleward undercurrent. While the bathymetric patterns of the macro- and meiobenthos have been previously described, latitudinal changes have received less attention. We found that, in parallel with opposed latitudinal gradients of oxygen and fresh organic matter load in the sediments, there is a latitudinal sequence in the distribution of macrofaunal biomass, diversity and density, as described by the Pearson and Rosenberg model (1978) for the macrobenthic response under organic enrichment. This latitudinal pattern is complemented by the maximum development of the giant sulphur nitrate vacuolated bacteria Thioploca spp. under more extreme oxygen deficiency (but not anoxia) and organic load than the macrofauna is able to cope with. Temporal variability in the OMZ also results in significant responses of the benthic communities, but there are differences in the sign and intensity according to the sedimentary environment. In the organic-rich sediments off the central Peruvian shelf, different benthic states, involving the macrobiotic components (infauna and Thioploca) and the meiofauna, are triggered by the varying subsurface oxygen regime.