Response of the epibenthic macrofaunal community to a strong upwelling-driven hypoxic event in a shallow bay of the southern Humboldt Current System.

In January 2008, most of the southern coastal zone of the Humboldt Current System was affected by an intense upwelling event. This caused an intrusion of equatorial sub-surface water into the coastal zone, generating severe hypoxic conditions (≤0.5 ml O(2) l(-1)) three days after the beginning of the event. A rapid, massive die-off of marine organisms occurred in Coliumo Bay on January 3rd, affecting zooplankton, mollusks, crustaceans and fishes. Normal oxygen concentrations were observed on January 10th, seven days after the hypoxic event. Here we analyze the response of the epibenthic macrofauna community using data spanning three years of sampling which encompass the short-term hypoxic disturbance in the bay. We found that (i) strong changes in total density, total biomass, and diversity occurred immediately after the hypoxic event, negatively affecting crustaceans and fishes, while gastropods were favored, (ii) initial changes were reverted over a period of three months, (iii) on an inter-annual time scale, species richness and diversity decreased following the hypoxic event. Total density increased strongly, but total biomass showed no clear inter-annual trend. These results show that, while initial recovery from hypoxia was fast, over longer time scales the community exhibited a shift to an alternative structure dominated principally by Nassariid scavenger species.