Oxygen depletion and benthic mortalities: The first in situ experimental approach to documenting an elusive phenomenon

Abstract

Oxygen depletion events and anoxia are a key threat to shallow marine coastal seas worldwide. The mortalities they trigger, however, are difficult to document in full. We developed an underwater device to experimentally induce hypoxia and anoxia on the seafloor. The EAGU (Experimental Anoxia Generating Unit) combines a time-lapse camera and flashes with an array of sensors and a datalogger. The unit was successfully deployed in 24 m depth in the Northern Adriatic Sea for 3 to 5 d and yielded detailed information on the behavior and sequence of mortality of macrobenthic organisms - both epi- and infauna - under decreasing oxygen and increasing H2S concentrations. This unit, designed as a chamber with an instrument lid, also can be deployed in an open configuration to document low dissolved oxygen (DO) events. The equipment can provide data for a catalog of behavioral patterns, define indicator species, help reconstruct past mortalities, and better gauge the stability and status of benthic communities.