The Experimental Anoxia Generating Unit (EAGU), especially designed and constructed by the Vienna team, experimentally obtains detailed information on the effects of oxygen deficiency on the behaviour and survival of macrobenthic assemblages in the Gulf of Trieste, Northern Adriatic Sea. The unit can be deployed for up to five days to autonomously generate oxygen crises and quantify both physico-chemical parameters and benthic responses. Naturally occurring oxygen depletions can also be documented with a second, open frame.
The EAGU creates anoxia by sealing a 50 x 50 x 50 cm volume of water off from the surrounding environment. The centerpiece is the separate plexiglass lid that houses
a time-lapse digital camera (Canon EOS 30D) mounted in an underwater housing (Bruder, Germany)
two flashes (Subtronic, Germany)
two battery packs (Werner Lightpower, Germany)
two oxygen sensors (2 cm and 20 cm above the bottom), one hydrogen sulphide-, and one temperature sensor along with the datalogger (Unisense, Denmark)
separate pH-sensor (WTW, Germany)
The instrument lid is positioned atop two different bases (see below). The time-lapse camera produces a series of images at 6 minute intervals. The sensor values are logged every minute.
The EAGU is delivered to the bottom by 2 divers and then positioned over benthic organisms.
The system is initially positioned for 24h in an “open” configuration (50 x 50 x 50 cm open-sided aluminium frame) over the fauna. Here, we document benthic behaviour during normoxic oxygen conditions.
Afterwards, the EAGU is switched to its “closed” configuration (plexiglass enclosure, same size as the frame) and repositioned over the same assemblage for another 48 to 72 h. Here, the behavioural responses to continuously decreasing oxygen- and increasing hydrogen sulphide concentrations are documented.
|Fig. 1 Experimental Anoxia Generating Unit (EAGU) with instrument lid positioned on top of plexiglass chamber (open on the bottom). Here, only one sensor is connected to the datalogger and inserted through a sensor port. ch: camera housing, dl: datalogger, eb: external battery, fl: flashes, mb: metal brackets, os: oxygen sensor, pc: plexiglass chamber (wall thickness: 6 mm), sp: sensor port.|| |
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Fig. 2 Test deployment of EAGU in shallow (12 m) depth. Crossbar (top center) between the two brackets serves to lift and transport the lid as well as an attachment site for buoys to provide neutral buoyancy during manipulations. Tube on left is a sheath for sensors during transport.