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 epiand 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.
*Present address: Department of Marine Biology, Faculty of Life Sciences,
University of Vienna, Althanstrasse 14, 1090 Vienna, Austria.
stachom5@univie.ac.at Acknowledgments Funding was provided by the Austrian
Science Fund (FWF; project P17655-B03). The concept behind this work was
discussed from the onset with Robert Diaz, Rutger Rosenberg, Heye Rumohr, and
Kay Vopel, all of whom provided important advice and encouragement, especially
Robert Diaz, whose own “dome of death” idea has come to fruition in our EAGU
instrument. We would like to thank Valentin Perlinger (workshop), Gregor Eder
(photography), Sabine Maringer (chemistry), Philipp Steiner, Alexandra
Haselmair, and Ivo Gallmetzer (technical and diving support). Frank Bruder
modified the camera housing, Markus Moll and Wolfgang Tick from Subtronic
adapted the flashes. Lars Damgaard from Unisense answered our many questions
about the sensors and the datalogger. Toshiba sponsored a field-notebook for
the duration of the project. Special thanks to Joerg Ott for organizing and
helping re-equip our boat, to Lado Celestina for refitting and maintaining the
boat, and to the director (Alenka Malej) and staff (Gaspar Polajnar, Branko
Cermelj, Janez Forte, Tihomir Makovec, Mira Avcin) of the Marine Biology
Station (MBS) Piran, Slovenia, for their hospitality and support in matters
large and small. Finally, an insightful anonymous reviewer provided a wealth of
‘spot on’ suggestions that we were happy to incorporate. Limnol. Oceanogr.:
Methods 5, 2007, 344–352 © 2007, by the American Society of Limnology and
Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODS Stachowitsch et al.
Continuous documentation of anoxia 345 team of sport divers to record the
extent of an ongoing oxygen depletion event. Additional benthic mortality
events were also discovered in 1980, 1983, and 1989 during routine fieldwork
(Stachowitsch 1991). Although seasonal anoxia in the northern hemisphere occurs
mostly in late summer/fall (Pearson and Rosenberg 1978; Stachowitsch and Avcin
1988; Druon et al. 2004), its actual timing is related to local weather
conditions. The onset and extent of such disturbances are difficult to predict
and tend to elude investigation in the field. Finally, mortality events often
run their course within a few days (Stachowitsch 1984), further hindering their
full documentation. Laboratory chamber/aquarium experiments on respiration and
responses to decreasing oxygen concentrations typically involve individual
specimens or species (Renaud 1986; de Zwaan 2001; Miller et al. 2002; Matozzo
et al 2005; Shimps et al. 2005). Their results, while physiologically accurate,
do not combine all the relevant information about actual behavioral responses,
intraand interspecific interactions, mortality sequences, and community-level
processes in the natural environment. We addressed this dilemma by developing a
device that can create and fully document small-scale experimental anoxia, in
situ, as well as document the sequence of benthic mortalities. This instrument
combines photo-documentation with detailed chemo-physical analyses and allows
the behaviors and mortalities of benthic organisms to be analyzed during an
oxygen depletion event from the onset. The focus is on the macrofauna because
macroepiand infauna are widely used to detect and monitor community responses to
environmental change. Here, as in the past, we refer to the macrofauna as those
organisms that are visible in situ to the naked eye and to the camera, although
in certain other habitats, e.g., the deep-sea benthos, such organisms may be
referred to as megafauna. Many benthic organisms are sedentary and longlived,
and the community structure therefore reflects environmental conditions
integrated over extended periods (Bilyard 1987; Gray et al. 1988; Bourget et
al. 2003; Ragua-Gil et al. 2004). Moreover, the benthos in the Northern
Adriatic – via re-colonization and succession – can store information on prior
disturbances over years or even decades and, therefore. can be regarded as a
long-term memory of the overall system (Stachowitsch 1992). Materials and procedures
Design of the experimental anoxia generating unit (EAGU)— The EAGU (Fig. 1)
creates anoxia by sealing a 50 × 50 × 50 cm volume of water off from the
surrounding environment. The instrument lid is positioned atop two different
bases. The first is the “open” configuration (hereafter referred to as
“frame”), a 2 cm aluminum-profile frame, (L × W × H = 50 × 50 × 50 cm) that is
positioned over selected benthic organisms on the sediment surface. This
configuration permits full water exchange and does not disrupt normal
bottom-water currents. We observed no sediment accumulation or scouring of the
seabed adjacent to the frame. This configuration is used to document animal
behavior under normoxic conditions (as a control before reconfiguring to
generate anoxia) or to record oxygen depletion events. The second, “closed”
configuration (hereafter referred to as “chamber”) also consists of an
aluminum-profile frame of the same size, but with 6-mm-thick plexiglass plates
on its four vertical sides. This cube-like chamber is open above and below. The
lower plexiglass edges are strengthened with sharpened aluminum elements. This
chamber is pushed approximately 2 cm into the sediment to hinder water exchange
through the substrate (Fig. 2). The watertight lid (simple rubber seal around
upper edge of chamber) prevents exchange with the water column. This
configuration is used to document behavioral responses to decreasing oxygen
concentrations. The four lower corners of both configurations are equipped with
removable 7-cm-long tapered metal tips that help stabilize the device in the
sediment. The chamber is also equipped with two 50-cm-long handles to
facilitate transportation and manipulations. The lid consists of a 12-mm-thick
plexiglass plate measuring 51 × 70 cm and bears the equipment described below.
Fig. 1. Experimental Anoxia Generating Unit (EAGU) with instrument lid
positioned on top of plexiglass chamber. 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, sp: sensor port. Stachowitsch et al. Continuous
documentation of anoxia 346 Camera equipment: A digital camera (Canon EOS 30D)
with a zoom lens (Canon EFS 10-22mm, f/3.5-4.5 USM), mounted in an underwater
carbon-fiber housing (Fig. 1) with a dome port (both Bruder). The camera’s
number of effective pixels is 8.2 MP. The time-lapse function is effected by a
Canon Timer Remote Controller (TC-80N3), and a 1 GB flashcard is used. The lens
and its setting (14 mm) were chosen to provide an optimal combination of
distortion-free images, a view of the entire 50 × 50 cm sediment area along
with a portion of the vertical plexiglass walls, and to position the camera as
close to the bottom as possible. This provided clearer images in turbid
conditions (frame) and reduced the water volume in the chamber. Two underwater
flashes (“midi analog,” series 11897; Subtronic). The flashes are modified to
be adjusted manually (we used the 1/16 setting) and are attached to the lid by
PVCswivel arms on two adjoining sides (Fig. 1). Two external battery packs
power both the camera and the flashes (akku-safe 9Ah Panasonic; Werner light
power Unterwassertechnik). The camera housing is positioned such that it lies
centrally over the frame or chamber. The camera housing port fits snugly into
an O-ring-equipped opening, with the dome projecting below the lid. The housing
is further attached to the lid with an L-shaped aluminum bracket. The housing
has four sockets: two for the flashes and two for the battery packs. Available
power is usually the limiting factor in stand-alone long-term measurements. A
special electronic control circuit (Fig. 3) was developed in order to run the equipment
for at least 72 h with sufficiently small and light external batteries in
combination with a commercially available camera and flash. The circuit was
built on a small board (12 × 3 cm) using standard CMOS integrated circuits for
logic functions and transistors for switching. The following functions were
implemented: (1) A monitoring circuit (ICL7665 + Power Transistor) interrupts
the 12 V supply power when the voltage falls below 10.2 V to prevent damage to
batteries and electronics; (2) A stabilizing circuit (LM 317) provides a
constant 7.5 V to the Canon camera. The camera automatically switches itself
off 1 min after each shot; (3) A charging circuit (resistor + diode) constantly
recharges the internal batteries in the flashes.