The ECHOES system consisted of an Acoustic Doppler Velocimeter (ADV, Nortek, USA) that was coupled to a FirestingO2-Mini fiber-optic O2 meter with a fluorescence-based fast-response (< 0.3 s) 430 µm tip diameter optode (Pyroscience, GE) and a fast-response (< 0.6 s) H+ Ion-Selective Field Effect Transistor and controller (ISFET; Microsens, CH) (Figure 2). The ADV had the capability to log both external signals (O2 and H+) and also provided output power to the sensors.

The H+ ISFET sensors were adapted to EC by using a flow-through design across the sensing surface of the ISFET sensor (Figure 2). The ISFET sensors are light-sensitive and must therefore be used in a dark environment. To prevent light interference, and to sample in the same measuring volume as the other sensors, a 40 mm long section of 19 gauge (0.69 mm ID) stainless steel tube was attached to the ISFET sensor to create a micro-flow cell for measuring pH. One side of the tubing was machined flat over a 5 mm length and epoxied over the sensing surface of the ISFET (Figure 2). A Mag-Drive gear pump (Micropump, USA) was used to pull fluid from the measuring volume to the sensor at a flow rate of 0.1 L min-1. The pump was located in a separate underwater housing with a dedicated constant-voltage battery source. The O2 optode and the H+ ISFET sensor were co-located just outside the ~ 2 cm3 measuring volume of the ADV, located 0.15 m below the ADV sensor head. The height of the ADV measuring volume above the sediment surface was 0.30 m.

The H+ ISFET sensor works in conjunction with a reference electrode. The reference electrodes were constructed from 8mm outside diameter alkali-resistant glass (Schott, GE) with a micro-porous glass frit tip (Princeton Applied Research, USA), filled with a saturated solution of potassium chloride, and fitted with a silver chloride coated silver wire. The reference and ISFET sensors were pre-conditioned in seawater overnight to reduce equilibration time in the field. An ISFET controller (Microsens, CH) operated the ISFET at a constant drain-source voltage and current relative to the source voltage provided by the reference electrode (see Martz et al. 2010) and had an analog output of approximately 55mV per pH unit. To better resolve the signal it was amplified 50 times using a custom signal amplifier with an adjustable voltage offset (Alligator Technologies, USA).

The O2 optodes were calibrated pre- and post-deployment using water-saturated air and saturated ascorbic acid solution (anoxic). A reference, or baseline, fluorescence signal was automatically subtracted from each O2 measurement by the Firesting O2 meter to remove any signal contributions due to ambient light variations. The fiber optic cable was securely fastened to the frame with tape throughout its length to prevent signal bias due to cable movement.

The ADV provided power for and logged the signal from both the H+ ISFET and the O2 optode at a frequency of 64 Hz. All instrumentation was mounted to a custom carbon-fiber frame with small-diameter legs (1 cm) to limit hydrodynamic interference. The lightweight construction allowed for ease of handling but each frame leg needed to be weighted and secured using anchors.