This work presents a novel design for an optical bubble size sensor that is rugged, economical to build, and capable of accurately measuring methane bubble sizes in aquatic environments over long deployment periods. The sensor intercepts rising gas bubbles, elongates them in a thin glass tube, and routes elongated bubbles past an optical detector. The optical detector records information on bubble rise velocity and travel time, which can be combined with the flow path geometry to calculate bubble volume at flow rates up to 3 bubbles/second. The sensor circuitry is powered by 6‐V C alkaline battery packs and is cased in a waterproof housing built from commercially available PVC pipe fittings. Laboratory testing indicates the sensor can accurately measure bubble volumes up to 1 mL in volume. We deployed the sensor in a lake with a history of methane ebullition and gathered data on bubble size distributions (most bubbles were between 0.025 mL and 0.2 mL) as well as the precise timing of bubbling events. The sensor also includes an optional gas collection system to allow for bulk gas sampling.

A compact field-deployable optical instrument using fluorescence, absorbance, and scattering to identify and quantify contaminants and natural substances in water bodies is described. The instrument is capable of deployment on autonomous underwater and surface vehicles, manned vehicles, fixed platforms such as buoys, or access points in water supply or drainage networks. The instrument comprises (1) a flowcell, (2) multiple optical systems, (3) a data logger, (4) a power control board and computer, and (5) a battery. The instrument has been packaged in a cylindrical pressure case of 200 mm diameter and 300 mm length for electrically and mechanically seamless insertion as a STARFISH AUV payload module. The same module can be fitted with watertight end caps for use aboard other platforms, or simpler packaging can be employed for use in less demanding environments. For spectrofluorometry, the system uses six (expandable to twelve) electronically switchable excitation sources, allowing the construction of fluorescence excitation-emission matrices (EEMs). A deuterium-tungsten light source (185 to 1100 nm) is used in making UV-VIS absorbance measurements. Turbidity can be measured by nephelometry, using observations of light scattering at each excitation wavelength. The absorbance and turbidity capabilities provide useful water quality information and can also be used for correction of inner shielding effects. Validation of the instrument includes (1) comparison with a commercial luminescence spectrometer in measuring both standards and field samples, (2) comparisons of observed spectra with published optical characteristics for several chemicals, and (3) field demonstration aboard an AUV.

Assessing the aquatic chemistry of water bodies through sample collection is labor- and time-intensive with limits on discrete spatial coverage that may not provide a detailed representation of the system. A practical approach is to develop in situ sensors deployed aboard autonomous underwater vehicles (AUVs) for three-dimensional water chemistry mapping. For this purpose, a compact optical instrument (LEDIF) measuring fluorescence, absorbance, and scattering to quantify contaminants and natural substances in water bodies is packaged inside a pressure hull and attached to a highly modular and flexible AUV (Small Team of Autonomous Robotic FISH (STARFISH)). LEDIF-STARFISH was deployed at a reservoir in Singapore for in situ real-time chlorophyll a and turbidity data collection. Locations of potential algal hot spots were observed, providing unprecedented insight into the plankton biomass distribution of the reservoir at different times. The results showcase the instrument's potential in tracking spatiotemporal variability of substances in large water bodies.