Environ. Sci. Technol. 2007, 41, 6210-6215
In Situ Applications of a New Diver-Operated Motorized Microsensor Profiler M I R I A M W E B E R , * ,†,‡ P A U L F A E R B E R , † V O L K E R M E Y E R , † C H R I S T I A N L O T T , †,‡ GABRIELE EICKERT,† KATHARINA E. FABRICIUS,§ AND DIRK DE BEER† Max-Planck-Institute for Marine Microbiology, Celsiusstrasse 1, 28359 Bremen, Germany, HYDRA Institute for Marine Sciences, Elba Field Station, Via del Forno 80, 57034 Campo nell’Elba (LI), Italy, and Australian Institute of Marine Science, PMB No 3, Townsville, Queensland 4810, Australia
Microsensors are powerful tools for microenvironment studies, however their use has often been restricted to laboratory applications due to the lack of adequate equipment for in situ deployments. Here we report on new features, construction details, and examples of applications of an improved diver-operated motorized microsensor profiler for underwater field operation to a water depth of 25 m. The new motorized profiler has a final precision of 5 µm, and can accommodate amperometric Clark-type microsensors for oxygen and hydrogen sulfide, potentiometric microsensors (e.g., for pH, Ca2+), and fiber-optic irradiance microsensors. The profiler is interfaced by a logger with a signal display, and has pushbuttons for underwater operation. The system can be pre-programmed to autonomous operation or interactively operated by divers. Internal batteries supply power for up to 24 h of measurements and 36 h of data storage (max. 64 million data points). Two flexible stands were developed for deployment on uneven or fragile surfaces, such as coral reefs. Three experimental pilot studies are presented, where (1) the oxygen distribution in a sand ripple was 3-D-mapped, (2) the microenvironment of sediment accumulated on a stony coral was studied, and (3) oxygen dynamics during an experimental sedimentation were investigated. This system allows SCUBA divers to perform a wide array of in situ measurements, with deployment precision and duration similar to those possible in the laboratory.
Introduction On water-solid interfaces (surfaces of sediment, organisms, etc.), high conversion rates by microorganisms or chemical processes combined with mass transfer resistance often lead to steep concentration gradients of metabolic substrates and products. High spatial resolution techniques are needed to investigate these gradients on the surfaces of sediments, biofilms, and benthic organisms. Microsensors have the required spatial resolution (5-100 µm) and are widely used * Corresponding author phone: +390565988027; fax: +390565988090; e-mail:
[email protected]. † Max-Planck-Institute for Marine Microbiology. ‡ HYDRA Institute for Marine Sciences. § Australian Institute of Marine Science. 6210
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to study such chemical and physical microenvironments. The accuracy of flux and conversion measurements in benthic microlayers depends on minimal disturbance of systems under investigation. A significant advantage of microsensors is that they are minimally invasive because of their small tip diameters (1-20 µm). However, the transport of samples to the laboratory may change their fluxes and metabolic rates. For example, sediment retrieval leads to altered oxygen uptake because of changed pressure, flow, or advective transport conditions (1, 2), and stress due to retrieval can potentially alter metabolic rates in macro-organisms. Therefore, laboratory measurements should preferably be compared against field measurements. Previously microsensor field applications were conducted either close to the water surface where a laboratory microsensor profiler could be used without submersion or using submersible systems. Autonomous microsensor profiling systems were mounted on free-falling landers or deployed by remotely operated vehicles (3-5). Diver-operated microsensor submersible systems have also been used before. For example the MiniProfiler (Unisense, Denmark), a portable unit for field measurements to a depth of 50 m, was previously used for seagrass studies (6, 7), while others placed laboratory instruments into custom-made housings to study the oxygen dynamics in sediment burrows of a mud shrimp (8, 9). Further, a commercial profiler (Unisense, Denmark) was used by divers to study wave-induced hydrogen sulfide fluxes around a sessile ciliate colony, oxygen profiles in seagrass, and Antarctic microbial mats (6, 10, 11). In these studies the microsensor was fixed to a micromanipulator on a monopod, positioned manually (description see ref 10) and left stationary for up to 4 h. To investigate algae below sea ice a buoyant up-side-down tripod was used under water and connected by cable to the control and data acquisition instruments which were put onto the ice surface (12). All previously described in situ systems so far lacked the possibility to interact with the measuring procedure, to be deployed on uneven fragile surfaces, and depended on external storage capabilities and power supplies either by extra battery packs or from a power source, e.g., on a boat. We have built a system that overcomes the limitations described above. Our purpose was to investigate the effects of sedimentation by different types of sediment on scleractinian corals, which required detailed in situ measurements of microenvironments and fluxes within the coral-sediment interface. None of the previously used profilers was suitable for our studies. The purpose of our developments was to extend the capabilities of in situ microsensor applications, by providing for (1) manual and very precise micropositioning, to avoid breakage of the microsensor on the coral skeleton, (2) choice of motorized and autonomous profiling operation, in order to avoid disturbance by the diver as well as allowing measurements beyond diving times, (3) control by a diver over all functions in order to react to the natural setting such as, e.g., sediment depth, and (4) deployment on uneven and fragile environments by flexible stands. Various features of the equipment, e.g., the flexibility of our two new stands, the motorized profiling of the amperometric oxygen and potentiometric pH microsensors, and the ability of the diver to interact with the system during measurements are shown by three examples of field applications. We addressed the following hypothesis that (1) oxygen concentration in a sand ripple differs among peak, slope, and trough due to wave-induced differential porewater flow; (2) oxygen is depleted and pH is reduced in muddy 10.1021/es070200b CCC: $37.00
2007 American Chemical Society Published on Web 07/18/2007
FIGURE 1. Schematic diagram of the diver-operated motorized microsensor profiler with the small stand. sediment accumulated on corals as consequence of microbial activity, harming corals irreversibly; and (3) oxygen at the surface of an alga is depleted immediately after a sedimentation event. We also demonstrate that this profiler is highly suitable for other underwater studies.
Experimental Section The profiler consists of a microsensor, a micromanipulator with motor, the stand, the amplifier, and the logger. The microsensor is mounted onto a 3-D-micromanipulator and connected to an amplifier by a cable. The motor is also mounted onto the micromanipulator and connected by a cable to the logger, the control and data acquisition unit of the system. Another cable connects the logger to the amplifier. The micromanipulator is fixed onto an extension arm, which is mounted onto a ball-head onto the stand. The flexible stands have individually adjustable legs, with holders for lead weights to stabilize the profiler (Figures 1 and 2). Microsensors. Amperometric Clark-type oxygen and spherical fiber-optic irradiance microsensors were prepared as described previously (13, 14). Liquid ion-exchange membrane pH microsensors (15) were modified by combining a pH-reference electrode into the sensor (Figure S1; for detailed description see Supporting Information). All microsensors had a tip diameter of 10-50 µm and a stirring sensitivity of
