Assessment of sensor performance

There is an international commitment to develop a comprehensive, coordinated and sustained ocean observation system. However, a foundation for any observing, monitoring or research effort is effective and reliable in situ sensor technologies that accurately measure key environmental parameters. Ultimately, the data used for modelling efforts, management decisions and rapid responses to ocean hazards are only as good as the instruments that collect them. There is also a compelling need to develop and incorporate new or novel technologies to improve all aspects of existing observing systems and meet various emerging challenges. Assessment of Sensor Performance was a cross-cutting issues session at the international OceanSensors08 workshop in Warnemünde, Germany, which also has penetrated some of the papers published as a result of the workshop (Denuault, 2009; Kröger et al., 2009; Zielinski et al., 2009). The discussions were focused on how best to classify and validate the instruments required for effective and reliable ocean observations and research. The following is a summary of the discussions and conclusions drawn from this workshop, which specifically addresses the characterisation of sensor systems, technology readiness levels, verification of sensor performance and quality management of sensor systems

Trajectories of sinking particles and the catchment areas above sediment traps in the northeast Atlantic

A Lagrangian analysis of particles sinking through a velocity field observed by Eulerian frame measurements was used to evaluate the effects of horizontal advection and particle sinking speed on particle fluxes as measured by moored sediment traps. Characteristics of the statistical funnel above moored deep-ocean sediment traps at the German JGOFS quasi-time series station at 47N, 20W (Biotrans site) were determined. The analysis suggests that the distance and direction between a given sediment trap and the region at the surface where the particles were produced depends on the mean sinking velocity of the particles, the horizontal velocity field above the trap and the deployment depth of the trap. Traps moored at different depths at a given mooring site can collect particles originating from different, separated regions at the surface ocean. Catchment areas for a given trap vary between different years. Typical distances between catchment areas of traps from different water depth but for a given time period (e.g., the spring season) are similar or even larger compared to typical length scales of mesoscale variability of phytoplankton biomass observed in the temperate northeast Atlantic. This implies that particles sampled at a certain time at different depth horizons may originate from completely independent epipelagic systems. Furthermore catchment areas move with time according to changes in the horizontal flow field which jeopardizes the common treatment of interpreting a series of particle flux measurements as a simple time series. The results presented in this work demonstrate that the knowledge of the temporal and spatial variability of the velocity field above deep-ocean sediment traps is of great importance to the interpretation of particle flux measurements. Therefore, the one-dimensional interpretation of particle flux observations should be taken with care

Development of a reliable microelectrode dissolved oxygen sensor

This article reports the results of a careful experimental and analytical investigation which led to the development of an accurate and reproducible microelectrode dissolved oxygen sensor. Primarily designed for oceanographic applications but also applicable to environmental and water process monitoring, the sensor measures the diffusion controlled current to a bare Pt microdisc electrode for the reduction of oxygen. A successful reconditioning potential waveform is reported which yields a very stable amperometric response over continuous operation, with a maximum deviation of the limiting current under 1.5% over 24 h. An automated calibration method developed to accurately characterise the electrodes is described. Excellent linearity is obtained for all electrodes tested and in each case, the number of apparent electrons for the reduction of oxygen is reported. As an alternative to calibration, an analytical treatment which accounts for temperature and salinity effects is given to calculate the dissolved oxygen concentration directly from the limiting current. While the analytical approach yields a concentration relative error circa 11% for a 50 ?m diameter Pt disc, the calibration, has lower errors and yields a detection limit down to 0.9 ?M with the same disc. Although this investigation builds on established principles, this article describes, for the first time, the conditions required to obtain accurate and reproducible measurements and provides an estimate of their precision. Preliminary field trials to measure oxygen depth profiles in the ocean have proved very encouraging [R. Prien, R. Pascal, M. Mowlem, G. Denuault, M. Sosna, Development and first results of a new fast response microelectrode DO-sensor, in Oceans 2005—Europe, Vols. 1 and 2, 2005, pp. 744–747]

Hydrothermal activity on the southern Mid-Atlantic Ridge: Tectonically- and volcanically-controlled venting at 4–5°S

We report results from an investigation of the geologic processes controlling hydrothermal activity along the previously-unstudied southern Mid-Atlantic Ridge (3–7°S). Our study employed the NOC (UK) deep-tow sidescan sonar instrument, TOBI, in concert with the WHOI (USA) autonomous underwater vehicle, ABE, to collect information concerning hydrothermal plume distributions in the water column co-registered with geologic investigations of the underlying seafloor. Two areas of high-temperature hydrothermal venting were identified. The first was situated in a non-transform discontinuity (NTD) between two adjacent second-order ridge-segments near 4°02?S, distant from any neovolcanic activity. This geologic setting is very similar to that of the ultramafic-hosted and tectonically-controlled Rainbow vent-site on the northern Mid-Atlantic Ridge. The second site was located at 4°48?S at the axial-summit centre of a second-order ridge-segment. There, high-temperature venting is hosted in an 18 km2 area of young lava flows which in some cases are observed to have flowed over and engulfed pre-existing chemosynthetic vent-fauna. In both appearance and extent, these lava flows are directly reminiscent of those emplaced in Winter 2005?06 at the East Pacific Rise, 9°50?N and reference to global seismic catalogues reveals that a swarm of large (M 4.6?5.6) seismic events was centred on the 5°S segment over a 24 h period in late June 2002, perhaps indicating the precise timing of this volcanic eruptive episode. Temperature measurements at one of the vents found directly adjacent to the fresh lava flows at 5°S MAR (Turtle Pits) have subsequently revealed vent-fluids that are actively phase separating under conditions very close to the Critical Point for seawater, at 3000 m depth and 407 °C: the hottest vent-fluids yet reported from anywhere along the global ridge crest.<br/