Feature ArticlesUV Spectrometer ProPS Conducts Marine Research Field Trials
By Eberhard Kopiske
Chief Technical Officer
Technical and Scientific Officer
TriOS Me'- und
For the German Federal Ministry of Education and Research's Nitrate Continuous Observation Sensor (NICO) project, TriOS was challenged to adapt its ProPS ultraviolet (UV) photometer'originally developed for nitrate, nitrite, chemical oxygen demand and total organic carbon analysis in industrial applications'to the needs of marine research.
The project's requirements were a nitrate detection limit of about one micromole per liter and an algorithm capable of handling different environments, like coastal waters with a higher turbidity or open-ocean regions with different salinities. The depth rating for the standard housing also needed to be increased from 1,500 meters to 6,000 meters, and to grant autonomous operation while moored, the sensor was equipped with an external battery pack and intelligent data-logging unit.
These new features have been tested on several cruises for the project on the German research vessels Heincke, Poseidon, Meteor and Polarstern in several regions of interest, including north of the Canary Islands at the mooring site Estaci'n Europea de Series Temporales del Oceano, Islas Canarias; near Cap Blanc in Mauritania; the Hakon Mosby mud volcano north of Norway; the German Bight in the North Sea; and other areas in the Atlantic Ocean.
During these cruises, ProPS sensors were used on board the ships for analyzing water samples in situ or from Niskin bottles in the laboratory; tied to a conductivity, temperature and depth (CTD) sensor for profiling; and installed on different remotely operated vehicles (ROVs).
The UV photometer ProPS operates within a range of 190 to 360 nanometers. Changing environmental conditions can be handled with an adjustable path length of 10 to 60 millimeters.
The measured spectrum can be analyzed online with the TriOS software MSDA_XE, as well as seen and rated simultaneously by the user. A multicomponent analysis tool developed by TriOS allows the user to determine substance concentrations from the measured spectrum through the use of reference spectra. Reference spectra for other substances like colored dissolved organic matter (CDOM) or a different salinity can be added and will optimize and extend the analysis with respect to changing measurement conditions.
An external battery pack and a data-logging unit allow the sensor to be operated on moorings. Intelligent data and power management, together with nanocoated windows that provide pollution-free and energy-neutral protection against biofouling, grant long operating time. Besides the online calculation of spectra, all gathered spectra can be analyzed later with more complex and individual reference spectra and analysis algorithms. Data from other sensors, like multiparameter probes or fluorometers, can be connected to the same data-logging unit to extend the data sets.
The option to use the ProPS sensor on ROVs (or other platforms like autonomous underwater vehicles) requires the implementation of a standardized interface for power and data transfer.
The sensor has been used on several cruises in the southern and northern Atlantic, the Norwegian Sea, the German Bight, and the Elbe and Weser estuaries on the coast of the German North Sea, taking continuous measurements in flow-through cuvettes for onboard laboratory analysis and conducting vertical profiling mounted on a CTD rosette and on board ROVs.
Water samples from Niskin bottles were analyzed in standard or flow-through cuvettes. Reference measurements were performed either directly on board or samples were deep-frozen for later analysis. Underway measurements during research cruises were performed by pumping seawater permanently through a flow-through cuvette or basin on board. In order to evaluate sensor performance (i.e., biofouling, data accuracy and stability) for long-term applications, such as volunteer observing ships, a six-week measurement campaign was carried out on the Polarstern (meridional section from 54' S to 54' N).
The sensor was also used in several vertical profiles in combination with a CTD sensor and Niskin sampler. A power and data connection separate from the CTD or sampler cable was required for the sensor, meaning the maximum depth for CTD-mounted profiles was 100 meters. Each sample was triggered manually, depending on real-time data obtained by the CTD unit.
Online Measurements on ROVs
MARUM's two ROVs feature the Shilling Robotics (Davis, California) SeaNet Interface as standard for the scientific payload. The ProPS sensor has been adapted to this interface so that the user can directly control it near the seafloor via the ROV's telemetry from the laboratory, allowing it to make measurements down to the ROV's full depth rating.
A dive on the ROV Cherokee reached a depth of 400 meters with continuous ProPS measurements on the Poseidon's cruise No. 344 in November 2006.
The sensor has not only been used for nitrate measurements, but also for CDOM and chlorophyll detection in an algal bloom area. Frozen reference samples were sent to the Leibniz Institut f'r Meeresforschung at the University of Kiel (IFM-GEOMAR) in Kiel, Germany, for nitrate analysis. The results show a strong correlation between the in-situ measurement and the reference analysis.
Online on the ROV Quest
On the Long-Term Observations on Mud-Volcano Eruptions project's Polarstern cruise ARKXXIV-2 to the Hakon Mosby mud volcano in July, the ProPS sensor was used on dives down to 1,300 meters. Spectra were measured every five minutes and displayed on a screen for scientific analysis in real time. A decrease in absorbance from 215 to 260 nanometers with increasing depth was observed. No significant change was seen from zero to 400 meters, but some effect was visible from 400 to 1,265 meters.
Interpreting these spectra required detailed analysis in the laboratory, but they can also be displayed as a difference against a local reference, making slight changes visible and helping to identify locations for water sampling or push cores.
The online calculation of concentrations and display of the spectra can be useful for the detection of seeps containing dissolved substances. This instrument allows the user to view a cross section of an area of interest with a direct analysis of the spectra on board the ship, supporting a decision on the exact position for push cores or for the deployment of a microprofiler on the seafloor.
Performance in Field Tests
The ProPS sensor worked reliably throughout all of the tests. The reference measurements of water samples for nitrate show a close correlation. However, the optical absorbance changed with the temperature of the sample, requiring a correction for temperature for in-situ measurements or maintenance of the samples at a constant temperature for measurements in the lab. This did not cause a significant problem, because the temperature was measured throughout the tests by CTD profiles, so the temperature could be corrected later using these data. Avoiding high temperature gradients by storing the device in water with a similar temperature shortly before the profile starts will reduce temperature effects on the spectra and improve the quality of the gathered data.
The sensor also worked reliably on the MARUM ROVs. The only problem the team encountered was that when the lamp ignited, there was a high inrush current from the ProPS, requiring an adaptation of the software fuses in the ROV.
An underwater flow-through adapter is also available for the ProPS. This enables an exact definition of the sampling position with the manipulator and a tube fixed to it. The instrument is stable and robust; the only maintenance that might be necessary during a cruise is to clean the windows. Some reference measurements with artificial seawater or demineralized water are recommended to verify the stability of the optical performance.
The ProPS UV spectrometer is an instrument for online nitrate measurements in the water column, moorings and shipborne laboratories. No chemical reagents are required except demineralized water for cleaning and reference measurements, making long-term deployments much easier.
The sensor's nanocoated windows require minimal maintenance. This feature's effectiveness at preventing biofouling has been verified in long-term deployments in the biologically active German Bight.
Testing has shown the instrument to be reliable as a nitrate meter, but it is recommended that the user take a temperature measurement in parallel in order to apply a temperature correction to the results. A dependency of the optical spectra on ambient pressure has not yet been verified.
Another big potential of the sensor is the access to raw spectra. Any substance can be measured as a reference and can be selected for the online multisubstance analysis. The online availability of the analysis and spectra is very useful for integrating the sensor on an ROV.
The authors wish to thank the research partners of the NICO project funded by the German Federal Ministry of Education and Research under Grant 03F0444A, Dr. Arne K'rtzinger, Bj'rn Fiedler and their team at IFM-GEOMAR and Dr. Oliver Zielinski and his team at the University of Applied Science in Bremerhaven, Germany. The authors would also like to thank the captains and crew members of the participating research vessels.
Eberhard Kopiske studied physics at the University of Oldenburg and later founded Thales Instruments GmbH with two colleagues. He worked for two years as a system engineer at OHB System GmbH on an International Space Station project, and since 2002, he has developed and operated deepsea research equipment for MARUM.
Karin Munderloh studied at the University of Oldenburg with a focus on marine physics and optics. She then worked for the Institute of Environmental Physics at the University of Bremen in the Differential Optical Absorption Spectroscopy group under professor John Burrows. She has been chief technical officer at TriOS since 2000.
Steffen Schwalfenberg studied marine measuring and plant technology at the University of Applied Science Bremerhaven. He currently works as a technical and scientific researcher in the research and development and support departments of TriOS.