Following satisfactory laboratory tests of the Unisense nitrite and NOx biosensors (see Annual/Interim Project Report for Period 2005/2006), field trials were conducted during the current project year. For the first field test, the sensors were taken to sea on RV Cefas Endeavour during July 2006 (National Marine Monitoring Programme (NMMP) cruise). Two sea-going scientists were fully trained to set up and operate the instrument within 2 h as the biosensors are relatively straightforward to use. The highlight of the trial was a successful change of biochambers on-board the ship, an operation which requires sterile working and considerable care due to the fragility of the small diameter glass tip of the transducer, and thus is not easy to carry out at sea without prior experience. Also encouraging was that the corresponding sensors performed well for at least 1 week on board and even after our return to the laboratory, despite encountering, at times, not strictly controlled conditions (lack of aeration, no regular maintenance, temperature variations). Results were however difficult to interpret, mainly because of the backdrop of the low nutrient values expected in July which are close to the limit of detection of the biosensors. The lack of temperature control during this trial also affected the biosensor’s signal. However, when a very high value was recorded in the Tees estuary, an excellent correlation between sensor signal and analysis using traditional wet chemistry was found (Skalar 440 mM TOxN, sensor 442 mM NOx). Additionally, the biosensor’s robustness was found to be satisfactory, as no strong drift or loss of activity occurred over the one-week period during which numerous stations were visited and calibrations performed.
A second field trial on RV Cefas Endeavour took place during February 2007, when nutrient levels were expected to be much higher than in the summer. As a means of controlling temperature, the sensors and samples were submerged in a waterbath thermostated at 30°C. Samples were collected while leaving from Lowestoft harbour sailing towards the Gabbard (southern North Sea) and again on the return journey to Lowestoft. Where matching samples have been collected for analysis by Skalar, excellent correlations were found between values calculated from the NOx sensor response and TOXN concentrations. Given the success of this trial it is regrettable that more samples could not be analysed, but the instrument was also being used to record sediment oxygen profiles during the cruise, and so was not always available.
Both field trials were carried out using batch analysis, as the very small diameter flow cells (inner volume 5 ml) obtained from Unisense were only suitable for analysis of very clean solutions within the laboratory, where tests returned good results. However, the small size of the flow cell meant that they were not easy to connect to a continuous water supply on the ship, where small particles, gas bubbles and other disruptions add difficulties in maintaining a steady flow. Should further field studies be planned with continuous rather than batch measurements, a set-up more desirable when following events such as plumes released after a disturbance, the construction of a more robust, custom designed thermostatic flow-through system is recommended and should constitute future work.
