Evaluation of the Aanderaa Oxygen Optode in continuous use in the NOC Portsmouth Bilbao FerryBox system 2005, 2006, with an assessment of the likely errors in the estimation of oxygen concentration anomalies

An Aanderaa Oxygen Optode has been used as part of the “FerryBox” system on the P&O Ferries vessel the MV Pride of Bilbao since the beginning of 2005. This report covers data collected through 2005 and 2006. The accuracy, precision and stability of the Optode have been compared with (1) measurements of oxygen in water samples collected and processed on board the ship by Winkler titration and (2) estimates of the oxygen anomaly. The comparison of the Winkler data with the output from the Optode suggests that the accuracy of the Optode in both years was close to 98% of the true value when compared to the Winkler measurements. The data suggest that the precision based on the comparison of the two data sets was equivalent to a standard deviation of 5 ?M or 2% of the average value in 2005 and 2.9 or 1% of the average value in 2006. The resolution of the recorded a data is better than 1 ?M. Two sources of error in measurement of temperature which are significant for the calculation of the oxygen anomaly have been identified. Temperatures measured using sensors in board the ship are higher than “true” temperatures. The error changes through the year as the difference in temperature between inside the ship and the external water temperature changes. This error may be as large as 0.5 °C. The anomaly has to be calculated using temperature measurement in the water flowing past the Optode. This is because of delays and mixing in the water being pumped into the ship have smoothing effect on the data, and can enhance the difference between the internal and external temperature to being as high as 1.0 °C. Consequently to avoid this large error due to the timing of the measurements the oxygen anomaly should be calculated from the temperature measured in the pumped water stream adjusted by an offset to the “true” outside temperature

A regional analysis of new production on the northwest European shelf using oxygen fluxes and a ship of opportunity

Seasonal new production (g C m?2) estimates obtained from dissolved oxygen and nitrate concentrations in surface waters (5 m depth) along a track between the UK (Portsmouth) and northern Spain (Bilbao) are compared. An oxygen flux method, in combination with a ship of opportunity (SOO), was tested on the northwest European shelf for its value in distinguishing high production in frontal regions. Dissolved oxygen, nitrate and chlorophyll a samples were collected monthly from February to July 2004, alongside continuous autonomous measurements of salinity, temperature and chlorophyll fluorescence. Depth integrated new production estimates for all the individually analysed hydrographic regions of the route were produced.Results from three widely used gas-exchange parameterizations gave seasonal (February–July) new production estimates of 54–68 g C m?2 for the Ushant region of the western English Channel and 31–40 g C m?2 for the shelf slope, averaging 24–31 g C m?2 for the route. This is double the route average obtained using the nitrate assimilation method (17 g C m?2) and within the ranges of previous estimates in the same region. The oxygen flux method gave a fivefold enhancement compared to the nitrate method in the Ushant frontal region and a threefold enhancement in the English Channel and shelf break regions. Determining oxygen fluxes to estimate new production may be more reliable than nitrate assimilation in active tidal or frontal regions of shelves where nitrate may be added to the system post-winter through advection or entrainment. <br/

A study of gas exchange during the transition from deep winter mixing to spring bloom in the Bay of Biscay measured by continuous observation from a ship of opportunity

Monitoring from ships of opportunity allows a wide range of parameters to be measured, thereby extending the coverage of operational oceanographic studies. Observation of dissolved oxygen using new sensors offers an effective way of monitoring changes in biological production. The limits of the application were tested following the transition from winter storms to the spring bloom (2007). Calculation of dissolved nitrogen enables changes in gas saturation due to physical and biological processes to be separated. By combining these measurements with data from numerical models and Argo floats the critical role of subsurface processes in determining rates of change at the surface can be assessed