M01 SUNA - Nitrate Observations Corrected

Various nitrate sensors including the SUNA V1, SUNA V2 and ISUS (Sea-Bird Scientific) were deployed on NERACOOS Buoy M (Latitude: 43° 29.44'N and Longitude: 67° 52.79'W) from June 23, 2013 to March 23, 2021. This time period corresponds to University of Maine buoy deployments M0124, M0125, M0126, M0127, M0129, and M130 (http://gyre.umeoce.maine.edu/data/gomoos/buoy/html/M01.html). These sensors were placed at the surface, 1m, 50m, 100m, 150m and 250m and programmed to collect nitrate measurements six times per day (0045, 0445, 0845, 1245, 1645 and 2045 GMT) over a fixed sampling window of 30 seconds at approximately 1.4 readings per second. The number of sensors and their depths varied for each deployment depending on the number of available sensors. Five measurements recorded in the middle of the 30 second data stream were transmitted back to the University of Maine Physical Oceanography Group via cell phone. The mean concentration and standard deviation were calculated for each specific sampling time. If the standard deviation for a mean was greater than 2µM, the values were eliminated. Surviving data were then quality controlled (QCed) by removing outliers assessed as values less than -0.5µM and greater than 30µM. Whenever possible at the time of buoy deployment and recovery, a standard CTD cast was conducted with a carousel water sampler equipped with Niskin bottles. Ground truth water samples were collected at the same depth as the nitrate sensors. These samples were analyzed for nitrate concentrations at the University of Maine using a Bran Luebbe AA3 Autoanalyzer and standard techniques. Ground truth corrections were added or subtracted as an offset only if the difference between the nitrate concentration of the water sample collected and the mean nitrate sensor reading at approximately the same time was greater than 2 µM. Upon recovery of the instruments, the data was processed using the software provided by Sea-Bird (UCI.com) and compared with the real-time measurements as a check for accuracy. In the event that the sensor was not able to transmit in real-time, the processed data was QCed. Please contact David Townsend, davidt@maine.edu, University of Maine, School of Marine Sciences if you have any questions.