DOI: 10.5281/zenodo.14908032
title: A 5-year Validated Nitrate Dataset from the Ocean Observatories Initiative Pioneer - New England Shelf Array

description: The Ocean Observatories Initiative (OOI) deployed both the In-Situ Ultraviolet Spectrophotometer (ISUS) and Submersible Underwater Nitrate Sensor (SUNA) for continuous, in-situ measurement of nitrate. At the Pioneer-New England Shelf Array (Pioneer-NES), ISUS/SUNA sensors were deployed at 7-meters depth at the Inshore (ISSM), Central (CNSM), and Offshore (OSSM) Surface Mooring locations. The SUNA sensor replaced the ISUS sensors spring 2018. The SUNA was a major improvement in technology, with significant improvements in accuracy and precision. However, it still suffers from calibration drift due to lamp fatigue and biofouling as well as spectral interference due to bromide and fluorometric CDOM. Drift is corrected by application of post-cruise calibrations to recalculate the temperature-and-salinity corrected nitrate concentration following Sakamoto (2009a) and estimating a linear drift between pre-and-post cruise deployments. Validation is performed by comparison with discrete water samples collected during deployment/recovery of the sensors. These datasets include the nitrate data from the Pioneer-NES ISSM (CP03ISSM-RID26-07-NUTNRB000.nc), CNSM (CP01CNSM-RID26-07-NUTNRB000.nc), and OSSM (CP04OSSM-RID26-07-NUTNRB000.nc) SUNA instruments spanning Spring 2018 through Fall 2022. Each dataset contains the measured nitrate, the temperature-salinity corrected nitrate, the drift-corrected nitrate, and the nitrate following validation with bottle samples.
description: Datasets

SUNA datasets are known as “NUTNR” on OOI (Nutrient Sensor). NUTNR datasets predating spring 2018 are from the ISUS instrument and not used, due to known measurement issues that make a quantitative data quality assessment difficult. SUNA datasets at the Pioneer-NES array begin with deployment 9 for CNSM and deployment 8 for both ISSM and OSSM. SUNA datasets from the three moorings cover from March 2018 through November 2022, when the Pioneer-NES array was retired.

The SUNA datasets are downloaded on a deployment-by-deployment basis from the OOI Gold Copy THREDDs catalog. Each dataset additionally contains the practical salinity and seawater temperature measured by the collocated CTD on the mooring NSIF. This data is interpolated to the SUNA dataset timestamps and integrated into the dataset by OOI. Salinity and temperature are necessary to calculate the Sakamoto (2009) nitrate correction for salinity interference.

 

Apply the Drift Correction

Drift in the observed nitrate is known to occur due to lamp aging and biofouling. The impact of lamp aging can be corrected for using a post-cruise calibration and calculating a linear change between the pre-and-post deployment calibrations following Palevsky et al (2023):

$$\frac{\Delta NO_{3}}{day} = \frac{NO_{3}^{predeployment \ cal} - NO_{3}^{postdeployment \ cal}}{Predeployment \ date - postdeployment \ date}$$

In practice, we don’t have the DI water dataset for the pre-and-post deployment calibrations, just the calibration files themselves. Instead, we recalculate the nitrate concentration timeseries using the post-deployment calibration and assume that the difference between the timeseries calculated with the pre-deployment calibration and post-deployment calibration at the start of the deployment is $\Delta NO_{3}$. This assumption is reasonable because, apart from burn-in, the sensor is lightly used pre-deployment and drift due to lamp aging is a function of lamp-use.

 

Bottle Correction

One of the primary purposes of the discrete water sampling data is for the evaluation and validation of OOI instrumentation.  Following the methods described in Palevsky et al. (2023), we use the bottle data collected during deployment and recovery of the SUNA sensors to correct for offsets and drift. This is done with the following equation:

$$\frac{\Delta NO_{3}}{day}=\frac{(NO_{3}^{bottle}-NO_{3}^{SUNA})_{deployment}-(NO_{3}^{bottle}-NO_{3}^{SUNA})_{recovery}}{Recovery\ date-deployment\ date}$$

and

$$\Delta NO_{3}^{offset}=(NO_{3}^{bottle}-NO_{3}^{SUNA})_{deployment}$$

which yields the following equation for calculating the bottle-adjusted nitrate:

$$NO_{3}^{adj}(t)=NO_{3}^{obs}(t)+\frac{\Delta NO_{3}}{day}*\Delta t+\Delta NO_{3}^{offset}$$

 

Apply Bottle Offsets

Next, the timeseries is smoothed using a 6-hour centered rolling average to reduce noise while preserving daily nutrient cycling patterns. Then, the deployment and recovery bottle nitrate and smoothed SUNA nitrate are matched based on time. If there is no SUNA nitrate data available within 3 days of the bottle data at recovery, we do not calculate a drift based on the bottle data. Instead, we apply only the offset correction using the $\Delta NO_{3}^{offset}$ value. Otherwise, we go ahead and calculate $\frac{\Delta NO_{3}}{day}$ and apply the above equation to arrive at our bottle-corrected nitrate $NO_{3}^{adj}$. 

 

Create the Final Datasets

Lastly, we want to take the datasets we've constructed above, clean them up, and save only the parameters of interest to us. In addition to the different nitrate concentration time series generated by the above steps, we also retained the salinity, sea water temperature, and the median-absolute-deviation for the nitrate value generated during the burst-averaging step.
description: File name: CP04OSSM-RID26-07-NUTNRB000.nc



Geographic  Coverage



Latitude range: 39.9391 to 39.9391

Longitude range: -70.87643 to -70.87643



Time coverage



Start time: 2018-03-24T14:45:00 UTC

End time: 2022-10-23T17:00:00 UTC



Parameters within file: "serial_number", "deployment", "sea_water_practical_salinity", "sea_water_temperature", "nitrate_concentration", "nitrate_concentration_qc_flag", "corrected_nitrate_concentration", "corrected_nitrate_concentration_qc_flag", "drift_corrected_nitrate", "drift_corrected_nitrate_qc_flag", "bottle_corrected_nitrate", "bottle_corrected_nitrate_qc_flag", "burst_median_absolute_deviation", "time"


File name: CP03ISSM-RID26-07-NUTNRB000.nc



Geographic  Coverage



Latitude range: 40.36718 to 40.36718

Longitude range: -70.88215 to -70.88215



Time coverage



Start time: 2018-03-25T15:30:00 UTC

End time: 2022-11-13T11:45:00 UTC



Parameters within file: "serial_number", "deployment", "sea_water_practical_salinity", "sea_water_temperature", "nitrate_concentration", "nitrate_concentration_qc_flag", "corrected_nitrate_concentration", "corrected_nitrate_concentration_qc_flag", "drift_corrected_nitrate", "drift_corrected_nitrate_qc_flag", "bottle_corrected_nitrate", "bottle_corrected_nitrate_qc_flag", "burst_median_absolute_deviation", "time"


File name: CP01CNSM-RID26-07-NUTNRB000.nc



Geographic  Coverage



Latitude range: 40.13975 to 40.13975

Longitude range: -70.77128 to -70.77128



Time coverage



Start time: 2018-03-24T21:30:00 UTC

End time: 2022-11-11T13:00:00 UTC



Parameters within file: "serial_number", "deployment", "sea_water_practical_salinity", "sea_water_temperature", "nitrate_concentration", "nitrate_concentration_qc_flag", "corrected_nitrate_concentration", "corrected_nitrate_concentration_qc_flag", "drift_corrected_nitrate", "drift_corrected_nitrate_qc_flag", "bottle_corrected_nitrate", "bottle_corrected_nitrate_qc_flag", "burst_median_absolute_deviation", "time"