DOI: 10.5281/zenodo.10247019
title: Multiple years of Seaglider observations of hydrography, dissolved oxygen, chlorophyll a, and optical backscatter at Station ALOHA

description: File descriptions:
Seaglider missions are identified as GLIDER_MISSION (i.e. sg148_12 is glider 148, mission 12) and each have three files associated. For example:

sg148_12_qc_pass.xlsx contains only quality controlled (QC flags = 1) core data for an entire mission. Core data may include temperature, conductivity, salinity, potential density anomaly, calibrated dissolved oxygen concentrations, calibrated chlorophyll a concentrations, and the backscattering coefficient due to particles (bbp) at up to three wavelengths (470, either 650 or 660, and 700 nm) and spike flags. Bbp data is corrected with an in situ dark subtraction from near 200 m deep. Associated metadata (datetime, latitude, longitude, depth, dive number, and vertical profile direction) is also included.
sg148_12_alldata.nc contains all data (i.e. all QC flag levels) and associated quality control flags. In addition to core and metadata, factory-only calibrated observations (e.g. dissolved oxygen concentrations, chlorophyll a concentrations, and bbp) are listed. 
sg148_12_qctests.nc contains all quality control test values (pass: QC = 1, input flag: QC = 2, questionable data QC = 3, bad data: QC = 4). The maximum test QC flag value (e.g. out of range, density inversions, bioflouling, etc.) was passed to the variable QC flag (e.g chla_qcflag or salin_qcflag). . 
 
Dataset description:
The SCOPE-ALOHA Seaglider dataset was designed to monitor the spatial and temporal variability of physical and biogeochemical properties around the long term sampling site Station ALOHA (22°45′N, 158°W). Seagliders are autonomous underwater vehicles that take high frequency (up to 0.2 Hz in our dataset), depth-resolved observations over several months and can be used to map large spatial features. The gliders depicted in this study were equipped with sensors to measure temperature, salinity, pressure, dissolved oxygen concentration (O2), chlorophyll a concentration (Chl a) from fluorescence (excitation/emission lambda = 470/695 nm), and the particulate backscattering coefficient (bbp) at three wavelengths (lambda = 470 nm, 700 nm, and either 650 or 660 nm depending upon mission). Vertical profiles down to at least 200 m were collected for all sensors over periods of several months per mission. This dataset comprises 18 missions between 2008 and 2023 centered on Station ALOHA, totaling over 20,000 depth profiles. Chlorophyll a and oxygen concentrations are calibrated with discrete observations. Particulate backscattering coefficients are corrected with an additional dark subtraction. This dataset is an improvement on the raw data files as they are quality controlled, calibrated, and corrected.
Raw data files can be found at https://hahana.soest.hawaii.edu/seagliders/index.php.
Methods:
Code for all processing steps is on GitHub (https://github.com/cathygarcia/SeagliderDataprocessing). The steps listed here are a brief summary. 
Temperature, Conductivity, Salinity, and Potential Density Anomaly

Both temperature and conductivity profiles were lag corrected.
Practical salinity was calculated using the Gibbs Seawater Toolbox (gsw_SP_from_C.m), and then converted to absolute salinity (gsw_SA_from_SP.m). 
Potential density anomaly was calculated with respect to a reference water pressure of 0 db using the Gibbs Seawater Toolbox (gsw_sigma0.m).
Dissolved oxygen concentrations

Raw optode phase values proceeded through a series of corrections to account for the effects of temperature, salinity, pressure, and time response in addition to sensor drift (Bittig et al., 2018, Barone et al., 2019).
 Optode phase values were converted to dissolved oxygen concentrations, and re-calibrated using discrete Winkler measurements. 
Chlorophyll a

Factory-calibrated chlorophyll a observations were re-calibrated using discrete measurements of either HPLC chlorophyll a (16 missions) or fluorometric chlorophyll a (2 missions).
Daytime chlorophyll a values are not quench corrected, and may be lower than actual values. It is recommended to use nighttime profiles near the surface. 
Additionally, a spike flag is included based on published protocol (Briggs et al., 2011).
Backscattering coefficient due to particles (bbp)

Factory-calibrated bbp values could have a large offset, that was not expected based on natural variability.
A mission-specific deep dark correction (1st percentile of bbp at 190-200 m) was subtracted for each bbp dataset. Both the uncorrected and corrected data are available.
Additionally, a spike flag is included based on published protocol (Briggs et al., 2011).