ALOHA Wirewalkers

The ALOHA Wirewalker project is a collection of autonomous oceanographic observations from the North Pacific Ocean that are obtained with two wave-powered drifting profilers (Wirewalker™, Del Mar Oceanographic) since June 2017. The autonomous profilers are deployed in the open ocean where they move vertically along a wire that is suspended between a float at the sea surface and a weight placed at 400 m depth. The system horizontal displacement follows the ocean currents. These observations allow the investigation of the diel ecosystem variability and the study of the formation and maintenance of relatively thin layers of plankton accumulation thanks to the high resolution both vertically (~0.1 m) and temporally (1/6 s).

The Wirewalkers are deployed during expeditions of the Hawaii Ocean Time-series (HOT) where they complement shipboard measurements of the physical and biogeochemical characteristics of the ocean. Additionally, the Wirewalkers are deployed during the expeditions organized by the Simons Collaboration on Ocean Processes and Ecology ( SCOPE).

Measurements collected within the ALOHA Wirewalker project have been processed using a consistent procedure in order to facilitate the joint analysis of the observations from different deployments. The ALOHA Wirewalker dataset from June 2017 to February 2020 is now public and accessible as doi:10.5281/zenodo.3750468.

This project is supported by the Simons Foundation

Instruments

The Wirewalkers collect vertically-resolved observations every ~30 minutes from the sea surface to a maximum depth of 400 m. Measurements include hydrographical, biogeochemical, and optical parameters. Hydrographical measurements are collected with a RBR Maestro and include temperature, salinity, and depth. The concentration of dissolved oxygen is measured using a fast response optode (Rinko III, JFE). An ECO Puck sensor (Sea-Bird) measures proxies for particle concentration (scattering at 124^o and 650 nm), pigment concentration (chlorophyll fluorescence), and the concentration of chromophoric dissolved organic matter (CDOM) through fluorescence. A c-star transmissometer (Sea-Bird) measures beam attenuation at 650 nm, which is a second proxy for particle concentration. Downwelling irradiance in four spectral bands (PAR, 380, 412, and 490 nm) is measured with an OCR-504 (Sea-Bird). The coordinates of the Wirewalker are recorded using Pacific Gyre and Xeos tracking devices with Iridium satellite communication.

Changes in sensor performance are monitored and tracked by recording the signal in controlled conditions before deployment: dark counts are recorded for the ECO Puck and the OCR-504; dark and air counts are recorded for the c-star; a two-point calibration is done for the Rinko III.

Beam attenuation measurements are not shared in the first release of the Wirewalker dataset for two main reasons: 1) problems with water condensation on the instrument optics impacted several of our deployments; 2) there is no protocol to normalize beam attenuation profiles to account for sensor cleanliness (the subtraction of the minimum value from each profile is a common approach, but it does not insure that different deployments are comparable).

Deployments

In the period between June 2017 and May 2023, the ALOHA Wirewalkers were deployed 52 times, for a cumulative duration of 173 days during which they collected a total of 8605 vertical profiles from 0 to 400 m (Table 1). Most deployments were in the North Pacific Ocean, north of the Hawaiian archipelago. Most deployments took place during HOT cruises and lasted between 1.2 and 4.0 days. Sixteen deployments took place during SCOPE cruises (MESO-SCOPE, Eddy Experiment, Gradients 3, PARAGON 1, Gradients 4, PARAGON 2 & Gradients 5) and lasted up to 13.1 days.

Two Wirewalkers were used for all of our deployments. The Wirewalker mounting the data logger RBR Maestro 80330 was deployed 19 times, while the Wirewalker mounting RBR Maestro 80331 was deployed 33 times.

Calibrations

Processing

The first processing of Wirewalker measurements is done by the RBR routines of the data logging system (RBR Maestro). The RBR processing follows the scaling procedures indicated by the manufacturers of the different instruments with few exceptions:

• Factory calibrations are not applied for the ECO Puck sensors.
• The Rinko III processing uses the logger temperature rather than the instrument temperature to calculate oxygen saturation and no pressure correction is applied.
• Beam attenuation is measured with respect to air instead of water.

Besides the RBR processing, data are further processed using MATLAB. As a first step, we merge the coordinates collected with the positioning sensor to the underwater observations collected by the Wirewalkers. We then select only observations collected during the ascent phase of the profile, which are less noisy due to the relatively constant vertical velocity of the profiler.

The hydrographical processing involves calculating several hydrographical parameters including absolute salinity, practical salinity, conservative temperature, depth, and potential density anomaly with respect to a water pressure of 0 decibar. For these calculations, we use the routines from the Gibbs-SeaWater Oceanographic Toolbox (http://www.teos-10.org/software.htm).

Processing of the bio-optical measurements starts with applying the linear scaling coefficients of the ECO Puck sensors based on the factory calibration. Furthermore, measurements of scattering at 124^o are scaled to obtain the particle backscattering coefficient, bbp, by subtracting the scattering due to seawater, and by scaling to scattering at all backward directions. Seawater scattering is calculated using the routines provided by Zhang et al. (2009), and the geometrical scaling was done by using the conversion coefficient χ = 1.076 proposed by Sullivan et al. (2013).

The most extensive processing is done on measurements of oxygen saturation from the Rinko III sensor. This process included four steps:

1. modification of linear calibration coefficients
2. time-response correction
3. pressure correction
4. alignment with Winkler oxygen measurements

The modification of the linear calibration coefficients is based on repeated tracking of instrument performance with two-point calibrations. From each two-point calibration we calculate new slope and intercept for the regression to calculate oxygen saturation. These parameters are tracked over time and we noticed a sharp change in the intercept value approximately in March 2018 for both sensors. For sake of simplicity, we assumed a stepwise change in sensor performance with different slopes and intercepts for the period before March 2018 and for the period starting in March 2018 (Figure 2a-d). We then correct for the instrument response time by using the inverse filtering algorithm proposed by Bittig et al. (2014). We assigned the instrument response time iteratively by aligning the observations collected during ascent and descent, as previously done by Barone et al. (2019) for underwater glider measurements. To keep the process simple, we assumed that the response time of the instrument did not change over time, and we assigned it a value of 12 seconds (even though there are indications that the response time increased from our initial deployments). Furthermore, since inverse filtering amplifies the noise, we applied a 2 seconds running mean to smooth the output signal. After this step, we corrected oxygen saturation to account for the effect of pressure, based on the formula provided by the manufacturer. We then calculated the solubility of oxygen in seawater and obtained dissolved oxygen concentration from measurements of oxygen saturation. This signal is then compared with dissolved oxygen measurements using the Winkler technique from nearby ship operations during each deployment. For this comparison, we only consider shipboard measurements in the upper 50 m of the water column, which were collected less than 0.3 days apart, and a distance lower than 10 km from the Wirewalker. We consider Winkler measurements to be more accurate than the optode measurements so we correct the latter by subtracting the average offset with the Winkler for each deployment. We then recalculate oxygen saturation from the offset-corrected oxygen concentration.

In the first release of the ALOHA Wirewalker dataset, some measurements were discarded because of problems with the sensors. Specifically, oxygen is not reported for the two MESO-SCOPE deployments because they were characterized by a non-linear drift in sensor performance. These were the first deployments of the Rinko III, which might be subject to faster drift when the sensing foil is new.

Backscattering was anomalously high during HOT-302 and HOT-303. We discarded those measurements because the deep water value appeared largely inconsistent with all other observations, and the sensor started recording more typical values towards the end of the HOT-303 deployment.

The OCR-504 sensor was not mounted on Wirewalkers during both MESO-SCOPE deployments and during HOT cruises 296, 297, and 298. For those deployments, irradiance was not measured at 380, 412, and 490 nm, while PAR irradiance was measured with a Biospherical QCP cosine sensor. Furthermore, the OCR-504 did not record any observations during HOT-314.

References

• Barone, B., Nicholson, D., Ferrón, S., Firing, E. and Karl, D., 2019. The estimation of gross oxygen production and community respiration from autonomous time‐series measurements in the oligotrophic ocean. Limnology and Oceanography: Methods, 17(12), pp.650-664.

• Bittig, H.C., Fiedler, B., Scholz, R., Krahmann, G. and Körtzinger, A., 2014. Time response of oxygen optodes on profiling platforms and its dependence on flow speed and temperature. Limnology and Oceanography: Methods, 12(8), pp.617-636.

• Sullivan, J.M., Twardowski, M.S., Zaneveld, J.R.V. and Moore, C.C., 2013. Measuring optical backscattering in water. In Light scattering reviews 7 (pp. 189-224). Springer, Berlin, Heidelberg.

• Zhang, X., Hu, L. and He, M.X., 2009. Scattering by pure seawater: effect of salinity. Optics Express, 17(7), pp.5698-5710.

Figure caption: Performance of the two optode oxygen sensors over time. Intercepts and slopes from the two-point calibrations are depicted as blue points in panels a,c for the sensor connected to Maestro 80330, and panels b,d for the sensor connected to Maestro 80331. Red lines depict the coefficients applied to the sensors for the deployments taking place on dates marked with red dots. Panels e,f depict the offset between the dissolved O₂ concentration measured by Wirewalkers and the concentration measured on the ship using the Winkler method.

Data

The ALOHA Wirewalker dataset from June 2017 to February 2020 is now public and accessible at doi:10.5281/zenodo.3750468 and from July 2020 to May 2023 at doi:10.5281/zenodo.10065303 .

Data are also available through the Collaborative Marine Atlas Project (CMAP).

Plots

Validation

To verify the accuracy of the Wirewalker measurements, we compared the HOT deployments with the observations collected with the shipboard CTD system, which were calibrated using laboratory analyses on water collected with Niskin-like bottles. Considering that shipboard measurements and Wirewalker measurements were collected at a distance of several km, we did not expect that each deployment showed complete consistency between the two sets of observations. However, we did not expect systematic discrepancies for all deployments. For this comparison, we considered only shipboard observations collected during the time when the Wirewalker was deployed.

The comparison shows good consistency between shipboard and Wirewalker measurements of temperature and salinity, but not of O₂ concentration. Specifically, we observed O₂ differences that increased with depth and changed with time, with larger differences measured in the most recent deployments. This observation seems to indicate a drift in the performance of the Rinko III sensor.

Comparison of average profiles measured during HOT cruises by Wirewalkers (solid black lines) and shipboard instruments (dashed red lines) for temperature (top panel), absolute salinity (SA, middle panel), and O₂ (bottom panel). To better visualize the observations, we added a cumulative offset to each profile (from the previous deployment) of 5^o C, 0.5 g kg^-1, and 50 mmol m^-3. As a result, the last deployment has a total offset of 75^o C, 7.5 g kg^-1, and 750 mmol m^-3 from the first deployment.

• Benedetto Barone - Research fellow
• Timothy Burrell - Research specialist
• Tara Clemente - Field operations coordinator
• Lance Fujieki - Computer specialist
• David Karl - Principal investigator
• Ryan Tabata - Research specialist

Daniel K. Inouye Center for Microbial Oceanography: Research and Education (C-MORE)
University of Hawaii at Manoa
1950 East-West Road, Honolulu, Hawaii 96822

For information contact Benedetto Barone