Accurate observations of the Earth system are required to understand how our planet is changing and to help manage its resources. The aquatic environment—including lakes, rivers, wetlands, estuaries, coastal and open oceans—is a fundamental component of the Earth system controlling key physical, biological, and chemical processes that allow life to flourish. Yet, this environment is critically undersampled in both time and space. New and cost-effective sampling solutions are urgently needed. Here, we highlight the potential to improve aquatic sampling by tapping into recreation. We draw attention to the vast number of participants that engage in aquatic recreational activities and argue, based on current technological developments and recent research, that the time is right to employ recreational citizens to improve large-scale aquatic sampling efforts. We discuss the challenges that need to be addressed for this strategy to be successful (e.g., sensor integration, data quality, and citizen motivation), the steps needed to realize its potential, and additional societal benefits that arise when engaging citizens in scientific sampling.

Ion‐sensitive field effect transistor‐based pH sensors have been shown to perform well in high frequency and long‐term ocean sampling regimes. The Honeywell Durafet is widely used due to its stability, fast response, and characterization over a large range of oceanic conditions. However, potentiometric pH monitoring is inherently complicated by the fact that the sensors require careful calibration. Offsets in calibration coefficients have been observed when comparing laboratory to field‐based calibrations and prior work has led to the recommendation that an in situ calibration be performed based on comparison to discrete samples. Here, we describe our work toward a self‐calibration apparatus integrated into a SeapHOx pH, dissolved oxygen, and CTD sensor package. This Self‐Calibrating SeapHOx is capable of autonomously recording calibration values from a high quality, traceable, primary reference standard: equimolar tris buffer. The Self‐Calibrating SeapHOx's functionality was demonstrated in a 6‐d test in a seawater tank at Scripps Institution of Oceanography (La Jolla, California, U.S.A.) and was successfully deployed for 2 weeks on a shallow, coral reef flat (Lizard Island, Australia). During the latter deployment, the tris‐based self‐calibration using 15 on‐board samples exhibited superior reproducibility to the standard spectrophotometric pH‐based calibration using > 100 discrete samples. Standard deviations of calibration pH using tris ranged from 0.002 to 0.005 whereas they ranged from 0.006 to 0.009 for the standard spectrophotometric pH‐based method; the two independent calibration methods resulted in a mean pH difference of 0.008. We anticipate that the Self‐Calibrating SeapHOx will be capable of autonomously providing climate quality pH data, directly linked to a primary seawater pH standard, and with improvements over standard calibration techniques.

We use autonomous nitrate (NO₃⁻), oxygen (O₂), and dissolved inorganic carbon (DIC) observations to examine the relationship between ratios of C:N:O at an upwelling site in the Southern California Current System. Mean ratios and 95% confidence intervals observed by sensors over 8 months were NO₃⁻:O₂ = −0.11 ± 0.002, NO₃⁻:DIC = 0.14 ± 0.001, and DIC:O₂ = −0.83 ± 0.01, in good agreement with Redfield ratios. Variability in the ratios on the weekly time scale is attributable to shifts in biological demand and nutrient availability and shown to exhibit a spectrum of values ranging from near 100% New Production to 100% Regenerated Production.

A low-cost (~$100), do-it-yourself (DIY) ultrasonic water level sensor/datalogger has been designed, constructed, and tested for use in education, citizen/community science, and research settings. The sensor package comprises an ultrasonic distance sensor, a microcontroller running Arduino firmware, a micro-SD card for datalogging, a real-time clock for timekeeping and sleep functions, and an OLED screen for real-time display. Electronics are housed in low-cost custom containers using either upcycled plastic containers or laser-cut acrylic. Reported ultrasonic sensor accuracy is 5 cm across a range of 15–645 cm, with an estimated power budget of 76 days of operation on a rechargeable 10 amp-hr battery. The DIY sensor has been field tested alongside two commercial sensors for 18 days in Wilmington, North Carolina, including during Tropical Storm Colin, with all sensor measurements in close agreement (e.g., root mean squared error of 1.5 cm between the DIY sensor and a proven commercial unit). Potential applications, design and construction, and deployment recommendations for the sensors are described, as well as simple and inexpensive modifications to the sensor and its packaging that could further improve performance. Preliminary lesson plans written to accompany the device for high school- and undergraduate-level educational projects are available.

As ocean acidification (OA) sensor technology develops and improves, in situ deployment of such sensors is becoming more widespread. However, the scientific value of these data depends on the development and application of best practices for calibration, validation, and quality assurance as well as on further development and optimization of the measurement technologies themselves. Here, we summarize the results of a 2-day workshop on OA sensor best practices held in February 2018, in Victoria, British Columbia, Canada, drawing on the collective experience and perspectives of the participants. The workshop on in situ Sensors for OA Research was organized around three basic questions: 1) What are the factors limiting the precision, accuracy and reliability of sensor data? 2) What can we do to facilitate the quality assurance/quality control (QA/QC) process and optimize the utility of these data? and 3) What sort of data or metadata are needed for these data to be most useful to future users? A synthesis of the discussion of these questions among workshop participants and conclusions drawn is presented in this paper.