Water quality measurements
In situ water quality sensors can be used to directly relate water quality attributes to satellite remote sensing reflectance and/or be used in combination with the aforementioned IOPs and AOPs to validate bio-optical models. Near real-time and real-time water quality measurements can complement traditional grab samples in generating satellite data products, such as in the development of developing the real-time water quality monitoring and forecasting system of the Australian AquaWatch mission (Dekker et al., 2018). In situ sensors (see table below) can measure many water quality attributes, such as turbidity, Chla, accessory algal pigments, and CDOM, based on the absorbance, scattering, or fluorescence principles associated with a given water quality parameter.
Turbidity can be measured using absorbance or scattering-based in-situ sensors, though scattering-based approaches are more common. Common units include nephelometric turbidity ratio units (NTU) and formazin nephelometric units (FNU). Scattering-based turbidity sensors are now so robust and accurate that low-cost, do-it-yourself turbidity sensors rival commercial sensors in’ accuracy and usability (Eidam et al., 2021; Droujko et al., 2022). To obtain physical units of mass/volume (e.g., mg/L), turbidity sensors can be calibrated with field samples that are analyzed in a laboratory for SPM concentration, typically by filtering the water and weighing the dried contents on the filter.
Chlorophyll a and accessory algal pigments, such as phycocyanin and phycoeryithrin, are typically measured using fluorescence-based in-situ sensors. Fluorescence is an inelastic scattering process and that occurs when a material is irradiated with light of a certain wavelength, which is absorbed and re-emitted at a longer wavelength. Most commercial Chla fluorescence sensors output concentrations in μg/L using laboratory-based calibrations with Chla standards, but may need ambient temperature corrections (Watras et al., 2017).
CDOM is a complex mixture of molecules with a range of measurement methods and units, and can be treated as both an IOP and a water quality attribute. In situ CDOM sensors typically use fluorescence with units of relative fluorescence units (RFU) or quinine sulfate dihydrate units (QSU), and require site-specific calibration using field samples and temperature corrections (Watras et al., 2011). Absorbance based measurements using an in-situ or laboratory spectrophotometer are also common, traditionally measuring absorbance at one or more wavebands and reporting the derived absorption coefficient in units of per meter (m-1) together with its wavelength (such as aCDOM(440)).
Examples of water quality attributes and in-situ sensors used for measuring them, including pros and cons for each sensor type. Note, that this is not an exhaustive list, and we do not endorse any particular sensors or companies. Specific sensors named here are simply to provide examples.
|
Sensor types |
Example Sensor |
Pros |
Cons |
|
Turbidity |
NIR bb-based turbidity sensors (e.g., Sea-Bird Scientific ECO and ECO Puck) |
Sensors are commonly used for nearshore and open ocean water quality monitoring Sensors can be used for bb-based turbidity measuring providing an alternative to 90° IR scattering-based turbidity sensors ECO sensors can be customized for bb, bb-based, and/or fluorescence-based water quality attribute measuring in up to three channels ECO Puck sensors are specifically designed for use in AUVs, profiling floats, and Slocum gliders |
Default calibrations of ECO sensors can lead to channel saturation in some waters Sampling location specific range adjustments or calibrations can be needed for bb– and fluorescence-based water quality attribute measuring Comparability to turbidity measurements using 90° IR scattering-based turbidity sensors can be limited |
|
90° IR scattering-based turbidity sensors (e.g., TriOS TTurb, Eureka Water Probes Trimeter and Manta+, Turner Designs C3, C6P, C-FLUOR, and Cyclops-7F, YSI EXO, ProDSS, and ProSwap + probes/sensors) |
Sensors are commonly used for inland water quality monitoring Sensors adhere to national and international standards for turbidity measuring Multiparameter probes find widespread use and can include temperature, depth, turbidity, fluorescence, and other sensors Individual and multiparameter probes can be used with in-situ deployment options ranging from hand-held sampling and profiling to autonomous profiling and mooring |
Comparability to turbidity measurements from NIR bb-based turbidity sensors can be limited |
|
|
Particle size distribution |
Particle size analyzers (e.g., Sequoia Scientific LISST-200X and LISST-HAB) |
Sensors are commonly used in ocean optics LISST-200X can be used for particle size, concentration, beam attenuation, VSF, depth, and temperature measuring LISST-HAB is based on the LISST-200X and integrates Turner Designs Cyclops-7F fluorometers for additional PC, PE, and Chla measuring Sensors can be used with different in-situ deployment options including complementing CTDs |
Sensors might exceed practical needs for most satellite data validation studies in inland waters in terms of measured IOPs and water quality attributes and associated costs VSF is measured at a single wavelength |
| Chla, PC, PE, and CDOM | Absorption and attenuation sensors (e.g., Sea-Bird Scientific ac-s), absorption sensors (e.g., TriOS OSCAR), or attenuation sensors (e.g., TriOS VIPER) | Sensors can be used for laboratory and/or in-situ absorption and/or attenuation measuring including profiling
Sensors can be calibrated for water quality attribute measuring Sensors can be used for many water quality attributes beyond bb– and fluorescence-based sensors including for NO3-N, TOC, and DOC measuring Some sensors are adapted to online water quality monitoring |
|
| Fluorescence sensors (e.g. Sea-Bird Scientific ECO and ECO Puck, TriOS matrixFlu VIS and nanoFlu, Eureka Water Probes Trimeter and Manta+, Turner Designs C3, C6P, C-FLUOR, and Cyclops-7F, YSI EXO + probes/sensors, bbe Moldaenke FluoroProbe and PhycoProbe) |
Sensors are commonly used from nearshore and open ocean to inland water quality monitoring
Sensors can be used for Chla, PC, PE, CDOM, fluorescence and rhodamine (tracing applications) measuring Sensors can be re-calibrated by users via software if sampling location specific calibrations are desired for increased accuracy Re-calibrations of most sensors are more straightforward than for absorption and attenuation sensors Some individual and multiparameter probes can be used in wired or wireless configurations |
References
Dekker, A.G., MacLeod, A., 2021. End User Consultation Report (No. SmartSat Technical Report AQW-2). Adelaide, Australia.
Eidam, E.F., Langhorst, T., Goldstein, E.B., McLean, M., 2022. OPENOBS : Open‐source, low‐cost optical backscatter sensors for water quality and sediment‐transport research. Limnology & Ocean Methods 20, 46–59. https://doi.org/10.1002/lom3.10469
Droujko, J., Molnar, P., 2022. Open-source, low-cost, in-situ turbidity sensor for river network monitoring. Sci Rep 1210 12, 10341. https://doi.org/10.1038/s41598-022-14228-4
Watras, C.J., Morrison, K.A., Rubsam, J.L., Hanson, P.C., Watras, A.J., LaLiberte, G.D., Milewski, P., 2017. A temperature compensation method for chlorophyll and phycocyanin fluorescence sensors in freshwater. Limnology & Ocean Methods 15, 642–652. https://doi.org/10.1002/lom3.10188
Watras, C.J., Hanson, P.C., Stacy, T.L., Morrison, K.M., Mather, J., Hu, Y. ‐H., Milewski, P., 2011. A temperature compensation method for CDOM fluorescence sensors in freshwater. Limnology & Ocean Methods 9, 1544 296–301. https://doi.org/10.4319/lom.2011.9.296