# Floating Photovoltaic Plant Monitoring: A Review of Requirements and Feasible Technologies
## 4 Sensors for Water Quality Monitoring  
Proper water quality monitoring (WQM) is necessary before installation, during operation, and after decommissioning of an FPV to investigate its environmental impact [26]. This paragraph provides an overview of commercial off-the-shelf sensors used for water quality monitoring in research applications. Water bodies can be monitored using static sensors submerged at fixed locations and depths (fixed sensors), or by using underwater robots with integrated onboard sensors (mobile sensors), or, finally, by collecting water samples for laboratory analysis. The first two approaches are preferred to increase the autonomy of water monitoring activities, and benefit from compact and lightweight sensors. It is worth noting though, that the general tradeoff involving spatial and temporal sampling frequency when considering mobile and fixed monitoring stations is also relevant. Mobile platforms are capable of increased and readily tunable spatial coverage and density, while fixed stations offer better temporal density performances. Cost considerations may heavily impact the choice. The preferred sensing approaches generally rely on multiparametric probes consisting of a main body with a data logger and inputs for multiple sensor integrations, enabling simultaneous or consecutive measurement of several parameters such as temperature (T), conductivity (\(\sigma\)), dissolved oxygen (DO), turbidity (NTU/FNU), pH, light intensity (I), and chlorophyll-a (Chl-a). A depth or pressure sensor is also required to normalize measurements with specific depths in the water column. Some multiparametric probes are also equipped with wipers to clean the sensor regularly, reducing maintenance efforts by lowering the post-deployment intervention frequency. As we have seen, water body monitoring can be implemented through fixed, networked installations or with the use of underwater vehicles. The dimensions and weight of the probes can impact the maneuverability of underwater vehicles [73] as well as the positioning in fixed networked deployments. For this reason, compact and light probes have evident advantages for both uses (fixed and mobile). The maximum operating depth of the sensors varies depending on the model and manufacturer. FPV installation can occur in water bodies of varying depths. According to the literature, WQM under FPV has been performed at different nodes of selected water columns, reaching the bottom of the water body ([PERSON] 2021). Thus, a multiparametric probe has to take into account the maximum depth of the water body to be monitored. Here, the scientific literature using multiparametric probes for WQM in FPV installations has been reviewed. The cited multiparameter probes, their manufacturers, their sensors, and their main features are summarized in Table 2, where additional papers on general WQM (not specific to FPV) were included for the sake of completeness. The review identified six papers that used nine different multiparameter probes (EXO3, YSI 6920, QAM300-DE, UltraPen PT1, Troll 9500, CTD Diver, AP2000, MiniDOT Loggger) for monitoring water bodies with FPV. These are listed in Table 2 (column \"Ref\") in order of decreasing publication year. Works related to FPV are marked with an asterisk under the second column (\"FPV\"). The first such study appeared in 2020 [78] when FPV technology and awareness of its potential environmental effects were sufficiently mature to be studied experimentally. [PERSON] and colleagues in 2020 and 2021 proposed the idea of using small robotic vehicles equipped with sensors and multiparametric probes for FPV applications [78]. EXO3 (YSI) and YSI 6920, used by [PERSON] et al. in 2023 [68] and Yang 2022 [60], respectively, can measure the highest number of parameters (seven out of eight), with Light Intensity being the only missing parameter (see Table 2, \"Available Sensors\"). Troll 9500 by In Situ (USA) measures six out of eight parameters, excluding Light Intensity and Chlorophyll-a.