toxins (neurotoxins, cytotoxins, endotoxins, or hepatotoxins), collectively known as cyanotoxins. The relations between nutrients feeding in function of hydrologic conditions (storm and non-storm) and cyanobacteria death are still delicate to understand (Hartshorn et al., 2016). Indeed, proliferation of toxin-producing cyanobacterial blooms is attributed to a large number of environmental factors, including: unbalanced N and P inputs (leading to silicate starvation and the disappearance of diatoms with siliceous frustules); increased temperature, water residence times, vertical stratification, and pH. All of this will be exacerbated by climate change (Howard et al., 2017). The toxin releases pose a danger to humans and animals, in the wetland themselves as well as in the water bodies downstream (Griffiths and Mitsch, 2020).
1.13 Wetland Monitoring
1.13.1 Monitoring Large-Scale CWs
CWs can be monitored by setting probes and sampling devices at the inlet(s) and outlet(s), and sometimes within the water body itself, depending upon the accessibility. Some classical probes with self-storage capacity for long-term in situ monitoring measure temperature, pressure (for water level monitoring) (Auderset Joye and Boissezon, 2017), conductivity, dissolved oxygen, or light (Gardner et al., 2019). Drake et al. (2018) have monitored nitrates at the inlet and outlet of a large CW at high frequency (15 min) using a UV probe over periods of six months (the probes had to be removed in winter). The probes were able to catch the nitrate variations during rain events, which is not possible by manual low-frequency sampling. It was therefore possible to estimate with high accuracy the nitrogen removal potential of the CW (Drake et al., 2018).
For parameters which cannot be measured by in situ probes and which necessitate analysis in a laboratory, it is possible to take samples from the shore when the CW is not too large. It is more difficult for large-scale systems, especially in case of shallow zones. Aquatic drones (i.e., UAV or unmanned aquatic vehicle) which can be fitted with various probes (e.g., temperature, dissolved oxygen, and conductivity) and take samples while being controlled from the shore or running in autonomous drive are helpful in such conditions. Such an UAV is regularly used to monitor a 2-ha 50-cm-deep FSF-CW located downstream the urban wastewater treatment plant in Reims (France) and designed to polish the reclaimed wastewater (Figure 1.6). It is part of a set of three 2-ha FSF-CWs where the effect of emerged vegetation is currently investigated. The CW is rectangular (50 m × 350 m) and free from emerged vegetation. The UAV, fitted with a dissolved oxygen probe, a temperature sensor and a GPS is controlled from the shore and navigate along the centerline, while bringing water samples to the shore regularly. The dissolved oxygen profile collected in March 2020 is shown in Figure 1.7, together with the dissolved organic carbon and dissolved total nitrogen in the water samples (analyzed in the laboratory after the mission). A sharp increase of the dissolved oxygen concentration is observed 100 m downstream the inlet and corresponds to a large over saturation (150%) of dissolved oxygen: oxygen production is due to the photosynthesis performed by submerged aquatic plants (Ceratophyllum sp.) and algae. Nitrogen removal is observed between the inlet and the outlet of the CW, but dissolved organic carbon remains unchanged.
Figure 1.6 (a) The 2-ha FSF-CW (Reims, France) in March 2020; (b) the UAV (Spyboat, CT2MC), Petite-bourgeoise, France collecting samples and monitoring dissolved oxygen and temperature.
Figure 1.7 Monitoring the Reims FSF-CW with an aquatic drone. (a) Dissolved organic carbon (DOC,
UASs (Unmanned Aerial Systems, i.e., aerial drone) have also been proposed to collect water samples from the sky (Lally et al., 2019). The payload is still rather limited (200 to 300 ml approximately), especially for small UASs and the success rate of water capture needs to be increased. Nevertheless, development is progressing in that field: Benson et al. have reported the successful collection of small sterile water samples (50 ml) for microbial identification in water bodies (Benson et al., 2019).
1.13.2 Vegetation Monitoring
Aquatic plants are an essential element of CW but the monitoring of their development (e.g., growth, senescence) is not easy, especially on large-scale systems. Accurate field surveys can be organized from the ground but there are time-consuming and labour-intensive. Very large systems cannot be monitored completely. Depending upon the water depth, some areas might not be accessible by boat. Recent progress in aerial imagery by UAS (i.e., aerial) can be a solution to these problems. UAS can be fitted with classical RGB cameras as well as more sophisticated sensors to cover the light spectrum down to NIR: they can detect free water areas, emerged as well as submerged aquatic plants (Chabot et al., 2017; Chabot et al., 2018). After calibration with ground sensors, an estimation of dissolved oxygen, turbidity (Wang et al., 2020a) or Chlorophyll-a (Gitelson et al., 2007) can also be provided. However, dedicated flights should be organized with licensed UAS pilots and under proper weather conditions (no rain and low wind). Furthermore, wildlife disturbance and even collisions with birds should be considered (Jarrett et al., 2020). Satellite images can also be used even if the spatial resolution is lower.
Saberioon et al. (2020) have successfully monitored Chlorophyll-a and total suspended solids in fishponds and sandpit lakes of various surfaces (tens to hundreds of hectares) and depths (from 2 m for fishponds to 10 m for sandpit lakes) using the Sentinel-2 constellation. The two Sentinel-2 satellites are working at high spatial resolution (10 to 60 m) with a revisit cycle of 5 days. Sentinel-2 satellites are fitted with a multi-spectral imager working with 13 bands (from visible to mid-infra-red) and provide a high temporal resolution series of images, if the sky is cloud-free (Saberioon et al., 2020). These images are freely available on the Copernicus-ESA website (https://scihub.copernicus.eu/).
In spite of its small size, the development of the vegetation in the 1.5-ha CW constructed downstream the St Just–St Nazaire de Pézan wastewater treatment plant (5,000 persons equivalent) in France can be monitored using the Sentinel-2 constellation true colour images (Figure 1.8).
The intensity of the green channel (560 nm) reflects the development of emerged and free-floating vegetation, as shown in Figure 1.9. It is a global assessment as the CW is composed of different zones (e.g., phytoplankton, reedbeds, or free-surface area) (pole-zhi.org, 2013). The vegetation starts to grow at the end of March and reaches its maximum (as monitored though the green channel) early June.
Figure 1.8 Sentinel-2 True colour images of the St Just–St
