Optical and biogeochemical properties of Belgian inland and coastal waters Supplementary material to :

From 2017 to 2019, an extensive sampling campaign was conducted in Belgian inland and coastal waters, aimed at providing paired data of optical and biogeochemical properties to support research into optical monitoring of aquatic systems. The campaign was focused on inland waters, with sampling of four lakes and a coastal lagoon along the growth season, in addition to samples of opportunity of other four lakes. Campaigns also included the Scheldt estuary over a tidal cycle and two sampling campaigns in the Belgian coastal zone. Measured parameters include inherent optical properties (ab5 sorption, scattering and attenuation coefficients, near-forward volume scattering function, turbidity), apparent optical properties (Secchi disk depth, substrate and water-leaving Lambert-equivalent bi-hemispherical reflectance), and biogeochemical properties (suspended particulate matter, mineral fraction of particle mass, particle size distribution, pigment concentration, DNA metabarcoding, flow microscopy counts, and bottom type classification). The diversity of water bodies and environmental conditions covered a wide range of system states. The chlorophyll a concentration varied from 0.63 mg m−3 to 10 382.72 mg m−3, while the suspended particulate matter concentration varied from 1.02 g m−3 to 791.19 g m−3, with mineral fraction varying from 0 to 0.95. Depending on system and season, phytoplankton assemblages were dominated by cyanobacteria, green algae (Mamiellophyceae, Pyramimonadophyceae) or diatoms. The dataset is available from Castagna et al. (2022), https://www.pangaea.de/tok/c67200d99ea9bbbeadd9edec9690f937b5bacbff.


Figure S1
Evaluation of the salinity effect on the determination of a g from contrasting limits (salinity of 0 and 35). Absolute values are expressed in terms of attenuation coefficient, c, as they include the optical effects of the quartz cuvette (baseline in air). The difference of the attenuation from artificial seawater to pure water was subtracted from a g of marine and brackish samples for an approximate correction.

Figure S2
Example of samples for determination of particle absorption coefficient. Sample of station ZL_03 shows cyanobacterial filamentous colonies forming non-homogeneous deposition on the filter surface. The filter is shown before and after the pigment chemical oxidation with sodium hypochlorite.

Figure S3
Evaluation of the salinity effect on the determination of c nw from contrasting limits (salinity of 0 and 35). Absolute values are expressed in terms of attenuation coefficient, c, as they include the optical effects of the quartz cuvette (baseline in air). The difference of the attenuation from artificial seawater to pure water was subtracted from c nw of marine and brackish samples for an approximate correction.

Figure S4
Detailed example of the validation of the particle beam attenuation coefficient, c p , measured with the spectrophotometer at an acceptance angle of 0.074º against the values measured by the LISST instruments at an acceptance angle of 0.018º. The difference between c p measured by the different instruments (A) is accounted by the scattering within the difference of acceptance angle of the instruments, as estimated from the particle volume scattering function, β p (B).

Figure S5
Example of the measurement setup for on water radiometery from small platforms using a single spectrometer. The global (direct + diffuse) downwelling plane irrandiance was estimated from the exitant radiance from a reference sintered PTFE plaque, while the water leaving radiance was estimated at 0.5 m from the platform, alligned to the Sun azimuth, by placing the opening of the lens' shield at 2.5 cm below the water surface.

Figure S6
Example of the shadowing simulations calculated for the setup of the measurements of reflectance spectroscopy using the on water method. The example here shows the shadowing for the diffuse fraction with the Sun at 30º zenith angle (clear skies; Castagna et al., 2019) and optically deep waters. (A) Total shadowing error as a function of the total absorption coefficient, a t , and the total beam attenuation, c t ; (B) The difference between the total shadowing with the shadowing caused by the tip of the lens' shield (skylight-blocking apparatus), showing that the plaform shadowing (boat) is small and only relevant at low a t values.

Figure S7
Example of the setup for measurement of reflectance spectroscopy of the sediment surface, sampled with cores taken from the Spuikom. Measurements were performed under a water layer of 5 cm. Note that the images show different cores.

Figure S8
Example of the floating biofilm patches observed in the Spuikom in July, 2018. Reflectance spectroscopy were performed in situ without disturbing the floating mats. Microscopy observations revealed an assemblage of benthic diatoms inclusing species of the genera Pleurosigma, Gyrosigma and Navicula.

Figure S9
Example of the setup for measurements of reflectance spectroscopy of macroalgae. Description of the measurement, instruments and auxiliary materials are provided in the main text.