Welcome to a CIRFA seminar on optical remote sensing organized by the Earth observation group at UiT and CIRFA.

WHEN: Jan. 23, 2020 from 14:00 to 15:00
WHERE: CIRFA, SIVA Innovation Centre (Forskningsparken)

Follow the seminar online: https://connect.uninett.no/cirfa-seminar


Light in the dark: retrieving underwater irradiance in shallow eutrophic waters from absorption and beam attenuation spectral measurements

Light is essential for primary production and, therefore, its attenuation controls the vertical distribution of plants and phytoplankton over the water column. The diffuse attenuation of downwelling irradiance (Kd) is mainly governed by the attenuation by the water itself and the concentrations of inherent optical properties (IOPs, e.g., phytoplankton, inorganic particles and colored dissolved molecules), which makes it an important parameter for eutrophication monitoring. Over the past century, Denmark has had recurrent eutrophication events, with extreme episodic cases where anoxic conditions were observed. Since the 1980’s, eutrophication in Danish waters has been monitored with regards to the Kof photosynthetically active radiation (PAR, 400–700 nm), Kd(PAR). However, radiometric measurements in Denmark are limited by a low solar

zenith angle and by only few light hours during winter. On top of that, surface radiometric measurements are highly affected by light refraction influenced by waves, compromising the monitoring of shallow turbid waters as in Denmark. Therefore, we employed a semi-analytical model valid for the shallow Danish waters to retrieve the underwater light field and estimate the Kd(PAR) from a spectral AC instrument (AC-S). We used a monitoring dataset where Kd(PAR) was estimated using two distinct approaches based on equations from the literature. Results show that modelled PAR profiles follow the overall shape exhibited by in situ radiometric profiles, presentingsmoother profiles, especially in the surface layer (2–5 m). Along with that, the model provided robust Kd(PAR) estimates, that were strongly correlated to the reference Kd(PAR) values and with low root mean square error (RMSE). Thus, AC-S data can be efficiently applied to retrieve the underwater light field and Kd(PAR). This will make possible to retrieve Kd(PAR) in the absence of light and, therefore, will allow for environmental monitoring outside the daylight hours making it more efficient, and will provide insights into the factors controlling light attenuation.