Estimation of diffuse light attenuation for Danish waters from measurements of inherent optical properties

TY - ABST

T1 - Estimation of diffuse light attenuation for Danish waters from measurements of inherent optical properties

AU - Gonçalves-Araujo, Rafael

AU - Markager, Stiig

PY - 2019

Y1 - 2019

N2 - Light is essential for primary production and, therefore, its attenuation controls the vertical distribution of plants and phytoplankton over the water column. Additionally, diffuse light attenuation (kd) is mainly governed by the attenuation by the water itself and the concentrations of inherent optical properties (IOPs) of e.g. phytoplankton, inorganic particles and colored dissolved molecules, what makes it an important parameter for eutrophication monitoring. Given that Denmark has recurrent eutrophication episodes and few light hours during winter, this study aims at developing a model valid for the shallow Danish waters to estimate the kd of photosynthetic active radiation (PAR, 400-700 nm), kdPAR), from a spectral AC instrument (AC-S). The possibility of retrieving kd(PAR) in the absence of light will allow for environmental monitoring outside the daylight hours making it more efficient, and will provide insights into the factors controlling light attenuation. We used a monitoring dataset where kd(PAR) was estimated using two distinct approaches: (1) using a spectrally resolved calculation over the PAR range; and (2) using values integrated for the PAR wavelength range. Results show that modelled kd(PAR) profiles follow the overall shape exhibited by in situ radiometric profiles. Moreover, the modelled kd(PAR) is strongly correlated to the reference kd(PAR) values, however, with some values deviating from 1:1 regression line. We attribute such an overestimation to the fact that the upwelling light is not accounted in the model and there might be an influence from bottom reflection and solar-induced fluorescence from phytoplankton. Finally, with respect to the two methods applied, results from the spectrally resolved approach presented better performance. This is mainly because the spectrally resolved calculations take into account the stronger absorption, mostly in the blue and red bands, that is not accounted when computing kd from PAR data.

AB - Light is essential for primary production and, therefore, its attenuation controls the vertical distribution of plants and phytoplankton over the water column. Additionally, diffuse light attenuation (kd) is mainly governed by the attenuation by the water itself and the concentrations of inherent optical properties (IOPs) of e.g. phytoplankton, inorganic particles and colored dissolved molecules, what makes it an important parameter for eutrophication monitoring. Given that Denmark has recurrent eutrophication episodes and few light hours during winter, this study aims at developing a model valid for the shallow Danish waters to estimate the kd of photosynthetic active radiation (PAR, 400-700 nm), kdPAR), from a spectral AC instrument (AC-S). The possibility of retrieving kd(PAR) in the absence of light will allow for environmental monitoring outside the daylight hours making it more efficient, and will provide insights into the factors controlling light attenuation. We used a monitoring dataset where kd(PAR) was estimated using two distinct approaches: (1) using a spectrally resolved calculation over the PAR range; and (2) using values integrated for the PAR wavelength range. Results show that modelled kd(PAR) profiles follow the overall shape exhibited by in situ radiometric profiles. Moreover, the modelled kd(PAR) is strongly correlated to the reference kd(PAR) values, however, with some values deviating from 1:1 regression line. We attribute such an overestimation to the fact that the upwelling light is not accounted in the model and there might be an influence from bottom reflection and solar-induced fluorescence from phytoplankton. Finally, with respect to the two methods applied, results from the spectrally resolved approach presented better performance. This is mainly because the spectrally resolved calculations take into account the stronger absorption, mostly in the blue and red bands, that is not accounted when computing kd from PAR data.

M3 - Conference abstract for conference

T2 - 20. Danske Havforskermøde

Y2 - 23 January 2019 through 25 January 2019

ER -