Browsing by Subject "Pohjanpitäjänlahti"

Light is crucial for aquatic ecosystems, as photosynthesis supports the higher trophic levels. Light attenuates in water due to absorption and scattering by optically active substances (OAS), the main ones in coastal environments being chlorophyll-a (chl-a), particulate matter and coloured dissolved organic matter (CDOM). Light attenuation measurements are also important indicators of many environmental changes. Traditionally, Secchi depth has been used to monitor changes in light attenuation. However, Secchi depth (ZS) is a not an accurate estimate of light attenuation, and conversion to light attenuation coefficient (Kd) is needed to study light conditions as aquatic organisms observe them. Therefore, calibration between methods is important. In this study, light attenuation was estimated with Secchi disc, a light meter and light loggers to scrutinize the possibilities of autonomous measurements in this context. The aims of the study were 1) to compare three methods for estimating light attenuation and 2) to identify the environmental drivers affecting the relationships and accuracy of these methods, and 3) to assess the possibility of using conductivity as a proxy for light attenuation. The main hypotheses were that the differences in conversion between ZS and the two sensor-based methods in different concentrations of optically active substances can be explained by contribution of scattering to light attenuation, and that variation in different Kd-estimates is due to the different measurement spectra. The effect of three OASs (chl-a, particulate matter, CDOM) on differences among light attenuation estimates were quantified in Pohjanpitäjänlahti bay. Light attenuation was mainly controlled by CDOM, followed by particulate matter and chl-a. Conductivity was found to be a good proxy for light attenuation. However, there was a mixed signal if the differences among methods could be explained by OAS concentrations or conductivity, as the hypotheses were supported with one device pair but not with the other. Therefore, the differences among light measurement methods might stem from the characteristics of the devices or measurement errors. However, all the methods were found suitable for tracking changes in light attenuation and a summary table of the advantages and disadvantages of each method is presented to help choose a suitable method to estimate light attenuation e.g., in future studies or environmental monitoring.