Baltic Sea transparency from ships and satellites: centennial trends

Abstract

Water transparency can be measured with optical instruments and estimated with satellite sensors, but such measurements have been widely available for only a few decades. Estimates of water transparency using a white disk called a Secchi disk have been made for over a century and can be used to estimate long-term trends. However, historic in situ measurements of the Secchi depth (ZSd) were irregular in space and time and are difficult to interpret in regular time series due to biases introduced by changing locations and the timing of measurements. Satellite data time series, on the other hand, have consistent resolution in both space and time but cover too short a time to resolve climate-scale trends. We normalized historic ZSd measurements in the Baltic Sea with a satellite-derived mean climatology at 5 d temporal and 4 km spatial resolutions and created a merged time series of ZSd for the last century. The mean ZSd in the Baltic Sea from 1927-2020 decreased by 4.2 ± 0.6 m at a rate of 0.045 ± 0.06 m yr-1. Most of the change happened before 1987, and a further decrease was evident primarily in the satellite data during the 1998-2008 period. After 2008, no significant trend in ZSd and or the coefficient of diffuse light attenuation was detected in the Baltic Sea. However, in some sub-basins of the Baltic Sea, the decrease in ZSd continued even after that. The decrease in spectral water transparency in recent decades was highest in the 412 nm band, indicating an increase in the concentration of chromophoric dissolved organic matter.

Multidecadal changes in ocean transparency: Decrease in a coastal upwelling region and increase offshore

“…The Lee et al (2015) Z SD model has been applied in a number of regions (Shang et al 2016; Liu et al 2020 a , b ; Kahru et al 2022; Brewin et al 2023). We validated the satellite‐derived Z SD datasets at daily and 4.5‐km resolutions.…”

“…The algorithm of Lee et al (2015), on the other hand, is based on new theoretical interpretation of sighting a Secchi disk in water, which adapts to the change in the spectral composition of subsurface light. The algorithm and the model have been validated in different locations with different sensors (Lee et al 2015(Lee et al , 2018Liu et al 2019;Kahru et al 2022;Brewin et al 2023).…”

“…However, such effects are not uniform in space and time. Kahru et al (2022) removed both the mean spatial and seasonal variability in Z SD using satellite‐derived Z SD climatology to further investigate centennial trends in the Baltic.…”

“…Here, we adopt the same approach as Kahru et al (2022) and normalize historic shipborne Z SD measurements by subtracting the mean 10‐d seasonal Z SD value of the nearest 4.5‐km satellite pixel from each Z SD measurement. We then compare in situ and satellite time series and merge them, in order to reconstruct a seven‐decade record of water transparency in the CCE.…”

Tracking freshwater browning and coastal water darkening from boreal forests to the Arctic Ocean

“…For the North Sea, Skagerrak Sea, and Baltic Sea, several studies have found that light attenuation has been increasing throughout the 20 th century (Sandén and Håkansson 1996;Aarup 2002;Dupont and Aksnes 2013;Capuzzo et al 2015;Opdal et al 2019;Kahru et al 2022), inferred from a gradual decreasing Secchi disk depth-a phenomenon often referred to as coastal water darkening. Interestingly, the term darkening originally referred to the freshening of coastal waters, and associated increase in light attenuation (Aksnes et al 2009), but is more commonly used as any nonbiotic driver of reduced light penetration in coastal waters, including both dissolved and suspended matter (Frigstad et al 2013;McGovern et al 2019;Blain et al 2021;Konik et al 2021).…”