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MEPS 697:1-13 (2022)  -  DOI:

Baltic Sea transparency from ships and satellites: centennial trends

Mati Kahru1,*, Henry Bittig2, Ragnar Elmgren3, Vivi Fleming4, Zhongping Lee5, Gregor Rehder2

1Scripps Institution of Oceanography, UCSD, La Jolla, CA 92092-0218, USA
2Leibniz Institute for Baltic Sea Research Warnemünde (IOW), Seestrasse 15, 18119 Rostock, Germany
3Department of Ecology, Environment and Plant Sciences, Stockholm University, 10691 Stockholm, Sweden
4SYKE Marine Research Centre, PO Box 140, 00251 Helsinki, Finland
5School for the Environment, University of Massachusetts Boston, Boston, MA 02125, USA
*Corresponding author:

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.

KEY WORDS: Water transparency · Baltic Sea · Eutrophication · Secchi depth · Light attenuation · kd490 · CDOM · Chromophoric dissolved organic matter · Climate variability

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Cite this article as: Kahru M, Bittig H, Elmgren R, Fleming V, Lee Z, Rehder G (2022) Baltic Sea transparency from ships and satellites: centennial trends. Mar Ecol Prog Ser 697:1-13.

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