Journal cover Journal topic
Biogeosciences An interactive open-access journal of the European Geosciences Union
Biogeosciences, 13, 1009-1018, 2016
http://www.biogeosciences.net/13/1009/2016/
doi:10.5194/bg-13-1009-2016
© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
23 Feb 2016
Changing seasonality of the Baltic Sea
Mati Kahru1, Ragnar Elmgren2, and Oleg P. Savchuk3 1Scripps Institution of Oceanography, University of California San Diego, La Jolla, California
2Department of Ecology, Environment and Plant Sciences, Stockholm University, Stockholm, Sweden
3Baltic Nest Institute, Stockholm University, Stockholm, Sweden
Abstract. Changes in the phenology of physical and ecological variables associated with climate change are likely to have significant effect on many aspects of the Baltic ecosystem. We apply a set of phenological indicators to multiple environmental variables measured by satellite sensors for 17–36 years to detect possible changes in the seasonality in the Baltic Sea environment. We detect significant temporal changes, such as earlier start of the summer season and prolongation of the productive season, in several variables ranging from basic physical drivers to ecological status indicators. While increasing trends in the absolute values of variables like sea-surface temperature (SST), diffuse attenuation of light (Ked490) and satellite-detected chlorophyll concentration (CHL) are detectable, the corresponding changes in their seasonal cycles are more dramatic. For example, the cumulative sum of 30 000 W m−2 of surface incoming shortwave irradiance (SIS) was reached 23 days earlier in 2014 compared to the beginning of the time series in 1983. The period of the year with SST of at least 17 °C has almost doubled (from 29 days in 1982 to 56 days in 2014), and the period with Ked490 over 0.4 m−1 has increased from about 60 days in 1998 to 240 days in 2013 – i.e., quadrupled. The period with satellite-estimated CHL of at least 3 mg m−3 has doubled from approximately 110 days in 1998 to 220 days in 2013. While the timing of both the phytoplankton spring and summer blooms have advanced, the annual CHL maximum that in the 1980s corresponded to the spring diatom bloom in May has now shifted to the summer cyanobacteria bloom in July.

Citation: Kahru, M., Elmgren, R., and Savchuk, O. P.: Changing seasonality of the Baltic Sea, Biogeosciences, 13, 1009-1018, doi:10.5194/bg-13-1009-2016, 2016.
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Short summary
Using satellite-derived data sets we have found drastic changes in the timing of the annual cycle in physical and ecological variables of the Baltic Sea over the last 30 years. The summer season starts earlier and extends longer. The period with sea-surface temperature of at least 17 ˚C has doubled; the period with high water turbidity has quadrupled. While both the phytoplankton spring and summer blooms have become earlier, the annual maximum has switched to the summer cyanobacteria bloom.
Using satellite-derived data sets we have found drastic changes in the timing of the annual...
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