Photo: Maciej Tomczak

Published: 2015-12-11

Changing seasonality of the Baltic Sea

Summer season in the Baltic Sea starts earlier. The number of days with sea water warmer than 17°C have almost doubled over the past 30 years. The productive season has also been prolonged. These findings are based on satellite data and the results demonstrate how such indicators of seasonality can be used to detect long-term changes.

A discussion paper under review in Biogeosciences has already attracted attention among researchers. Changing seasonality of the Baltic Sea is an interesting topic, notes the authors from University of California and Stockholm University.

Several indicators of seasonality were estimated from environmental variables that have been measured in the Baltic Sea by satellite sensors for 17–35 years. Such phenological indicators were often found to be more sensitive for detection of long-term changes than absolute values of the variables.

Earlier summer and warmer water

An earlier start of the summer season and a prolongation of the productive season were detected in multiple variables ranging from basic physical drivers to ecological status indicators. For instance, the period of the year with sea surface temperature higher than 17 °C has almost doubled (from 29 days in 1982 to 56 days in 2014), as has the period with satellite-estimated chlorophyll concentration higher than 3 mg m-3 (from approximately 110 days in 1998 to 220 days in 2013).

Changes in phytoplankton blooms

Both the phytoplankton spring and summer blooms have advanced, and the annual chlorophyll maximum that in the 1980s corresponded to the spring diatom bloom in May now occurs during the summer cyanobacteria bloom in July.

Because these analyses are based on satellite data, which are available for most of the world ocean, such studies can be made for most marine areas, not only for data-rich areas like the Baltic Sea.

Link to the study:

Changing seasonality of the Baltic Sea


USA: Mati Kahru, University of California San Diego

Sweden: Ragnar Elmgren and Oleg Savchuk, Stockholm University

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Web editor: Marmar Nekoro

Updated: 2015-12-11
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