Dataset from a mesocosm experiment on brownification in the Baltic Sea

Area of Science:

• Marine ecology
• Biogeochemistry
• Climate change impacts

Background:

• Coastal seas face increasing brownification due to terrestrial organic matter runoff, a consequence of climate change.
• This phenomenon alters light penetration and nutrient availability in marine ecosystems.
• Understanding the impact on planktonic communities is crucial for predicting ecosystem responses.

Purpose of the Study:

• To investigate the effects of increased organic matter and nutrient loading on a coastal planktonic ecosystem.
• To assess how brownification influences key ecological parameters such as primary production and community structure.
• To differentiate the impacts of organic carbon versus inorganic nutrient additions.

Main Methods:

• A 15-day mesocosm experiment using 2.2 m³ plastic bags in Finland's SW coast.
• Four treatments were applied: control, organic carbon (HuminFeed), inorganic nutrients, and combined additions.
• Daily water sampling measured transparency, nutrients, chlorophyll a, primary and bacterial production, and particle counts via flow cytometry.

Main Results:

• Organic matter addition (HuminFeed) significantly reduced water transparency.
• Nutrient addition stimulated chlorophyll a concentrations and primary production.
• The combined treatment showed complex interactions, with altered bacterial production and particle composition.

Conclusions:

• Increased terrestrial organic matter runoff, driven by climate change, can lead to coastal sea brownification, impacting light availability.
• Simultaneous nutrient enrichment can exacerbate or mitigate these effects, depending on the planktonic community's response.
• Mesocosm experiments provide valuable insights into the complex responses of planktonic ecosystems to multiple environmental stressors.