Ecosystem responses to increased organic carbon concentration: comparing results based on long-term monitoring and whole-lake experimentation

ABSTRACT

Recent increases in terrestrial dissolved organic carbon (DOC) concentrations in northern inland waters have many ecological consequences. We examined available data on carbon cycles and food webs of 2 boreal headwater lakes in southern Finland. Basic limnology and catchment characteristics of a pristine lake, Valkea-Kotinen (VK), were monitored over the past 25 years while the lake has undergone browning and DOC increased from ∼11 to 13 mg L⁻¹. Pronounced changes in the early 2000s represent a regime shift in DOC concentration and color. Lake Alinen Mustajärvi (AM) was manipulated for 2 years by additions of labile DOC (cane sugar), raising the DOC concentration from ∼10 to 12 mg L⁻¹, but not changing light conditions. The 2 different approaches both revealed increased concentrations and efflux of carbon dioxide (CO₂) from the lakes and thus net heterotrophy and changes in the pelagic community structure following an increase in DOC concentration. Long-term monitoring of VK revealed a decline in phytoplankton primary production (PP) along with browning, which was reflected in retarded growth of young (1–2-year-old) perch. In the experimentally manipulated lake (AM), PP was not affected, and the growth of young perch was more variable. The results suggested the importance of a pathway from labile DOC via benthic invertebrates to perch. Although provided with this extra resource, the food chain based on DOC proved inefficient. Long-term monitoring and whole-lake experimentation are complementary approaches for revealing how freshwater ecosystems respond to climate and/or atmospheric deposition-induced changes, such as browning.

KEYWORDS:

• boreal lakes
• browning
• CO₂ flux
• food webs
• lake metabolism
• terrestrial organic carbon

Acknowledgements

Long-term monitoring of Valkea-Kotinen was funded by Ministry of Environment and Lammi Biological Station, University of Helsinki. Fish studies were funded by the Natural Resources Institute Finland. The carbon addition experiment in Alinen Mustajärvi was funded by Academy of Finland project grant 114604 to professor Roger I. Jones. We thank 2 anonymous reviewers for the constructive criticism of the earlier versions of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

ORCID

Paula Kankaala http://orcid.org/0000-0002-1422-2806

Minna Hiltunen http://orcid.org/0000-0002-0003-3000

Roger I. Jones http://orcid.org/0000-0002-0835-8264

Elina Peltomaa http://orcid.org/0000-0002-2858-8358