Mixed effects of climate warming on stream processes

Posted on September 24, 2012

Although fresh waters play a major role in the global carbon cycle, we’re largely ignorant of the effects of climate change on biological processes in streams.  In principle, because rates of respiration accelerate faster than rates of photosynthesis with rising temperature, climatic warming should tip the metabolic balance in favour of respiration rather than production.  More respired CO2 would then be released to the atmosphere, encouraging positive feedback in the greenhouse effect. To test these ideas a natural experiment was performed in a geothermal area of southwest Iceland.  Measurements of oxygen inflows and outflows were made along stretches of 13 naturally-heated groundwater-fed streams which varied widely in temperature (5-25oC).   As predicted, respiration rates exceeded production rates and respiration was more closely correlated with  temperature than was production.  While further studies are needed to improve the accuracy of predictions, the results indicated that a temperature rise of five degrees (from 13 to 18oC) would double stream carbon emissions.  Parallel experiments involving the addition of nitrogen and phosphorus to the study streams showed that higher metabolic rates in the warm streams were supported by faster rates of nutrient cycling, which means that  climate warming could have the effect of reducing the supply of nutrients to downstream ecosystems.  In other words, the impacts of warming could be both positive (reduced eutrophication) and negative (higher carbon emissions). 

Reference:  Demars, B.O.L. et al.  2011.  Temperature and the metabolic balance of streams.   Freshwater Biology 56, 1106–1121.  http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2427.2010.02554.x/pdf