Method development

Whole stream metabolism tells us something about the rate of energy capture (photosynthesis) and respiration at the ecosystem scale (a river reach). While conceptually it is just a simple mass balance of oxygen, in practice it is important to understand the underlying principles before applying one of the available methods. I spent a lot of time thinking about it with Russ Manson (Stockton University, USA) and suggested a new (pragmatic) method for spatially heterogeneous streams, available as an Excel workbook (Demars et al 2011 Freshwater Biology). This new method outperformed all previous methods in spatially heterogeneous streams and is now also available as R code (with example data), thanks to Joshua Thompson (Smithsonian Environmental Research Centre, USA). Russ Manson developed a different approach to partition the metabolism of the main channel with that of the water transient storage zone. Finally, I also derived a very simple set of equations for students to understand the basic principles of the method and provided an excel spreadsheet where it is easy to play with the parameters of the equations and see what are the effects on the diel change in dissolved oxygen concentration, net metabolism and oxygen flux at the water-air interface. See the review by Demars et al (2015) and its supplementary files.

Carbon, temperature and the metabolic balance of streams

Using a model system in Iceland with geothermal streams, I have tested the hypothesis that stream respiration will increase faster than photosynthesis with warming, leading to increasing stream CO2 emissions (positive feedback loop) - Demars et al 2011 Freshwater Biology. This study was part of a larger investigation led by Nikolai Friberg (now in NIVA, Norway), Gisli Gislason (University of Iceland, Reykjavik) and Jon Olafsson (Institute of Freshwater Fisheries, Reykjavik). It also benefitted from collaboration with Russ Manson (Stockton University, USA) and Guy Woodward's group (now Imperial College London, England).

Subsequent studies included testing the interaction effect of temperature x water transient storage (Demars et al 2011 Knowledge and Management of Aquatic Ecosystems), testing the response of respiration to temperature in the laboratory using epiphytic biofilms with different temperature history (Perkins et al 2011), and testing the effect of seasonality on ecosystem energy capture (O'Gorman et al 2012).

Stream thermal reaction norm

Streams and rivers process a significant part of the carbon leaching out terrestrial ecosystems, and contribute an estimated 86 percent of the global carbon dioxide (CO2) emissions from inland waters. One major control of these CO2 emissions is the metabolic balance between photosynthesis, fixing CO2 to produce organic carbon, and respiration converting organic carbon into CO2. Carbon emissions from rivers could increase with warming, independently of organic carbon inputs, because the apparent activation energy is predicted to be higher for respiration than photosynthesis. The apparent activation energy of aquatic photosynthesis remains controversial, however, because it does not consider the effect of CO2 concentrating mechanisms. We reported in Nature Geoscience (Demars et al. 2016) the thermal response of aquatic photosynthesis from streams in the temperature range 4-70°C located in geothermal areas of North America, Iceland and Kamchatka. We showed, with a thermodynamic theory of enzyme kinetics, that the apparent activation energy of aquatic ecosystem photosynthesis, within 4-45°C, is approximately 0.57 electron volts (eV), which is similar to respiration. We found a temperature invariance of the metabolic balance during the summer from a global synthesis including 222 streams across biomes. Our mechanistic approach warned, however, of a possible direct effect of temperature on annual CO2 emissions from streams, which may be amplified with the concomitant increase in organic carbon supply.