| 19 Feb 2026
Controls on Steppe River Metabolism Vary by Scale and Network Location
Abstract. We explore geographic scaling of metabolism estimates derived from dissolved oxygen measurements with data from 75 sites in three corresponding ecoregions across the temperate steppes of Mongolia and the United States. We used a nested analysis with descending spatial scales (country, ecoregion, river basin, upper or lower watershed, and wide or constrained valleys) to assess spatial heterogeneity. We then linked estimates of metabolism with reach-to-watershed-scale metrics representing geomorphology, topography, climate, and anthropogenic activity to provide possible explanations for spatial scaling dependencies. We evaluated gross primary production (GPP) and ecosystem respiration (ER) at in-situ water temperature and after standardizing them to 20 °C (GPP20 and ER20). There was no significant effect of scaling on ER, and river basin explained only modest variation in GPP. In contrast, GPP20 varied significantly with ecoregion, river basin, basin position (upper vs. lower), and valley morphology (constrained vs. wide). ER20 had no significant spatial predictors. Best regression models for GPP included positive relationships with water velocity and median basin slope and for ER included mean basin air temperature, percentage of urban land use in the basin, and GPP. Best subset regression models for GPP20 included depth, water velocity, and basin slope and for ER20 included depth and mean basin air temperature. The proportion of the watershed in urban or cropland was explanatory of ER, but not GPP. We conclude that fundamental components of ecosystem metabolism respond to different watershed scales and to distinct environmental controls. Thus, macrosystem-scale studies require multi-scale assessment to predict and capture variation in aquatic metabolism. This suggests that universal models of river metabolism are unlikely to perform as well as models built to match the specific scale of inquiry or management.
This is a timely and interesting paper examining the controls on stream metabolism across a large range of spatial scales for dry, temperate rivers in the USA and Mongolia. The Mongolian data - and the comparisons with the US data - are most important given the near lack of such information on streams from this region. I enjoyed reading it.
The paper is well-written and the methodology and key findings are clearly explained. The figures are appropriate for illustrating the related points in the text, and presuming they are reproduced in the original colors, visually acceptable.
I do have several comments and suggestions they authors may wish to consider when revising the text. These are generally in order of appearance in the paper rather than in order of importance.
In the Introduction, it may be worth adding that this methodology may be used at these - or similar - sites to assess how climate change effects may temper the regional controls on metabolism, especially noting the later comment on the role of GPP as an effective indicator of stream health.
I am not familiar at all with rivers in Mongolia. Are they clear water or turbid? I'm presuming the USA sites are clear water - is that correct? In particular, is water clarity sufficient for PAR to reach the benthos in the thalweg, or just in littoral zones? Although stream metabolism doesn't distinguish benthic and pelagic metabolism, the importance of benthic production can be inferred from knowing whether water depth is less or greater than euphotic depth.
Line 163: I would like to see some commentary on the limitations and possible data artifacts that may affect subsequent modelling and meta-analysis arising from the very short (1 - 7 day) deployment at each site. I totally understand the logistical constraints. How is a very overcast day, or consecutive days, considered for example. This will result in GPP suppressed well below 'normal' behavior at this site. Is this overcome to some extent simply by the number of sites examined?
Line 175: What were these metrics? (i.e. what criteria?)
Line 184: What is "unreasonably high"? Vague
Line 217: Nutrients are discussed later but I would have liked to see some data on concentrations of bio-available N and P as well as DOC in the rivers, as factors potentially directly affecting GPP and ER. Is there much of a difference across the various spatial scales? Even spot measurements during the deployments would provide some insight. These can be related to, but not inferred from, land use for example.
Line 252: How many sites are there with urban influence? How is this defined? Is it the same definition in both countries? Urbanization is not listed amongst the factors in lines 246-249. Is that just an oversight?
Line 267: Was the significant water depth effect related to turbidity and euphotic depth, as noted above?
Line 278: 'strong' meaning here?
Line 329: Suggest citing the finding of Hall (2013) that a substantial fraction (estimate of 44% on average) of newly created organic matter is respired before it is taken up by higher trophic levels. Freshwater Science, 32(2): 507-516 (2013)
Line 350: Are the constrained valley effects (partly) through topographic shading?
Line 378: Turbidity again. "...for metabolism in drier temperate areas, results from one country can likely be applied to another". I'm not convinced at all about this. My thinking - and I'm very happy to be convinced otherwise - is that parts of Central Asia, southwest USA, Australia, southern Africa etc where light penetration into a turbid water column is a major constraint on GPP may disrupt this universality of behavior.