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Land cover changes alter hydrologic (e.g., infiltration-runoff), biochemical (e.g., nutrient loads), and ecological processes (e.g., stream metabolism). We quantified differences in aquatic ecosystem respiration in two contrasting stream reaches from a forested watershed in Colorado (1st-order reach) and an agricultural watershed in Iowa (3rd-order reach). We conducted two rounds of experiments in each of these reaches, featuring four sets of continuous injections of Cl as a conservative tracer, resazurin as a proxy for aerobic respiration, and one of the following nutrient treatments: (a) N, (b) N + C, (c) N + P, and (d) C + N + P. With those methods providing consistent information about solute transport, stream respiration, and nutrient processing at the same spatiotemporal scales, we sought to address: (1) Are respiration rates correlated with conservative transport metrics in forested or agricultural streams? and (2) Can short-term modifications of stoichiometric conditions (C:N:P ratios) override respiration patterns, or do long-term physicochemical conditions control those patterns? We found greater respiration in the reach located in the forested watershed but no correlations between respiration, discharge, and advective or transient storage timescales. All the experiments conducted in the agricultural stream featured a reaction-limited transformation of resazurin, suggesting the existence of nutrient or carbon limitations on respiration that our short-term nutrient treatments did not remove. In contrast, the forested stream was characterized by nearly balanced transformation and transient storage timescales. We also found that our short-lived nutrient treatments had minimal influence on the significantly different respiration patterns observed between reaches, which are most likely driven by the longer-term and highly contrasting ambient nutrient concentrations at each site. Our experimental results agree with large-scale analyses suggesting greater microbial respiration in headwater streams in the U.S. Western Mountains region than in second-to-third-order streams in the U.S. Temperate Plains region.
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Physical and stoichiometric controls on stream respiration in a headwater stream
Abstract. Many studies in ecohydrology focusing on hydrologictransport argue that longer residence times across a stream ecosystem shouldconsistently result in higher biological uptake of carbon, nutrients, andoxygen. This consideration does not incorporate the potential forbiologically mediated reactions to be limited by stoichiometric imbalances.Based on the relevance and co-dependences between hydrologic exchange,stoichiometry, and biological uptake and acknowledging the limited amountof field studies available to determine their net effects on the retentionand export of resources, we quantified how microbial respiration iscontrolled by the interactions between and the supply of essential nutrients (C, N, and P)in a headwater stream in Colorado, USA. For this, we conducted two rounds ofnutrient experiments, each consisting of four sets of continuous injectionsof Cl− as a conservative tracer, resazurin as a proxy for aerobicrespiration, and one of the following nutrient treatments: (a) N, (b) N+C,(c) N+P, or (d) C+N+P. Nutrient treatments were considered to be knownsystem modifications that alter metabolism, and statistical tests helpedidentify the relationships between reach-scale hydrologic transport andrespiration metrics. We found that as discharge changed significantlybetween rounds and across stoichiometric treatments, (a) transient storagemainly occurred in pools lateral to the main channel and was proportional todischarge, and (b) microbial respiration remained similar between rounds andacross stoichiometric treatments. Our results contradict the notion thathydrologic transport alone is a dominant control on biogeochemicalprocessing and suggest that complex interactions between hydrology, resourcesupply, and biological community function are responsible for drivingin-stream respiration.
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- PAR ID:
- 10440438
- Date Published:
- Journal Name:
- Biogeosciences
- Volume:
- 20
- Issue:
- 15
- ISSN:
- 1726-4189
- Page Range / eLocation ID:
- 3353 to 3366
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/bg-20-3353-2023
