作者：C. Gutierrez-Canovas, D. von Schiller, G. Pace, L. Gomez-Gener and C. Pascoal

Streams are significant contributors of greenhouse gases (GHG) to the atmosphere, and the increasing number of stressors degrading freshwaters may exacerbate this process, posing a threat to climatic stability. However, it is unclear whether the influence of multiple stressors on GHG concentrations in streams results from increases of in-situ metabolism (i.e., local processes) or from changes in upstream and terrestrial GHG production (i.e., distal processes). Here, we hypothesize that the mechanisms controlling multiple stressor effects vary between carbon dioxide (CO2) and methane (CH4), with the latter being more influenced by changes in local stream metabolism, and the former mainly responding to distal processes. To test this hypothesis, we measured stream metabolism and the concentrations of CO2 (pCO2) and CH4 (pCH4) in 50 stream sites that encompass gradients of nutrient enrichment, oxygen depletion, thermal stress, riparian degradation and discharge. Our results indicate that these stressors had additive effects on stream metabolism and GHG concentrations, with stressor interactions explaining limited variance. Nutrient enrichment was associated with higher stream heterotrophy and pCO2, whereas pCH4 increased with oxygen depletion and water temperature. Discharge was positively linked to primary production, respiration and heterotrophy but correlated negatively with pCO2. Our models indicate that CO2-equivalent concentrations can more than double in streams that experience high nutrient enrichment and oxygen depletion, compared to those with oligotrophic and oxic conditions. Structural equation models revealed that the effects of nutrient enrichment and discharge on pCO2 were related to distal processes rather than local metabolism. In contrast, pCH4 responses to nutrient enrichment, discharge and temperature were related to both local metabolism and distal processes. Collectively, our study illustrates potential climatic feedbacks resulting from freshwater degradation and provides insight into the processes mediating stressor impacts on the production of GHG in streams. Os rios s & atilde;o grandes emissores de gases com efeito de estufa (GEE) para a atmosfera, e o crescente n & uacute;mero de agentes de stress que degradam os rios pode exacerbar este processo, e constituir uma amea & ccedil;a & agrave; estabilidade clim & aacute;tica. No entanto, n & atilde;o & eacute; claro se o efeito dos impactos humanos nas concentra & ccedil;& otilde;es de GEE na & aacute;gua est & aacute; associado ao aumento do metabolismo local do rio (processos locais) ou ao aumento da produ & ccedil;& atilde;o de GEE nas zonas a montante dos rios ou nas zonas terrestres adjacentes (processos distais). A nossa hip & oacute;tese & eacute; que os mecanismos que controlam os efeitos dos impactos humanos na emiss & atilde;o de GEE variam entre o di & oacute;xido de carbono (CO2) e o metano (CH4). A nossa previs & atilde;o & eacute; que o CO2 responde principalmente a processos distais, enquanto o CH4 & eacute; mais influenciado por altera & ccedil;& otilde;es no metabolismo local dos cursos de & aacute;gua. Para avaliar esta hip & oacute;tese, medimos o metabolismo aqu & aacute;tico e as concentra & ccedil;& otilde;es de CO2 (pCO2) e CH4 (pCH4) em 50 rios que abrangem gradientes de enriquecimento em nutrientes, deple & ccedil;& atilde;o de oxig & eacute;nio, stress t & eacute;rmico, degrada & ccedil;& atilde;o da zona ribeirinha e caudal. Os nossos resultados indicam que estes agentes de stress tiveram efeitos aditivos no metabolismo e nas concentra & ccedil;& otilde;es de GEE nos rios, e que as intera & ccedil;& otilde;es entre os agentes de stress tiveram pouca capacidade preditiva. O enriquecimento em nutrientes foi associado a um aumento da heterotrofia e pCO2, enquanto o pCH4 aumentou com a deple & ccedil;& atilde;o de oxig & eacute;nio e com a temperatura da & aacute;gua. O caudal estava positivamente correlacionado com a produ & ccedil;& atilde;o prim & aacute;ria, a respira & ccedil;& atilde;o e a heterotrofia, mas negativamente correlacionado com o pCO2. Os nossos modelos indicam que as concentra & ccedil;& otilde;es equivalentes de CO2 podem duplicar em rios eutrofizados e com baixa concentra & ccedil;& atilde;o de oxig & eacute;nio, em compara & ccedil;& atilde;o com os rios oligotr & oacute;ficos e com & aacute;guas bem oxigenadas. A aplica & ccedil;& atilde;o de modelos de equa & ccedil;& otilde;es estruturais mostrou que os efeitos do enriquecimento em nutrientes e do caudal no pCO2 estavam relacionados com processos distais e n & atilde;o com o metabolismo local. Em contrapartida, as respostas do pCH4 ao enriquecimento de nutrientes, ao caudal e & agrave; temperatura estavam relacionadas tanto com o metabolismo local como com processos distais. O nosso estudo demonstra que a degrada & ccedil;& atilde;o dos rios e dos ecossistemas ribeirinhos pode ter efeitos negativos na estabilidade clim & aacute;tica e fornece informa & ccedil;& atilde;o relevante sobre os processos biogeoqu & iacute;micos que medeiam os impactos humanos na produ & ccedil;& atilde;o de GEE nos rios. We found that the simultaneous occurrence of several human-induced stressors in streams increases the concentrations of greenhouse gases in their waters, potentially exacerbating climate crisis. Our results also indicate that the scale at which human impacts occur has different effects on carbon dioxide and methane concentrations. Collectively, our findings suggest that reducing nitrogen inputs and addressing hypoxia in streams could provide additional co-benefits, aiding in the fight against climate change.

来源：Global Change Biology 2024 Vol. 30 Issue 5. DOI: 10.1111/gcb.17301