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Abstract
Restoring wetlands will reduce nitrogen contamination from excess fertilization but estimates of the efficacy of the strategy vary widely. The intervention is often described as effective for reducing nitrogen export from watersheds to mediate bottom-level hypoxia threatening marine ecosystems. Other research points to the necessity of applying a suite of interventions, including wetland restoration to mitigate meaningful quantities of nitrogen export. Here, we use process-based physical modeling to evaluate the effects of two hypothetical, but plausible large-scale wetland restoration programs intended to reduce nutrient export to the Gulf of Mexico. We show that full adoption of the two programs currently in place can meet as little as 10% to as much as 60% of nutrient reduction targets to reduce the Gulf of Mexico dead zone. These reductions are lower than prior estimates for three reasons. First, net storage of leachate in the subsurface precludes interception and thereby dampens the percent decline in nitrogen export caused by the policy. Unlike previous studies, we first constrained riverine fluxes to match observed fluxes throughout the basin. Second, the locations of many restorable lands are geographically disconnected from heavily fertilized croplands, limiting interception of runoff. Third, daily resolution of the model simulations captured the seasonal and stormflow dynamics that inhibit wetland nutrient removal because peak wetland effectiveness does not coincide with the timing of nutrient inputs. To improve the health of the Gulf of Mexico efforts to eliminate excess nutrient, loading should be implemented beyond the field-margin wetland strategies investigated here.

Streams play a key role in the global carbon cycle. The balance between carbon intake through photosynthesis and carbon release via respiration influences carbon emissions from streams and depends on temperature. However, the lack of a comprehensive analysis of the temperature sensitivity of the metabolic balance in inland waters across latitudes and local climate conditions hinders an accurate projection of carbon emissions in a warmer future. Here, we use a model of diel dissolved oxygen dynamics, combined with high-frequency measurements of dissolved oxygen, light and temperature, to estimate the temperature sensitivities of gross primary production and ecosystem respiration in streams across six biomes, from the tropics to the arctic tundra. We find that the change in metabolic balance, that is, the ratio of gross primary production to ecosystem respiration, is a function of stream temperature and current metabolic balance. Applying this relationship to the global compilation of stream metabolism data, we find that a 1 degrees C increase in stream temperature leads to a convergence of metabolic balance and to a 23.6% overall decline in net ecosystem productivity across the streams studied. We suggest that if the relationship holds for similarly sized streams around the globe, the warming-induced shifts in metabolic balance will result in an increase of 0.0194 Pg carbon emitted from such streams every year. ; National Science Foundation (NSF)National Science Foundation (NSF) [EF-1258994]; NSFNational Science Foundation (NSF) [EF-1065255, EF-1065286, EF-1065055, EF-1065682, EF-1065267, EF-1064998, EF-1065377]; Northern Australian Environmental Resources Hub of the National Environmental Science Program; Scale, Consumers and Lotic Ecosystem Rates project ; The authors thank K. Gido for his contribution in obtaining funding and designing the field experiments. K. Gido, J. Drake, C. Osenberg and J. Minucci provided comments on earlier versions of this paper. Georgia Advanced Computing Resource Center provided the computing facility. This study was supported by the National Science Foundation (NSF, grant EF-1258994) and is part of the Scale, Consumers and Lotic Ecosystem Rates project supported by NSF grant EF-1065255. Data collection at each site was supported by NSF grants EF-1065286, EF-1065055, EF-1065682, EF-1065267, EF-1064998 and EF-1065377, and the Northern Australian Environmental Resources Hub of the National Environmental Science Program. ; Public domain authored by a U.S. government employee