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Temporal patterns in stream chemistry provide integrated signals describing the hydrological and ecological state of whole catchments. However, stream chemistry integrates multi-scale signals of processes occurring in both the catchment and stream. Deconvoluting these signals could identify mechanisms of solute transport and transformation and provide a basis for monitoring ecosystem change. We applied trend analysis, wavelet decomposition, multivariate autoregressive state-space modeling, and analysis of concentration–discharge relationships to assess temporal patterns in high-frequency (15 min) stream chemistry from permafrost-influenced boreal catchments in Interior Alaska at diel, storm, and seasonal time scales. We compared catchments that varied in spatial extent of permafrost to identify characteristic biogeochemical signals. Catchments with higher spatial extents of permafrost were characterized by increasing nitrate concentration through the thaw season, an abrupt increase in nitrate and fluorescent dissolved organic matter (fDOM) and declining conductivity in late summer, and flushing of nitrate and fDOM during summer rainstorms. In contrast, these patterns were absent, of lower magnitude, or reversed in catchments with lower permafrost extent. Solute dynamics revealed a positive influence of permafrost on fDOM export and the role of shallow, seasonally dynamic flowpaths in delivering solutes from high-permafrost catchments to streams. Lower spatial extent of permafrost resulted in static delivery of nitrate and limited transport of fDOM to streams. Shifts in concentration–discharge relationships and seasonal trends in stream chemistry toward less temporally dynamic patterns might therefore indicate reorganized catchment hydrology and biogeochemistry due to permafrost thaw. 

River networks regulate carbon and nutrient exchange between continents, atmosphere, and oceans. However, contributions of riverine processing are poorly constrained at continental scales. Scaling relationships of cumulative biogeochemical function with watershed size (allometric scaling) provide an approach for quantifying the contributions of fluvial networks in the Earth system. Here we show that allometric scaling of cumulative riverine function with watershed area ranges from linear to superlinear, with scaling exponents constrained by network shape, hydrological conditions, and biogeochemical process rates. Allometric scaling is superlinear for processes that are largely independent of substrate concentration (e.g., gross primary production) due to superlinear scaling of river network surface area with watershed area. Allometric scaling for typically substrate-limited processes (e.g., denitrification) is linear in river networks with high biogeochemical activity or low river discharge but becomes increasingly superlinear under lower biogeochemical activity or high discharge, conditions that are widely prevalent in river networks. The frequent occurrence of superlinear scaling indicates that biogeochemical activity in large rivers contributes disproportionately to the function of river networks in the Earth system.

 

Production of animal biomass and the number of trophic levels supported by an ecosystem depend in part on rates of primary production, disturbance, predator–prey interactions, and the efficiency of energy flow through food webs. Of these factors, food web efficiency has been among the most difficult to quantify empirically. Thus, both the drivers and consequences of variation in food web efficiency remain largely unstudied in field settings. We estimated food web efficiency in nine desert streams spanning gradients of flash flood recurrence, resource availability, and trophic structure. Food web efficiency was estimated as fish community production relative to gross primary production at an annual timescale, based on quarterly observations of fish biomass and stream metabolism. Gross primary production was greatest in streams characterized by flashier flow regimes and greater relative light, temperature, and nitrogen availability. Fish production ranged from 0.02 to 0.50 g C m⁻² yr⁻¹, food web efficiency ranged from 9.5 × 10⁻⁵to 1.8 × 10⁻², and both properties decreased with flashier flow regime, light, temperature, and nitrogen availability, but were not associated with food chain length. These results, combined with opposite effects of environmental variation on primary vs. fish production, indicated that the effects of disturbance regime (i.e., scouring floods), light, and temperature on fish production were not strongly mediated by bottom‐up controls. Estimates of food web efficiency under ambient disturbance and resource regimes suggest that a decoupling of energy flow from primary producers to upper trophic levels may prevail in hydrologically dynamic desert stream ecosystems.

 

Environmental regimes, which encompass decadal‐scale or longer variation in climate and disturbance, shape communities by selecting for adaptive life histories, behaviors, and morphologies. In turn, at ecological timescales, extreme events may cause short‐term changes in composition and structure via mortality and recolonization of the species pool. Here, we illustrate how short‐term variation in desert stream fish communities following floods and droughts depends on the context of the long‐term flow regime through ecological filtering of life history strategies. Using quarterly measures of fish populations in streams spanning a 10‐fold gradient in flow variation in Arizona, USA, we quantified temporal change in community composition and life history strategies. In streams with highly variable flow regimes, fish communities were less diverse, fluctuation in species richness was the principle mechanism of temporal change in diversity, and communities were dominated by opportunistic life history strategies. Conversely, relatively stable flow regimes resulted in more diverse communities with greater species replacement and dominance of periodic and equilibrium strategies. Importantly, the effects of anomalous high‐ and low‐flow events depended on flow regime. Under more stable flow regimes, fish diversity was lower following large floods than after seasons without floods, whereas diversity was independent of high‐flow events in streams with flashier flow regimes. Likewise, community life history composition was more dependent on antecedent anomalous events in stable compared to more temporally variable regimes. These findings indicate that extreme events may be a second‐level filter on community composition, with effects contingent on the long‐term properties of the disturbance regime (e.g., overall degree of variation) in which extremes take place. Ongoing changes to global environmental regimes will likely drive new patterns of community response to extreme events.

 

Experimental and ambient warming of Arctic tundra results in emissions of greenhouse gases to the atmosphere, contributing to a positive feedback to climate warming. Estimates of gas emissions from lakes and terrestrial tundra confirm the significance of aquatic fluxes in greenhouse gas budgets, whereas few estimates describe emissions from fluvial networks. We measured dissolved gas concentrations and estimated emissions of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) from water tracks, vegetated depressions that hydrologically connect hillslope soils to lakes and streams. Concentrations of trace gases generally increased as ground thaw deepened through the growing season, indicating active production of greenhouse gases in thawed soils. Wet antecedent conditions were correlated with a decline in CO₂and CH₄concentrations. Dissolved N₂O in excess of atmospheric equilibrium occurred in drier water tracks, but on average water tracks took up N₂O from the atmosphere at low rates. Estimated CO₂emission rates for water tracks were among the highest observed for Arctic aquatic ecosystems, whereas CH₄emissions were of similar magnitude to streams. Despite occupying less than 1% of total catchment area, surface waters within water tracks were an estimated source of up to 53–85% of total CH₄emissions from their catchments and offset the terrestrial C sink by 5–9% during the growing season. Water tracks are abundant features of tundra landscapes that contain warmer soils and incur deeper thaw than adjacent terrestrial ecosystems and as such might contribute to ongoing and accelerating release of greenhouse gases from permafrost soils to the atmosphere.