Louisiana Department of Natural Resources

Wildfires are an increasingly important factor in boreal forest biome. The fire cycle have significant effects on the cycling of nutrients within and through an ecosystem. Past research shows that major elements, nutrients, and organic matter in stream water can be altered for decades after a wildfire. Here we examine how stream chemistry might be used to characterize biogeochemical effects of fire across the continuous permafrost taiga of the Central Siberia. By comparing the results of laboratory combustion of organic horizons, soil solution chemistry in burned and unburned watersheds, and stream chemistry in a chronosequence of watersheds with a well-described fire history, we assess possible proxies in stream water chemistry for wildfire severity and timing in this landscape. Our results suggest that the time since last wildfire can be estimated by sulfates (SO 4 2− ) and nitrates (NO 3 − ) concentrations, and that the soil active layer thickness (ALT, the soil depth at which soil thaws annually) increase by wildfires is reflected in bicarbonates (HCO 3 − ) concentrations. Phosphate (PO 4 3− ), although it is released from organic matter by combustion, is unchanged in stream water following fire. Chloride (Cl − ) produces a useful marker for hydrologic conditions at the time of sampling, allowing a relatively small number of stream water samples to be used to characterize past fire regime. Concentrations of SO 4 2− , NO 3 − and HCO 3 − in stream runoff can thus be used as a specific fingerprint of fire history in the northern taiga larch and birch forests of the Central Siberia, since an increase in their concentrations in stream runoff reflects fire occurrence in the catchment area. • Wildfires increase concentrations of major ions in pyrogenic organic layers, topsoil and streams. • Phosphorus is liberated by organic matter combustion, but retained in subsoil. • Wildfires affect stream HCO 3 − , SO 4 2− , Cl − and NO 3 − concentrations at decadal scales. • Stream bicarbonate can serve as a proxy for the active layer depth and wildfire timing • Sulfate in stream water is the most reliable indicator of time since fire disturbance in the catchments.

Abstract Rivers are increasingly fragmented and degraded, yet the European Union Nature Restoration Regulation calls for restoring at least 25,000 kilometres of free-flowing rivers by 2030. Translating this ambition into effective implementation remains challenging because restoration priorities differ across ecological, social, economic, and governance contexts. Here, we synthesize expert knowledge from 45 countries through a structured, multi-step prioritization process to identify research priorities for restoring free-flowing rivers in Europe. We identified 27 priorities and analysed how expert background and spatial context influenced their ranking. Restoration priorities differed systematically depending on whether experts emphasized ecological integrity, community engagement, economic considerations, or governance capacity, revealing clear patterns in how disciplinary and professional perspectives shape implementation pathways. This demonstrates that restoration strategies cannot be universal but must be adapted to local and regional political, institutional, and ecological conditions. Building on these findings, we propose a structured prioritisation framework that links barrier removal, connectivity restoration, governance mechanisms, and policy instruments to context-specific needs. Together, our results provide an empirically grounded and implementation-oriented roadmap to support European Union Member States in delivering ambitious river restoration targets in a context-sensitive and socially robust manner.

Abstract Estuaries are threatened by eutrophication due to anthropogenic nutrient loading. The Great Bay Estuary of New Hampshire and Maine has been designated as nitrogen impaired primarily due to a 65% loss in seagrass coverage between 1996 and 2023. This region has also experienced multiple consecutive years of low annual precipitation and more recent record precipitation years. The loss of seagrass, increased precipitation variability, and continued anthropogenic influence have biogeochemical consequences for the estuarine filter. Water quality budgets for a subregion of Great Bay Estuary were developed at annual timescales for nitrogen, orthophosphate (PO 4 3− ), dissolved organic carbon (DOC), and total suspended solids (TSS). Results show annual total nitrogen (TN) input loads, including tidal flux into Great Bay, (3,900 kg ha −1 year −1 ) are less than output loads whereas dissolved inorganic nitrogen (DIN) inputs (1,410 kg ha −1 year −1 ) exceed outputs, indicating net TN export and net DIN retention. PO 4 3− loads were nearly balanced, with a mean input and output of 257 kg ha −1 year −1 and 238 kg ha −1 year −1 , respectively. Seagrass acreage negatively correlated with DOC inputs (33,300 kg ha −1 year −1 ). High TSS output loads (227,000 kg ha −1 year −1 ) resulted in net TSS export. Despite critical seagrass loss and recent point source nitrogen load reductions, the system continues to retain bioavailable forms of nutrients, suggesting the importance of other primary producers to estuarine biogeochemical cycles. Water quality budgets for Great Bay provide useful insight into the biogeochemical capacity of an estuary during a time of habitat degradation and subsequent coastal management efforts.

Forest canopy near-infrared reflectance and mass-based canopy nitrogen concentration (canopy %N) have been shown to be positively correlated. While the mechanisms underpinning this relationship remain unresolved, the broad range of wavelengths involved points to structural properties that influence scattering and covary with %N. Despite this, efforts that have focused on commonly measured structural properties such as leaf area index (LAI) have failed to identify a causal mechanism. Here, we sought to understand how lidar-derived canopy traits related to additional properties of foliar arrangement and structural complexity modulate the effects of leaf spectra and leaf area index (LAI) on canopy reflectance. We developed a leaf layer spectra model to explore how canopy reflectance would change if complex foliage arrangements were removed, compressing the canopy into optically dense, uniform stacked layers while maintaining the same leaf area index. Model results showed that LAI-weighted leaf reflectance saturates at a leaf area index of approximately two for needleleaf species and four for broadleaf species. When upscaled to estimate plot-level canopy reflectance in the absence of structural complexity (NIRrLAI), results showed a strong positive relationship with canopy %N (r2 = 0.86), despite a negative relationship for individual leaves or “big-leaf” canopies with an LAI of one (NIRrL, r2 = 0.78). This result implies that the relationship between canopy near-infrared reflectance and canopy %N results from the integrated effects of canopy complexity acting on differences in leaf-level optical properties. We introduced an index of relative reflectance (IRr) that shows that the relative contribution of structural complexity to canopy near-infrared reflectance (NIRrC) is related to canopy %N (r2 = 0.55), with a three-fold reduction from potential canopy near-infrared reflectance observed in stands with low %N compared to a two-fold reduction in stands with high %N. These findings support the hypothesis that the correlation between canopy %N and canopy reflectance is the result of interactions between leaf traits and canopy structural complexity.

<strong class="journal-contentHeaderColor">Abstract.</strong> Flowing stream networks expand and contract in response to dynamic groundwater levels. Field studies generally associate greater flowing network length (L) with higher streamflow (Q), but this neglects potential hysteresis caused by nonequilibrium groundwater flow after rain and snowmelt. Using a new version of the Distributed Hydrology Soil Vegetation Model (DHSVM), we predict that groundwater hysteresis may decouple L from Q across large (&gt;<span>&thinsp;</span>100<span>&thinsp;</span>%) variations in Q. In a 27<span>&thinsp;</span>km² snowy volcanic watershed, seasonal anomalies in measured stream ionic concentration indicate an outsized contribution from longer subsurface flowpaths during recession, supporting our L-Q hysteresis hypothesis and refining our model calibration. The model can reproduce observed stream network elasticity (from field surveys), and the predicted network length anomaly mirrors seasonal anomalies in measured stream ionic concentration (r<span>&thinsp;</span>=<span>&thinsp;&minus;</span>0.92), suggesting that the model can capture the seasonal reconfiguration of groundwater flowpaths. A warmer climate is expected to cause a partial transition from snow to rain resulting in flashier streamflow, but our simulations predict that seasonal groundwater hysteresis would dampen storm-scale stream network elasticity, thereby significantly increasing L-Q hysteresis on daily to monthly timescales (p<span>&thinsp;</span>&lt;<span>&thinsp;</span>0.01). Conceptual models of stream networks should consider the potential effects of groundwater hysteresis in headwaters catchments, especially in a changing environment. More broadly, our investigation highlights how spatially distributed process-based hydrological modeling can sometimes reveal emergent hydrological behaviors that are not apparent from sparse field data.

ABSTRACT Woody plant encroachment and subsequent thicketization is a global phenomenon that has been demonstrated to reduce groundwater recharge, yet it is unknown whether reducing woody plant cover will restore groundwater recharge rates to pre‐encroachment levels. We investigated how savanna restoration influences soil moisture dynamics and deep drainage in the Post Oak Savanna ecoregion of Texas, USA, which overlies the recharge zone of the underlying Carrizo–Wilcox Aquifer. We compared soil moisture and deep drainage responses in water years 2023 and 2024 across the following three vegetation states: restored savanna, woodland mosaic and thicketized woodland. We found that savanna had higher profile mean volumetric water content than both woodland mosaic and thicketized woodland. Deep drainage rates were consistently higher in the savanna than in the woodland mosaic, and those in the intercanopy area were higher than those in the undercanopy areas. A water balance analysis provided independent validation of the matric potential based drainage estimates, demonstrating consistent temporal patterns and overlapping ranges. These findings indicate the effectiveness of thicketized savanna restoration in enhancing deep drainage and sustaining groundwater resources in subhumid landscapes.

Climate change is heating surface waters while deoxygenating bottom waters of lakes, compressing vertical habitat for cold-water fish. Resulting increases in the volume of water lacking dissolved oxygen (anoxia) could facilitate the formation of toxic, bioaccumulating methylmercury (MeHg). Oxythermal squeezes, where thermally stressful surface water pushes cold-water fish species against or into low-oxygen deep water, could enhance mercury bioaccumulation. Here, we test this hypothesis using high-resolution oxygen data and brook trout muscle mercury data for 16 neighboring lakes (Adirondack Park, New York, USA) collected in the fall of two consecutive years. In four anoxic lakes that underwent oxythermal squeezes in both years, muscle mercury concentrations were consistently elevated (0.40 μg/g mean (0.08 μg/g SD) length-adjusted wet weight concentration), and averaged more than double the levels in well-oxygenated lakes. Importantly, lakes with substantial anoxic zones yet no oxythermal habitat compression showed intermediate concentrations, indicating that forcing fish to reside next to oxygen-depleted water plays a key role in boosting their methylmercury exposure. Climate change is pushing many lakes toward such oxythermal habitat squeezes; hence, our findings suggest that higher fish mercury contamination is to be expected, even when external loading of mercury to aquatic ecosystems is stable or declining.