Dissolved organic carbon (DOC) export from watersheds by streams is an important and changing component of the global carbon cycle. We examined controls on DOC export by quantifying changes in the DOC concentration-discharge relationship between 1992 and 2022 for nine forested headwater catchments at the Hubbard Brook Experimental Forest (northeastern United States), which historically received high atmospheric acid deposition. We observed an increase in the intercept of the log-log concentration-discharge relationship and a weaker increase in the slope of that relationship, along with seasonal and watershed-level differences. The intercept, which indicates the average stream DOC concentration at a given discharge, was inversely related to ionic strength of the soil solution as predicted by electrolyte solubility theory. This relationship varied between the two watersheds where soil solution chemistry data were available in response to soil pH. The intercept was not strongly related to annual precipitation or air temperature. DOC export ranged from 13 to 156 kg C ha -1 y -1 among study watersheds and years, and was correlated with annual precipitation and discharge. Historical data suggest that DOC export has increased over the past 50 years, likely due both to increases in precipitation and runoff and to increases in the intercept and slope of the concentration-discharge relationship. Our results suggest the potential for long-term legacy effects of acidification on DOC solubility and stream DOC concentrations in acid-impacted watersheds, despite reductions in acid deposition, as mineral weathering slowly replenishes the ionic strength and buffering capacity of soil solutions.

Historically, recreational fisheries have been managed through a single-species framework where fish species are considered in isolation. This single-species framework can lead to unintended consequences for fisheries, potentially resulting in fishery collapses or regime shifts. A common factor leading to regime shifts and the prevention of population recoveries are interspecific interactions between the collapsed species and their competitors. Increasingly, ecosystem-based management has been advocated in recreational fisheries where users have a diverse set of goals. Although, in practice, ecosystem-based management can be difficult in systems subject to non-linear dynamics. We used a modeled recreational fishery to describe how interactions between two harvested species could drive shifts in the stable state of the system and sought to understand how interactions could be leveraged for efficient management. Our experiments explored: 1) the effectiveness of single-species management actions at maintaining the desired stable state as compared to multi-species management; 2) the diversity of decision making paths that could lead to positive outcomes when leveraging certain interspecific interactions; and 3) how interspecific interactions could be leveraged to maintain a system in a safe-operating-space despite stable state drivers outside of a manager’s control. Our model demonstrated how interspecific interactions within a system could lead to non-linear outcomes, and when these interactions were unaccounted for, resulted in regime shifts. Accounting for interspecific interactions allowed decision makers to meet their goals through a diverse and cost-effective combination of direct (i.e., managing the focal species through stocking and harvest limitation) and indirect (i.e., managing the competitor) approaches.

Experimentation in natural science is commonplace, and the reasons and methods for undertaking it are well understood. The use of experimental methods has risen remarkably over the last 25 years in the field of economics and has had a tremendous impact on the way economic research is carried out. This raises the question; how can economic experiments advance our understanding of recreational fishing. Here, we argue that many opportunities and benefits exist for the application. There have already been several successful experiments (in the lab or the field) to understand the drivers of recreational fisher behaviour. Despite these efforts, there remains a raft of opportunities to use experiments to help understand cause and effect in terms of recreational fisher behaviour. In this chapter, we provide a taxonomy of economic experiments and highlight their application to recreational fisheries. We sketch how controlled and uncontrolled interventions in recreational fisheries are learning opportunities to better understand fisher behaviour and social-ecological dynamics. Further, we highlight practical aspects of experimentation that will help its implementation and suggest opportunities for future experiments in recreational fisheries.

The browning of freshwater ecosystems is increasingly evident in temperate and northern regions, with widespread ramifications for lake physics, chemistry, and biology. Contrasting results on how freshwater browning may impact fish have been reported, but there has been no comprehensive examination of how browning may cause cascading effects on individual- to population- to community-level traits of freshwater fishes. We addressed this knowledge gap by summarizing the existing literature and conducting a series of original analyses to: (i) explore the effects of a brown water gradient on populations of eight economically important species of fish across 871 lakes; and (ii) examine how a brown water gradient may influence community trait compositions across 303 lakes. From our literature synthesis, we found that fish growth is often negatively associated with browner waters, despite browning generally showing no effect on fish foraging. We also demonstrated that browner waters had greater abundances of northern pike (Esox lucius) and walleye (Sander vitreus), but lower numbers of lake trout (Salvelinus namaycush), yellow perch (Perca flavescens), largemouth bass (Micropterus salmoides), smallmouth bass (M. dolomieu), and lake whitefish (Coregonus clupeaformis). Moreover, we showed that fish communities were significantly more likely to contain species with larger eyes in browner lakes. Lastly, we examined relationships between various metrics of browning (i.e. dissolved organic carbon, Secchi transparency, water colour) and present a framework for how the effects of freshwater browning on fish may scale from individuals to populations to communities.

The oxidation of organic matter from fuel combustion or vegetation emissions into organic acids is a major source of dissolved organic carbon (DOC) in precipitation. Long-term measurements of DOC in precipitation are rare, but the existing records mostly show decreases due to reduction in fuel combustion. Here, we show a recent, sudden increase in precipitation DOC concentration in a 27-year record from the Hubbard Brook Experimental Forest (HBEF) in northern New Hampshire, USA Starting in 2010, where mean annual DOC concentration increased from about 80 to 130 mu mol L-1 in 2022. No other solutes in precipitation showed a similar sudden change. The weekly DOC concentration was not clearly related to the 72-hr air mass trajectory characteristics or changes in trajectories. We assessed the feasibility of multiple possible causes for the DOC increase, including an increase in biogenic volatile organic compound (BVOC) emissions from the forest or from forest fires, changes in oxidation processes in the troposphere, and changes in gas-phase solubility due to increasing pH in precipitation. Further study of sudden changes in BVOC emissions in the region, possible causes, and air quality effects are warranted.

Headwater streams draining northern hardwood forests are vulnerable to environmental pressures, including climate change, atmospheric deposition, land-use changes, and recreational impacts. These streams play a crucial role in the forest ecosystem by influencing nutrient dynamics, water quality, and serving as refugia for aquatic species. This study focuses on epiphytic diatom communities that colonize bryophytes within Hubbard Brook Experimental Forest (HBEF), where extensive ecological research exists but with limited data on diatom community composition. Bryophytes provide stable, nutrient-rich microhabitats that support diatom growth, offering refugia from flood scour and accumulating stream sediments, making them useful for studying diatom dynamics. We compared diatom communities colonizing artificial bryophytes across seven streams weir ponds in HBEF in New Hampshire and explored the influence of various environmental factors on species richness. Distinct diatom communities were observed, along with increased species richness under higher light exposure. Further analysis reveals significant variability in diatom communities driven by watershed-specific environmental conditions and temporal changes. A total of 86 diatom taxa spanning 43 genera were identified, with species presence significantly associated with environmental variables such as light intensity (lux), dissolved organic carbon, pH, and total dissolved nitrogen. These findings reinforce the value of Diatoms are sensitive environmental indicators in this study, provide useful insights into ecosystem health and resilience of streams flowing through northern hardwood forests in the face of environmental stressors.

Freshwater ecosystems are affected by fluctuations in terrestrially derived dissolved organic carbon (DOC). Increased DOC runoff from watersheds can contribute to the “browning” of freshwater systems, altering water properties and disrupting food web dynamics, with potential impacts on fish. This study examined how carbon sources and the trophic ecology of temperate freshwater fishes varied with DOC concentrations. We used stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) to trace carbon sources in organisms at low trophic levels and to assess the trophic ecology of six fish species from 70 lakes in Sweden, Germany, Canada, and the United States. We found that both pelagic and benthic δ13C decreased as DOC increased, suggesting higher reliance on terrestrial carbon by lower trophic level organisms. Fish responses to elevated DOC were limited to certain species. Mid-trophic level European perch (Perca fluviatilis) increased pelagic diet proportions, while top predators like northern pike (Esox Lucius) and walleye (Sander vitreus) showed nonlinear shifts. Except for walleye, no effect of DOC was detected on fish trophic position. Our findings suggest that DOC alters energy use and diet but not trophic position in some freshwater fish.

The pace and trajectory of ecosystem development are governed by the availability and cycling of limiting nutrients, and anthropogenic disturbances such as acid rain and deforestation alter these trajectories by removing substantial quantities of nutrients via titration or harvest. Here, we use six decades of continuous chemical and hydrologic data from three adjacent headwater catchments in the Hubbard Brook Experimental Forest, New Hampshire-one deforested (W5), one CaSiO3-enriched (W1), and one reference (W6)-to quantify long-term nutrient and mineral fluxes. Acid deposition since 1900 drove pronounced depletion and export of base cations, particularly calcium, across all watersheds. Experimental deforestation of W5 intensified loss of biomass and nutrient cations and triggered sustained increases in streamwater pH, Ca2+, and SiO2 exports over nearly four decades, greatly exceeding the effects of direct CaSiO3 enrichment in both duration and magnitude. We detect no long-term changes in water yield or water flow paths in the experimental watersheds, and we attribute this multidecadal increase in weathering rates following deforestation to biological responses to severe nutrient limitation. Our evidence suggests that in the regrowing forest, plants are investing photosynthate into belowground processes that amplify mineral weathering to access phosphorus and micronutrients, consequently elevating the export of less limiting elements present in silicate parent material. Throughout decades of forest regrowth, enhanced biotic weathering has continued to deplete the acid buffering capacity of the terrestrial ecosystem while the export of weathering products has elevated the pH of the receiving stream.

Catch-and-release (C&R) angling is often used to maintain high catch rates but fish vulnerability to capture may decrease following hooking, thereby decreasing angler catch per unit effort (CPUE) (hyperdepletion). To determine if fish post-capture response affected recapture probability and population-level CPUE, individual capture histories of Largemouth Bass in two lakes were compared before and after doubling angling effort in a Before-After Control-Impact (BACI) analysis. Previous capture and day-of-season both affected recapture probability. Counteracting effects of previous capture and reduced late-season catch rates caused no hyperdepletion of angler CPUE. Our results highlight the complexity of fish behavioral responses to angling and suggest that hyperdepletion of angling catch rates may not be an issue in C&R fisheries.

Understanding controls on primary productivity is essential for describing ecosystems and their responses to environmental change. In lakes, pelagic gross primary productivity (GPP) is strongly controlled by inputs of nutrients and dissolved organic matter. Although past studies have developed process models of this nutrient-color paradigm (NCP), broad empirical tests of these models are scarce. We used data from 58 globally distributed, mostly temperate lakes to test such a model and improve understanding and prediction of the controls on lake primary production. The model includes three state variables-dissolved phosphorus, terrestrial dissolved organic carbon (DOC), and phytoplankton biomass-and generates realistic predictions for equilibrium rates of pelagic GPP. We calibrated our model using a Bayesian data assimilation technique on a subset of lakes where DOC and total phosphorus (TP) loads were known. We then asked how well the calibrated model performed with a larger set of lakes. Revised parameter estimates from the updated model aligned well with existing literature values. Observed GPP varied nonlinearly with both inflow DOC and TP concentrations in a manner consistent with increasing light limitation as DOC inputs increased and decreasing nutrient limitation as TP inputs increased. Furthermore, across these diverse lake ecosystems, model predictions of GPP were highly correlated with observed values derived from high-frequency sensor data. The GPP predictions using the updated parameters improved upon previous estimates, expanding the utility of a process model with simplified assumptions for water column mixing. Our analysis provides a model structure that may be broadly useful for understanding current and future patterns in lake primary production.

Stream bryophytes (mosses and liverworts) are widely recognized as important macroinvertebrate habitats, but their overall role in the stream ecosystem, particularly in nutrient cycling, remains understudied. Hubbard Brook Experimental Forest in New Hampshire, USA, contains some of the most extensively researched streams in the world, yet few studies mention their bryophytes. Perhaps this is because early estimates place bryophyte coverage in these streams at an insignificant 2%. However, data from 2019 show that contemporary coverage ranges from 4 to 40% among streams. To investigate how stream bryophyte cover may be changing over time and influencing stream nutrient stocks, we conducted field surveys, measured the mass of organic and inorganic bryophyte contents, and quantified nutrient uptake with bottle incubations of bryophyte mats. This study marks a novel attempt to map stream bryophyte coverage with estimates of C, P, and N stocks and fluxes. From our 2022 field surveys, we found that median bryophyte coverage varied across streams in the same catchment (0-41.4%) and shifted from just 3 y prior. We estimate that these bryophyte mats stored between 14 and 414 g of organic matter per m2 of stream in the form of live biomass and captured particulates. Within 12 h of light incubation, 35 out of 36 bryophyte clump samples sorbed peak historical water-column concentrations of PO43-, as measured in the Hubbard Brook stream chemistry record. In Bear Brook, our scaled estimate of bryophyte mat NO3- uptake (2.3 g N/y) constitutes a substantial portion of previously estimated whole-stream NO3- uptake (12 g N/y). Cumulatively, our data demonstrate that bryophytes and their associated mineral substrates and biota-known as the bryosphere-are crucial in facilitating headwater stream nutrient cycling. These bryospheres may contribute significantly to interannual variability in stream nutrient concentrations within nutrient-poor streams, especially in climate-sensitive regions.

Widespread and increasing use of road deicing salt is a major driver of increasing lake chloride concentrations, which can negatively impact aquatic organisms and ecosystems. We used a simple model to explore the controls on road salt concentrations and predict equilibrium concentrations in lakes across the contiguous United States. The model suggests that equilibrium salt concentration depends on three quantities: salt application rate, road density, and runoff (precipitation minus evapotranspiration). High application combined with high road density leads to high equilibrium salt concentrations regardless of runoff. Yet if application can be held at current rates or reduced, concentrations in many lakes situated in lightly to moderately urbanized watersheds should equilibrate at levels below currently recommended thresholds. In particular, our model predicts that, given 2010–2015 road salt application rates, equilibrium chloride concentrations in the contiguous United States will exceed the current regulatory chronic exposure threshold of 230 mg L −1 in over 2000 lakes; will exceed 120 mg L −1 in over 9000 lakes; and will be below 120 mg L −1 in hundreds of thousands of lakes. Our analysis helps to contextualize current trends in road salt pollution of lakes, and suggests that stabilization of equilibrium chloride concentrations below thresholds designed to protect aquatic organisms should be an achievable goal.

Synthesis research in ecology and environmental science improves understanding, advances theory, identifies research priorities, and supports management strategies by linking data, ideas, and tools. Accelerating environmental challenges increases the need to focus synthesis science on the most pressing questions. To leverage input from the broader research community, we convened a virtual workshop with participants from many countries and disciplines to examine how and where synthesis can address key questions and themes in ecology and environmental science in the coming decade. Seven priority research topics emerged: (1) diversity, equity, inclusion, and justice (DEIJ), (2) human and natural systems, (3) actionable and use-inspired science, (4) scale, (5) generality, (6) complexity and resilience, and (7) predictability. Additionally, two issues regarding the general practice of synthesis emerged: the need for increased participant diversity and inclusive research practices; and increased and improved data flow, access, and skill-building. These topics and practices provide a strategic vision for future synthesis in ecology and environmental science.

Adapting to social and environmental change requires learning and governance that span ecological levels, political jurisdictions, and management challenges. Governance of these challenges is often comprised of public and private sector actors with overlapping jurisdictions that work together—termed polycentric governance. Polycentric governance systems have been found to improve adaptability through learning. In this paper, we compare how local organizations perceive a governance systems’ function and structure to help them learn and adapt to change. In our interviews with organization leaders in three north-central US states, we used expert elicitation to compare the degree to which the organizations’ partners help them experiment and learn to adapt to challenges. The challenges most frequently identified included social challenges like sharing knowledge and funding as well as ecological issues related to the resource. The associated polycentric governance systems’ structures varied by state. Independence and jurisdictional overlap—measures of polycentricity—differed by partner type, while consideration of partners’ best practices was similar for all partner types. Most partners were said to provide helpful information and respond to queries facilitating learning, but government partners were not always encouraging innovation or flexible implying less space for experimentation. We found that in each of the three states there is a mixture of actors at multiple scales partnering with the lake organizations at different frequencies and modes of interaction. We conclude that polycentric governance is beneficial for learning and experimentation, and that different structures may be beneficial to adapting within different contexts or problems definitions. The challenge for these systems is controlling areas of risk while providing flexibility to experiment and adapt to changing conditions.

The movement of water to and through aquatic ecosystems plays a major role in controlling rates and extents of biogeochemical transformations in those ecosystems. In this chapter, we describe the role of hydrology in the delivery of carbon and nutrients that fuels ecosystem processes such as lake metabolism (ecosystem respiration and primary production). We discuss how residence time ultimately controls the rates of biogeochemical processes within and across lake ecosystems. We draw on a legacy of theoretical and empirical research showing the relationship between hydrology, residence time, and lake ecosystem processes. Finally, we conclude by identifying important next steps for future work at the intersection of hydrology, ecosystem ecology, and limnology.

Soil is the largest terrestrial carbon (C) reservoir and a large potential source or sink of atmospheric CO ₂ . Soil C models have usually focused on refining representations of microbe‐mediated C turnover, whereas lateral hydrologic C fluxes have largely been ignored at regional and global scales. Here, we provide large‐scale estimates of hydrologic export of soil organic carbon (SOC) and its effects on bulk soil C turnover rates. Hydrologic export of SOC ranged from nearly 0 to 12 g C m −2 yr −1 amongst catchments across the conterminous United States, and total export across this region was 14 (95% CI 4‐41) Tg C/yr. The proportion of soil C turnover attributed to hydrologic export ranged from <1% to 20%, and averaged 0.97% (weighted by catchment area; 95% CI 0.3%–2.6%), with the lowest values in arid catchments. Ignoring hydrologic export in C cycle models might lead to overestimation of SOC stocks by 0.3–2.6 Pg C for the conterminous United States. High uncertainty in hydrologic C export fluxes and potentially substantial effects on soil C turnover illustrate the need for research aimed at improving our mechanistic understanding of the processes regulating hydrologic C export.