Dissolved organic matter (DOM) is a critical component of the global carbon cycle and various ecosystem functions. The complexity of its chemical structures and environmental processes requires an extensive analytical workflow for comprehensive characterization, whereas DOM implication in various ecological functions necessitates large-scale environmental investigation. Here, we measured the optical properties of chromophoric DOM (CDOM) and the molecular indices of solid-phase extractable DOM (SPE-DOM) in the vast land-to-ocean continuum of the St. Lawrence system, which encompasses diverse geological watersheds, land uses, vegetation, hydrology, water chemistry and ecosystem functions. The continental waters of Lake Ontario’s watersheds and the Saguenay Fjord are characterized by allochthonous DOM of high aromaticity, large molecular size, and low molecular lability. The molecular indices of SPE-DOM are in agreement, as indicated by the double-bond equivalent and aromaticity index. On the other hand, the DOM of larger water bodies and the marine endmember is smaller, less aromatic, more saturated, has lower O/C ratios, and contains more sulphur and phosphorus. While the results of the two approaches are consistent in fresh and marine waters, discrepancies occur in estuarine waters: SPE-DOM molecular indices varied in a conservative manner with increasing practical salinity, whereas CDOM optical properties remained quasi-constant. Such divergence warrants further investigation to calibrate better these two unique sets of techniques for the highly dynamic estuarine ecosystems. The interpretation of DOM dynamics using optical and molecular indices requires caution in integrating the results into a holistic perspective that encompasses the origins of DOM, its transport and transformation processes, and ultimately, CO2 emissions from continents to the global ocean.

Soil texture and climate are considered the major controls on soil organic carbon (SOC) storage globally, and model simulations suggest that cooler regions of the planet will be more sensitive to SOC losses caused by climate warming. To investigate this pattern, we measured SOC and geochemical properties in surface (0-10 cm) mineral soil at 198 forested sites across the Boreal Plains, Boreal Shield, and Taiga Shield Ecozones in central Canada, where mean annual air temperature (MAT) ranged from -6.0 to +0.7 degrees C. Across the five ecoregions, SOC was strongly related to soil organic matter (SOM) with SOC:SOM ratio of 0.47. Despite the substantial temperature gradient, we found that SOC was only weakly correlated with temperature, precipitation, and net primary productivity (NPP). Instead, SOC was strongly related (r > 0.7) to soil geochemical properties with SOC increasing in finer textured soils that had higher concentrations of aluminum (Al) and iron (Fe) and lower silica (Si) content. To extend the climate and soil geochemistry gradient, we expanded the analysis to include soils from temperate forests in the Mixedwood Plains Ecozone of southeastern Canada as well as published data from natural shrublands and grassland sites spanning the Southern Hemisphere and found that the strong correlations between SOC, Al + Fe, and Si persisted. These data suggest that soil texture and geochemical properties provide protection to SOC, and relationships with geochemistry must be incorporated in Earth System Models to improve spatial prediction of SOC stocks and their sensitivity to climate change.

There is growing recognition that management efforts to limit harmful algal bloom (HAB) production in lakes need to consider tributary loadings of both phosphorus (P) and nitrogen (N). This may be the case for Lake of the Woods (LoW), which experiences annual HABs, and has been historically monitored for P, but not N. Ongoing agricultural intensification within the basin, including expansion of tile-drained row crop production, is creating new sources of N that may be entering rivers to a greater extent in the winter/spring, when sampling is typically less frequent. To address this gap, we investigated seasonal P and N inputs to the Rainy River, the largest tributary that feeds the LoW, from seven tributaries that drain the Lower Rainy River basin. Total P (TP) concentrations were consistently high at all seven tributaries and exceeded water quality guidelines, and total Kjeldahl-N (TKN) levels, which include ammonium (NH4-N) but largely reflect organic N, were also high relative to reference conditions for the region. In contrast, nitrate (NO3-N) levels were generally low, especially in the growing season. Notably, NO3-N, TKN, and TP concentrations were highest in tributaries with more agricultural development. There were no clear seasonal patterns in TP or TKN, whereas NO3-N was up to 10 times higher in the winter compared with the growing season. Higher N losses from agricultural areas that were especially clear in the winter suggest that N export is sensitive to regional trends of agricultural intensification and winter warming and warrant increased scrutiny of N inputs to the basin.

The mosaic of urban, agricultural and natural covers that typifies most developed landscapes makes it challenging to identify the primary causes of stream flow perturbation in mixed landcover watersheds. This is especially true in southern Ontario, Canada, where approximately 1/3 of the Canadian population lives in urban areas surrounded by agriculture. Whilst previous studies have examined the impacts of urban or agricultural landcover on stream flow separately, they are rarely considered together. Furthermore, major expansions in tile-drained (TD) cropland in Ontario over the past several decades could affect the flow regime; however, this has never been examined. This study assessed the effect of landcover on flow regime at 19 proximal watersheds that varied in agriculture (0%-87%), natural (2%-97%) and urban landcover (2%-96%) using the Richards-Baker index (RBI), the coefficient of variation (CV) and a Baseflow index (BFI). Urbanized watersheds were consistently the most flashy (highest RBI and CV), regardless of season, whereas agricultural watersheds had moderately flashy conditions that varied between the growing (GS) and non-growing seasons (NGS). Natural watersheds were the least flashy and had the highest BFI. Watersheds dominated by TD cropland were flashier during the NGS compared with un-tiled agricultural watersheds. Furthermore, TD-streams were warmer in the NGS and cooler in the GS, such that the former could affect ice breakup. A 50-year analysis at three watersheds showed statistically significant increases in flashiness and decreases in BFI at an urbanizing watershed. In contrast, watersheds that remained agricultural or natural underwent small but significant declines in flashiness and increases in baseflow, potentially due to increases in precipitation and forest maturation. Our results suggest that continued expansions of urban and TD cropland may increase NGS flashiness. In contrast, enhanced soil moisture storage provided by TD could decrease the potential for flooding in the GS in southern Ontario. Southern Ontario watersheds have faced changing landcovers due to urban expansion and agricultural intensification, which affect hydrologic regimes. This research quantified flashiness in streams across a wide range of land covers through both a short-term and long-term analysis. Watersheds with more urban cover or tile drainage showed higher flashiness values in the short-term analysis, and urban expansion increased stream flashiness over a 50 year flow record. image

The use of wood ash as a soil amendment remains restricted in many parts of Canada. To better understand belowground biogeochemical responses to wood ash, soil solution chemistry was measured over 3 years following the application of wood ash (0, 2.5, 5.0, and 7.5 Mg·ha−1) at a hardwood stand in Ontario, after which soil microbial response was assessed using 16S rRNA gene and internal transcribed spacer sequencing. Metal concentrations in the locally sourced wood ash were below provincial regulatory limits. Significant increases in soil solution pH were observed within the forest floor in the first year of the trial, and significant increases in calcium and magnesium were also observed in later years of the trial. Concentrations of most metals in soil water either decreased or exhibited no significant change in response to wood ash. There was an increase in diversity and richness of soil prokaryotic groups in the FH horizon at the highest wood ash treatment that is most likely linked to the large increase in pH. This study indicates that wood ash has a strong ameliorative effect on soil and soil water chemistry without major changes to soil microbial communities and is a viable amendment to forest soils at dosages below 5 Mg·ha−1.

Freshwater systems in cold regions, including the Laurentian Great Lakes, are threatened by both eutrophication and salinization, due to excess nitrogen (N), phosphorus (P) and chloride (Cl-) delivered in agricultural and urban runoff. However, identifying the relative contribution of urban vs. agricultural development to water quality impairment is challenging in watersheds with mixed land cover, which typify most developed regions. In this study, a self-organizing map (SOM) analysis was used to evaluate the contributions of various forms of land cover to water quality impairment in southern Ontario, a population-dense, yet highly agricultural region in the Laurentian Great Lakes basin where urban expansion and agricultural intensification have been associated with continued water quality impairment. Watersheds were classified into eight spatial clusters, representing four categories of agriculture, one urban, one natural, and two mixed land use clusters. All four agricultural clusters had high nitrate-N concentrations, but levels were especially high in watersheds with extensive corn and soybean cultivation, where exceedances of the 3 mg L-1 water quality objective dramatically increased above a threshold of similar to 30 % watershed row crop cover. Maximum P concentrations also occurred in the most heavily tile-drained cash crop watersheds, but associations between P and land use were not as clear as for N. The most urbanized watersheds had the highest Cl- concentrations and expansions in urban area were mostly at the expense of surrounding agricultural land cover, which may drive intensification of remaining agricultural lands. Expansions in tile-drained corn and soybean area, often at the expense of mixed, lower intensity agriculture are not unique to this area and suggest that river nitrate-N levels will continue to increase in the future. The SOM approach provides a powerful means of simplifying heterogeneous land cover characteristics that can be associated with water quality patterns and identify problem areas to target management.

Characterizing streamflow and relationships with climate and watershed characteristics is an essential first step in the design of any monitoring program to assess basin response to changes in land use or climate. This is especially true for the international Lake of the Woods watershed, where recurrent algae blooms have been associated with nutrient inputs from the watershed and climate warming. Here, we present a basin-wide hydroclimatic analysis within the sparsely monitored Canadian portion of the basin. Spatial and temporal patterns in climate and runoff were assessed across the two major geo-zones: the Precambrian 'Shield zone', dominated by bedrock, forests and lakes, and the poorly drained 'Agassiz zone' where ditching and drainage for agriculture have substantially enhanced the hydrologic connectivity. While climate conditions were consistent across the watershed, Agassiz basins were flashy, highly variable, and more seasonal compared with Shield rivers, likely due to the moderating effect of lake storage in the Shield region. Temperatures increased across the basin (1910–2010), and there was more rainfall and runoff during the ice-covered months (Nov-Mar), suggesting a shift toward earlier snowmelt. Marked seasonality and large swings in flow extremes at the Agassiz rivers suggest this region is particularly sensitive to hydroclimatic change and that frequent monitoring is needed to capture important periods of nutrient export like spring runoff and storm events. In contrast, substantial storage within the Shield landscape suggests this zone is more hydrologically 'resilient' to climate extremes and that water quality and quantity measurements can be less frequent.

Atmospheric deposition is an important source of both phosphorus (P) and nitrogen (N) to lakes and their watersheds, but the two nutrients are rarely reported together. For the first time, we measured total P (TP) and N (total, ammonium, NH4-N and nitrate, NO3-N) bulk deposition to the Lake of the Woods (LoW) watershed, a large international lake that experiences frequent, lake-wide cyanobacterial blooms. Phosphorus deposition was highly variable both spatially and seasonally, with on average >75 % of annual deposition occurring in the spring and summer months, associated with local biogenic input. In contrast, winter TP deposition was relatively low and spatially invariant, and not affected by local sources. Importantly, these results suggest that P deposition may be a much larger source of input to the LoW as a result of its tortuous shoreline and abundance of forested islands (>5000), which are concentrated in the northern, Canadian waters and greatly extend the 'shoreline influence' within this lake. We suggest that a single annual TP load estimate is not appropriate for the LoW because winter deposition is likely much lower than past estimates whereas spring/summer deposition is substantially higher and could be an important, but previously unrecognized source, of bioavailable P to the nearshore waters. Further research is needed on the spatial and temporal patterns of P vs N deposition to the lake surface and their influence on primary productivity and algal species composition.& COPY; 2022 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.

Outside of the five Great Lakes, Lake of the Woods (LoW) is the largest transboundary lake shared by Canada and the United States. Over the last two decades, an international consortium of researchers has advanced our understanding of eutrophication science in this complex watershed. This introductory paper outlines eleven research articles included in this special section, within three main themes pertinent to LoW: external phosphorus sources to LoW and internal phosphorus loading, lake and watershed modeling, and new insights into cyanobacterial and harmful algal blooms (cHABs). We conclude with a roadmap to guide future transboundary water quality management in LoW, including remaining research gaps and future monitoring needs.

Lake of the Woods (LoW) is a large, transboundary lake that continues to experience harmful algal blooms despite large declines in phosphorus (P) inputs from point sources. Tributary runoff is considered the largest source of P to the lake; however, there are few monitoring data within the Canadian portion of the basin (-60% of total area), to guide management. To address this gap, we monitored five rivers of con-trasting land use within the lower Rainy River region, an acknowledged "hot spot" of P delivery in the basin. Total P (TP) concentrations were consistently high at all five rivers (volume-weighted range: 19-215 lg/l) despite differences in agriculture across sites (7-27%), suggesting 'natural' background P levels are high in this landscape. Furthermore, TP concentrations were strongly correlated with total sus-pended sediment and geogenic metals, iron (Fe) and aluminum, and TP was especially high during events (>400 lg/l) indicating erosion is an important source of P delivery. However, equally high TP and Fe dur -ing periods of slow, stagnant flow in the summer and under winter ice suggest erosion is not the only source of P. Instead, we suggest redox release of P from streambed sediment is also important. This obser-vation is significant, because internal P release within the tributaries, especially during the summer could 'seed' downstream algal blooms. The strong sensitivity of TP to both high and low flow conditions indi-cates that frequent, all-season, multi-year measurements are needed to understand the mechanisms of P delivery in this basin.Crown Copyright & COPY; 2022 Published by Elsevier B.V. on behalf of International Association for Great Lakes Research. All reserved.

Limestone is a common amendment used to counteract soil acidity and metal pollution. Understanding the legacy effects of a one-time soil limestone application and subsequent afforestation is needed to evaluate the long-term success of remediation efforts. In this study, soil and tree chemistry were measured across 15 limed sites that were treated and planted 14 to 37 years ago in Sudbury, Ontario, along with two untreated sites. Soil pH and exchangeable base cation (calcium (Ca) and magnesium (Mg)) concentrations were generally elevated especially in surface organic [FH] horizons up to 37-years post limestone treatment. High site-to-site variation however, obscured clear patterns over time and base cation mass budgets were generally unable to account for the mass of added Ca and Mg. Metal partitioning (Kd) in soil was most influenced by soil pH rather than organic matter (OM) showing that metal availability increases as liming effects fade. This study shows that the legacy effects of soil liming can persist for several decades and are most apparent in the forest floor (FH), but legacy effects are quite modest, and it is likely that a considerable amount of limestone has been lost through erosion.

Eutrophication remains the most widespread water quality impairment globally and is commonly associated with excess nitrogen (N) and phosphorus (P) inputs to surface waters from agricultural runoff. In southern Ontario, Canada, increases in nitrate (NO3-N) concentrations as well as declines in total phosphorus (TP) concentration have been observed over the past four decades at predominantly agricultural watersheds, where major expansions in row crop production at the expense of pasture and forage have occurred. This study used a space-for-time approach to test whether 'agricultural intensification', herein defined as increases in row crop area (primarily corn-soybean-winter wheat rotation) at the expense of mixed livestock and forage/pasture, could explain increases in NO3-N and declines in TP over time. We found a clear, positive relationship between the extent of row crop area within watersheds and NO3-N losses, such that tributary NO3-N concentrations and export were predicted to increase by ~0.4 mg/L and ~130 kg/km2 respectively, for every 10% expansion in row crop area. There was also a significant positive relationship between row crop area and total dissolved phosphorus (TDP) concentration, but not export, and TP was not correlated with any form of landcover. Instead, TP was strongly associated with storm events, and was more sensitive to hydrologic condition than to landcover. These results suggest that pervasive shifts toward tile-drained corn and soybean production could explain increases in tributary NO3-N levels in this region. The relationship between changes in agriculture and P is less clear, but the significant association between dissolved P and row crop area suggests that increased adoption of reduced tillage practices and tile drainage may enhance subsurface losses of P.

Southern Ontario is home to over a third of the Canadian population and is also one of the most productive agricultural areas in the country. This mosaic of a large and growing urban population and prime agricultural land creates particular challenges for soil and water resource management. While urban areas continue to expand in southern Ontario, changes in agricultural cover and practices within the headwaters are also important to consider. There have been dramatic increases in tile-drained cash crop production (principally grain, corn, and soybean) in southern Ontario over the past few decades, largely at the expense of pasture and forage land. Urban populations will continue to expand into the future, but there is considerable scope for further agricultural change in the headwaters as well. Expansions in urban land cover and intensification of agriculture affect the hydrologic response to extreme events as well as water quality and nitrate leaching in particular. It is important to consider the effects of shifts in both types of land cover on stream flow and water quality in the variable landscape and climatic conditions of the lower Great Lakes.

Soil mineral surface area is regarded as a key uncertainty in the estimation of base cation weathering rates, yet is rarely measured. Acidification studies rely heavily on pedotransfer functions (PTFs) that use widely available soil data to estimate mineral surface area. This study examined the relationship between soil properties andmineral surface area in soils (n = 25) from Kitimat, British Columbia, an area that is receiving elevated sulphur (S) deposition due to recent modernization of an aluminum (Al) smelter. Mineral surface area was measured on bulk soil samples using BET (Brunaeur, Emmett and Teller) gas-adsorption. Previously published particle size-based PTFswere a poor predictor of surface area in Kitimat soils (R-2 between 0.42 and 0.66). Instead, mineral surface areawas best predicted using a regionally-specific PTF (R-2 = 0.81), which used particle size as well as the concentration of kaolinite, the most abundant clay mineral in the region. Surface area values estimated using the regionally-specific PTF were applied to the PROFILE model to calculate weathering rates for critical load estimates. These estimates predicted that none of the sites received S deposition in exceedance of their critical load for acidity. However, as surface area is largely related to kaolinite content (a mineral that does not largely contribute toweathering rates), the applicability of using surface area functions forweathering rates is questionable. Further, the texture-based PTF developed for Kitimat did not provide accurate estimates of measured surface area for other soils in Canada, particularly at surface area values exceeding 2.5 m(2) g(-1). (C) 2019 Elsevier B.V. All rights reserved.

In many regions, chemical recovery in lakes from acidic deposition has been generally slower than expected due to a variety of factors, including continued soil acidification, climate-induced sulphate (SO4) loading to lakes and increases in organic acidity. In central Ontario, Canada, atmospheric sulphur (S) deposition decreased by approximately two-thirds between 1982 and 2015, with half of this reduction occurring between 2005 and 2015. Chemical recovery in the seven lakes was limited prior to 2005, with only small increases in pH, Gran alkalinity and charge-balance ANC (acid-neutralizing capacity). This was because lake SO4 concentrations closely followed changes in S deposition, and decreases in base cation concentration closely matched declines in SO4. However, decreases in S deposition and lake SO4 were more pronounced post-2005, and much smaller decreases in lake base cation concentrations relative to SO4 resulted in large and rapid increases in pH, alkalinity and ANC. Dissolved organic carbon concentrations in lakes increased over the study period, but had a limited effect on lake recovery. Clear chemical recovery of these lakes only occurred after 2005, coinciding with a period of dramatic declines in S deposition.

Winter is an understudied but key period for the socioecological systems of northeastern North American forests. A growing awareness of the importance of the winter season to forest ecosystems and surrounding communities has inspired several decades of research, both across the northern forest and at other mid- and high-latitude ecosystems around the globe. Despite these efforts, we lack a synthetic understanding of how winter climate change may impact hydrological and biogeochemical processes and the social and economic activities they support. Here, we take advantage of 100 years of meteorological observations across the northern forest region of the northeastern United States and eastern Canada to develop a suite of indicators that enable a cross-cutting understanding of (1) how winter temperatures and snow cover have been changing and (2) how these shifts may impact both ecosystems and surrounding human communities. We show that cold and snow covered conditions have generally decreased over the past 100 years. These trends suggest positive outcomes for tree health as related to reduced fine root mortality and nutrient loss associated with winter frost but negative outcomes as related to the northward advancement and proliferation of forest insect pests. In addition to effects on vegetation, reductions in cold temperatures and snow cover are likely to have negative impacts on the ecology of the northern forest through impacts on water, soils, and wildlife. The overall loss of coldness and snow cover may also have negative consequences for logging and forest products, vector-borne diseases, and human health, recreation, and tourism, and cultural practices, which together represent important social and economic dimensions for the northern forest region. These findings advance our understanding of how our changing winters may transform the socioecological system of a region that has been defined by the contrasting rhythm of the seasons. Our research also identifies a trajectory of change that informs our expectations for the future as the climate continues to warm.

Urbanization is generally recognized as the most widespread form of landuse/landcover change (LULC) within populated regions, including southern Ontario, and is often at the expense of surrounding agricultural land. However, changes in agricultural LULC within these peripheral regions should be considered when interpreting water quality changes in watersheds containing mixed LULC The objectives of this study were to first, quantify changes in LULC within twelve Lake Ontario tributaries between 1971 and 2010, and secondly, to determine whether these changes co-occurred with changes in total phosphorus (TP) and nitrate nitrogen (NO3-N) concentrations in streams. Water quality data were obtained from the Ontario Provincial Water Quality Monitoring Network while historical land use was reconstructed from agricultural census reports, historical land cover maps, and modern remotely sensed datasets. Urban cover increased, although percent increases in urban cover were small in the most agriculturally dominated watersheds (3-8%). The area of agriculture declined across all watersheds, yet the proportion of agricultural land dedicated to crop (corn, soybean, and wheat) production increased, including in the most urbanized watersheds (e.g. Mimico 89% urban; 2009-11). Total P concentrations in streams were highest at the urbanized watersheds, particularly in the 1970s, before declining in recent decades. In contrast, NO3-N concentrations were highest (>1.5 mg/L; 2000-10) within the most agricultural watersheds (e.g. Gages 71% agriculture) and have increased over the same period of row crop expansion. Further research is needed to determine the mechanisms behind the potential relationship between expanding row crop cover and stream NO3-N concentrations in southern Ontario. (C) 2019 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.

Stream chemistry is often used to infer catchment‐scale biogeochemical processes. However, biogeochemical cycling in the near‐stream zone or hydrologically connected areas may exert a stronger influence on stream chemistry compared with cycling processes occurring in more distal parts of the catchment, particularly in dry seasons and in dry years. In this study, we tested the hypotheses that near‐stream wetland proportion is a better predictor of seasonal (winter, spring, summer, and fall) stream chemistry compared with whole‐catchment averages and that these relationships are stronger in dryer periods with lower hydrologic connectivity. We evaluated relationships between catchment wetland proportion and 16‐year average seasonal flow‐weighted concentrations of both biogeochemically active nutrients, dissolved organic carbon (DOC), nitrate (NO3‐N), total phosphorus (TP), as well as weathering products, calcium (Ca), magnesium (Mg), at ten headwater (<200 ha) forested catchments in south‐central Ontario, Canada. Wetland proportion across the entire catchment was the best predictor of DOC and TP in all seasons and years, whereas predictions of NO3‐N concentrations improved when only the proportion of wetland within the near‐stream zone was considered. This was particularly the case during dry years and dry seasons such as summer. In contrast, Ca and Mg showed no relationship with catchment wetland proportion at any scale or in any season. In forested headwater catchments, variable hydrologic connectivity of source areas to streams alters the role of the near‐stream zone environment, particularly during dry periods. The results also suggest that extent of riparian zone control may vary under changing patterns of hydrological connectivity. Predictions of biogeochemically active nutrients, particularly NO3‐N, can be improved by including near‐stream zone catchment morphology in landscape models.

Sugar maple (Acer saccharum Marsh.) is a commercially important species throughout much of eastern North America and concerns have been raised regarding its successful regeneration owing to nutrient limitation, especially calcium (Ca) caused by acidic deposition. Timber harvesting can alter nutrient availability in soil, but the short-term effects of selection harvesting on sugar maple seedling chemistry have not been studied. In this study, sugar maple seedlings were collected for elemental analysis from unharvested areas, canopy gaps and skid trails one and two years post selection harvest in a mixed hardwood forest in central Ontario, Canada. Plant community composition and sugar maple seedling morphology were measured along with soil physical and chemical properties. While there were few measurable differences in soil physical or chemical characteristics amongst sites, understory vegetation was markedly different and Carex spp. were dominant in skid trails. In addition, sugar maple seedlings in the disturbed sites (gaps and skid trails) were morphologically different (decreased height:diameter ratio) and had lower foliar concentrations of essential plant nutrients including nitrogen (N), magnesium (Mg), potassium (K), and phosphorus (P) compared with unharvested sites. Calcium concentrations in seedlings were unaffected by harvesting, perhaps due to low soil Ca levels across all sites. In contrast to Ca, foliar concentrations of K and P in seedlings growing in gaps and skid trails were below or approached reported nutrient deficiency thresholds for mature trees, which may have potential consequences for forest regeneration that have not been previously considered.