Henry F. Wilson

Ecosystem and Seasonal Control of Stream Dissolved Organic Carbon Along a Gradient of Land Use

We examined the influence of watershed land use and morphology on dissolved organic carbon (DOC) concentration in 32 south-central Ontario streams having varying agricultural land-use intensities in their catchments. For streams in this region, both univariate and multivariate regression models identify the proportion of the watershed with poorly drained soils (r2 up to 0.67) as a better predictor of stream DOC concentrations than any other landscape characteristic, including the proportion of the watershed as wetland. Agricultural land use did not strongly influence DOC concentrations in our study area; however, we do show that land-use changes could significantly alter the delivery of DOC to streams in the region. We also identify how landscape–DOC relationships change over a 2-year time period, as related to season, regional climatic conditions, soil moisture, and hydrology. Our results indicate that the relationships between landscape predictors and stream DOC concentrations are temporally dynamic. Strong temporal trends are shown seasonally and in association with climate, through its control of modelled soil moisture conditions. During periods of positive and negative deviation from normal soil moisture conditions, the relationships of DOC concentrations with landscape characteristics become less predictable. We show that these dominant patterns are likely a function of varying flow paths and that anthropogenic changes that affect soil moisture conditions or flow path will in turn strongly influence DOC dynamics.

Effects of land use on water column bacterial activity and enzyme stoichiometry in stream ecosystems

Fifty streams, located in southern Ontario, Canada, were visited in September 2008 to investigate the effect of varying land use, land cover, and associated resource inputs on water column bacterial abundance (BACT), production (BP), and extracellular enzyme activity and stoichiometry. Principle components analysis was used to summarize landscape data, producing three components (PCs), which explained 79.2% of the variability in the data. The PCs grouped into the following gradients: (PC1) urban land use and continuous annual cropping to wetland-like cover, (PC2) rotational cropping to forest-like cover, and (PC3) increasing rural and agricultural land uses with increasing watershed size. These landscape gradients created imbalanced resource availability. Nutrient resources were more abundant in streams with more intensive anthropogenic land uses, but carbon availability was primarily controlled by the abundance of natural land covers (wetland and wooded areas). BACT, BP, and enzyme activities were positively related primarily to nutrient availability and/or anthropogenic land use (Stepwise R2 range: 0.33–0.73). The ratio of β-glucosidase to alkaline phosphatase activity approached a 1:1 balance with increasing anthropogenic land use, decreased wetland and forest cover, and increased total dissolved nitrogen. The ratio of leucine-aminopeptidase to alkaline phosphatase activity approached 1:1 with both increased dissolved organic carbon and nitrogen. Moreover, enzyme C:N:P ratios moved closer to 1:1:1 with faster water column bacterial turnover times. These results suggest that water column microbial communities are better able to balance resource availability with growth in streams receiving nutrient subsidies from anthropogenic sources and under these conditions when carbon resources increase.

Land use controls nutrient excretion by stream invertebrates along a gradient of agriculture

Abstract We examined the relationship between land use in the watershed and nutrient release by mayflies in streams in southcentral Ontario (Canada). Mayfly excretion rates and molar ratios of dissolved organic C (DOC), NH4+, and soluble reactive P (SRP) were measured in 6 streams flowing through watersheds with contrasting agricultural land use. Mass-specific release rates of NH4+ and SRP by mayflies and % agricultural land use (% agriculture) in the watershed were strongly positively related (r2 = 0.48, p < 0.01 and r2 = 0.58, p < 0.05, respectively). Changes in excretion rates and ratios probably were caused by lower periphyton C:N and C:P ratios associated with elevated total dissolved N and total P concentrations (r2= 0.30, p < 0.05 and r2= 0.38, p < 0.001, respectively) in streams in watersheds with high % agriculture. Mayfly mass-specific excretion rates and periphyton elemental composition were related to % agriculture, but the relatively constant body C, N, and P content in mayflies across all streams suggests strong homeostasis in these consumers. Watershed land use alters nutrient loading, which influences periphyton elemental composition and, ultimately, stoichiometry of nutrient fluxes through invertebrate consumers.

Effects of crop rotation and management system on water-extractable organic matter concentration, structure, and bioavailability in a chernozemic agricultural soil.

Water-extractable organic matter (WEOM) in soil affects contaminant mobility and toxicity, heterotrophic production, and nutrient cycling in terrestrial and aquatic ecosystems. This study focuses on the influences of land use history and agricultural management practices on the water extractability of organic matter and nutrients from soils. Water-extractable organic matter was extracted from soils under different crop rotations (an annual rotation of wheat-pea/bean-wheat-flax or a perennial-based rotation of wheat-alfalfa-alfalfa-flax) and management systems (organic or conventional) and examined for its concentration, composition, and biodegradability. The results show that crop rotations including perennial legumes increased the concentration of water-extractable organic carbon (WEOC) and water-extractable organic nitrogen (WEON) and the biodegradability of WEOC in soil but depleted the quantity of water-extractable organic phosphorus (WEOP) and water-extractable reactive phosphorus. The 30-d incubation experiments showed that bioavailable WEOC varied from 12.5% in annual systems to 22% for perennial systems. The value of bioavailable WEOC was found to positively correlate with WEON concentrations and to negatively correlate with C:N ratio and the specific ultraviolet absorbance of WEOM. No significant treatment effect was present with the conventional and organic management practices, which suggested that WEOM, as the relatively labile pool in soil organic matter, is more responsive to the change in crop rotation than to mineral fertilizer application. Our results indicated that agricultural landscapes with contrasting crop rotations are likely to differentially affect rates of microbial cycling of organic matter leached to soil waters.

Increases in humic and bioavailable dissolved organic matter in a forested New England headwater stream with increasing discharge

Understanding the processes controlling the transfer of organic matter from terrestrial to aquatic ecosystems is of fundamental importance for the aquatic sciences. Over the course of a full year, fluorescence, absorbance and bioavailability of dissolved organic matter (DOM) were characterised in Bigelow Brook, a forested headwater stream in Massachusetts, USA. Parallel factor analysis (PARAFAC) identified a four-component model to describe observed DOM fluorescence (C1–C4). Component C2 exhibited the characteristics of a more humic-like fluorophore, with a potentially more reduced redox state and increased with discharge, whereas more fulvic-like (C1) and protein-like (C3, C4) fluorophores decreased. Under both dark and light-exposed conditions, percentage bioavailable dissolved organic carbon (%BDOC) increased with discharge (R2 = 0.37 and R2 = 0.56). C2 and specific absorptivity (SUVA) were reduced following BDOC incubations, whereas C1, C3 and C4 increased. These changes to DOM characteristics with increasing discharge were observed under both baseflow and stormflow conditions, indicating that with rising watertable, loading from a large riparian or hyporheic pool of organic matter is likely occurring. Other headwater streams, where loading is controlled by hillslope processes, are likely to exhibit a similar pattern of increasing export of more humic and bioavailable DOM during hydrologic events.

Riverine Export of Aged Carbon Driven by Flow Path Depth and Residence Time

The flux of terrestrial C to rivers has increased relative to preindustrial levels, a fraction of which is aged dissolved organic C (DOC). In rivers, C is stored in sediments, exported to the ocean, or (bio)chemically processed and released as CO2. Disturbance changes land cover and hydrology, shifting potential sources and processing of DOC. To investigate the likely sources of aged DOC, we analyzed radiocarbon ages, chemical, and spectral properties of DOC and major ions from 19 rivers draining the coterminous U.S. and Arctic. DOC optics indicated that the majority is exported as aromatic, high molecular weight, modern molecules while aged DOC tended to consist of smaller, microbial degradation products. Aged DOC exports, observed regularly in arid basins and during base flow in arctic rivers, are associated with higher proportion of mineral weathering products, suggesting deeper flows paths. These patterns also indicate potential for production of microbial byproducts as DOC ages in soil and water with longer periods of time between production and transport. Thus, changes in hydrology associated with landscape alteration (e.g., tilling or shifting climates) that can result in deeper flow paths or longer residence times will likely lead to a greater proportion of aged carbon in riverine exports.

Groundwater-Driven Wetland-Stream Connectivity in the Prairie Pothole Region: Inferences Based on Electrical Conductivity Data

This study examined the potential for electrical conductivity (EC) to serve as an indicator of groundwater-driven wetland-stream connectivity in the Prairie Pothole Region. Focus was on the Broughton’s Creek Watershed (Manitoba, Canada) where thirteen wetlands and a creek were monitored in 2013–2014. A connectivity index (CI), computed by incorporating EC data in a hyperbolic solute export model, identified a potential for both shallow and deep groundwater-driven wetland-stream connectivity to occur, although shallower connections were rarer. Both raw EC and CI values were strongly correlated to wetland volume capacity, indicating the importance of storage and flow generation processes for wetland-stream connectivity potential. The proposed CI was instrumental in reaching that conclusion, making it a simple yet physically-based metric of wetland behavior that should be tested in multiple environments to confirm or infirm its validity.

Phosphorus export dynamics and hydrobiogeochemical controls across gradients of scale, topography and human impact

Concentration-discharge (c-Q) plots are routinely used as an integrated signal of watershed response to infer solute sources and travel pathways. However, the interpretation of c-Q data can be difficult unless these data are fitted using statistical models. Such models are frequently applied for geogenic solutes, but it is unclear to what extent they might aid in the investigation of nutrient export patterns, particularly for total dissolved phosphorus (TDP), which is a critical driver of downstream eutrophication problems. The goal of the present study was therefore to statistically model c-Q relations (where c is TDP concentrations) in a set of contrasting watersheds in the Northern Great Plains – ranging in size from 0.2 to 1000+ km2 – to assess the controls of landscape properties on TDP transport dynamics. Six statistical models were fitted to c-Q data, notably (i) one linear model, (ii) one model assuming that c-Q relations are driven by the mixing of end-member waters from different landscape locations (i.e., hydrograph separation), (iii) one model relying on a biogeochemical stationarity hypothesis (i.e., power law), (iv) one model hypothesizing that c-Q relations change as a function of the solute subsurface contact time (i.e., hyperbolic model), and (v) two models assuming that solute fluxes are mostly dependent on reaction rates (i.e., chemical models). Model performance ranged from mediocre (R2 0.9), but the hydrograph separation model seemed most universal. No watershed was found to exhibit chemostatic behavior but many showed signs of dilution or enrichment behavior. A tendency toward a multi-model fit and better model performance was observed for watersheds with moderate slope and higher effective drainage area. The relatively poor model performance obtained outside these conditions illustrates the likely importance of controls on TDP concentrations in the region that are independent of flow dynamics.

A global database of nitrogen and phosphorus excretion rates of aquatic animals

Animals can be important in modulating ecosystem-level nutrient cycling, although their importance varies greatly among species and ecosystems. Nutrient cycling rates of individual animals represent valuable data for testing the predictions of important frameworks such as the Metabolic Theory of Ecology (MTE) and ecological stoichiometry (ES). They also represent an important set of functional traits that may reflect both environmental and phylogenetic influences. Over the past two decades, studies of animal-mediated nutrient cycling have increased dramatically, especially in aquatic ecosystems. Here we present a global compilation of aquatic animal nutrient excretion rates. The dataset includes 10,534 observations from freshwater and marine animals of N and/or P excretion rates. These observations represent 491 species, including most aquatic phyla. Coverage varies greatly among phyla and other taxonomic levels. The dataset includes information on animal body size, ambient temperature, taxonomic affiliations, and animal body N:P. This data set was used to test predictions of MTE and ES, as described in Vanni and McIntyre (2016; Ecology DOI: 10.1002/ecy.1582).

Before the storm: Antecedent conditions as regulators of hydrologic and biogeochemical response to extreme climate events

While the influence of antecedent conditions on watershed function is widely recognized under typical hydrologic regimes, gaps remain in the context of extreme climate events (ECEs). ECEs are those events that far exceed seasonal norms of intensity, duration, or impact upon the physical environment or ecosystem. In this synthesis, we discuss the role of source availability and hydrologic connectivity on antecedent conditions and propose a conceptual framework to characterize system response to ECEs at the watershed scale. We present four case studies in detail that span a range of types of antecedent conditions and type of ECE to highlight important controls and feedbacks. Because ECEs have the potential to export large amounts of water and materials, their occurrence in sequence can disproportionately amplify the response. In fact, multiple events may not be considered extreme in isolation, but when they occur in close sequence they may lead to extreme responses in terms of both supply and transport capacity. Therefore, to advance our understanding of these complexities, we need continued development of a mechanistic understanding of how antecedent conditions set the stage for ECE response across multiple regions and climates, particularly since monitoring of these rare events is costly and difficult to obtain. Through focused monitoring of critical ecosystems during rare events we will also be able to extend and validate modeling studies. Cross-regional comparisons are also needed to define characteristics of resilient systems. These monitoring, modeling, and synthesis efforts are more critical than ever in light of changing climate regimes, intensification of human modifications of the landscape, and the disproportionate impact of ECEs in highly populated regions.