Towards a unified model of stream water quality generation: Elucidating the role of hotspots, landscape heterogeneity and scale in a nested stream network

Abstract:

Summary

The objective of this paper, which is the first of two companion papers from a nested boreal catchment, is to present a conceptual model for how scale-dependent processes regulate stream water quality across a heterogeneous landscape. To do this we summarize a decade of research from the Krycklan Catchment Study and highlight a few contrasting chemical elements that illustrate the role of hot spots, landscape structure, and changes in drainage basin size as determinants of stream water biogeochemistry. This long-term, large-scale and multi-disciplinary research project provides a framework for understanding how diverse elements are transformed, mobilized, and transported through the terrestrial environment to adjacent headwater streams and downstream to larger rivers. These results show how the heterogeneous nature of stream water biogeochemistry can be used to better understand how different catchment types will respond to changes in land-use, long range transport of air pollutants, and climate.

Background

Streams and rivers make up an infinitesimal fraction of the land surface but are of fundamental importance for transporting fresh water, carbon, nutrients, and toxicants from terrestrial ecosystems to lakes and oceans. In the boreal region, stream networks are embedded in a patchy mosaic of wetlands that are interspersed throughout the forest landscape. This mosaic results in spatially and temporally complex and dynamic water chemistry patterns that depend on stream size, catchment characteristics, and flow. Designing modeling tools that operate effectively at different scales is therefore a challenging task. These tools must include scale-dependent aspects but also bridge temporal dynamics, different landscape elements, and potential changes introduced by human perturbation.

Uncertainties regarding the controls of surface water quality and their susceptibility to human perturbation are related to the interaction between hydrological pathways and biogeochemical processes. While the role of each factor is well established and of fundamental importance, our understanding of how these factors interact to regulate water quality across scales remains rather undeveloped. This is problematic given that pressures on water resources are increasing at an unprecedented rate as a result of intensified land-use, atmospheric deposition of pollutants, and climate change.

While hydrological pathways are controlled primarily by physical processes determined by soil conditions, topography, and climate; biogeochemical transformations are more conditioned by a combination of physical, chemical and biological factors encountered en route as water and solutes travel from precipitation inputs to the stream. In the boreal landscape, a fundamental aspect of this biogeochemical regulation is the presence of organic soils, which provide interfaces with the atmosphere, mineral soils, and surface waters. These transition zones include organic rich humus layers in forest soils, riparian soils lining streams and lakes, and wetlands composed of large peat deposits in valley bottoms. Although the role of natural transition zones have been highlighted in many previous studies, the influence of these organic soil interfaces on stream biogeochemistry have previously not been adequately appreciated for the large suite of compounds that make up the soup of surface water quality.

Here we show how combining hydrological and biogeochemical process based research across large spatial and temporal scales can help address the question of how diverse chemical compounds are regulated in stream networks. While the conceptual model presented here has been developed primarily to elucidate the regulation of natural and anthropogenic compounds it can also provide a basis for improving our ability to infer how these systems can be affected in the future by intensified land-use and/or changing climate.

Study site and data availability

This presentation provides a synthesis of the hydrology, biogeochemistry, soil science, and ecology research conducted over the past 30 years in the Krycklan field research infrastructure that to date has resulted in close to 500 peer-reviewed publications. The extension of the research program in 2002 from the 50 ha Krycklan sub-catchment called Svartberget to the present 6790 ha research catchment has provided unique opportunities to study the role of scaling, landscape organization, and climate as drivers of water quality (Laudon et al. 2013). The Krycklan Catchment Study presently consists of 17 continuously monitored streams, 500 soil water lysimeters, 25 deep groundwater wells, a 150 m ICOS tower (Integrated Carbon Observatory System; http://www.icos-sweden.se/) for carbon and water exchange measurements, as well as a long-term climate monitoring program. From this network an open access database (www.slu.se/Krycklan) has been created with information from over ten thousand stream and soil/groundwater samples, and now includes over 10 million unique data points available for research on water quality, soil water, and groundwater.

Results and discussion

One of the more novel findings from the Krycklan catchment study has been that stream biogeochemistry in the boreal landscape can be organized along two major environmental gradients: (1) proportion of wetlands, and (2) drainage size. These findings are the basis of this synthesis (Fig. 1). They can be conceptualized as the interplay between organic soil interfaces that regulate many natural and anthropogenically-derived solutes, and water transit times that increase with catchment size. The effects of these gradients combine to influence patterns of stream pH and hence the speciation of many metals.

A primary focus of the long-term research in the Krycklan catchment has been on dissolved organic carbon (DOC), which resulted in a conceptual landscape model presented by Laudon et al. (2011). This model demonstrated the strong mechanistic linkage between hydrological pathways and catchment characteristics as regulators of stream chemistry. Based on this concept, Lidman et al. (2014) improved its applicability for several other solutes, ranging from base cations to trace metals originating from mineral weathering, by incorporating element specific affinity to soil organic matter (SOM). That development demonstrated that most naturally derived metals (derived from in-situ mineral weathering) are negatively correlated to the proportion of wetlands in the catchment, and that the slope of the relationship was steeper for elements with higher affinity to SOM. The main mechanism underlying these watershed processes is that elements with higher affinity to SOM accumulate to a much greater degree within the mineral soil/peat interface (Fig. 1B; Lidman et al. 2012).

While elements originating from mineral weathering are negatively related to wetlands, anthropogenically derived elements demonstrate the opposite pattern. Lead (Klaminder et al. 2008), mercury (Lee et al. 1995), and persistent organic pollutants (Bergknut et al. 2010) that are all dominated by atmospheric deposition are in fact many times higher in streams draining wetland dominated catchments. Again this is related to SOM interfaces, but in this case it is at the humus layer where the high accumulations of both lead (Fig. 1a; Klaminder et al. 2008) and persistent organic pollutants (Bergknut et al. 2011) occur. In contrast to the weathering products described above, elements deposited on wetlands are directly connected to the stream, and hence are exported at a much higher rate, because of rapid water routing both as overland flow and through preferential pathways.

Whereas the forest/wetland gradient can explain most of the spatial variability among elements with high affinity to SOM, large downstream biogeochemical changes are also observed in this drainage network. The annual volumetric average pH of stream water increases by more than 1.5 pH units from the headwater streams (Although much remains to be explained in order to gain a full mechanistic understanding of the biogeochemical regulation of natural streams, especially for elements sensitive to redox conditions, nutrients that undergo rapid uptake and metals that experience kinetically limited transformation, this conceptual model is a first step toward a unified model of stream water biogeochemical regulation. The insight from this work has been that it is possible to make general predictions of water quality, which are valid for a wide range of substances with contrasting properties, based on a sound process-based understanding of both hydrology and biogeochemistry. The basis for this work can of course be improved with a more mechanistic perspective of how landscape scale processes regulate stream biogeochemistry (See the submitted companion paper by Tiwari et al. to this session) but this approach does provide a simple first attempt to reach a unified model of stream water quality generation in a nested boreal stream network.References

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