A spatially explicit model of iron loading to lakes

Abstract

Terrestrial and aquatic ecosystems are intimately linked by the export of elements from watersheds. Although export is influenced by land cover within watersheds, few models evaluate how the spatial configuration of land cover influences loading. In this study we examined spatial variation of land cover at a 10 ϫ 10 m resolution by developing a mass balance, maximum likelihood model of lake iron (Fe) concentrations in 93 watersheds. The model estimated lake iron concentrations based on loading, within-lake processes and losses. Two models were developed. One considered loading from eight land cover types, whereas the second model included the distance of each grid cell to account for Fe losses along flow paths to the lake. In-lake production and losses were accounted for as a function of lake area, water color, and discharge. If we treated watersheds as homogeneous source areas, export was estimated as 450 mg Fe m Ϫ2 yr Ϫ1 ; however, in spatial models export varied from negligible to 5,400 mg Fe m Ϫ2 yr Ϫ1 based on differential loadings from eight cover types. Accounting for losses of Fe based on distance from the lake did not improve the model. Although areal export of Fe was greater from wetlands, upland forests dominate the landscape and thus accounted for on average 75% of the total Fe load. Fe losses from lakes were primarily regulated by discharge; however, water color and lake depth were also important. Overall, the analysis revealed that lake Fe concentrations are related to land cover based on strong differential Fe loadings.

Nitrogen deposition and lake nitrogen concentrations: a regional analysis of terrestrial controls and aquatic linkages

“…As a result, areas of the same cover type high in the watershed contribute the same amount to loading as areas low in the watershed. This result is consistent with our previous studies of DOC and iron (Canham et al 2004, Maranger et al 2006 where the entire watershed, not just areas nearest lakes, provided source areas for export to lakes. To explore this point further, we considered if flowpaths from upland areas that passed through wetlands would show evidence of removal (or possibly enhancement) of N loading.…”

“…As in our previous work (Canham et al 2004, Maranger et al 2006, we have assumed that k is independent of stream length. Note that since the analysis is recursive, inputs from lakes located further up a lake chain are already taken into account when calculating the discharge from the immediately upstream lake.…”

“…Our approach was based on the principles of mass-balance, in which variation in lake total nitrogen (TN) concentration is a balance between total inputs to the lake, primarily from the surrounding watershed, and net losses, primarily as a result of in-lake degradation and output in lake discharge (Canham et al 2004, Maranger et al 2006). Inputs to the lake are assumed to be independent of in-lake concentration, while losses are assumed to be proportional to in-lake concentration.…”

Widespread Increases in Iron Concentration in European and North American Freshwaters

“…Hence, the fact that peatlands and coniferous forests act as important Fe sources (Bjorkvald et al, ; Kortelainen et al, ) is related to their relatively high DOM concentrations when compared to other types of land cover (Camino‐Serrano et al, ; Kalbitz et al, ). Peatland and forest soils often yield more dissolved Fe than mineral soils, although the latter soil type contains more bound Fe (Dillon & Molot, ; Kortelainen et al, ; Maranger et al, ). The Fe transport capacity is also affected by the organic matter composition, whereby high molecular weight DOM—such as fulvic and humic acids—act as important Fe carriers due to their high affinity for Fe (Laglera & van den Berg, ; Riise, ).…”

Increasing iron concentrations in surface waters – a factor behind brownification?

“…In fact, Fe is soluble only under anoxic conditions, as FeII, or at strongly acidic pH, and it is the interaction between Fe and OM that maintain Fe in solution under conditions prevailing in these rivers out of which it would otherwise precipitate (Shapiro, 1966). Accordingly, an important pathway for Fe into surface waters is leaching from organic forest and wetland soils in complex with OM (Maranger et al, 2006). There is a spatial relationship between Fe and OM measured both as COD and TOC among the rivers.…”