Nitrogen and phosphorus loads to temperate seepage lakes associated with allochthonous dissolved organic carbon loads

Abstract

Terrestrial loads of dissolved organic matter (DOM) have increased in recent years in many north temperate lakes. While much of the focus on the “browning” phenomena has been on its consequences for carbon cycling, much less is known about how it influences nutrient loading to lakes. We characterize potential loads of nitrogen and phosphorus to seepage lakes in northern Wisconsin, USA, based on a laboratory soil leaching experiment and a model that includes landscape cover and watershed area. In these seepage lakes, nutrient concentrations are positively correlated with dissolved organic carbon concentrations (nitrogen: r = 0.68, phosphorus: r = 0.54). Using long‐term records of browning, we found that dissolved organic matter‐associated nutrient loadings may have resulted in substantial increases in nitrogen and phosphorus in seepage lakes and could account for currently observed nutrient concentrations in the lake. “Silent” nutrient loadings to brown‐water lakes may lead to future water‐quality concerns. PLAIN LANGUAGE SUMMARY: The color of many temperate lakes is changing; some lakes are becoming more darkly stained brown. The tea‐colored stain is due to dissolved organic matter from the surrounding landscape. Much of the research related to the causes and consequences of increased staining, or “brownification,” relate to its connection to the carbon cycle. However, by examining long‐term lake chemical records, analyzing the properties of the organic compounds, and modeling potential flows of the compounds, we find that carbon is not the only element that is influenced by browning. Nitrogen and phosphorus, two nutrients important to growth of organisms at the base of the food web, may also be increasing in lakes due to brownification.