Abstract
Quantifying nutrient attenuation at watershed scales requires long-term water chemistry data, water discharge, and detailed nutrient input chronicles. Consequently, nutrient attenuation estimates are largely limited to long-term research areas or modeling studies, constraining understanding of the ecological characteristics controlling nutrient attenuation and complicating efforts to protect or restore water quality in developed and developing regions. Here, we combined long-term data and a broad suite of biogeochemical parameters from 49 watersheds in northwestern France to test how well instantaneous measurements can predict nitrogen (N) and phosphorus (P) attenuation at watershed scales. We evaluated 13 biogeochemical and 12 hydrological proxies of hydrological flowpaths, residence time, and biogeochemical transformation. Across the 49 watersheds, nutrient attenuation ranged from 88 to −2% for N and 99-96% for P. The strongest biogeochemical proxies of N attenuation were NO − 3 isotopes, rare earth elements (REEs), radon, and turbidity, together explaining 75% of observed variation. For P attenuation, REEs, NO − 3 isotopes, molecular weight of dissolved organic matter, and radon were the strongest proxies, but only explained 27% of observed variation. However, a single hydrological parameter-annual runoff-explained 91% of N attenuation and the relative abundance of schist bedrock explained 56% of P attenuation. We discuss how runoff both controls and reflects watershed hydrology, biogeochemistry, and nutrient attenuation. For example, runoff was correlated with long-term decreases in nutrient concentration, demonstrating how leakier watersheds recover more quickly from nutrient saturation. Given the immense fertilization capacity of modern society, we propose that eutrophication can only be solved by reducing nutrient inputs, though hydrochemical proxies can provide valuable information on where to carry out essential food production activities.
Highlights
Since the Industrial Revolution, humans have more than doubled reactive nitrogen (N) inputs (Gruber and Galloway, 2008) and quadrupled phosphorus (P) inputs (Elser and Bennett, 2011) into the Earth’s ecosystems (Seitzinger et al, 2010; Foley et al, 2011; Abbott et al, 2018a)
The three samplings captured distinct hydrological conditions, with low but variable discharge among sites in the fall sampling (November of 2015), highest discharge in the spring (March of 2016), and low and consistent discharge among sites in the summer (June of 2018; Figure S1)
We observed a negative relationship between dissolved organic carbon (DOC) and NO−3 and a positive relationship between DOC and Molybdate-Reactive Phosphorus (MRP), which varied somewhat seasonally (Figure 2b, Figure S1)
Summary
Since the Industrial Revolution, humans have more than doubled reactive nitrogen (N) inputs (Gruber and Galloway, 2008) and quadrupled phosphorus (P) inputs (Elser and Bennett, 2011) into the Earth’s ecosystems (Seitzinger et al, 2010; Foley et al, 2011; Abbott et al, 2018a). Watersheds with similar nutrient inputs often have completely different nutrient export regimes, for headwater watersheds that make up most of the terrestrial-aquatic interface (Bishop et al, 2008; Abbott et al, 2018b; Helton et al, 2018; Wollheim et al, 2018) This variability in nutrient attenuation capacity is likely associated with differences in surface and subsurface characteristics, including differences in water storage capacity and residence times, abundance and activity of biotic nutrient sinks (e.g., plant or microbial assimilation, dissimilatory microbial metabolism), and abiotic factors (e.g., high sorption capacity in soils, mineral precipitation, presence of chemical reducers in bedrock) (Hansen et al, 2002; Aquilina et al, 2012, 2018; Thomas and Abbott, 2018; Kolbe et al, 2019). All these factors are influenced by changes in hydrological connectivity, uneven distribution of reactants and organisms due to the coevolution of surface and subsurface characteristics, and the stochastic nature of human and natural disturbance (Hansen et al, 2000; Thomas et al, 2015; Covino, 2017; Moatar et al, 2017)
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