Assessing nonpoint‐source nitrogen loading and nitrogen fixation in lakes using δ¹⁵N and nutrient stoichiometry

Abstract

Runoff from human-dominated watersheds has greatly altered nitrogen (N) and phosphorus (P) cycling in lakes. Nutrients from human sources are distinct from those from undisturbed ecosystems in several ways including lower N : P ratios, which can drive ecosystems to N-limited conditions, and enriched stable N isotope ratios. In this study, we used these distinct characteristics to estimate shifts in N sources to 27 lakes across a human density gradient in western Washington. We compared an N stable isotope two-source mixing model with a mixing model that coupled N stable isotopes to N : P stoichiometry and included N fixation. We found that a two-source mixing model (human and watershed sources) did not explain observed variation in d 15 N of particulate organic matter (POM) and primary consumers (R 2 5 0.60) as well as a model that included a third N source (N fixation; R 2 5 0.72). When fixed N was facultatively added to the ecosystem below a critical N : P ratio, the more complex mixing model captured the observed patterns in POM and primary-consumer d 15 N among lakes extremely well. In lakes with P concentrations . 20 mg L 21 (N : P mass ratio , 15.3), N fixation became an increasingly important component of the N cycle, accounting for . 50% of lake N budgets. This model provides a novel way to estimate the contribution of nonpoint N sources and N fixation to lakes in watersheds subject to human nutrient inputs.

Suburbanization alters small pond ecosystems: shifts in nitrogen and food web dynamics

“…Specifically, septic systems release nitrate to the 342 environment, whereas the dominant N form in forested ponds is ammonium (Capps et al 2014 Overall, our study provides clear evidence that suburbanization alters nitrogen dynamics 353 in small ponds, and that wastewater N likely subsidizes the N pool. While this pattern has also 354 been seen in lakes and rivers (Jankowski et al 2012;Kaushal et al 2006;Vander Zanden et al 355 2005), our study is unique in that we show that anthropogenic N can have a big impact at a small 356 spatial scale, where individual septic systems act as point sources of pollution (Moore et al 357 2014;Steffy and Kilham 2004). Wastewater intrusion from septic systems could indicate the 358 potential for long-term eutrophication, and likely means that other solutes also reach suburban 359 ponds, such as pharmaceutical and personal care products (PPCPs).…”

“…Suburbanization can enhance nutrient loading in runoff and 70 groundwater, altering the source and amount of nutrients available to the food web (Carey et al 71 2013;Harrison et al 2012;Withers et al 2011). In this study, we compared N stable isotopes 72 and animal waste (see Methods; Jankowski et al 2012;Moore et al 2014). As agricultural land 76 cover near our study ponds was negligible (<5% within one pond's buffer), an increase in δ 15 N 77 would indicate human wastewater incursion.…”

Cyanobacteria as biological drivers of lake nitrogen and phosphorus cycling

“…For this initial model, we assumed that cyanobacterial addition of ''new'' N via N fixation increases linearly with P v www.esajournals.org concentration, since P is needed to build the enzymes required in N fixation (Elser et al 1996). This formulation is simpler than the asymptotic association reported by Jankowski et al (2012) but provides a tractable starting point. We also assumed, based on cyanobacterial abundance patterns (Smith 1983, Gophen et al 1999, that N fixation rates follow a logistic function of the N:P ratio (by mass) in which N fixation is highest at low N:P ratios, decreases as N:P ratios begin to increase, and declines to zero at high N:P ratios.…”

Sources of sediment-bound organic matter infiltrating spawning gravels during the incubation and emergence life stages of salmonids

“…Papanicolaou et al, 2003;Dazell et al, 2005;Guo and Macdonald, 2006;Fox and Papanicolaou, 2008;Fox, 2009;Jacinthe et al, 2009;Fox et al, 2010;Mukundan et al, 2010Mukundan et al, , 2012Jankowski et al, 2012). The isotopic ranges in Table 2 for the farm yard manure/ slurry samples reflected those typically reported for grass ( in the farm yard manure and slurries collected from the Aran, Lugg and Test catchments suggest input from livestock fed a maize diet, which may be the source of the 13 C-enriched values in the interstitial sediments sampled from the artificial redds of the Rede and Test (although local direct inputs from soils under maize cultivation cannot be ruled out).…”