Watershed vs. within‐lake drivers of nitrogen: phosphorus dynamics in shallow lakes

Abstract

Research on lake eutrophication often identifies variables affecting amounts of phosphorus (P) and nitrogen (N) in lakes, but understanding factors influencing N:P ratios is important given its influence on species composition and toxin production by cyanobacteria. We sampled 80 shallow lakes in Minnesota (USA) for three years to assess effects of watershed size, proportion of watershed as both row crop and natural area, fish biomass, and lake alternative state (turbid vs. clear) on total N : total P (TN : TP), ammonium, total dissolved phosphorus (TDP), and seston stoichiometry. We also examined N:P stoichiometry in 20 additional lakes that shifted states during the study. Last, we assessed the importance of denitrification by measuring denitrification rates in sediment cores from a subset of 34 lakes, and by measuring seston δ N in four additional experimental lakes before and after they were experimentally manipulated from turbid to clear states. Results showed alternative state had the largest influence on overall N:P stoichiometry in these systems, as it had the strongest relationship with TN : TP, seston C:N:P, ammonium, and TDP. Turbid lakes had higher N at given levels of P than clear lakes, with TN and ammonium 2-fold and 1.4-fold higher in turbid lakes, respectively. In lakes that shifted states, TN was 3-fold higher in turbid lakes, while TP was only 2-fold higher, supporting the notion N is more responsive to state shifts than is P. Seston δ N increased after lakes shifted to clear states, suggesting higher denitrification rates may be important for reducing N levels in clear states, and potential denitrification rates in sediment cores were among the highest recorded in the literature. Overall, our results indicate lake state was a primary driver of N:P dynamics in shallow lakes, and lakes in clear states had much lower N at a given level of P relative to turbid lakes, likely due to higher denitrification rates. Shallow lakes are often managed for the clear-water state due to increased value as wildlife habitat. However, our results indicate lake state also influences N biogeochemistry, such that managing shallow lakes for the clear-water state may also mitigate excess N levels at a landscape scale.

Transport of N and P in U.S. streams and rivers differs with land use and between dissolved and particulate forms

“…We used linear mixed‐effects models with log 10 TN, DIN, or PN, TP, DIP, or PP as the response variable. Models contained the proportion of row‐crop agriculture (pCRO), pasture (pHAY), urban (pURB), or forested (pFOR) land cover in the watershed as predictors, in addition to log 10 discharge (Q, L/s; Arbuckle and Downing 2001, Ginger et al 2017). Parameter estimates from the land‐use models described above can be interpreted as the effect of land use on the N or P concentration, where a positive parameter estimate for pAG would indicate higher concentrations of a given nutrient per unit increase in pAG.…”

“…In contrast, nutrient sources such as sewage effluent, runoff from urban watersheds, and amplified erosion of upland soils are likely to increase P relative to N (Downing and McCauley 1992, Withers and Jarvie 2008, Duan et al 2012). Although the predominant land use within a watershed is expected to affect stream N:P ratios, evidence for such shifts remains less explored in streams and rivers than in lake ecosystems (Downing and McCauley 1992, Vanni et al 2011, Ginger et al 2017, but see Maranger et al 2018, McDowell et al 2019). Distinct patterns of N and P loading likely prompt shifts in N vs. P limitation of critical ecosystem functions such as nutrient uptake, primary production and detrital breakdown (Rosemond et al, 2002, Tank and Dodds 2003, Dodds and Smith 2016), while also affecting the transport of N, P, and other elements downstream, eventually to coastal ecosystems.…”

Winter Oxygen Regimes in Clear and Turbid Shallow Lakes

“…Turbid lakes had significantly higher concentrations of total phosphorus (TP) and chl a than clear lakes ( p = <0.001 for TP, p = 0.00138 for chl a , Table 2). Turbid lakes also had significantly higher concentrations of total nitrogen (TN) ( p = 0.0239, Table 2), likely due to higher rates of denitrification in clear lakes, as well as uptake of nitrogen by macrophytes (Ginger et al., 2017). Dissolved organic carbon (DOC) was not significantly different between clear and turbid lakes ( p = 0.876, Table 2).…”

Using hidden Markov models to inform conservation and management strategies in ecosystems exhibiting alternative stable states

“…SAV sustains invertebrate communities that provide important food sources for waterfowl, but the abundance and diversity of SAV are reduced in the turbid, algae‐dominated state. Parasites associated with amphibian malformations likely have higher prevalence in turbid lakes (Johnson & Chase, 2004), and nitrogen may accumulate at higher rates (Ginger et al., 2017). To maintain high water quality and habitat for wildlife, it is essential to identify drivers of shallow lake state transitions and to assess the vulnerability of clear lakes to turbid shifts.…”