Ice duration drives winter nitrate accumulation in north temperate lakes

Abstract

The duration of winter ice cover on lakes varies substantially with climate variability, and has decreased over the last several decades in many temperate lakes. However, little is known of how changes in seasonal ice cover may affect biogeochemical processes under ice. We examined winter nitrogen (N) dynamics under ice using a 301 yr dataset from five oligotrophic/mesotrophic north temperate lakes to determine how changes in inorganic N species varied with ice duration. Nitrate accumulated during winter and was strongly related to the number of days since ice-on. Exogenous inputs accounted for less than 3% of nitrate accumulation in four of the five lakes, suggesting a paramount role of . Given that winters with shorter ice duration (< 120 d) have become more frequent in these lakes since the late 1990s, peak winter nitrate concentrations and cumulative nitrate production under ice may be declining. As ice extent and duration change, the physical and chemical conditions supporting life will shift. This research suggests we may expect changes in the form and amount of inorganic N, and altered dissolved nitrogen : phosphorus ratios, in lakes during winters with shorter ice duration.*Correspondence: steve.powers@wsu.edu Author Contribution Statement: SMP led the manuscript effort. SMP and EHS came up with the research questions and designed the study approach. SMP, SEH, and SGL contributed to statistical analyses. SMP and SGL contributed figures. EHS, NRL, and RJH contributed data. SMP, EHS, HMB, NRL, and SEH interpreted results. All authors contributed to writing of the paper.Data Availability Statement: Data are available in the North Temperate Lakes LTER repository at https://lter.limnology.wisc.edu/data. We used a merged winter data set of (1) This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Scientific Significance StatementMany freshwater lakes freeze during winter, yet little is known about the rates and controls on ecological processes under ice, such as nitrification, or how these processes may be affected by ongoing and future changes in ice cover. To address these knowledge gaps, we analyzed three decades of winter chemistry data related to the nitrogen cycle in five seasonally ice-covered north temperate lakes. We found that nitrate accumulated during winter as a result of internal lake processes and was strongly related to the number of days since ice-on. Consequently, winters with shorter ice duration may result in reduced total nitrification and lower peak nitrate concentrations, with potentially important implications for annual lake biogeochemical budgets, primary productivity, and biological communities.

Nitrification contributes to winter oxygen depletion in seasonally frozen forested lakes

“…The variation in water chemistry between depths implies at least some degree of stratification, and thus the presence of NO 3 -N may reflect depth-specific nitrification. However, in three lakes, surface and middle NO 3 -N coincided with NH 4 -N accumulation in deep waters (Allequash, Big Musky, Sparkling, Powers et al 2017), suggesting some vertical transfer of N and/or DO, especially during late winter. Below 4.0°°C, water density increases as it warms (Wetzel 2001), and relatively small density gradients under ice may promote circulation (Bruesewitz et al 2014), perhaps delivering oxygenated water to benthic nitrifiers; this provides a possible explanation for the maintenance of DO concentrations above 2.5 mg L -1 in deep water of Crystal and Trout, the two deepest lakes ([ 20 m), which in turn had the highest deepwater NO 3 -N accumulation rates.…”

“…Following from work by Striegl et al (2001), a surge of recent research has confirmed that biogenic gases often accumulate over winter in seasonally frozen lakes (Ducharme-Riel et al 2015;Denfeld et al 2016) along with oxidized solutes including nitrate, sulfate, and carbonate/bicarbonate from methane oxidation (Gammons et al 2014;Hanson et al 2006;Powers et al 2017). These findings point to active benthic and planktonic communities (Bertilsson et al 2013;Hampton et al 2017) that affect whole-lake chemistry through sustained aerobic and anaerobic biological processes under ice-which may not be surprising given similar observations from permanently frozen lakes (Morgan-Kiss et al 2016;Powers and Hampton 2016).…”

“…Recently Powers et al (2017) found that nitrate accumulated overwinter, using [ 30 years of underice measurements in five northern oligotrophic and mesotrophic lakes. The results suggested strong seasonal forcing by the number of ice covered days.…”

“…This research builds on findings of winter nitrate accumulation in Powers et al (2017), again using the number of days since iceon to account for seasonal forcing, with the addition of new oxygen data. We also include new data sets for two bog lakes that routinely experience winter DO concentrations below 1.0 mg L -1 , enabling comparisons between lakes that vary in trophic status.…”

How Does Changing Ice-Out Affect Arctic versus Boreal Lakes? A Comparison Using Two Years with Ice-Out that Differed by More Than Three Weeks

“…Data were available for both years to compare lake These pathways that drive physical changes in lake ecosystems may also contribute to chemical changes that influence phytoplankton ( Figure 1). Earlier ice-out may lead to increased nutrient loading [34], or conversely, reductions in the duration of winter ice cover may contribute to reduced under-ice nitrate production [35], thus links between ice-out and changes in nutrients remain unclear. In addition, increased light exposure from earlier ice-out can alter dissolved organic carbon concentrations and quality [36].…”