Under-ice and open-water ecosystem metabolism in temperate water bodies

Abstract

Winter, historically a largely un-monitored season, is important and changing. There is evidence of the importance of under-ice phytoplankton in temperate lakes, but it is currently unknown if the often high winter phytoplankton biomass translates to high productivity and what influence it has on year-round lake metabolism. Winters are getting shorter, but our ability to forecast change is hindered by our limited understanding of what happens under the ice. Here, we compare under-ice and open-water rates of areal gross production (AGP) and areal respiration (AR) from 3 Canadian reservoirs and one large lake using oxygen (O2) changes in light-dark bottle experiments, δ18O-O2 models, and fluorometry. During the open-water season, AGP was 81× greater than under-ice rates, with AR rates 8× higher than measured during winter. Open-water samples indicated autotrophy (P:R= 1.10). Consistent with current assumptions, the cold under-ice environment is associated with low primary productivity. Our results challenge the assumption that mean water column irradiance is lowest during the winter in dimictic water bodies; we find similar light conditions during the open-water season. Winter mean light is regulated by snow thickness; upon manual snow removal, we observe a 67 % increase in under-ice mean water column irradiance. The first-ever under-ice application of the δ18O2-method indicated that AGP responded to improvements in light. This study reveals further insights into the importance of under-ice metabolism on year-round processes in a changing climate.

Variability in Ice Cover Does Not Affect Annual Metabolism Estimates in a Small Eutrophic Reservoir

“…Despite this unexpected relationship, other studies have found that high concentrations of ammonium can suppress algal growth rates (Glibert et al., 2016), and can lead to decreases in N‐fixing cyanobacteria (Yang et al., 2023), which often dominate FCR's phytoplankton community in summer months (Lofton et al., 2022). Light was additionally found to be a driver of GPP (Table 2), which was consistent with previous studies (Hu et al., 2015; North et al., 2023), and highlights the important role ice may play altering GPP rates in FCR (Leppäranta et al., 2012).…”

“…Second, in our analysis of the effects of winter ice cover on metabolism, we were limited by the longest ice duration observed during our study period (35 days in 2016), which was two to four months shorter than previous winter metabolism studies (Brentrup et al, 2021;Obertegger et al, 2017). Given the shorter ice cover duration in FCR, our results are inherently different from prior ice-cover studies that experienced deep snowpack on the ice that altered light penetration and GPP rates (Leppäranta et al, 2012;North et al, 2023). Third, our monitoring data did not include records for ice thickness or transparency, which would have improved understanding of the underwater light environment in the winter and subsequent effects on GPP rates (Leppäranta et al, 2012), but were logistically impossible to collect due to the danger of sampling intermittent ice conditions.…”

“…In some cases, similar or even higher levels of phytoplankton biomass can persist under-ice when compared to ice-free conditions, resulting in increased GPP (Hampton et al, 2017;Jewson et al, 2009;Twiss et al, 2012). However, under other conditions (e.g., opaque ice or snow cover), phytoplankton productivity may be limited, decreasing winter GPP and resulting in net heterotrophy (Jewson et al, 2009;Leppäranta et al, 2012;North et al, 2023). These variable responses highlight the need for characterizing metabolism during ice-covered periods as ice cover and thickness is decreasing on lakes worldwide (Magnuson et al, 2000;Sharma et al, 2021;Weyhenmeyer et al, 2022).…”

“…Changing climate and the shrinking duration of winter are altering the role of freshwater ecosystems in the global carbon cycle (IPCC, 2022), which can be quantified using functional ecosystem metrics (Brentrup et al., 2021; North et al., 2023; Palmer & Richardson, 2009). One commonly used functional metric is net ecosystem production (NEP), the balance of whole ecosystem gross primary production (GPP) and respiration (R), which can elucidate whether freshwaters are net sources or sinks of organic carbon.…”

“…Studies quantifying ecosystem metabolism rates (i.e., NEP, GPP, and R) have primarily focused on the metabolism of naturally‐formed lakes in the summer months (e.g., Richardson et al., 2017; Solomon et al., 2013; but see Williamson et al., 2021), leaving substantial uncertainty as to how these rates vary among seasons and years in the face of climate change. Multiple studies in north temperate lakes have shown that winter metabolism rates are more heterotrophic than summer rates (Brentrup et al., 2021; North et al., 2023; Rabaey et al., 2021), and that the inclusion of winter rates in annual estimates can shift lakes from autotrophic to heterotrophic (Brentrup et al., 2021), highlighting the importance of monitoring lake and reservoir metabolism year‐round.…”