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

Abstract

Temperate reservoirs and lakes worldwide are experiencing decreases in ice cover, which will likely alter the net balance of gross primary production (GPP) and respiration (R) in these ecosystems. However, most metabolism studies to date have focused on summer dynamics, thereby excluding winter dynamics from annual metabolism budgets. To address this gap, we analyzed 6 years of year-round high-frequency dissolved oxygen data to estimate daily rates of net ecosystem production (NEP), GPP, and R in a eutrophic, dimictic reservoir that has intermittent ice cover. Over 6 years, the reservoir exhibited slight heterotrophy during both summer and winter. We found winter and summer metabolism rates to be similar: summer NEP had a median rate of -0.06 mg O2 L-1 day-1 (range: -15.86 to 3.20 mg O2 L-1 day-1), while median winter NEP was -0.02 mg O2 L-1 day-1 (range: -8.19 to 0.53 mg O2 L-1 day-1). Despite large differences in the duration of ice cover among years, there were minimal differences in NEP among winters. Overall, the inclusion of winter data had a limited effect on annual metabolism estimates in a eutrophic reservoir, likely due to short winter periods in this reservoir (ice durations 0-35 days), relative to higher-latitude lakes. Our work reveals a smaller difference between winter and summer NEP than in lakes with continuous ice cover. Ultimately, our work underscores the importance of studying full-year metabolism dynamics in a range of aquatic ecosystems to help anticipate the effects of declining ice cover across lakes worldwide. Lakes and reservoirs around the world are experiencing decreases in ice cover duration, with many waterbodies starting to experience non-continuous ice cover throughout the winter. These changes in ice duration have the potential to influence carbon cycling, but to date few long-term studies have included winter data. We analyzed 6 years of minute-resolution oxygen data from a small reservoir that experiences non-continuous ice cover to estimate whether the surface water was a source or sink of carbon at daily, seasonal, and annual scales. We found that the reservoir was often a source of carbon to the atmosphere, regardless of whether data from winter were included. Our results differed from previous studies conducted in higher-latitude lakes that experience continuous ice cover throughout the winter, potentially due to the already-short duration of ice cover in this reservoir. As the duration of ice cover continues to decrease across lakes and reservoirs worldwide, our work highlights the need for studying how changing winter conditions-especially non-continuous ice cover-affects year-round carbon cycling. Winter data have rarely been included in lake metabolism studies, limiting our understanding of how ice affects metabolism estimates Annual metabolism estimates were similar across 6 years with widely varying ice cover Water chemistry explained variability in daily gross primary production, but not respiration or net ecosystem production, over 6 years