Do riverine heatwaves impact ecosystem metabolism?

In response to climate change, research on extreme conditions is accelerating. One such condition is the aquatic heatwave. With an expected rise in the occurrence, severity, and span of heatwaves, aquatic ecosystem processes will be affected, but the impacts are understudied and uncertain. This study addressed the effects of aquatic heatwaves on gross primary production (GPP), ecosystem respiration (ER), and their difference defined as net ecosystem metabolism (NEM) in a riverine ecosystem. Using long-term data from a site in the James River, water temperature time series were analyzed for heatwave metrics. In addition, by using dissolved oxygen time series, daily rates of GPP and ER were estimated. Of the 40 heatwaves observed during this study, 70% were of moderate severity and 30% were of strong severity, as determined based on peak temperatures. The average (± SD) frequency of heatwaves was 2 ± 2 events per year and ranged up to 5 events per year. The average duration and maximum intensity of a heatwave was 8 ± 3 days and 5.22 ± 1.36°C. GPP was significantly higher during moderate heatwaves (1.01 ± 1.30 g O2 m-2 d-1, p-value = 0.003) compared to non-heatwave conditions (0.70 ± 0.96 g O2 m-2 d-1). GPP significantly declined during strong heatwaves (0.49 ± 1.13 g O2 m-2 d-1) relative to moderate heatwaves (p-value = 0.002), which suggests unfavorable conditions for primary producers as heatwaves become more severe. ER and NEM were not significantly different during heatwave and non-heatwave conditions, nor during moderate versus strong heatwaves. Overall, these results suggest that GPP will increase with increasing water temperature until a thermal maximum is reached and then begin to decline. This may result in increased CO2 release to the atmosphere as rivers become increasingly heterotrophic under persistent and strong heatwaves.