Continuous monitoring reveals multiple controls on ecosystem metabolism in a suburban stream

Abstract

Summary 1. Primary production and respiration in streams, collectively referred to as stream ecosystem metabolism, are fundamental processes that determine trophic structure, biomass and nutrient cycling. Few studies have used high‐frequency measurements of gross primary production (GPP) and ecosystem respiration (ER) over extended periods to characterise the factors that control stream ecosystem metabolism at hourly, daily, seasonal and annual scales. 2. We measured ecosystem metabolism at 5‐min intervals for 23 months in Shepherd Creek, a small suburban stream in Cincinnati, Ohio (U.S.A.). 3. Daily GPP was best predicted by a model containing light and its synergistic interaction with water temperature. Water temperature alone was not significantly related to daily GPP, rather high temperatures enhanced the capacity of autotrophs to use available light. 4. The relationship between GPP and light was further explored using photosynthesis–irradiance curves (P–I curves). Light saturation of GPP was evident throughout the winter and spring and the P–I curve frequently exhibited strong counterclockwise hysteresis. Hysteresis occurred when water temperatures were greater in the afternoon than in the morning, although light was similar, further suggesting that light availability interacts synergistically with water temperature. 5. Storm flows strongly depressed GPP in the spring while desiccation arrested aquatic GPP and ER in late summer and autumn. 6. Ecosystem respiration was best predicted by GPP, water temperature and the rate of water exchange between the surface channel and transient storage zones. We estimate that c. 70% of newly fixed carbon was immediately respired by autotrophs and closely associated heterotrophs. 7. Interannual, seasonal, daily and hourly variability in ecosystem metabolism was attributable to a combination of light availability, water temperature, storm flow dynamics and desiccation. Human activities affect all these factors in urban and suburban streams, suggesting stream ecosystem processes are likely to respond in complex ways to changing land use and climate.

Hydromorphological restoration stimulates river ecosystem metabolism

“…Rates of ER mirrored those of GPP, suggesting that autotrophic respiration largely drove temporal patterns in ER, despite an overall ratio of GPP : ER < 1 and a slightly negative NEP during most of the measurement period. Similar patterns were found in streams in the US (Beaulieu et al, 2013;Hall et al, 2016). The average GPP : ER ratio was significantly higher downstream of the restored reaches in our study (0.86 and 0.97, respectively) and in the combined restored reach (1.16) than in the upstream degraded river (0.66), indicating an increase in autotrophic processes following restoration.…”

“…The obtained metabolic rates should be directly comparable to metabolic rates of the upstream, degraded river (measured at station D with the onestation method) as results obtained with the one-station and the two-station methods often agree remarkably well (e.g., Bernot et al, 2010;Beaulieu et al, 2013). The total restored reach (R1 + R2) showed higher GPP, ER, NEP, and GPP : ER than the upstream degraded river.…”

“…The change in DO was estimated as the difference between consecutive 5 min readings of the same probe for the one-station method and as the difference between upstream (top of reach R1) and downstream probes (end of reach R2) for the two-station method (Roberts et al, 2007;Beaulieu et al, 2013).…”

“…Ecosystem metabolism is a measure of the production and use of organic matter within a river reach by all biota. Therefore, it provides key information about a river's trophic and energetic base (relative contribution of allochthonous and autochthonous carbon) (Young et al, 2008;Beaulieu et al, 2013). The majority of stream ecosystem metabolism work has investigated natural changes, such as effects of floods and droughts (e.g., Uehlinger, 2000), seasonal or interannual changes (e.g., Uehlinger, 2006;Beaulieu et al, 2013), interbiome differences (e.g., Mulholland et al, 2001), or landuse change (e.g., Gücker et al, 2009;Silva-Junior et al, 2014).…”

“…Therefore, it provides key information about a river's trophic and energetic base (relative contribution of allochthonous and autochthonous carbon) (Young et al, 2008;Beaulieu et al, 2013). The majority of stream ecosystem metabolism work has investigated natural changes, such as effects of floods and droughts (e.g., Uehlinger, 2000), seasonal or interannual changes (e.g., Uehlinger, 2006;Beaulieu et al, 2013), interbiome differences (e.g., Mulholland et al, 2001), or landuse change (e.g., Gücker et al, 2009;Silva-Junior et al, 2014). The majority of these studies have focused on smaller streams, while only few studies have measured metabolism of larger streams and rivers (e.g., Uehlinger, 2006;Dodds et al, 2013;Hall et al, 2015Hall et al, , 2016.…”