Enhancement of primary production during drought in a temperate watershed is greater in larger rivers than headwater streams

Abstract

Drought is common in rivers, yet how this disturbance regulates metabolic activity across network scales is largely unknown. Drought often lowers gross primary production (GPP) and ecosystem respiration (ER) in small headwaters but by contrast can enhance GPP and cause algal blooms in downstream estuaries. We estimated ecosystem metabolism across a nested network of 13 reaches from headwaters to the main stem of the Connecticut River from 2015 through 2017, which encompassed a pronounced drought. During drought, GPP and ER increased, but with greater enhancement in larger rivers. Responses of GPP and ER were partially due to warmer temperatures associated with drought, particularly in the larger rivers where temperatures during summer drought were > 10 C higher than typical summer baseflow. The larger rivers also had low canopy cover, which allowed primary producers to take advantage of lower turbidity and fewer cloudy days during drought. We conclude that GPP is enhanced by higher temperature, lower turbidity, and longer water residence times that are all a function of low discharge, but ecosystem response in temperate watersheds to these drivers depends on light availability regulated by riparian canopy cover. In larger rivers, GPP increased more than ER during drought, even leading to temporary autotrophy, an otherwise rare event in the typically light-limited heterotrophic Connecticut River main stem. With climate change, rivers and streams may become warmer and drought frequency and severity may increase. Such changes may increase autotrophy in rivers with broad implications for carbon cycling and water quality in aquatic ecosystems.Droughts are a disturbance to river ecosystems with farreaching effects on their structure and function. As freshwaters enter drought, the physical template (e.g., temperature, turbidity, conductivity, and pH) of stream and river ecosystems changes. These fundamental habitat alterations affect all components of the food web and the physicochemical environment (Stanley et al. 1997;Dahm et al. 2003). Many of the changes to river networks during drought are stressful to biota,

Drought alters the biogeochemistry of boreal stream networks

“…Specifically, drought caused a significant reduction in aerobic respiration along the experimental stream reach, which decreased from −403 ± 172 to −130 ± 81 mmol O 2 m −2 d −1 . Previous studies testing the effects of drought, either experimentally or under natural conditions, have shown that low flows can either enhance 31,32 or reduce 33 rates of stream aerobic respiration. In our case, aerobic respiration decreased nonlinearly with WRT (r 2 = 0.41; p < 0.001; n = 111), ostensibly because the biochemical O 2 demand driven by aerobic respiration greatly exceeded the resupply of O 2 to hyporheic sediments as drought ensued.…”

“…To normalize and compare Q from the studied streams with different catchment areas, we report specific discharge (mm day −1 ). To isolate drought hydrological conditions from the rest of the periods, we delineated the specific discharge and grouped studied responses based on percentile distributions of the historical (1980-2018) discharge records in the catchment 32 : drought (0th-10th percentile; n = 59), low flow (10th-20th percentile; n = 22), baseflow (20th-50th percentile; n = 90), and high flow (50th-100th percentile; n = 193).…”

Thinking like a consumer: Linking aquatic basal metabolism and consumer dynamics

“…Using continuous measures of environmental conditions such as flow and integrating those measures with high‐resolution time series of ecosystem metabolism, we have the opportunity to determine when and where the impacts of physical constraints and disturbance on energy resources may constrain community membership or reduce the total consumer production and biomass. For example, studies using high‐frequency measurements of ecosystem metabolism and GPP have already revealed that in many systems low flows coincide with periods of high light availability (Hensley et al 2019; Hosen et al 2019) resulting in higher rates of photosynthetic autochthonous production (Hall et al 2015; Hosen et al 2020). These integrated studies suggest that autochthonous production may counterbalance allochthonous production in temperate stream networks with potential implications for consumer production and biomass.…”

River network travel time is correlated with dissolved organic matter composition in rivers of the contiguous United States

“…Hydrology influences autochthonous production via channel depth and water clarity. Riverine water clarity increases during low flow/long travel time conditions due to lower levels of highly chromophoric allochthonous organic matter (Hensley et al, 2019) and particulates (Hosen et al, 2019). Increased light availability during low flow conditions favours increased aquatic photosynthesis (Hall et al, 2015; Hosen et al, 2019).…”

“…Riverine water clarity increases during low flow/long travel time conditions due to lower levels of highly chromophoric allochthonous organic matter (Hensley et al, 2019) and particulates (Hosen et al, 2019). Increased light availability during low flow conditions favours increased aquatic photosynthesis (Hall et al, 2015; Hosen et al, 2019). Greater travel time during low flows allows more time for photosynthetic communities to develop biomass (Glibert et al, 2014; Paerl et al, 1998; Vannote et al, 1980).…”