Reach‐scale river metabolism across contrasting sub‐catchment geologies: Effect of light and hydrology

Abstract

We investigated the seasonal dynamics of in‐stream metabolism at the reach scale (∼ 150 m) of headwaters across contrasting geological sub‐catchments: clay, Greensand, and Chalk of the upper River Avon (UK). Benthic metabolic activity was quantified by aquatic eddy co‐variance while water column activity was assessed by bottle incubations. Seasonal dynamics across reaches were specific for the three types of geologies. During the spring, all reaches were net autotrophic, with rates of up to 290 mmol C m−2 d−1 in the clay reach. During the remaining seasons, the clay and Greensand reaches were net heterotrophic, with peak oxygen consumption of 206 mmol m−2 d−1 during the autumn, while the Chalk reach was net heterotrophic only in winter. Overall, the water column alone still contributed to ∼ 25% of the annual respiration and primary production in all reaches. Net ecosystem metabolism (NEM) across seasons and reaches followed a general linear relationship with increasing stream light availability. Sub‐catchment specific NEM proved to be linearly related to the local hydrological connectivity, quantified as the ratio between base flow and stream discharge, and expressed on a timescale of 9 d on average. This timescale apparently represents the average period of hydrological imprint for carbon turnover within the reaches. Combining a general light response and sub‐catchment specific base flow ratio provided a robust functional relationship for predicting NEM at the reach scale. The novel approach proposed in this study can help facilitate spatial and temporal upscaling of riverine metabolism that may be applicable to a broader spectrum of catchments.

Headwater gas exchange quantified from O2 mass balances at the reach scale

“…The average shear velocity, u * , computed as u * = u ( C D ) 1/2 , with C D being the drag coefficient (Wüest and Lorke ). An average C D of 3.3 × 10 −3 was used for both sites based on further surveys of the River Avon sub‐catchments (Rovelli et al ). The average slope, 0.002 m m −1 , was estimated from GPS measurements during the respective field campaigns.…”

“…Similarly, estimates of O 2 gas transfer velocity were also standardized to a Schmidt number of 600 ( k 600 ) based on Jahne et al () as where Sc T ≈ 0.00086842 T 4 − 0.10115 T 3 + 4.8055 T 2 − 124.34 T + 1745.1 is the Schmidt number for O 2 in freshwater at the local water temperature T ( see Wanninkhof ). Stream temperature time series were obtained from background measurements of physical parameters during the observational period at each site (Rovelli et al ).…”

“… Obtained from depth and flow velocity transects performed during the observational period under stable hydrograph ( see Rovelli et al ).…”

Geographically widespread 13C‐depletion of grazing caddis larvae: A third way of fuelling stream food webs?

“…All the evidence so far comes from a number of southern English chalk streams, which are groundwater‐fed, free‐flowing systems with very high alkalinity and primary production (Rovelli et al., ). Chalk is a form of very soft limestone formed by the shells of coccolithophores laid down in shallow Cretaceous seas and forming highly fractured and extremely permeable thick beds.…”

“…All the evidence so far comes from a number of southern English chalk streams, which are groundwater-fed, free-flowing systems with very high alkalinity and primary production (Rovelli et al, 2017).…”