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.
Highlights
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)
Mean discharge during each campaign varied by less than 12% from seasonal monthly discharges during the campaigns, indicating that the hydrological conditions during the campaigns were representative of each season
Light availability at the Chalk river Wylye (CW) reach during the summer was comparable to conditions during the spring, and the diurnal light dynamics were clearly reflected in the benthic oxygen fluxes (Fig. 3), which ranged from 29.4 mmol m22 h21 at nighttime to 26.3 mmol m22 h21 at daytime
Summary
The study was performed within the upper reaches of the 1650 km catchment of the (Hampshire) River Avon in southern England (Fig. 1A). Benthic rate Water column rate Combined benthic and water column rate Acoustic Doppler velocimeter Aquatic eddy co-variance Base flow index Base flow ratio O2 concentration Time-averaged O2 concentration O2 concentration fluctuations Bottom drag coefficient Clay river Sem site (AS2) Chalk river Wylye site (CW2) Conductivity-temperature-depth Compensation irradiance Light saturation parameter Ecosystem respiration AEC-based turbulent oxygen flux Greensand river Nadder site (GN1) Gross primary production AEC measurement height von Karman constant Net ecosystem metabolism Net ecosystem primary production Dissolved oxygen Production Photosynthesis–irradiance Photosynthetically active radiation Maximum production rate Respiration Flow-velocity magnitude Longitudinal flow fluctuations Vertical velocity Time-averaged vertical velocity Vertical velocity fluctuations Sediment surface roughness parameter. Stream metabolism Benthic and water column respiration rates, ERb and ERw (in mmol m22 d21), were calculated as the average of oxygen fluxes measured during the nighttime (in mmol m22 h21) and scaled to 24 h. Complementary continuous discharge monitoring was performed in the vicinity of our sampling locations over approximately 2 yr from June 2013 to June 2016 (see Heppell and Binley 2016a3; Heppell et al 2017)
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