In situ oxygen dynamics and carbon turnover in an intertidal sediment (Skallingen, Denmark

Intertidal areas are considered to be productive habitats, but due to the challenging and dynamic environment, in situ assessments of their performance remain scarce. Here, we aimed to quantify the production and turnover of organic material in an intertidal sediment across several consecutive diel/tidal cycles and to evaluate the importance of key drivers affecting the community performance. Time-series measurements of the oxygen (O sub(2)) microdistribution across 2 sites at a silty sandflat documented extreme variability, which was mainly driven by light availability and flow conditions. Diffusion dominated the interstitial solute transport, but advective porewater transport was observed during 15% of the 9 d study period. Photosynthetic activity never exhibited light inhibition and average daytime net photosynthesis ranged between 1.01 and 11.15 mmol m super(-2) d super(-1) depending on the daily light availability, while the assessed gross primary production was ~80% higher. When irradiance exceeded 17.2 mol photons m super(-2) d super(-1), daytime net autotrophy more than balanced the heterotrophic activity during the night (average night-time respiration, ER sub(n): -4.01 mmol m super(-2) d super(-1)). However, despite intense primary production, the integrated activity during the target autumn period was net heterotrophic with an average net ecosystem metabolism of -2.21 (span: -7.93 to 1.48) mmol O sub(2) m super(-2) d super(-1). This study highlights the extreme temporal and spatial variation of intertidal sediments and the importance of accounting for natural in situ dynamics to correctly assess their performance and contribution to system production.