Mineralisation of organic matter in coastal sediments at different frequency and duration of resuspension

Coastal sediments represent sites of major importance for many biogeochemical processes, including organic matter mineralisation. These sediments are frequently subjected to intermittent physical forcing resulting in resuspension, which potentially influences sediment processes. In this study we investigated how the frequency and duration of resuspension events affect organic matter mineralisation rates, by creating conditions where the resuspension effect was as isolated as possible from other factors possibly affecting the mineralisation rate. Results show that continuous resuspension or resuspension in 12 h intervals double the mineralisation rates compared to sediments not subjected to water turbulence (2.0 ± 0.2 vs. 1.1 ± 0.3 μmol ΣCO2 (g d.w.)−1 d−1). However, when subjected to short resuspension events (5 s) once every 24 or 48 h the sediment mineralisation rate were enhanced even more, to 5.2 ± 0.3 μmol ΣCO2 (g d.w.)−1 d−1. Longer intervals between resuspension events (72–96 h) did not affect the mineralisation rate compared to no water turbulence. This indicates that resuspension enhances mineralisation rates, and that even very short resuspension events can influence sediment carbon and nutrient cycling to a large extent if occurring often enough. Hence, sediment mineralisation rate measurements without resuspension may significantly underestimate mineralisation rates. However, given our results, it is possible that continuous low-level shear stress in coastal areas may be enough to stimulate mineralisation, and then specific events with increased shear stress and resuspension may not cause any additional enhancement. Therefore, to illuminate potential effects of resuspension on mineralisation under field conditions, more information about the level of shear stress that is required to affect mineralisation rates is needed.