A similar effect was observed in

experiments where fresh organic material was added to simulate sedimentation of the phytoplankton spring bloom Selleck PF 2341066 (e.g. Jensen et al. 1990, Conley & Johnstone 1995). In the case of the spring phytoplankton bloom deposition Jensen et al. (1990) argues strongly that the influx of NOx− into the sediments is due to the suppression of nitrification resulting from an oxygen deficit in sediments, which in turn is related to increased microorganism activity in response to the deposition of fresh organic material. As a result, diffusion from the water is the predominant NOx− source for denitrification. Furthermore, several studies suggest a higher ammonium efflux from sediments under hypoxic conditions, e.g. Chesapeake Bay (Kemp et al. 1990),

the Louisiana shelf (McCarthy et al. 2008) and Danish coastal systems (Conley et al. 2007) due not only to suppressed nitrification efficiency, but also to elevated levels of the dissimilatory nitrate reduction to ammonium (DNRA). DNRA has also been called Screening Library concentration a ‘short circuit in the biological N cycle’ (Cole & Brown 1980), since it allows the direct transformation of NO3− and NO2− to NH4+ (Rütting et al. 2011). In our study the NH4+ accumulation rate at 2 mg O2 l−1 (Figure 4) was higher than that given by the model; it is not clear whether this is a sign of nitrification limitation or the start-up of DNRA. Instead of NH4+ utilisation by nitrification and its subsequent contribution to denitrification, the NH4+ is effluxed out of the sediments, indicating the production of bioavailable forms of N under hypoxic conditions. It is clear that the presence of one of these competing processes cannot be explained Carbohydrate solely by nutrient measurements. It should also be mentioned that several authors have concluded that a decrease in bottom water O2 concentration might even stimulate denitrification by shortening the physical distance between NOx− production and reduction zones ( Stockenberg & Johnstone 1997, Hietanen & Kuparinen 2008). However, according

to long-term observations by Kristensen (2000), persistently hypoxic bottom water conditions and high O2 consumption within the sediment surface decrease NOx− supplies and consequently hamper denitrification. Biogeochemical models that include simulation of sediment phosphorus transformation and flux (e.g. Savchuk & Wulff 2009, Eilola et al. 2009) show a clear pattern of reducing PO43− flux out of sediments with increasing oxygen concentration and thus increasing PO43− adsorption in sediments. This pattern is also reproduced in our model. Figure 3 demonstrates stable simulated flux rates of phosphate under hypoxic conditions, a smooth decline under oxygenated conditions and stable low flux rates at high oxygen concentrations, which is in good agreement with the median values of the observed experimental fluxes.