Vertical distribution of benthic ciliates in response to the oxygen concentration in an intertidal North Sea sediment

We investigated the vertical distribution of benthic ciliates in response to changes in the oxygen conditions in an intertidal North Sea sediment. Sediment samples originating from a sandy flat were placed in a small temperature-controlled seawater flow-through system in the laboratory. Oxygen concentrations in the sediment were manipulated by enhancing or repressing photosynthetic activity of sediment diatoms and cyanobacteria by varying light intensities in 3 parallel setups (continuous darkness, permanent light intensity of 400 mu E m(-2) s(-1), and 7 h light and 7 h dark switch). Oxygen profiles s were measured employing high resolution electrochemical oxygen microsensors. Simultaneously, small sediment cores were collected from the respective treatments, and divided into 1 or 2 mm thin slices. The ciliates from each vertical layer were identified and enumerated employing the quantitative protargol stain method. The ciliate community could be divided into 2 distinct groups. Epibenthic ciliates remained close to the sediment surface at all times and were rarely found in anoxic layers. In contrast, the majority of the ciliates adapted to an interstitial life style were almost always found in anoxic sediment layers and appeared to migrate away from high oxygen concentrations. There may be some strictly anaerobic ciliates among the ciliate assemblage; however, most of the ciliates we found and describe are microaerophilic and therefore rely on a permanent access to oxygen. The potential implications of these findings for the nutrient cycling across the oxic/anoxic boundary within sediments are discussed in light of possible limitations of the employed techniques.