Sediments

We study the microbial conversions and transport phenomena in intertidal sediments with a combination of in situ microsensor and laboratory incubation techniques. The focus of our studies is on porous sand plates in the Wadden Sea. Some measurements were performed on carbonate sediments in a coral lagoon. Intertidal areas are subjected to highly variable conditions. Interfacial and internal transport is determined by the hydrodynamics (tides, waves, and currents), conversions by varying temperatures, light (photosynthesis) and organic input. We have tried to get insight in the temporal and spatial scales of the environmental conditions, their effects on transport phenomena and microbial activities. In permeable sediments advective processes lead to an enhanced exchange of particles and solutes with the overlaying water. The availability of e-donors and e-acceptors such as oxygen and organic material is therefore highly fluctuating in time and in depth of the sediments. Porous sediments can be compared to biofilters, which efficiently remove and turn over particles and solutes from the water body. Oxygen consumption exceeds sulfate reduction by far at all investigated stations during all seasons.
Measurements were performed during tidal cycles in different seasons by using microsensors attached to an autonomous in-situ measuring device (PROFILUR), on two locations, a sand- and a mixed flat in the backbarrier area of Spiekeroog and a very coarse grained sand flat near Sylt. Sulfate reduction rates were measured by radiotracer incubations. Aerobic respiration rates were determined on intact sediment cores, by per fusing the cores with oxygenated seawater and following locally the initial oxygen consumption rate, either by microsensors or by planar optodes. By comparing potential respiration rates with oxygen penetration rates (measured with PROFILUR), areal rates were obtained.
In March, on the top of the sand flat a dynamic O2-penetration depth was observed in March. Deepest penetration of 2-3 cm was reached just after maximum high tide. During summer the penetration depth was reduced to values between a few mm and 1 cm, due to higher oxygen demand. Increased transport of O2 into the sediments at high tide increased microbial respiration rates considerably. This is clearly demonstrated in Fig. 1, showing the oxygen consumption rates at different stages of the tidal cycle.
In the area near the low water line, smelly black anoxic surface sediments develop during low water, independent of the season (see photograph). These seeps are the result of local drainage of anoxic pore water from the plate. Microsensor measurements performed in the seep areas demonstrated a very strong tidal influence at this site (see contour plots). During low tide oxygen disappeared from the overlying water of the sediments, H2S and nutrients are released into the overlying water. PO4 concentrations of 400 µmol/L were observed, more than 100 times higher than in ambient seawater. Also gaseous methane release was observed. The black surface sediments at the lower edges of the plates seem exchange channels, connecting the anoxic and the oxic world. They may play an important role for the carbon and nutrient cycles in intertidal ecosystems. The seepage of anoxic, sulfidic and nutrient rich water has a strong effect on the local microbiology. For example, instead of the omnipresent diatoms we found in the surface sediments only green flagellates, the sediments are rich in sulfide oxidizers forming white sulfur mats.
Sulfate reduction increased with distance from the high-water line. Higher rates close to the low waterline were attributed to higher supply of organic material during the longer inundation during the tidal cycle. SRR varied strongly with season.
A method for measuring potential 2D oxygen consumption rates (OCR) of undisturbed permeable sediment cores at high spatial and temporal resolution was developed with L. Polerecky and D. de Beer. Oxygen rich water is flushed though a squared sediment core until a homogenous oxygen distribution in the sediment core is reached. The water flow is than stopped and the total 2D oxygen change over time is followed with the planar O2 optode.
Only few studies on the temperature dependence of aerobic respiration in sediments have been carried out, although it is an important pathway of carbon oxidation, typically estimated to account for 50 % of the carbon mineralisation in coastal sediments. Therefore the sum of chemical and biological OCR from 3 intact Sylt Hausstrand sediment cores from one location were investigated at different temperatures (4-28°C) in the laboratory. Total OCR`s of a single sediment core are highly heterogeneous and variability between replicates is also very high.
Temperature dependence of total OCR`s of different sediment horizons were quantified by the activation energy which was calculated from Arrhenius Plots. Activation energies from the uppermost 3 horizons (0-5cm) were in the range of 40-50 kJ mol-1, whereas the activation energy of the deepest horizon (7-8cm) was 20-30 kJ mol-1. Q10 values calculated from the activation energies are 2 for the 0-5 cm sediment depth and 1.6 for the deepest sediment horizon. We can conclude that the ratio biological:chemical oxygen consumption is higher in the top layer than in the bottom layer. Heron Island: The rates of various microbial processes (oxygen respiration, photosynthesis, nitrogen fixation and sulfate reduction) in permeable carbonate reef sediment in carbon turnover was investigated at Heron Island, Australia. In situ oxygen dynamics were monitored during 24 h cycles. Oxygen penetration depth, SRR and oxygen consumption rates are showing pronounced differences between the stations. Although oxygen consumption rates are exceeding SRR at all stations, SRR can be high and are at the two stations close to the island among the highest measured in carbonate reef sediments so far. Nitrogen fixation occurred only in the light.

E. Walpersdorf, U. Werner, L. Polerecky, U. Franke