Bio irrigation is the active pumping of water through the burrows in the sediment, this is done by the organisms that live in the sediment. Bio-irrigation can be measured by adding bromide or uranine to the overlying water. The organisms will pump the bromide or uranine dye into the sediment that causes a decrease in concentration in the overlying water. Bio-irrigation measurements with bromide are expensive and discrete samples need to be taken. Uranine, on the other hand can be measured continuously for a better insight in the animal activity. A disadvantage of uranine is that it probably adsorbs to organic material and can be influenced by pH because it can get protonated. The main question is to what extent is it possible to measure the bio-irrigation flux with the fluorescent dye uranine?The influences of the pH were determined experimentally by adding HCl drop-by-drop while measuring the fluorescence with different pH values. The extent of adsorption were determined by different experiments with a change in amount of algae, uranine concentration, different salinities and different sediment like mud and unwashed construction sand. Also, there were some experiments done with “cores”. Defaunated sand and mud were used as a control and there were experiments done with animals added to the sand and mud. Also, intact cores from the field locations Dortsman and Zandkreek were used for experiments. The results show that algae has a big influence on the adsorption of uranine. The higher the amount of algae, the more adsorption of uranine. The uranine concentration has also an influence on the adsorption. The higher the concentration uranine, the more uranine will be adsorbed. Salinity has no clear influence on the adsorption. The different sediment types show almost no clear influence on the adsorption of uranine. The pH results show that with a pH of around 7.5, less uranine is measured than added to the samples. Uranine can protonate which changes the structure and leads to a different excitation wavelength for the protonated molecules. Not all molecules come in the excited state and will not give an emission wavelength that is measured. To control the pH, the adsorption experiments were done over again with buffered seawater. The results show that there is less adsorption. The results of the defaunated cores show that defaunated sand has no adsorption over 24 hours of incubation. The defaunated mud cores show that there is a little adsorption of 0.5 μg/L uranine to the mud. Defaunated sand with lugworms (Arenicola marina) added to the samples show that there is bio-irrigation. Defaunated mud with a clam (Scrobicularia plana) show that there is a decrease in uranine concentration. The adsorption needs to be corrected before the bio-irrigation can be determined for this sample. The cores from Dortsman show that there is bio-irrigation in the samples. The cores from Zandkreek show that there is no adsorption or bio-irrigation, which is strange because the defaunated muddy samples from the same site show that there is some adsorption. Probably the mud from the field is very thick and there were only two or three small worms per core, which is not good enough to measure bio-irrigation. The defaunated mud was also washed out and probably looser than the intact mud from the field. So to measure the bio-irrigation flux correctly and accurately the pH needs to be checked so that it is not coming below 7.5. And the adsorption experiments need to be done with buffered seawater to keep the pH controlled. Also, a lower uranine concentration is the best to use, because that causes less adsorption.Experiments with defaunated sediment need to be done regularly to measure the adsorption. Also, the influences of the pH can be checked again to have more insight in what is happening into the samples and also to make the samples more basic to see what is happening in basic solutions.
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