Phosphate under the form of K2HPO4 was added to low nutrient North Atlantic seawater diluted with Milli-Q water to a salinity of 27 psu at a
ovalbumin 324-338 of 8 (± 0.05). Six solutions were prepared with concentrations of dissolved PO4 ranging from to 150 μM (for 20 g l? 1 particle concentration), to 200 μM (for 100 g l? 1 particle concentration) and to 250 μM (for 200 g l? 1 particle concentration), which is well within the range found in Wadden Sea sediments, which goes up to 1200 μM (Leote et al., in preparation). Polypropylene Falcon tubes were filled with 50 ml of each solution and dry Fe coated sand was added to a final particle concentration of 20, 100, and 200 g l? 1. Prior to incubation, the artificial sediment was equilibrated for a week in a PO4 containing seawater solution (220 μM K2HPO4, 27 psu), yielding an average pre-loading of 0.93 μmol PO4 g? 1, which is within the range of the 0.02–1.78 μmol PO4 g? 1 found in the North Sea and the 0.23–2.25 μmol PO4 g? 1 found in the Wadden Sea tidal flats (van Raaphorst and Kloosterhuis, 1994 and Slomp et al., 199
cool . This procedure was followed to reproduce the native loading of natural marine sediments and resemble our experiments to natural conditions. The tubes were placed in a vertical tube rotator at a defined speed (1, 2.5, 11 or 27 rpm) and incubated at a temperature of 10–12 °C, for a maximum duration of 14 days. At regular intervals, the tubes were left standing for approximately a minute, to allow the sand particles to settle and 2 ml samples were collected, filtered (0.2 μm pore diameter Acrodisc filter, Supor? membrane), acidified with 2 μl of 5 N Suprapure HCl, and kept at 4 °C until analysis of PO4 and Fe(II), using the methods described above. Samples were replaced by equal volumes of low nutrient North Atlantic seawater (27 psu), to maintain a similar particle concentration throughout the experiment. Identical incubations were also performed using the upper centimeter of the intertidal sediments. However, the initial PO4 concentrations used were lower (ranging from to 100 μM), as the sorption capacity was expected to be less than for the Fe coated sand.
pH was measured in the beginning and after 2 weeks of incubation. No consistent change was observed and variation throughout the experiment was lower than 0.1.
