In situ microsensor studies of a shallow water hydrothermal vent at Milos, Greece

The microenvironment and microcirculation of a shallow water hydrothermal vent system was studied together with the benthic primary production at Milos, Greece. In situ microprofiles of O2, pH, H2S and temperature were obtained using a miniaturised version of a profiling instrument. The sediment temperature increased toward the centre of the vent system, reaching a surface maximum of 100°C in the central yellow coloured sulfidic area. The oxygen penetration depth decreased from the unaffected sediment surrounding the vent system towards the vent centre; however, at the inner vent area the O2 penetration increased again. Similar results were obtained during laboratory measurements. H2S concentrations increased rapidly beneath the oxygenated zone in the different vent areas and reached values of approximately 900 μM at sediment depths of 7–17 mm in the central vent areas. The microprofiles resolved a microcirculative pattern where local pressure differences caused by outflowing seep fluids induced a downward transport of oxygenated water, creating small convective cells which efficiently reoxidised H2S of the seep fluid. Patches of benthic diatoms covered the sediment surface in the areas surrounding the vent system. The net photosynthesis of this community increased from 25 to 41.8 mmol O2 m−2 d−1 from early morning to midday. The amount of carbon fixed daily, as calculated from the in situ oxygen microprofiles, accounted for 0.67 mmol C m−2 d−1. Laboratory incubations indicated that photosynthesis was not carbon limited and consequently the excess dissolved inorganic carbon contained in the vent fluids presumably had no effect on benthic primary production.