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

The microenvironment and microcirculation of a shallow water hydrothermal v ent system was studied together with the benthic primary production at Mile s, Greece. In situ microprofiles of O-2, pH, H2S and temperature were obtai ned using a miniaturised version of a profiling instrument, The sediment te mperature increased toward the centre of the vent system, reaching a surfac e maximum of 100 degrees C in the central yellow coloured sulfidic area. Th e oxygen penetration depth decreased from the unaffected sediment surroundi ng the vent system towards the vent centre; however, at the inner vent area the O-2 penetration increased again. Similar results were obtained during laboratory measurements. H2S concentrations increased rapidly beneath the o xygenated zone in the different vent areas and reached values of approximat ely 900 mu M at sediment depths of 7-17 mm in the central vent areas. The m icroprofiles resolved a microcirculative pattern where local pressure diffe rences caused by outflowing seep fluids induced a downward transport of oxy genated water, creating small convective cells which efficiently reoxidised H2S of the seep fluid. Patches of benthic diatoms covered the sediment sur face in the areas surrounding the vent system. The net photosynthesis of th is community increased from 25 to 41.8 mmol O-2 m(-2) d(-1) from early morn ing 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). Laborator y incubations indicated that photosynthesis was not carbon limited and cons equently the excess dissolved inorganic carbon contained in the vent fluids presumably had no effect on benthic primary production. (C) 2000 Elsevier Science B.V. All rights reserved.