Variability of physico-chemical conditions in 9°50′N EPR diffuse flow vent habitats

The physico-chemical characteristics of habitats have been considered to be one of the prime determinants of animal distribution within hydrothermal vent sites. However, the relative importance of abiotic to biotic influences is still debated. The primary aim of this study is twofold. The first is to determine and compare the ranges of physico-chemical conditions that characterize mussel-dominated and tubeworm-dominated communities at different sites within the vent field of the 9°50′N segment on the East Pacific Rise. The second is to better understand the processes that determine the variability of physico-chemical conditions in these habitats. In situ chemical and temperature measurements confirmed the high variability on small spatial and temporal scales within single aggregations of animals. The correlation of temperature and sulfide or pH revealed substantial differences between similar habitat-types at different sites, which cannot be attributed to changes in the extent of fluid dilution in the subseafloor. Further investigation of habitat variability within individual sites highlighted specific chemical features for the four sites studied, emphasizing the importance of an extensive in situ chemical analyses survey before using temperature as a proxy for chemical conditions. At Tica and Biovent, the fluid source characteristics were shown to vary only slightly within the sites, among aggregations discretely distributed over several meters distance. The variability of the total sulfide concentration and chemical speciation with temperature in the various habitats at each of these sites, can be reasonably simulated from the conservative mixing model of a single source fluid with seawater. The small variation of the empirical trends between different types of faunal assemblages at Tica suggests that changes in the fluid chemistry are not a prime determinant of the temporal succession of mussels and tubeworms. The findings of very similar sulfide–temperature relationships in two sites of different age and faunal compositions, Biovent and Tica, further support this idea. The two other sites, Mussel Bed and Riftia Field, differ both in the animal communities present and their chemistries show significant discrepancies from the predictions of the conservative mixing model. At Riftia Field, elevated iron concentrations and relatively low sulfide levels were correlated to unusually low pH, which could not be fully explained by conservative mixing of a typical diffuse vent fluid with seawater. The increased acidity results in a dramatic reduction in the anionic form, HS−. This is the form preferentially assimilated by the tubeworms and could explain the decline of the tubeworms at this site. At Mussel Bed the relationship between the sulfide concentration and temperature varied substantially among the aggregations surveyed and may reflect the influence of associated free-living microbial communities on the chemistry of mussel habitats. This study emphasizes the complex interplay of diffuse fluid formation in subsurface, chemical reactivity in the mixing zone, and biological activity in controlling of characteristics of vent habitats.