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Chevaldonne, 1&2, Desbruyeres, D.2, Shank, T. M.1, Levai, G.1, & Lutz, R. A.1
1 Institute of Marine & Coastal Sciences, Rutgers University, New Brunswick, NJ 08903, USA
2 Département Environnement Profond, IFREMER Brest, B.P. 70, 29280 Plouzané, France
Animal communities thriving around deep-sea hydrothermal vents benefit from the characteristics of the emitted fluid, since it is the basis for their food supply through chemoautotrophic processes accomplished by microbes. The success of a species' installation at vents, however, is the result of a trade-off between the search for food (or "fuel"), and the adaptability to the harsh environmental conditions. These conditions typically have been described as hot, acidic, anoxic, radioactive, and toxic due to the abundance of heavy metals and hydrogen sulfide. Since vent communities are actually established within the mixing zone between the hydrothermal fluid and the ambient seawater, they have been shown to primarily experience highly variable environmental conditions. The analysis of time-series obtained from temperature probes deployed in different environmental settings, is a particularly adequate approach since it allows the ability to apprehend both temporal and spatial characteristics of a parameter believed to be a good tracer of the degree of mixing between seawater and the fluid.
We report here the results and synthesis of 10 years of efforts to obtain such time-series from diverse hydrothermal settings. We have analysed recordings from latitudes ranging from 22oS to 38oN, on two different ridge systems (EPR/Guaymas and MAR) and in two back-arc basins. More than 15 different locations were studied, including vent sites from one of the deepest (3525 m) and the shallowest (865 m) vent areas known so far. The instruments deployed ranged from simple autonomous Hobo probes to multi-probe recorders and benthic stations. In one instance, the temperature recorder was coupled with a current meter. Deployments lasted from a few hours, to record at short time intervals, to up to a year. The entire range of the hydrothermal ecosystem's habitats was probed, from the hottest alvinellid worm colonies found on the walls of active smokers, to the peripheral communities where temperature anomalies are hardly detectable.
Our data indicate a tremendous temporal and spatial variability at almost all scales of time and space. Both processes are highly intricate and a change in space usually induces a shift in the temporal processes, thus demonstrating the need for studies integrating both aspects. Chaotic and random variability is important and primarily results from: (1) the turbulent mixing between the two fluids; (2) volcanic and tectonic processes; (3) the fragility of some of the populated substrates, such as sulfide chimneys. However, the most important feature detected by the spectral analysis of the series, is the omnipresence of a periodic variability of tidal origin. We therefore confirm earlier reports that had detected such a tidal influence on the vent environment, and we demonstrate that tidal periodicity is the rule rather than the exception, and that wherever vent communities are found, the primary periodic signal is derived from the tides and their harmonics (ca. 3, 6, 9, 12, 24 hours). It appears, however, that the tidal influence is dampened by non-periodic processes when communities are closer to the venting source.
The way temperature is affected by tides has been discussed in the past, and it has been proposed that it could result either from a direct pressure effect on venting, or from the action of tidal cross-currents on the emitted fluid. We here demonstrate, based on current meter data, that the latter hypothesis is much more likely. The impact of these results on the global hydrothermal fauna is discussed.
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