Temporal variation of currents, particulate flux and organism supply at two deep-sea hydrothermal fields of the Azores Triple Junction

Abstract

In order to provide information on the production and the distribution of hydrothermal particulate and biological material produced by deep-sea hydrothermal vents, three moorings with sediment traps and currentmeters were deployed for one year in the Azores Triple Junction region. Two moorings were positioned close to two hydrothermal vent fields at the Mid-Atlantic Ridge (Rainbow, 36°14′N, 33°54′W, 2250 m depth and Lucky Strike 37°17′N, 32°17′W, 1600 m depth). A third was moored out of the influence of the Rainbow vents representing the reference of the pelagic flux in this area. At the Rainbow field, particulate flux (range 341-634 g m−2 d−1) showed a high temporal variation controlled essentially by the current direction. The particles were characterized by a high concentration of total sulfur (16.4%) and copper (8.6%) and a low concentration of organic carbon (1.4%). Bivalves collected in the trap belonged to the hydrothermal mytilid Bathymodiolus azoricus and were all in the prejuvenile prodissoconch II stage of their development. Their density decreased strongly between July and September, indicating a possible discontinuous reproduction of this species. The particles at Lucky Strike were rich in barium and the calculated flux was lower than at Rainbow (range 52-308 g m−2 d−1) because of the translucent fluid poor in particles emitted at this shallower station. However, a peak in the flux was observed in May corresponding to the seasonal phytoplankton bloom. Two different bivalve species were sampled in this trap. The most abundant was B. azoricus. However, their development stages showed higher variations compared to the Rainbow trap. The abundance of mussels in the trap was very low throughout the year and does not allow one to show significant spawning periodicity.

Comparison of a sediment trap and plankton pump for time‐series sampling of larvae near deep‐sea hydrothermal vents

“…This result is consistent with the expectation that, in the absence of behavior, shelled gastropod larvae would have higher sinking velocities than similar-sized polychaetes, particularly after depleting lipid stores. Selectivity for certain larvae has been reported previously for tube traps of various designs (e.g., Yund et al 1991;Todd et al 2006) and suggested for a recent study with sediment traps (Khripounoff et al 2008). Yund et al (1991) noted that differences in swimming behavior among taxa, and between larval stages within a species, might influence collection by a passive sediment trap.…”

“…1), within the expected accuracy of the LBL navigation (Soule et al 2008). Although more precise positioning can be attained by moving a mooring using a submersible (e.g., Mullineaux et al 2005;Khripounoff et al 2008), the spatial separation (~40 m) between pump and trap moorings was unavoidable because of risk of entanglement. The current experimental design could be improved further by deploying multiple trap/pump pairs at different locations (i.e., to avoid spatial pseudoreplication), but the expense of instrumentation and ship time would likely be prohibitive.…”

“…In the future, with the design of a high-volume pump system with internal sample preservation, a platform could be deployed on the seafloor to hold a timeseries sediment trap and multiple pumps (or, ideally, a timeseries pump system). Such a platform would also enable sampling larvae closer to the seafloor (e.g., 2.5 mab in Khripounoff et al 2008).…”

“…Several recent studies have used sediment traps for collection of larvae near deep-sea hydrothermal vents, including tube traps which yield single samples per deployment (Metaxas 2004) and time-series sediment traps (Comtet et al 2000;Adams and Mullineaux 2008;Khripounoff et al 2008). Tube traps, set into or just above the seafloor, have been used in coastal waters for determining larval supply to the benthos (e.g., Yund et al 1991;Bertness et al 1992;Gaines and Bertness 1993;Todd et al 2006).…”

“…Tube traps, set into or just above the seafloor, have been used in coastal waters for determining larval supply to the benthos (e.g., Yund et al 1991;Bertness et al 1992;Gaines and Bertness 1993;Todd et al 2006). Time-series sediment traps, used regularly over the past two decades to estimate sinking particulate fluxes to deep water (e.g., Knauer and Asper 1989) and for particulates from hydrothermal plumes (e.g., Khripounoff et al 2001Khripounoff et al , 2008, collect a sequence of samples and may be deployed for periods on the order of a year (Honjo and Doherty 1988). Traps are passive collectors, and their shape and configuration in combination with the ambient flow regime influence their collecting properties, sometimes with hydrodynamic biases in the estimation of sinking particulate fluxes (Butman et al 1986;Knauer and Asper 1989;Jürg 1996).…”