Temporal and spatial patterns of biological community development at nascent deep-sea hydrothermal vents (9°50?N, East Pacific Rise)
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The April 1991 discovery of newly formed hydrothermal vents in areas of recent volcanic eruption between 9 degrees 45’N and 9 degrees 52’N on the East Pacific Rise provided a unique opportunity to follow temporal changes in biological community structure from the “birth” of numerous deep-sea hydrothermal vents. In March 1992, DSV Alvin was used to deploy an on-bottom observatory, the Biologic-Geologic Transect, to monitor faunal succession along a 1.37 km segment of the axial summit caldera between 9 degrees 49.61’N and 9 degrees 50.36’N (depth similar to 2520 m). Photo- and videographic documentation of megafaunal colonization and chemical analyses of diffuse hydrothermal fluids associated with many of these developing communities within the Transect were performed in March 1992, December 1993, October 1993, and November 1995. Photographic and chemical time-series analyses revealed the following sequence of events in low-temperature venting areas. (1) Immediately following the 1991 eruption, hydrogen sulfide and iron concentrations in diffuse fluids were extremely high (>1 mmol kg(-1))and microbially derived material blanketed active areas of venting in the form of thick microbial mats. (2) Mobile vent fauna (e.g. amphipods, copepods, octopods, and galatheid and brachyuran crabs) and non-vent fauna (e.g. nematocarcinid shrimp) proliferated in response to this increased biological production. (3) Within 1 yr of the eruption, areal coverage of microbial mats was reduced by similar to 60% and individuals of the vestimentiferan tube worm Tevnia jerichonana settled gregariously in areas where diffuse flow was most intense. (4) Two years after the eruption, maximum levels of H2S decreased by almost half(from 1.90 to 0.97 mmol kg(-1)) and dense thickets of the vestimentiferan Riftia pachyptila dominated vent openings previously inhabited by Tevnia jerichonana. (5) Three years after the eruption, maximum hydrogen sulfide levels declined further to 0.88 mmol kg(-1) and mussels (Bathymodiolus thermophilus) were observed on basaltic substrates, (6) Four years after the eruption, galatheid crabs and serpulid polychaetes increased in abundance and were observed close to active vent openings as maximum hydrogen levels decreased to 0.72 mmol kg(-1). Also by this time mussels had colonized on to tubes of Riftia pachyptila. (7) Between 3 and 5 yr after the eruption, there was a 2- to 3-fold increase in the number of species in the faunal assemblages. In the absence of additional volcanic/tectonic disturbance, we predict that mytilid and vesicomyid bivalves will gradually replace vestimentiferans as the dominant megafauna 5-10 yr following the eruption. We also anticipate that the abundance of suspension feeders will decline during this period while the abundance of carnivores will increase. We hypothesize that the above series of events (1-7) represents a general sequence of biological successional changes that will occur at newly formed low-temperature deep-sea hydrothermal vents along the northern East Pacific Rise and contiguous ridge axes. Megafaunal colonization at deep-sea hydrothermal vents is considered to be the consequence of an intimate interaction of the life-history strategies of individual species, physical oceanographic processes, and the dynamic hydrothermal environment. Our observations indicate that the successful sequential colonization of dominant megafaunal vent species, from Tevnia jerichonana to Riftia pachyptila to Bathymodiolus thermophilus, also may be strongly influenced by temporal changes in geochemical conditions. Additional evidence demonstrating the close link between diffuse vent flux, fluid geochemistry, and faunal succession included the rapid death of several newly formed biological assemblages coincident with abrupt changes in the geochemical composition of the venting fluid and the local refocusing or cessation of vent flow. These correlations suggest that future models of faunal succession at hydrothermal vents along intermediate to fast-spreading mid-ocean ridges should consider not only the interplay of species-specific life-history strategies, community productivity, and physical oceanographic processes, but also the influence of changing geochemical conditions on the sequential colonization of megafaunal species. (C) 1998 Elsevier Science Ltd. All rights reserved.