TEMPORAL AND SPATIAL PATTERNS OF BIOLOGICAL COMMUNITY-DEVELOPMENT AT NASCENT DEEP-SEA HYDROTHERMAL VENTS (9-DEGREES-50'N, EAST PACIFIC RISE)

The April 1991 discovery of newly formed hydrothermal vents in areas o f recent volcanic eruption between 9 degrees 45'N and 9 degrees 52'N o n the East Pacific Rise provided a unique opportunity to follow tempor al changes in biological community structure from the ''birth'' of num erous deep-sea hydrothermal vents. In March 1992, DSV Alvin was used t o 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 colon ization and chemical analyses of diffuse hydrothermal fluids associate d with many of these developing communities within the Transect were p erformed in March 1992, December 1993, October 1993, and November 1995 . Photographic and chemical time-series analyses revealed the followin g 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 thi ck microbial mats. (2) Mobile vent fauna (e.g. amphipods, copepods, oc topods, and galatheid and brachyuran crabs) and non-vent fauna (e.g. n ematocarcinid shrimp) proliferated in response to this increased biolo gical production. (3) Within 1 yr of the eruption, areal coverage of m icrobial mats was reduced by similar to 60% and individuals of the ves timentiferan tube worm Tevnia jerichonana settled gregariously in area s where diffuse flow was most intense. (4) Two years after the eruptio n, maximum levels of H2S decreased by almost half(from 1.90 to 0.97 mm ol 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 dec lined further to 0.88 mmol kg(-1) and mussels (Bathymodiolus thermophi lus) were observed on basaltic substrates, (6) Four years after the er uption, galatheid crabs and serpulid polychaetes increased in abundanc e 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 co lonized 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 spec ies in the faunal assemblages. In the absence of additional volcanic/t ectonic disturbance, we predict that mytilid and vesicomyid bivalves w ill gradually replace vestimentiferans as the dominant megafauna 5-10 yr following the eruption. We also anticipate that the abundance of su spension feeders will decline during this period while the abundance o f carnivores will increase. We hypothesize that the above series of ev ents (1-7) represents a general sequence of biological successional ch anges that will occur at newly formed low-temperature deep-sea hydroth ermal vents along the northern East Pacific Rise and contiguous ridge axes. Megafaunal colonization at deep-sea hydrothermal vents is consid ered to be the consequence of an intimate interaction of the life-hist ory strategies of individual species, physical oceanographic processes , and the dynamic hydrothermal environment. Our observations indicate that the successful sequential colonization of dominant megafaunal ven t species, from Tevnia jerichonana to Riftia pachyptila to Bathymodiol us thermophilus, also may be strongly influenced by temporal changes i n geochemical conditions. Additional evidence demonstrating the close link between diffuse vent flux, fluid geochemistry, and faunal success ion included the rapid death of several newly formed biological assemb lages 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 rid ges should consider not only the interplay of species-specific life-hi story strategies, community productivity, and physical oceanographic p rocesses, but also the influence of changing geochemical conditions on the sequential colonization of megafaunal species. (C) 1998 Elsevier Science Ltd. All rights reserved.