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Dispersal Boundaries and Species Composition of Hydrothermal Ecosystems on the Southern Mid-Atlantic Ridge
To What Extent do the Equatorial Atlantic Fracture Zones Impact the Dispersion, Biogeography and Evolution of Vent-Endemic Fauna?

The tight coupling between geological processes and living organisms at hydrothermal vents provides a singular opportunity to study how fundamental planetary processes shape the evolution of life. These tightly linked geological-biological systems have yielded a global network of extraordinarily productive chemosynthetic communities. The spatial and temporal variability within this network has created distinct biogeographic provinces that are influenced by a variety of underlying forces. Significant advances in our understanding of how ridge crest processes shape life can be achieved by identifying key mid-ocean ridge localities that will yield large first order "jumps" in our knowledge of faunal evolution and biogeography.

Fracture zones of the equatorial Atlantic
Fracture Zones of the equatorial Atlantic. The Romanche FZ separates the N. Atlantic from the S. Atlantic. The large offsets in the ridge crest caused by the Romanche and Chain Fracture Zone are posited to be the isolating mechanism between the Northern MAR and Indian Ocean fauna. The most southern known vent site in the Atlantic, Logatchev (14°45N; red circle; Gebruk et al. 2001), is the closest known Atlantic site to the Kairei/Edmond fields on the Central Indian Ridge. Field area (indicated) of the proposed studies contains the only hydrothermal signal known in the South Atlantic.
Biogeographic patterns suggested by the recent discovery of Indian Ocean vent fauna raise fundamental hypotheses to address the marked “disconnect” between North Atlantic fauna and other ridge crest hydrothermal communities. In 2005, we will test our hypotheses that the major fracture zones of the Equatorial Atlantic prevent or inhibit along-axis transport and communication of vent-endemic fauna into (and out of) the South Atlantic. Specifically, the large geographic ridge offsets posed by the Romanche and Chain Fracture Zones and the strong regional unidirectional current flow across and through the Equatorial Atlantic Fracture Zones represent a hydrographic, bathymetric, and biogeographic barrier that bisects the Mid-Atlantic Ridge into northern and southern provinces.

location of 13 CTD stations
Location of 13 CTD stations (circles) within the segments (enumerated) taken during a SMAR cruise (PI Chris German) in October 2001) between the Chain and Boca Verde FZs. While evidence for hydrothermal venting was observed at several stations, the A1 segment plume signal anomalies (red circle) were equivalent to plume signals from known sites on the MAR
We will determine the biological character of vent fields on the unexplored Southern Mid-Atlantic Ridge. We also will determine how these potentially unique ecosystems relate to the global biogeographic framework as we investigate dispersal and evolutionary patterns of vent fauna, by assess the role of the Equatorial Atlantic Fracture Zones as barriers to species dispersal, and influencing the evolution and biogeography of South Atlantic vent fauna. South Atlantic vent communities will be characterized by determining the precise location of active venting and quantitatively assessing the distribution, abundance, and variation in microhabitat structure as they relate to hydrothermal activity and geological features via detailed downlooking imaging surveys using the autonomous vehicle ABE (Autonomous Benthic Explorer). Anticipated sampling of these characterized vent ecosystems on equatorial ridges connecting Atlantic Ocean basins will provide novel insights into processes controlling basin-scale biogeographic and evolutionary patterns. Manipulative experiments are focused on removal and subsequent exclusion of mussels from selected regions of mussel-dominated assemblages, coupled with detailed geochemical and microbiological characterization of associated microhabitats before and after mussel removal. Subsequent recruitment and colonization within exclusion cages in denuded and control areas are being followed through successive removal and analysis of deployed basalt blocks over time. Concomitant chemical, physical and microbiological analyses of fluids permit correlation of these parameters with temporal changes in community structure.