Prog. Oceanog. Vol. 24, pp. 15-24. 1990. Printed in Great Britain Au rights %smed. 0079 - aall/ So.00 + .50 em90 PeXgamon Press plc Fine-scale distribution of methanotrophic mussels at a Louisiana cold seep IAN R. MACDONALD~, W. RUSSELL CALLENDER~, zyxwvutsrqponmlkjihgfedcbaZYXWV ROGER zyxwvutsrqponmlkjihgfedcbaZYXWV A. BURKE, JR.~, SUSANNE J. MCDONALD’, and ROBERT S. CARNEY~ ‘Department of Oceanography; zDepartment of Geology; ‘Geochemical and Environmental Research Group, Texas A&M University. College Station, Texas 77843 USA. ‘Coastal Ecology Institute, Louisiana State Universiiy, Baton Rouge, Louisiana, 73803, USA. Abstract - Extensive aggregations of methanotrophic mussels (Mytilidae: Bathymodiolus-like) were foundin a 6Ox3OOm zoneon the 640m isobath of the Louisiana slope. Within the aggregations, living mussels occurred in dense curvilinear clusters up to 5m in length. Defunct clusters, consisting of gaping and d&articulated valves, were also common. Comparison of length frequency distribu- tions and mean densities of mussel clusters demonstrated that recruitment of juvenile mussels was ongoing in certain clusters and completely lacking in others. Surface sediments within the zone were characterized by dark patches and linear depressions, apparently associated with seeping hypersaline fluids. Pore fluids in surface sediments contained elevated salt concentrations (289% of ambient sea water), concentrations of ammonia up to 1.6mM, and up to 3mM HS. 1. INTRODUCI’ION Mussels (Mytilidae: Bafhymodiolus-like) found at hydrocarbon seeps on the Louisiana slope have been shown to possess methanotrophic capability mediated by bacterial symbionts (CHIL- DRESS, FISHER, BROOKS, KENNIIxIT,BIDIGARIXUI~ ANDERSON, 1986; FISHER, CHILDFUZ~S OREMLAND and BILXGARE, 1987). They utilize methane that seeps to the surface from deep reservoirs for both energy for autotrophic carbon fixation and for nutritive carbon (BROOKS, KENNICUTT, FISHER, MACKO, COLE,GILDRESS, BIDIGARE and VETTER, 1987). Similar capability is indicated for a second species of mussels from the Florida Escarpment (CAVANAUGH, LEVERING, MAKI, MITCHELL and LIDSTROM, 1987); however, the source of methane in this habitat is thought to be the microbial degradation of organic material (PAULL, JULL, TOULIN and LINIcK,1985; CARY, FRY, FJZLBECK and VETTER, 1989). CARY, FISHER and FELBECK (1988) have shown that Louisiana slope seep mussels will grow in methane concentrations as low as 245pM and oxygen concentrations of 290pM. In aquaria, such methane concentrations can be maintained by bubbling the gas through aerated water. In the field, mussels occupy a narrow (<lOcm thick) microhabitat on the surface sediment, within which the concentration of methane seeping in from below is subject to continual dilution via advection and diffusion into the overlying water column. Methane-rich gas has been observed venting within Louisiana slope mussel beds (BROOKS, Cox, BRYANT, KENNI- CUTT, MANN and MCDONALD, 1986). Methane concentrations of up to 66.5j.~M were measured in water collected within the bubble streams and significant correlation between the local abundance of seep mussels and the concentration of methane was found (~~CDONALD,BOLAND, BAKER, BROOKS, KENNICUTT and BIDIGARE, 1989). However, the Slope mussel beds are often extensive 15