Succession in Gulf of Mexico cold seep vestimentiferan aggregations: The importance of spatial variability

The slow rate of change in hydrocarbon seep communities on the upper -Louisiana slope prevents the use of direct observation in studying successional trends. We used a chronosequence consisting of three presumed stages - juvenile, adult and senescent - to test a previous model which proposed that sulfide availability and vestimentiferan growth and physiological health decline over the lifespan of a vestimenti-feran aggregation. We replicated the chronosequence at two sites to simultaneously -explore the influence of spatial heterogeneity on the characteristics of these communities. We determined environmental sulfide concentrations and vestimentiferan growth and condition in at least two vestimentiferan aggregations representative of each stage at each of these two sites. Hydrogen sulfide concentrations were highly variable both above and below the sediment's surface, and sulfide was present in high concentrations to sediment depths of 70 cm. Vestimentiferan growth and condition varied significantly on multiple spatial scales from sites separated by tens of kilometers, to aggregations separated by tens to hundreds of meters within a site, to individual vestimentiferans -separated by tens of centimeters within an aggregation. The striking variability in both environmental sulfide and vestimentiferan growth and condition within individual -aggregations suggests a crucial role for microhabitat variability in the persistence of vestimentiferan aggregations at these sites. Few significant successional trends in -environmental sulfide or vestimentiferan growth and condition were found over the three stages tested.