Linking Prokaryotic Assemblages and Biogeochemistry to Long-Term Declines in Chemoautotrophy in the Cariaco Basin

Chemoautotrophic carbon fixation has been measured semiannually in the CARIACO Ocean Time Series from November 1996 to the present. Early observed rates were inexplicably rapid, rivaling euphotic zone primary productivity. Over the last 19 years, rates integrated across the redoxcline and measured by consistent methodology have slowed by 30 mgC m^-2 d^-1 y^-1 on average. Coincident with this trend, the suboxic zone has narrowed while penetration of O₂ to depth as well as distributions of deep water ammonium and phosphate have shoaled. Community analyses reveal that integrated inventories (250-450 m) of total prokaryotes have also declined in this layer. Time series of specific ecophysiotypes derived from fluorescent in situ hybridization (FISH) reveal a changing prokaryotic community. For example, representation of epsilon-proteobacteria appears to be declining as gamma-proteobacteria are in their ascendancy. In this talk, we examine biological responses to changing biogeochemical distributions and the environmental factors driving these changes.