Hydrocarbon seeps are typically viewed as seafloor phenomena: with vast quantities of juicy organic molecules streaming out of the crust, sediment-hosted microbes are the primary recipients of the energetic windfall. But at particularly high-flow seeps, hydrocarbons can emerge into the bottom water, and, running the gauntlet of benthic aerobic heterotrophs, even reach the water's surface layers. A new study from Nigel D'souza and colleagues demonstrates that these seafloor features actually result in measurable increases in surface-water microbial activity. By quantifying chlorophyll concentrations, the team shows that seafloor venting of many kinds may have a larger effect on the surface world than previously believed.

Abstract below, full paper [here].

Natural hydrocarbon seeps occur on the sea floor along continental margins, and account for up to 47% of the oil released into the oceans1. Hydrocarbon seeps are known to support local benthic productivity2, but little is known about their impact on photosynthetic organisms in the overlying water column. Here we present observations with high temporal and spatial resolution of chlorophyll concentrations in the northern Gulf of Mexico using in situ and shipboard flow-through fluorescence measurements from May to July 2012, as well as an analysis of ocean-colour satellite images from 1997 to 2007. All three methods reveal elevated chlorophyll concentrations in waters influenced by natural hydrocarbon seeps. Temperature and nutrient profiles above seep sites suggest that nutrient-rich water upwells from depth, which may facilitate phytoplankton growth and thus support the higher chlorophyll concentrations observed. Because upwelling occurs at natural seep locations around the world1, 2, 3, we conclude that offshore hydrocarbon seeps, and perhaps other types of deep ocean vents and seeps at depths exceeding 1,000 m, may influence biogeochemistry and productivity of the overlying water column.