The influence of hydrothermal plumes on midwater ecology, and a new method to assess pelagic biomass

Submarine hydrothermal vent fields introduce buoyant plumes of chemically altered seawater to the deep-sea water column. Chemoautotrophic microbes exploit this energy source, facilitating seafloor-based primary production that may transfer to pelagic consumers. This dissertation synthesizes a growing body of scientific evidence supporting the hypothesis that hydrothermal plumes are the energetic basis of unique deep-sea pelagic food webs, including secondary and tertiary consumers. Contemporary largescale plume events associated with periods of eruptive activity suggest that hydrothermal plumes may have influenced basin-scale ocean chemistry during periods of increased submarine volcanism. The author speculates that hydrothermal plumes are capable of driving both positive and negative impacts to pelagic communities on broad spatiotemporal scales. Increased research efforts, focused on high-resolution surveys of midwater biology relative to plume structures, are recommended to establish baseline conditions and aid in monitoring the impact of anthropogenic disturbances to the deepsea pelagic biosphere. Present-day plume environments constrained by seafloor bathymetry may be analogous to hypothetical large-scale plume events in geologic history. To investigate this relationship, biogeochemical exploration was conducted on Kavachi, a highly active submarine volcano in the Solomon Islands. This expedition was serendipitously timed with a rare lull in volcanic activity, allowing for observation of the inside of Kavachi’s active crater and its flanks. Our research efforts resulted in medium-resolution bathymetry of the main peak paired with benthic imagery, biological observations of multiple trophic levels inside the active crater, petrological and geochemical analysis of samples from the crater rim, measurements of water temperature and gas flux over the summit, and descriptions of the hydrothermal plume structure. Populations of gelatinous animals, small fish, and sharks were observed inside the active crater, raising new questions about the ecology of active submarine volcanoes and the extreme environments in which large marine animals can exist. Observing pelagic biology in high spatiotemporal resolution presents unique challenges, characterized by the difficult task of measuring freeswimming animals in a three-dimensional environment. Several methods have arisen using bioluminescence as an assay for total living biomass. Building upon these efforts, a scientific complementary-metal-oxidesemiconductor (sCMOS) microscopy camera was outfitted for deep-sea imaging of marine bioluminescence. This system was deployed on multiple platforms (manned submersible, remotely operated vehicle, and towed body) in three oceanic regions (Western Tropical Pacific, Eastern Equatorial Pacific, and Northwestern Atlantic) to depths up to 2500 m. Using light stimulation, bioluminescent responses were recorded at high frame rates and in high resolution, offering unprecedented low-light imagery of deep-sea bioluminescence in situ. The kinematics of light production in several zooplankton groups was observed, and luminescent responses at different depths were quantified as intensity vs. time. These initial results signify a clear advancement in the bioluminescent imaging methods available for observation and experimentation in the deep-sea. The presented method may be applicable to large-scale, high-resolution surveys of pelagic biology in the context of deep-sea hydrothermal plumes.