Using fluorescence and bioluminescence sensors to characterize auto- and heterotrophic plankton communities

High-resolution autonomous sensors routinely measure physical (temperature, salinity), chemical (oxygen, nutrients) and biological (fluorescence) parameters. However, while fluorescence provides a proxy for phytoplankton, heterotrophic populations remain challenging to monitor in real-time and at high resolution. Bathyphotometers, sensors which measure the light emitted by bioluminescent organisms when mechanically stimulated, provide the capability to identify bioluminescent dinoflagellates and zooplankton. In the coastal ocean, highly abundant dinoflagellates emitting low-intensity flashes generate a background bioluminescence signal, while rarer zooplankton emit bright flashes that can be individually resolved by high-frequency sensors. Bathyphotometers were deployed from ships and onboard autonomous underwater vehicles (AUVs) during three field campaigns in Monterey Bay, California. Ship-based in situ water samples were simultaneously collected and the plankton communities characterized. Plankton concentrations were matched with concurrent datasets of fluorescence and bioluminescence to develop proxies for autotrophic and heterotrophic dinoflagellates, other phytoplankton such as diatoms, copepods, larvaceans (appendicularians), and small jellies. The method extracts the bioluminescence background as a proxy for dinoflagellates, and exploits differences in bioluminescence flash intensity between several types of zooplankton to identify larvaceans, copepods and small jellies. Fluorescence is used to discriminate between autotrophic and heterotrophic dinoflagellates, and to identify other autotrophic plankton. Concurrent fluorometers and bathyphotometers onboard AUVs can thus provide a novel view of plankton diversity and phytoplankton/zooplankton interactions in the sea.