Submarine hydrothermal activity is responsible for heat and chemical
exchanges through the seafloor. Shallow-water hydrothermal systems
(SWHS), while identified around the globe, are often studied in a way
that is less comprehensive than their deep-ocean counterparts (e.g.,
along ridges), where systematic optical and acoustic mapping is more
prevalent and coupled to in situ observations and sampling. Using aerial
drones, an AUV, and temperature measurements at 10-40 cm subseafloor, we
investigated in 2019 one of the most extensive SWHS known to date, in
Paleochori and nearby Spathi and Agia Kyriaki Bays (south of Milos,
Greece). Hydrothermal venting, found from the shore to water depths of
almost 500 m, shows emissions of gases and high-temperature fluids,
often associated with bacterial mats and/or hydrothermal mineral
precipitates. This study provides extensive drone mapping coupled with
local AUV surveys for seafloor characterization and ground-truthing from
the shore to similar to 20 m water depth. Seafloor photomosaics also
provide a detailed context to samples, measurements and observations
carried in situ. We interpret the photomosaics to define distinct
seafloor types, linked to this hydrothermal activity. White hydrothermal
patches (WHPs) often show a clear polygonal organization, together with
outflow areas that are both more dispersed and distributed. Polygonal
patterns likely result from fluid convection in a sandy porous medium
heated from below. These WHPs display elevated subseafloor temperatures,
typically >50 degrees C, with maximum values of similar to 75 degrees C.
Photomosaics also display textures of biological origin, including
seagrass and bioturbation patterns. Widespread bioturbation by burrowing
shrimps is often associated with WHPs, bounding them, but also occurs on
sandy seafloor away from hydrothermal patterns. Subseafloor temperatures
at these bioturbated areas are of similar to 30-40 degrees C, and are
thus transitional between hot WHPs and sedimented seafloor unaffected by
hydrothermal activity (similar to 24 degrees C). In addition to linking
subseafloor temperature data and interpreted seafloor photomosaics, our
results provide a comprehensive general overview of this SWHS, of the
organization of its hydrothermal outflow through the seafloor, and of
the underlying subseafloor fluid circulation. This paper also gives the
first perspectives on the heat fluxes of the system, and constitutes a
background for other studies on the nature and distribution of microbial
communities, controlled by this hydrothermal activity
