Intense but variable autotrophic activity in a rapidly flushed shallow‐water hydrothermal plume (Kueishantao Islet,Taiwan)

Shallow‐water hydrothermal plumes concomitantly host both photosynthetic and chemoautotrophic organisms in a single biotope. Yet, rate measurements to quantify the contributions of different autotrophic activity types are scarce. Herein, we measured the light and dark dissolved inorganic carbon (DIC) uptake rates in the plume water of the Kueishantao hydrothermal field using the ¹³C‐labeling approach. Seventy percent of the plume‐water samples had chemoautotrophy as the dominant mode of carbon fixation, with the dark DIC uptake rates (up to 18.6 mg C/m³/h) within the range of the primary production in productive inner‐shelf waters. When considered alongside the geochemical and microbiological observations, the rate data reveal the distribution of different trophic activities in the hydrothermal plume. The autotrophic activity at the initial phase of plume dispersal is low. This is explained by the short response time the chemoautotrophs have to the stimulation from vent‐fluid discharge, and the harmful effects of hydrothermal substances on phytoplankton. As plume dispersal and mixing continue, chemoautotrophic activities begin to rise and peak in waters that have low‐to‐moderate Si(OH)₄ content. Toward the plume margin, chemoautotrophy declines to background levels, whereas photosynthesis by phytoplankton regains importance. Our results also provide preliminary indication to the loci of enhanced heterotrophy in the plume. Results of artificial mixing experiments suggest that previously formed plume water is the primary source of microbial inoculum for new plume water. This self‐inoculation mechanism, in combination with the intense DIC uptake, helps to sustain a distinct planktonic autotrophic community in this rapidly flushed hydrothermal plume.