Warming promotes bacterial and phytoplankton growth and grazing rates in a Mediterranean coastal lagoon: an in situ mesocosm experiment

Plankton communities and their interactions are expected to change under global warming. To study the responses of bacterio- and phytoplankton growth and grazing rates, an in situ mesocosm experiment was carried out to simulate warming in natural coastal Mediterranean waters (Thau lagoon, France) during 18 days in April 2018. To estimate changes in growth and grazing rates of different bacterio- and phyto-plankton groups, six dilution experiments were realized during the phytoplankton pre-bloom, bloom and post-bloom periods, which occurred in the course of the experiment. Water temperature increase enhances bacterial and small phytoplankton growth rates during the bloom but surprisingly also during post-bloom periods, suggesting that warming extends nutrient availability through regeneration during the post-bloom. Similarly, predators of both bacteria and small phytoplankton seemed positively affected by warming as the grazing rates strongly increased during both bloom and post-bloom periods. The balance between growth and grazing rates indicates that warming tends to reduce phytoplankton biomass accumulation during the bloom, while it favours heterotrophic bacteria biomass accumulation in both pre- and post-bloom periods, despite growth rates stimulation of both planktonic groups. In summary, results suggest that warming favours small microorganisms and trigger the planktonic community toward the predominance of heterotrophic ones.