Abstract
To investigate the responses of a natural microbial plankton community of coastal Mediterranean waters to warming, which are still poorly known, an in situ mesocosm experiment was carried out in Thau Lagoon during autumn 2018. Several microorganisms, including virio-, bacterio-, and phytoplankton < 10 µm in size, were monitored daily and analysed using flow cytometry for 19 consecutive days in six mesocosms. Three mesocosms (control) had the same natural water temperature as the lagoon, and the other three were warmed by + 3 °C in relation to the control temperature. The cytometric analyses revealed an unexpected community dominated by picophytoplanktonic cells, including Prochlorococcus-like and Picochlorum-like cells, which had not previously been found in Thau Lagoon. The experimental warming treatment increased the abundances of nanophytoplankton, cyanobacteria, bacteria and viruses during the experiment and triggered earlier blooms of cyanobacteria and picoeukaryotes. Only the abundance of Picochlorum-like cells was significantly reduced under warmer conditions. The growth and grazing rates of phytoplankton and bacterioplankton estimated on days 2 and 8 showed that warming enhanced the growth rates of most phytoplankton groups, while it reduced those of bacteria. Surprisingly, warming decreased grazing on phytoplankton and bacteria at the beginning of the experiment, while during the middle of the experiment it decreased the grazing on prokaryote only but increased it for eukaryotes. These results reveal that warming affected the Thau Lagoon plankton community from viruses to nanophytoplankton in fall, inducing changes in both dynamics and metabolic rates.
Highlights
Small phytoplankton, such as picophytoplankton and nanophytoplankton, and nonphotosynthetic prokaryotes, such as heterotrophic bacteria and archaea, are key components of microbial food webs (Pomeroy 1974; Azam et al 1983; Mostajir et al 2015a)
Warming decreased grazing on phytoplankton and bacteria at the beginning of the experiment, while during the middle of the experiment it decreased the grazing on prokaryote only but increased it for eukaryotes. These results reveal that warming affected the Thau Lagoon plankton community from viruses to nanophytoplankton in fall, inducing changes in both dynamics and metabolic rates
This study shows that the phytoplankton community investigated in October 2018 was quite different from what is usually expected at this season in Thau Lagoon
Summary
Small phytoplankton, such as picophytoplankton and nanophytoplankton, and nonphotosynthetic prokaryotes, such as heterotrophic bacteria and archaea (hereafter referred to as bacteria only), are key components of microbial food webs (Pomeroy 1974; Azam et al 1983; Mostajir et al 2015a). They play an important role in the production of biomass, which is transferred by microzooplankton to higher trophic levels through the marine food web. Changes in planktonic microorganism abundance are mainly shaped by individual growth and mortality rates (Calbet and Landry 2004).
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