Coupling between microbial assemblages and environmental drivers along a tropical estuarine gradient

The change in the community structure of phytoplankton and bacterioplankton, and in the degree of coupling between them as well as the environmental conditions, have substantial impacts on the transfer of energy to higher trophic levels and finally on the fate of organic matter. The microbial community structure, usually described only by the abundance of the different taxonomic or functional groups, can be extended to include other levels of descriptors, like physiological state and single-cell properties. These features play a role in the ecological regulation of microbial communities but are not generally studied as additional descriptors of the community structure. Here, we show the changes in abundance and single-cell characteristics based on flow cytometry measurements of picocyanobacteria, photoautotrophic pico- and nanoeukaryotes, and heterotrophic bacteria during the rainy and dry seasons along the estuarine gradient of the inner Gulf of Nicoya. The spatiotemporal distribution of these microbial assemblages showed different patterns in surface and bottom waters along the estuarine gradient and seasonally, both in their abundances and single-cell traits, which suggest differences in their ecological regulation. The changes in the structure of the microbial community along the estuary correlated most significantly with the changes in environmental variables during the dry season. This seems to occur due to changes in salinity, concentration and lability of DOC, concentration of DIN and PO₄^3- and net community production, largely affected by the differences in the river flow. In addition, during the dry season, small-size phytoplankton and bacterioplankton assemblages, characterised by abundance and single-cell traits, presented a higher level of coupling, leading to a more complex ecological network with respect to the rainy season.