Seasonal patterns of microbial plankton and periodicity of climatic and hydrographic conditions in a semi-desert macrotidal wetland

Macrotidal wetlands of temperate areas are highly productive systems where interactions between land and water are particularly dynamic. Typically, these systems display temporal shifts in their water properties not only because of annual fluctuations in the photoperiod and air temperature but also due to strong tidal regimes and climatic events. In contrast with the well-studied microtidal systems, the temporal fluctuations in structure and biomass of microbial communities of semi-desert, enclosed temperate macrotidal wetlands are scarcely known, and thus the coupling between primary producers and the bacterial community occurring within them remains poorly understood. In this study we analyze the fluctuations of unicellular plankton and hydrographic conditions on a fixed station located in San Antonio Bay (SAB, Northern Argentine Patagonia) from spring 2016 to early summer 2018 with the aim of detecting possible associations with the periodicity of some climatological variables and local meteorological events. Density and biomass of microbial size fractions (pico-, nano- and microplankton) along with their nutrition modes (autotrophic, potentially mixotrophic and heterotrophic) were assessed based on bi-weekly samplings. In addition, the study also examines the relationships of autotrophs and bacterioplankton with the frequency, duration, and magnitude of phytoplanktonic blooms, as well as the biomass contribution of dominant diatom species. We found a microbial-dominated planktonic system with increased densities of both autotrophic and heterotrophic picoplankton and flagellated protists. The biomass of potentially mixotrophs dominated over that of autotrophs and heterotrophs along the entire period. Autotrophic biomass showed a single annual maximum in summer, while chlorophyll-a displayed a biannual cycle. Neither chlorophyll-a nor autotrophic biomass reached high values as compared to other coastal systems with no clear dominant phytoplanktonic taxa. The annual phytoplanktonic biomass cycle in SAB appears not to be related to those of other coastal environments from North Patagonia. We infer that the strong macrotidal dynamics limits primary production and turns this system into a high nutrient/low chlorophyll one that exports surplus nutrients in tide-driven pulses to neighboring areas.