Abstract
Although previous studies have described opposing states in upwelling regions, i.e., the rise of cold nutrient-rich waters and prevalence of surface warm nutrient-poor waters, few have addressed the transition from one state to the other. This study aimed to describe the microbial and viral structure during this transition and was able to obtain the taxonomic and metabolic compositions as well as physical-chemical data. This integrated approach allowed for a better understanding of the dynamics of the downwelling upwelling transition, suggesting that a wealth of metabolic processes and ecological interactions are occurring in the minute fractions of the plankton (femto, pico, nano). These processes and interactions included evidence of microbial predominance during downwelling (with nitrogen recycling and aerobic anoxygenic photosynthesis), different viral predation pressures over primary production in different states (cyanobacteria vs eukaryotes), and a predominance of diatoms and selected bacterial and archaeal groups during upwelling (with the occurrence of a wealth of nitrogen metabolism involving ammonia). Thus, the results provided insights into which microbes, viruses and microbial-mediated processes are probably important in the functioning of upwelling systems.
Highlights
Upwelling regions sustain a high productivity of primary producers, such as diatoms and fish
The water temperature measured during sampling and wind direction data (S1 Fig) confirmed the occurrence of an upwelling event, as stated in the Methods (Sampling site and dates)
On January 18th, the higher viral and microbial counts and higher ammonia levels point to the aftereffects of a downwelling condition, with a microbial food web and nitrogen cycling
Summary
Upwelling regions sustain a high productivity of primary producers, such as diatoms and fish. The Brazilian coast is not among the five major upwelling coasts of the world, namely, the Canary current system off the Iberian Peninsula and northwestern Africa, the Benguela current system off southwestern Africa, the Peru current system off western South America, and the western coast of the United States [1, 2], the southeastern portion of the coast includes the South Brazil Bight, a high productivity zone characterized by upwelling events [3]. The passage of southern (S-SW) cold fronts interrupts the upwelling, and surface waters from the Continental Water and the Brazilian Current (Tropical Water) prevail [2,3,4]
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