Influences of physical forces on the distribution of biomass and diversity of bacterioplankton and phytoplankton in Northeastern Brazil

Abstract

The energy flux in marine plankton food webs heavily relies on phytoplankton and bacterioplankton. Despite their ecological significance, tropical plankton communities, particularly in the Southwestern Tropical Atlantic, are not fully understood. This Ph.D. thesis aims to improve our understanding of tropical phytoplankton and bacterioplankton communities in the Southwestern Tropical Atlantic, their potential impact on the metabolic structure and trophic pathways of plankton food webs, and the underlying environmental processes regulating their dynamics. The thesis was structured in three main manuscripts that explored (i) the influence of the thermohaline configuration on coarse scale patterns of phytoplankton communities’ structure and main groups driving the total phytoplankton biomass, (ii) changes in autotrophic:heterotrophic ratios and their use as indicators of trophic pathways, and (iii) bottom-up and top-down processes driven the structure of microbial communities. The data used here were obtained from a research program composed of two oceanographic campaigns (ABRACOS 1 and 2), which extended to around 300 km2, and were performed during contrasting environmental settings (austral spring and fall). In the first manuscript, this data was used to examine how thermohaline stratification influence phytoplankton biomass and structure (i.e., size) along a coastal-offshore gradient. The results highlighted the role played by thermohaline structure as the main regulator of community dynamics. During fall, shallower thermocline and nutricline led to a threefold increase in biomass in the upper layers of the oceanic region. Despite this seasonal increase, we observed a dominance of recycled production, along with an uncoupled dynamics between biomass and size structure were observed. Picophytoplankton and nanophytoplankton accounted for about 80% of the community in both seasons, likely due to nitrogen limitation. Subsequently, in the manuscript 2 we then investigated how changes in thermohaline structure affect the autotrophic and heterotrophic biomass proportions of picophytoplankton and nanoplankton. To do so, we used cytometry data from fall campaign (ABRACOS 2). We explored their potential as indicators of trophic pathways within the plankton food web and identified environmental and biotic factors shaping their distribution. Nitrogen-limited environments led to the dominance of heterotrophic bacterial biomass over autotrophic growth. However, in the oceanic region, at the deep chlorophyll maximum, increased nutrient availability favored an increase in picoeukaryote biomass. Overall, these results emphasize the importance of heterotrophic biomass in planktonic microbial communities, providing insights on carbon fluxes in oligotrophic marine ecosystems. In the third manuscript, we investigated whether the community structure of picoplankton and nanoplankton influences the distribution of planktonic microorganism biomass and explored the role of bottom-up and top-down controls in food web formation. Our results suggest a predominant bottom-up control in regulating microphytoplankton biomass through changes in silicate availability and subsequent impact on diatoms. In contrast, mixotrophic and autotrophic microphytoplankton exhibited contrasting relationships with silicate, thriving at low concentrations and dominating in nutrient-rich environments, respectively. We also found that the Tintinnins play a predominant top-down control on autotrophic phytoplankton distribution. These results highlight the role of thermohaline structure controlling the structure (i.e., size, richness) and productivity of microbial communities. Finally, this thesis represents an important initial step toward modeling microbial networks in the Southwestern Tropical Atlantic.