Changes in bacterial concentrations with depth, temperature, and chlorophyll fluorescence in the Equatorial Pacific Ocean

Abstract

It is important to consider the abundance of marine bacteria, as they play important roles in biogeochemical processes that are significant in the carbon cycle. Marine bacteria can give us insight about factors such as the amount of nutrients in the water, dissolved organic matter (DOM) production by phytoplankton, and photosynthesis occurring in the water. High bacterial abundance indicates that there are high levels of nutrients and organic matter, and examining bacterial abundances at different depths can reveal the state of the ocean such as nutrient levels and bioactivity. My study was carried out by collecting water samples from the Equatorial Pacific Ocean and processing the samples through a flow cytometer to get heterotrophic bacterial concentrations measured in cells/mL. Here we show that bacterial abundance is the highest in the top 200 meters of the ocean and decreases with depth. I found that bacterial abundance mirrors the presence of phytoplankton as heterotrophic bacteria absorb nutrients from dead phytoplankton. My results show that temperature and chlorophyll fluorescence share the same trend as bacteria, of increasing at the surface and decreasing at depth, which seems to relate to the availability of sunlight with depth in the ocean. In addition, bacterial abundance is higher south of the equator compared to north of the equator. This could be due to sunlight and the angle of the sun’s rays on the ocean surface being more direct on the water, resulting in increased sunlight available for photosynthesis in phytoplankton and thus more food for bacteria.