Effect of atmospheric input on marine microbial food web

Abstract

Due to intensifying human activities in recent decades, atmospheric deposition is now recognized as an important source of bioavailable nutrients to the ocean, altering the patterns of the marine biogeochemistry. This dissertation focuses on exploring the effect of atmospheric input on dynamics of marine microbial food web, using techniques on community, individual and molecular levels, by conducting experiments in two different ecosystems exposed to high atmospheric input: the South China Sea affected by the East Asian aerosol, and the Mediterranean Sea affected by the Sahara mineral dust and European aerosol. In the South China Sea, for the first time, the comprehensive response of phytoplankton and bacterial community composition as well as the trophic interactions between prey and predators to the East Asian aerosol input was evaluated. High levels of aerosol loading relieved phytoplankton nutrient limitation, increased phytoplankton biomass, enhanced their physiological conditions, and shifted phytoplankton assemblages from being dominated by picoplankton to microphytoplanton, especially diatoms. However, the accumulation of phytoplankton biomass was not apparent under low levels of aerosol loading, and the abundance of autotrophic cyanobacteria and heterotrophic bacteria even decreased after aerosol addition, due to enhanced significant enhancement in both rates were obtained after aerosol addition, with different degree of the impact on each size fraction, however. Larger increase in growth rate was obtained for micro-phytoplankton, while pico- and nano-sized cells suffered larger increase of grazing pressure in aerosol amended waters, consequently inducing a phytoplankton community structure shift in response to atmospheric aerosol input. Clear shifts in the phylogenetic composition of the bacterial assemblage were also observed, although bacterial abundance was little changed. This result was possibly due to a combination of bottom up (aerosol nutrient input and phytoplankton community structure change) and top-down (enhanced mortality loss and selective feeding of bacterivorous protists) effects of the aerosol input. Our results suggest that East Asian aerosol plays a very important role in regulating microbial food web dynamics and thus the carbon cycle in the South China Sea, and highlight the importance of considering the role of protist grazing when evaluating the effect of atmospheric deposition on marine planktonic ecosystems under the scenarios of increasing human activity, atmospheric input and natural perturbations. Succession in active bacterial composition was also observed in the ultra-oligotrophic eastern Mediterranean Sea after addition of Sahara dust and mixed European aerosol during a mesocosm experiment that mimic the realistic dust deposition process. The rapid change in bacterial assemblage change corresponded with an increase in bacterial production and the high nucleic acid (HNA) bacteria to total bacteria abundance ratio, indicating the strong link between bacterial community composition and carbon cycling in the oligotrophic oceanic ecosystems.

Date of Award	2014
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology