Effect of organic nutrient perturbation on microbial community dynamics and composition in Scottish coastal waters
This study investigates the effect of organic nutrient perturbation on the dynamics of marine microbial communities, adopting three different approaches. A field study was conducted on the west coast of Scotland, in two contrasting locations, including a fish farm anticipated as a point source of organic nutrients. Data collected from the stations sampled revealed that the marine microbial communities were highly dynamic. The comparison of the physico-chemical and biological variables measured at each station indicated that the abundances, diversities and productions of planktonic micro-organisms varied in time and space. These marine microbes also demonstrated strong ecological interactions such as competition or predation. The dissolved organic matter (DOM) was revealed to originate from various sources with production by phytoplankton being dominant. Laboratory based experiments were conducted to complement field observations and test different hypotheses, such as the effect of substrate organic stoichiometry on the growth of bacteria and their grazers, the effect of DOM produced by nutrient-stressed phytoplankton on the bacterioplankton dynamics or the factors limiting the natural microbial communities. These experiments highlighted the key role of bacteria within the microbial loop, efficiently regenerating DOM into inorganic nutrients, the importance of the quality and stoichiometry of the DOM in controlling bacterial growth. Additionally in these experiments, the manipulation of size-fractionated microbial food webs revealed the importance of micro-grazers in controlling the bacterioplankton dynamics. Finally, mathematical modelling was used to synthesise the field and experimental observations and to simulate ecological interactions concurrent to organic nutrient variations. An existing model was modified to include the bacterioplankton component as well as a more adequate DOM pool representation.
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