Autotrophic and heterotrophic metabolism of microbial planktonic communities in an oligotrophic coastal marine ecosystem: seasonal dynamics and episodic events

Abstract. A 18 month study was performed in the Bay of Villefranche to assess the episodic and seasonal variation of autotrophic and heterotrophic ecosystem processes. A typical spring bloom was encountered, where maximum of gross primary production (GPP) was followed by maxima of bacterial respiration (BR) and production (BP). The trophic balance (heterotrophy vs. autotrophy) of the system did not exhibit any seasonal trend although a strong intra-annual variability was observed. On average, the community tended to be net heterotrophic with a GPP threshold for a balanced metabolism of 1.1 μmol O₂ l⁻¹ d⁻¹. Extended forest fires in summer 2003 and a local episodic upwelling in July 2003 likely supplied orthophosphate and nitrate into the system. These events were associated with an enhanced bacterioplankton production (up to 2.4-fold), respiration (up to 4.5-fold) and growth efficiency (up to 2.9-fold) but had no effect on GPP. A Sahara dust wet deposition event in February 2004 stimulated bacterial abundance, production and growth efficiency but not GPP. Our study suggests that short-term disturbances such as wind-driven upwelling, forest fires and Sahara dust depositions can have a significant but previously not sufficiently considered influence on phytoplankton- and bacterioplankton-mediated ecosystem functions and can modify or even mask the seasonal dynamics. The study also indicates that atmospheric deposition of nutrients and particles not only impacts phytoplankton but also bacterioplankton and could, at times, also shift systems stronger towards net heterotrophy.