Shifts in picophytoplankton community structure influenced by changing upwelling conditions
Résumé
The influence of upwelling events on the structure of picophytoplankton communities was assessed at the annual scale from a station within the South Australian shelf region. In this region, local (wind) and global (La Niña/El Niño-Southern Oscillation) hydroclimatic conditions affect the development of upwelling over the austral summer. Using flow cytometry, changes in picophytoplankton community structure were investigated in relation to the properties of the water column when the nature and strength of upwelling event differed for the upwelling seasons of 2008, 2009, and 2010. In 2008, strong upwelling favorable southeasterlies were responsible for extensive upwelling and the dominance of picoeukaryotes. Alternatively, in 2009, the observed dominance of Prochlorococcus reflected the presence of oligotrophic conditions whilst southeasterlies were replaced by downwelling favorable north-westerlies that likely prohibited the full development of upwelling. In 2010, whilst southeasterlies remained relatively weak, particularly cold and low saline upwelled waters indicated enhanced upwelling events. This weak local wind field together with the occurrence of El Niño explained the observation of shallow upwelled waters below the warm surface layer and subsequent enhanced stratification. These conditions led to the dominance of Synechococcus in surface and fluorescence maximum depths, but of Prochlorococcus in bottom upwelled waters. The tight association between upwelling and stratification, i.e. whether upwelled waters reach shallower depths and/or mix with those of the surface as a result of variable climatic conditions, was suggested as the process driving the vertical heterogeneity of picophytoplankton populations. This study brings valuable information for changing picophytoplankton community structure with potential future changing hydroclimatic forcing.