How flow cytometry can monitor change in Australian coastal waters: identifying seasonal variability and the effects of extreme marine events on picophytoplankton abundance.

Picophytoplankton are photosynthetic cells (< 3µm diameter) that contribute most of chlorophyll biomass and productivity in the world’s oceans. Prochlorococcus dominates in oligotrophic tropical oceans while Synechococcus is most abundant in nutrient replete waters. With a large surface area to volume ratio making these cells responsive to their environment, the abundance of the two species make them good indicators of seasonal change and extreme events. We used flow cytometry to analyse monthly samples (2009 – 2011) from the Integrated Marine Observing System National Reference Stations (NRS) around Australia. Prochlorococcus and Synechococcus were discriminated using their chlorophyll-a versus phycoerythrin fluorescence. Although we have only 3.5 years of data, seasonal patterns in picoplankton abundance were evident from stations at Rottnest Island (WA), Yongala and North Stradbroke Island (QLD), Maria Island (TAS) and Port Hacking (NSW), due to seasonality in flows of the Leeuwin and East Australian currents. Furthermore, picoplankton abundance underwent major shifts following extreme weather events. At Rottnest Island, unseasonably high abundances of Prochlorococcus were recorded in February 2011, transported south during an abnormal marine heat wave. At North Stradbroke Island and Port Hacking, elevated abundances of Synechococcus followed the September 2009 dust storms, probably due to enhanced growth and production through iron deposition. At Yongala station, Synechococcus abundance increased abruptly in February 2011 following an extensive, nutrient-laden flood plume from the Burdekin River. This data has revealed insights into factors controlling annual and inter-annual variation in picoplanktonabundances, as well as dynamics in relation to extreme weather events.

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