Comparing the effect of nitrate and urea enrichment on oligotrophic phytoplankton assemblages

Friday, 19 December 2014
Daniel P Harrison, University of Sydney, Uiversity of Sydney Institute of Marine Science, Sydney, Australia
It has been speculated in the literature that “… urea enrichment would preferentially lead to the enhanced production of cyanobacteria, picoeukaryotes, and dinoflagellates, rather than diatoms.” (Gilbert et al. 2010).

Urea is the most manufactured chemical in the world (160 M tonnes p.a.) and the majority will eventually find its way into the marine environment, potentially contributing an annual nitrogen loading equivalent to 5% of New Primary Production (N), important global ramifications are to be expected if the hypothesis above is correct.

 The hypothesis was tested by enriching oligotrophic water samples collected from the Port Hacking National Reference Station, Australia with urea and nitrate in repeated experiments over an annual cycle during 2013. Biomass increased in all experiments, and had a higher incidence of diatoms to dinoflagellates in all experiments, with no significant difference between treatments for diatom cell count. In two instances dinoflagellate cell counts were significantly higher in nitrate treatments than in urea treatments, with no significant difference for the remaining experiments.

The evidence does not support the hypothesis that urea preferentially leads to the production of dinoflagellates rather than diatoms when compared with nitrate enrichment. In this presentation I will examine and compare the effects of nitrate and urea enrichment on natural oligotrophic assembledges of phytoplankton, under laboratory conditions.