Growth response of natural phytoplankton to enrichment of urea and other forms of dissolved nitrogen
The Broadwater of Myall Lakes, NWS Australia is brackish water which has a high variation in water quality in particular salinity and nutrient concentration. In these experiments, we determined the growth and species assemblages of natural phytoplankton community exposed to nutrient enrichment. Laboratory incubation condition was used to measure 10 days biomass and group-specific response of phytoplankton community. Four experimental occasions were conducted in autumn (April and May, 2005) and early summer (November and December, 2005) with two experiments each season. Biomass of phytoplankton was determined based on chlorophyll fluorescence, extracted chlorophyll a and cell abundance of phytoplankton, and community structure/species assemblages was based on manual identification until genus level using upright light microscope. During four experiment events, chlorophyll growth response was significantly higher in N+P treatment than control and N treatment only, with slow growth rate occurring 24 to 48 h following nutrient addition. There was an inconsistent trend of biomass in terms of cell abundance in respond to nutrient enrichment between experiment occasions, except for May experiment. Generally, our study found that the greatest difference of phytoplankton growth/biomass at the Broadwater of Myall Lakes was at site level instead of nutrient treatments. Our study also revealed that urea and other forms of dissolved N stimulated growth of group specific of phytoplankton, with P addition contributed considerably to changing in community structure of phytoplankton. The result of this study suggest that urea was not a factor for Cyanobacteria bloom as compared to the other dissolved N forms, consequently urea does not give a further enhance for Cyanobacteria bloom formation in The Broadwater of Myall Lakes system. This study revealed that enrichment of different forms of dissolved nitrogen stimulated the growth of phytoplankton taxa in ways that resulted in significant differences in species assemblages among treatments for most sites. Extra addition of P can initiate bloom conditions for cyanobacteria. In order to reduce this possibility, P flow from catchment areas has to be prevented.
Keywords: Phytoplankton, community structure, nutrient, urea.