Phytoplankton of the Swan-Canning Estuary: a comparison of nitrogen uptake by different bloom assemblages

Spring chlorophyte-, summer dinophyte- and autumn cryptophyte-dominated phytoplankton blooms occurring in the upper reaches of the Swan-Canning Estuary have been studied to compare species composition and diurnal distribution and to determine preferential uptake rates for different nitrogen sources. Species distribution patterns over diurnal cycles showed dinophyte and cryptophyte species exhibiting diurnal vertical migration patterns which changed over a three-week bloom period. The dinophyte and cryptophyte behaviour appeared to be related to measured changes in ambient nitrogen profiles over the course of the bloom. Chlorophyte species were not shown to be strongly migratory over diurnal periods. Absolute uptake rates (ρ, approximating P max) for nitrogen, in the form of 15NO3 -, 15NH4 + or 15N-urea, were compared between seasonal bloom assemblages and ranged between 0.02 and 12 μg N 1-1 h-1 (NO3 0.02-7; NH4 0.4-9; urea 0.1-12 μg N 1-1 h-1). Specific uptake rates (n) ranged between 2 and 1770 ng N μg Chla-1 h-1 (NO3 2-290; NH4 19-1770; urea 6-1730 ng N μg Chla-1 h-1). Relative preference indices (RPI) for NO3 (new N) vs. NH4 (recycled N) were calculated to determine whether allochthonous or autochthonous nitrogen sources were implicated in supporting these blooms. Results indicated no physiological preference at any depth for NO3 or NH4 during spring and autumn, and for summer surface assemblages. There was a strong preference for NH4 (RPI ~ 0.95) at mid and bottom depths during summer. Ambient NH4 uptake rates were compared with changes in ambient NH4 profiles to determine the importance of measuring fluxes against absolute parameter measurements in understanding the nitrogen dynamics supporting phytoplankton growth. From flux measurements it was found that apparent reappearance rates for NH4 ranging between 1% and 1030% of absolute uptake rates occurred during summer, with decreases equivalent to 100% of ambient uptake rate occurring at depth at night. Nutrient profiling alone, in the absence of knowledge of nutrient fluxes, was insufficient in assessing possible nutrient control over phytoplankton growth and biomass maintenance. Within this system there is the potential for phytoplankton growth and bloom maintenance to occur supported solely by recycling of NH4. Copyright © 2001 John Wiley and Sons, Ltd.