Nitrogen uptake by phytoplankton in the Huon Estuary : with special reference to the physiology of the toxic dinoflagellate Gymnodinium catenatum

The Huon Estuary is a micro-tidal estuary in south-east Tasmania that is an important area for salmon aquaculture. In 2008 the salmon aquaculture industry in Australia is worth $260M (AUD) per year. Salmon aquaculture began in the Huon Estuary in the 1980's and production has since increased significantly. The Huon Estuary is nitrogen (N) limited and salmon farming is a significant input of N to this ecosystem. Both industry and government regulators are alert to the potential for eutrophication and increased harmful algal blooms if the assimilative capacity for N of the estuary is exceeded. As part of a larger project on the ecology of the Huon Estuary, this PhD research has two main objectives; firstly to determine whether phytoplankton in the Huon Estuary are using nitrogen that had, primarily, an oceanic source (e.g. nitrate) or was more locally supplied or regenerated (e.g. ammonium and urea) and secondly to examine the physiology of G. catenatum a toxic dinoflagellate that dominates the summer and autumn Huon Estuary phytoplankton biomass in many years. Uptake rates of NO\\(_3\\)\\(^-\\), NH\\(_4\\)\\(^+\\) and urea were measured on four occasions (28-29 May 2003, 23-24 Sept 2003, 18-19 Nov 2003, and 24-25 Feb 2004) in the Huon Estuary using a \\(^{15}\\)N tracer technique. Uptake rates were measured at Garden Island and Hideaway Bay in the lower estuary and at 5 and 20 m during the day and also at 5 and 20 m during the night. The mean uptake rates (mean across time of year, site, time of day and depth) for NH\\(_4\\)\\(^+\\) (0.13˜í¬¿g N ˜í¬¿g chl ˜í¬± h\\(^{-1}\\) and urea (0.09 ˜í¬¿g N ˜í¬¿g chl ˜í¬± h\\(^{-1}\\)1) were 4.5 and 3.2 times higher than the uptake of NO\\(_3\\)\\(^-\\) ( 0.3 ˜í¬¿tg N ˜í¬¿g ch la h\\(^{-1}\\)). Overall NH\\(_4\\)\\(^+\\), NO\\(_3\\)\\(^-\\) and urea were responsible for 52, 37.5 and 10.5% of N uptake respectively. Gymnodinium catenatum is a toxic dinoflagellate that blooms periodically in the Huon Estuary and in years that it blooms it dominates the phytoplankton biomass and has caused closure of shellfish farms in the area. Laboratory experiments on effect of temperature and irradiance on growth rate, effect of different nitrogen species on growth rate and preferential uptake of different nitrogen species by G. catenatum were undertaken to better understand the physiology of this species and to test a hypothesis that G. catenatum vertically migrates to access NH\\(_4\\)\\(^+\\) at depth during summer. The effect of 12 different temperatures ranging from 11.9-25.2 ¬¨‚àûC and irradiances from 5-283 gmol photons m\\(^{-2}\\)s\\(^{-1}\\) on growth and biochemical composition of G. catenatum. The highest predicted growth rates (>0.2 d\\(^{-1}\\)) occurred during summer and autumn as might be expected based on observations of bloom dynamics of this species in the Huon Estuary which supports both a summer and autumn bloom in many years. G. catenatum was able to grow using NO\\(_3\\)\\(^-\\), NH\\(_4\\)\\(^+\\) or urea as its sole nitrogen source. There was no significant difference in growth on any of these nitrogen sources. Preferential uptake of NH\\(_4\\)\\(^+\\), NO\\(_3\\)\\(^-\\) or urea was examined by growing G. catenatum on a mixture of NO\\(_3\\)\\(^-\\), NH\\(_4\\)\\(^+\\) and urea. The results clearly showed that NH\\(_4\\)\\(^+\\) was taken up first, followed by NO\\(_3\\)\\(^-\\) and finally urea. Maximum mean uptake rates were 170, 98 and 30 pg cell\\(^{-1}\\) hour\\(^{-1}\\) respectively for NH\\(_4\\)\\(^+\\), NO\\(_3\\)\\(^-\\) and urea. In addition to the laboratory experiments the nitrogen uptake characteristics of a bloom of G. catenatum was examined at Pelican Island, Southport (30-31/03/2004) nearby the Huon Estuary. Mean urea uptake was greatest (0.045 ˜í¬¿gN ˜í¬¿g chl ˜í¬± h\\(^{-1}\\)) followed by NH\\(_4\\)\\(^+\\) (0.029 ˜í¬¿gN ˜í¬¿g chl ˜í¬± h\\(^{-1}\\)) and the lowest uptake was of NO\\(_3\\)\\(^-\\) (0.025 ˜í¬¿gN ˜í¬¿g chl ˜í¬± h\\(^{-1}\\)). For G. catenatum growing in the Huon Estuary it seems increasingly apparent that it functions as a nitrogen scavenger. When N concentrations are exhausted, it is able to migrate through the water column seeking whatever form of nitrogen is available.