Phytoplankton dynamics in the Hunter River and factors affecting them

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NO FULL TEXT AVAILABLE. Access is restricted indefinitely. ----- This thesis examines phytoplankton dynamics and the influences of environmental variables on them in the Hunter River, New South Wales. The environmental variables of flow (discharge), water velocity, water temperature, turbidity, electrical conductivity (conductivity), pH and nutrients (ammonia, oxidized nitrogen, reactive phosphorus, reactive silica, total nitrogen and total phosphorus) were examined. Dominance in phytoplankton and periodic succession and the application of the Intermediate Disturbance Hypothesis to the Hunter River were also investigated. The environmental variables of conductivity, turbidity and nutrients appear to be particularly influenced by flow at several sites, with increased flow causing increases in turbidity and nutrient levels. Baseline nutrients are, however, still high with all nutrients exceeding the ANZECC guideline concentrations for natural waters. These concentrations are also considered high enough to give the river a trophic status of eutrophic, with inputs into the system from local land uses of farming, irrigation, mining and town sewage treatment plants. The phytoplankton communities are dominated by diatoms, with subdominant populations of Chlorophyceae with periodic succession showing transitions from true r-strategists (e.g. Cyclotella meneghiniana and Oocystis spp.) to true K-strategists (e.g. Ceratium spp. and Closterium spp.) through a dominance of intermediate strategists of the pennate diatoms. While the Hunter River was dominated throughout by Fragilaria spp., Navicula spp. and Oocystis spp., the river is divided distinctly into two sections, an upper and lower section which are characterized by different successional dominance, notably by large populations of Cyclotella meneghiniana and Melosira varians in the lower section and dominant populations of Closterium spp. and Ceratium spp. in the upper section. The trophic status of the Hunter River shows phytoplankton similarities to mesotrophic and eutrophic lakes. Due to Fragilaria spp., Navicula spp. and Oocystis spp. dominating, and the lack of cyanobacterial populations, the Hunter River appears to be predominantly bordering on eutrophic, with changes back to mesotrophic during the summer periods in the lower Hunter, when Cyclotella meneghiniana becomes dominant. Flow appears to determine the overall residence time of phytoplankton in the river and has an overall influencing effect, as it is highly correlated with many other influences such as turbidity, conductivity and nutrients. Water temperature in the Hunter River appeared to influence seasonality of the phytoplankton communities. Other than this no individual influence on phytoplankton species was apparent. The lack of stratification and low water velocities resulted in only small populations of cyanobacteria in the upper Hunter River sites, and rarely observed populations in the lower Hunter River sites. The average water velocity ranged from 0.599 m/s (upper site) to 0.031 m/s (lowest site). Liddell Weir however experienced an average velocity of 0.0307 m/s and, under favourable environmental conditions, increased growth of cyanobacteria could be possible if this velocity was maintained for extended periods. Turbidity in the Hunter showed peaks during major flow events, and increases in the populations of the diatoms Fragilaria spp. and Navicula spp. showed that benthic/periphytic assemblages contributed to the phytoplankton within these highly turbid periods. Conductivity and pH overall showed no influence individually on phytoplankton, but together with flow and nutrients, conductivity has an influence with a change in dominance from Chlorophyceae to diatoms shown at Glenbawn Weir. Nutrients in the Hunter River appear to be non-limiting to phytoplankton growth. From the observations of the Hunter River, the Intermediate Disturbance Hypothesis tends to apply to the phytoplankton and environmental conditions. • Generally disturbances of intermediate intensities tended to promote higher diversities during, or after, the disturbance event. • In periods of equilibrium and during disturbances of high and low intensities, phytoplankton diversity generally decreases.
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