| Summary: | This research determined the changes in phytoplankton community composition in a shallow subtropical lake, influenced by urban surroundings. Specifically this research focused on the effects of seasonal progression and episodic events on the changing inorganic nutrient dynamics and the effects those dynamics had on the phytoplankton community composition and productivity. This research quantified gross primary production and respiration to determine if seasonality or episodic events were acting as forcing functions of the phytoplankton community composition. Water samples were collected weekly at three sites on the lakes perimeter, as well as following episodic events, to monitor nitrate (NO3), phosphate (PO4-3), ammonium (NH4), and silicate (Si) concentrations, and diagnostic pigment concentrations. Gross primary production and respiration was measured following a four-hour incubation period. Results show that seasonality was not significant in affecting the inorganic nutrient concentrations, but episodic events were significant in influencing the concentrations of NO3 and PO4-3. Gross primary production existed at a mean rate of 3.45 gram carbon/gram chlorophyll a/ hour (g C/g chl/hr), and the median respiration rate was 0.66 g C/g chl/hr. Primary production and respiration were not significantly affected by seasonal progression, but gross assimilation of oxygen was significantly increased following episodic events, and increases were dependent on the presence of phosphate and ammonium. The phytoplankton community composition was determined to be 51% chlorophyceae, 30% cyanobacteria, 10% diatoms, and 3.4% cryptophyceae, Chlorophytes and diatoms significantly affected by seasonality and episodic events, but only at particular test sites, and the cyanobacteria and diatom populations experience a negative linear growth relationship with one another. The changes in community composition were the result of both seasonality and episodic events, and fluctuations of ammonium and phosphates, while productivity was influenced solely by the occurrence of episodic events. Low biodiversity within the phytoplankton community exists in this lake as a result of urban runoff and eutrophication.
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