Date of Award


Document Type

Open Access Dissertation


Biological Sciences


College of Arts and Sciences

First Advisor

James L. Pinckney


Coastal regions and estuaries are particularly sensitive to the increase in nutrient loading and river runoff, threatening the ecosystems with possible spreads in harmful algal blooms (HABs). As an example, blooms of toxic Pseudo-nitzschia species can release acute concentrations of the neurotoxin domoic acid (DA) in the water column. When ingested at concentrations higher than 20 ppm, DA can cause the death of marine birds, marine mammals and even humans. The main objectives of my research were to determine the drivers of Pseudo-nitzschia abundance and toxicity and to assess how these drivers influenced the phytoplankton community structure and DA allelopathy. Through multiple bioassays using water collected from two sites, North Inlet (high salinity, nutrient deplete) and Winyah Bay (low salinity, nutrient replete), links between the environmental factors and Pseudo-nitzschia abundances and toxicity were investigated. Subsequent experiments determined the influence of salinity on DA allelopathy. Finally, the impact of moderate loadings of nitrate and phosphate on Pseudo-nitzschia abundance, DA allelopathy and on the entire phytoplankton community composition were examined. The major conclusions of this research are:

1. In North Inlet, increases in Pseudo-nitzschia abundance and toxicity were driven by low temperatures and high salinity. In Winyah Bay, Pseudo-nitzschia cells were present but had difficulties acclimating to low salinities.

2. DA allelopathy was more effective on salinity-stressed cells (i.e. high salinity in North Inlet, low salinity in Winyah Bay).

3. Although the addition of nutrients enhanced Pseudo-nitzschia abundance, it decreased the effect of DA on natural phytoplankton communities and mitigated the influence of salinity on DA allelopathy.

4. Significant shifts in the phytoplankton community composition with the addition of nutrients were observed although the threshold of 40 μg chl a l-1 wasn’t reached. But no shift towards a particular group was established.

My findings highlight how environmental factors such as salinity and nutrients can play important roles on Pseudo-nitzschia abundance as well as on DA allelopathy by alleviating or exacerbating its effect. Furthermore, with a possible increase in HABs due to eutrophication, my results demonstrate the ecological risk of using a subjective chl a concentration based on total phytoplankton biomass for water quality criteria. Relative abundances of algal groups and species should be included to be able to identify harmful species.