Date of Award

Summer 8-20-2020

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)


Marine Biology


David Townsend

Second Committee Member

Lee Karp-Boss

Third Committee Member

Jeffrey Runge


A series of three oceanographic survey cruises were conducted in June, July, and August of 2019 in the northeastern Gulf of Maine and Bay of Fundy. Surface water samples were collected and analyzed for enumerations of cell densities of the dinoflagellate Alexandrium catenella in relation to cell densities of diatoms. Hydrographic profiles of temperature, salinity, and nutrients (silicate and nitrate) were also made at each station. Data were analyzed to determine if there was any statistically significant evidence of allelopathic interference imparted by diatoms that impede A. catenella. A. catenella cells were most abundant in June, reaching 6,195 cells per liter at the surface (1 m), with the highest densities occurring at offshore stations. Diatoms were also most abundant in June (681,667 cells/L), reaching highest cell densities at inshore stations, spatially separated from A. catenella maximal densities, which suggested an allelopathic inhibition of A. catenella by diatoms, as had been suggested by earlier workers; however, there was no statistically significant inverse relationship (according to Pearson correlation analysis; r=-0.42, P=0.131). Distributions of A. catenella and diatoms were similar to one another in July, with the highest densities occurring at shallower inshore stations (3,378 A. catenella cells/L and 108,333 diatom cells/L). The August survey cruise was limited in coverage and occupied fewer stations. A. catenella cell densities were highest in the interior Bay of Fundy in August (867 cells/L) while diatoms were more abundant in shallower, coastal waters off of Maine, and Nova Scotia (66,111 cells/L). The highest surface cell densities of both A. catenella and diatoms occurred in waters low in both nitrate and silicate in all three months, which is consistent with previous observations.

The dominant diatom genera included (in order): a) in June: Thalassiosira, Chaetoceros, Cylindrotheca, Pseudonitzschia, Thalassionema, and Rhizosolenia; b) in July: Chaetoceros, Thalassiosira, Cylindrotheca, Pseudonitzschia, Rhizosolenia, Guinardia, Thalassionema; and c) in August: Skeletonema, Chaetoceros, Thalassiosira, Cylindrotheca, Pseudonitzschia, Guinardia, Cylindrotheca, and Achnanthes.

Pearson correlation analyses also showed that there were no statistically significant correlations between either A. catenella or diatom cell densities and surface concentrations of the nutrient silicate; however, in June, A. catenella did show a statistically significant inverse correlation with nitrate (P=0.0187).

Overall there was a seasonal decline from June to August, which contrasts with earlier reports that showed seasonal increases from June to August in the NE Gulf of Maine and Bay of Fundy. In June, there was a positive correlation between A. catenella and salinity, which corresponded with the time of greatest A. catenella densities offshore (r=0.44; p= 0.003). In July, A. catenella were significantly inversely correlated with temperature where cells are usually most abundant in the colder waters of the Eastern Maine Coastal Current (r=0.32; p=0.036). August had fewer stations making it difficult to draw conclusions. Overall the data did not support the original hypothesis of allelopathy, as there was no statistically significant Pearson correlation between diatoms and A. catenella for any of the three summer surveys.