Date of Award


Publication Type

Doctoral Thesis

Degree Name



Biological Sciences


Biology, General.




In recent years, increased biomass of Fragilaria crotonensis has been reported from the Pigeon Bay waters of western Lake Erie for reasons not fully understood. This apparent increase in F. crotonensis biomass has been influenced primarily by large summer "pulses". The purpose of this study was to identify the factors which contribute to F. crotonensis "pulses" in Pigeon Bay. Summer not only provides optimal water temperatures for growth of F. crotonensis, but peak incident solar radiation and water transparency. It is hypothesized that the emergence of F. crotonensis "pulses" during summer is dependent upon three major factors: (1) adequate supplies of nutrients (Si: via dissolution of biogenic silica and/or external loading; P: via luxury consumption and/or internal or external loading); (2) the presence of slight thermal stratification with concomitant wind velocities sufficient to keep F. crotonensis circulating in the surface mixed layer; and, (3) the presence of low turbidities with concomitant irradiance sufficient for growth. Once such physicochemical conditions exist in Pigeon Bay, it appears that F. crotonensis can grow rapidly (because it is adapted to low light and is a superior exploitative competitor for nutrients) and achieve densities as high as 950,000 cell L('-1). Such "pulses" may be terminated by lack of wind (which allows cells to sink out of the photic zone), high wind velocities (which increase turbidity and decrease light penetration), or depletion of nutrients. Zooplankton grazing on F. crotonensis is probably insignificant because of its large size and chain-forming morphology. In discussing the factors which contribute to F. crotonensis "pulses" in Pigeon Bay, one must maintain a historical perspective. For example, summer nitrate:soluble reactive phosphorus ratios have increased from approximately four in the late 1960's to over 40 in the late 1970's and early 1980's. During this same time period, the summer phytoplankton shifted from being primarily nitrogen limited to being primarily phosphorus limited. Blue-green algal biomass declined while F. crotonensis biomass apparently increased. It is possible that this increase in summer nitrate:soluble reactive phosphorus ratios in Pigeon Bay may provide a more optimal environment for F. crotonensis growth. Laboratory work has shown that F. crotonensis has a relatively high N:P ratio (25) which is consistent with its increased biomass with increased N:P ratios in Pigeon Bay.Dept. of Biological Sciences. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis1985 .H377. Source: Dissertation Abstracts International, Volume: 46-08, Section: B, page: 2527. Thesis (Ph.D.)--University of Windsor (Canada), 1985.