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UBC Theses and Dissertations

Seasonality, sinking and the chlorophyll maximum of an oligotrophic British Columbia lake Jackson, Leland J.


A field investigation was carried out over two seasonal periods on an oligotrophic coastal British Columbia lake to determine the role of sinking in the formation of the chlorophyll maximum as well as some aspects of phytoplankton seasonality. Sinking rates of two diatoms were measured and found to be highest in the epilimnion and lowest at the depth of the chlorophyll maximum. Light affected sinking rate as well as the position of the chlorophyll maximum. The chlorophyll maximum formed at 10-12 m following the onset of seasonal thermal stratification and descended to ca. 22 m for the summer. A major factor in the formation of the chlorophyll maximum is the decrease of phytoplankton sinking rate at depth. Rhizosolenia eriensis is one of the first phytoplankters to bloom in the spring. Small flagellates (3-15 um) and occasionally Dinobryon sp. were also important numerically. In the summer Cyclotella spp. displaced R. eriensis as the dominant diatom in the epilimnion. The relative timing of seasonal maxima of blooms of various species remained similar during the two years investigated. Lake fertilization affected the phytoplankton standing stock. R. eriensis did not greatly benefit from fertilization since it sank out of the epilimnion and became a major constituent of the chlorophyll maximum before fertilization. Because of its large size and low C : cell volume ratio due to a large vacuole, R. eriensis is probably not a good food source for zooplankton.

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