Phytoplankton community development in the St.
Lawrence River
Carmen Ulrich
Advisor: Dr. Michael Twiss
Clarkson University
July 21, 2008
Abstract
In order to understand the seasonal longitudinal patterns of phytoplankton community
composition and abundance in the International Section of the St. Lawrence River,
measurements were made of nutrients (nitrate, phosphorus, silicate), water velocity, dissolved
oxygen, light attenuation, and the amount of zooplankton present, to determine their influence on
the phytoplankton community. Samples and measurements were taken at twenty stations evenly
spaced along a 160 km transect between Lake Ontario and the Moses-Saunders Dam by
Massena, NY. Prior research has shown that a sharp decrease in phytoplankton abundance takes
place in the summer as the water moves from Lake Ontario downstream in the St. Lawrence
River. The reason for this reoccurring trend is currently unknown. Measurements taken this year
illustrated patterns consistent with those of 2007. However, seasonal changes could be
distinguished, in which the change in phytoplankton abundances was more pronounced in July
than in June. In addition to annual comparisons, analyses will be made on the seasonal
differences in nutrient availability, physical constraints, and existing organisms. We hypothesize
that the marked change in the phytoplankton community composition is a result of increased
benthic grazing that arises at approximately fifty kilometers into the St. Lawrence River from
Lake Ontario.
Senior, Biology, REU, Dr. Michael Twiss