Effects of Wind Stress, Wind Speed and Direction on Phytoplankton Abundance in the Nearshore Zone of Lake Michigan

Abstract

Phytoplankton data from a shore and offshore intake in the near-shore zone of Lake Michigan at Chicago were examined to determine the effects of wind speed and direction on phytoplankton density. Over the entire year, regression analysis indicated that a small (4.2 and 5.5 percent) but statistically significant portion of the daily variation in phytoplankton density at both sites occurred with densities increasing with increasing north winds. On days with only a north wind, wind speed accounted for 34.9 and 42.1 percent of the variation in phytoplankton abundance. During short periods ( < one month) of relatively constant water temperature (e.g., January), wind stress, independent of wind direction, explained nearly 50 percent of the daily variation at the shore intake with phytoplankton density increasing with increasing wind speed. In the Chicago area during periods of thermal stratification, southwesterly winds produced upwellings which were accompanied by higher densities of both diatoms and blue-green Oscillatoria. The higher densities of blue-green algae caused by upwellings have not, to our knowledge, been previously reported in Lake Michigan.