Short-term variability of chlorophyll a concentrations in Lake Ontario

Abstract

Research on aquatic ecosystems has shown the presence of diel (24 hour) variations of chlorophylla. In order to investigate this phenomenon in the Great Lakes, an IFYGL program was set up in which 8 cruises were made roughly monthly between April and October 1972. Two Lake Ontario Stations were chosen, one inshore near Oshawa, Ontario and the other mid-lake. Chlorophylla samples were taken approximately every two hours at depths of 1, 5 and 1o m at the inshore station and 1, 10 and 20 m at the mid-lake station.

Chlorophylla concentrations at the 1 m depth were reduced during periods of high light intensities, but this phenomenon was observed only during the late June and July cruises at the near-shore station. Apparently, during most cruises, light intensities were too low to produce this effect. Coefficients of variation of chlorophylla concentrations as great as too% occurred at the deeper sampling depth during periods when the thermocline was well developed. This variability was associated with a thermocline motion in which the fixed depths sampled moved from the epilimnion into the thermocline or the hypolimnion and back again over the sampling period. When the lake was not thermally stratified, variability of chlorophylla concentrations was also observed with coefficients of variations between 20–3o%. Horizontal advection by currents, vertical turbulence, and presence of Langmuir cells were possible mechanisms causing such chlorophylla variability.

In light of these observations, caution must be used in interpreting spatial and temporal distributions of chlorophylla in lakes. Special care must be taken when sampling at depths near the thermocline due to movements that take place. For this reason, water samples taken from the epilimnion during periods of thermal stratification are recommended as opposed to fixed depth or integrated samples which could be affected by thermocline movements.