The Phytoplankton Population Dynamics of a Small Reservoir

Abstract

<p>Time series analysis was used to study the phytoplankton population dynamics of a small reservoir (Guelph Lake, southern Ontario). Three stations were sampled daily for one season (May 8 to August 20, 1981) and weekly at one station for 2 years (fall/1980 to fall/1982). Significant environmental variability was detected at time scales of days to weeks.</p> <p>Time series analysis of the daily data from 3 stations (maximum separation - 1 km) demonstrated the existence of strong horizontal advection within the basin with a period of 2-3 days and 10-20 day cycles associated with changing weather systems. Rates and directions of horizontal advection varied among different variables. The algal biomass (chlorophyll a) at station 1 lagged that at station 3 by half a day. The time lag between station 1 and station 3 for Aphanizomenon flos-aquae was 7 days, whereas for Ceratium hirundinella station 1 led station 3 by 3 days indicating opposing motion by the species. Longer term (10-20 day) periodicities occurred in between and within station comparisons for soluble reactive phosphorus and total phosphorus. Periodicities in the average daily wind speed and water column stability showed that such lags or leads were due to vertical mixing.</p> <p>Sedimentation traps were used to examine the settling of diatoms and the recruitment of summer blue-green species. The relationship between physical variables such as water column stability (N², Richardson number) or wind stress and the vertical heterogeneity of chlorophyll a, nutrients and phytoplankton species on a daily basis was examined. High Richardson numbers were better predictors of high vertical heterogeneity than Iow wind speed or N² alone. Higher correlations between physical variables and vertical heterogeneity were obtained in late summer. Relationships between environmental variables and phytoplankton population dynamics were examined on a daily and a weekly basis. Time lags occurred between changes in environmental variables and the abundance of particular phytoplankton as well as the rate of phytoplankton community change.</p> <p>The results suggest that non-equilibrium conditions prevail in the phytoplankton community of Guelph Lake. Significant environmental variability was detected at time scales of the same magnitude as phytoplankton generation times, time lags occurred between changes in environmental variables and algal abundances and the majority of the increase in the rate of phytoplankton community change occurred in response to allogenic factors. Some of the problems encountered with the use of bivariate time series techniques in phytoplankton are then discussed.</p>