Factors contributing to the spatial and temporal variability of phytoplankton communities in the Rideau River (Ontario, Canada)
Hamilton, Paul B.; Lavoie, Isabelle; Ley, Linda M.; Poulin, Michel
Global freshwater drainage systems are impacted by anthropogenic activities. Small and simple human controlled waterways like the Rideau River make good lotic systems for studying biological stability under natural and anthropogenic stressors. This study assessed the structure and dynamics of phytoplankton communities along the Rideau River, from headwater to outflow (110 km), during periods of seasonal growth (May-September) over the years 1998 to 2000. Four reaches were identified along the Rideau River based on hydrological and chemical characteristics. In conditions of normal weather patterns (temperature, cooling degree days, rain) and river discharge as observed in 1998 and 1999, the phytoplankton community showed clear correlations with reach location along the river and environmental variables especially soluble reactive phosphorus (SRP) and conductivity. Even when the 2000 weather and discharge conditions changed, the phytoplankton community patterns were generally maintained. More specifically, relations over season and year, with varying physical (residence time) and chemical (conductivity, soluble reactive phosphorus, nitrite+nitrate) variables, showed significant differences between the four reaches, although additional significance within the multiple ANOVA interactions indicates that the specific reach effect was variable and depended on the season and the year of study. The phytoplankton taxonomic composition changed along the Rideau River from dominant Chrysophyceae- Cryptophyceae at the upper stream headwater reach to Cryptophyceae-Bacillariophyceae in the mid-reaches, and finally changing to dominant Bacillariophyceae-Chlorophyceae in the lower part of the river. Cluster analyses further confirmed the grouping of sites along the river based on phytoplankton community structure, and showed that these groups of sites generally matched with the four reaches previously identified based on physical and chemical characteristics. Species diversity also showed changes downstream, with a clear decrease in the lower section of the river. In contrast, taxonomic distinctness increased in the lower section of the river. The significance of less prominent but key species in identifying phytoplankton communities along these reaches was determined to be important in characterizing community changes downstream. Plagioselmis nannoplanctica was the most dominant species in all the reaches and a statistically important contributor to all the different reach communities. Taxa of significance that showed both reach and seasonal changes in composition included Aulacoseira granulata, Stephanodiscus parvus, Skeletonema potamos, and Spermatozopsis exsultans. Taxa associated with benthic disturbances, Cocconeis placentula and Nitzschia palea, were key taxa within reach-4 located within the inner limits of the City of Ottawa.