Effect of discharge on the temporal dynamics of periphyton in an agriculturally influenced river

Periphyton in an agriculturally enriched river (Boyer River, Quebec, Canada) was sampled from mid-May to the end of September 1999 to evaluate the temporal succession of periphyton and to identify potential controlling variables. The river is located on the south shore of the St. Lawrence River and discharges into it approximately 30 kilometres downstream (east) of Quebec City. Land-use in the Boyer River watershed is 60% farmland and 40% forests. The site was chosen for its intense farming, accessibility and proximity to an automatic sampling station for water quality operated by the Quebec Ministry of the Environment that continuously recorded pH, temperature, dissolved oxygen and discharge. Event water samples were collected for nutrients, turbidity, conductivity and suspended solids according to discharge. Periphyton growth was scraped every two weeks from rocks over a 10 meter reach, between mid-May and the end of September using a template, blade and toothbrush. Samples for chlorophyll a (Chl a) and ash-free-dry-weight (AFDW) were filtered on to Whatman GF/C glass fiber filters the same day and additional samples were preserved with paraformaldehyde-glutaraldehyde for taxonomic analysis. Chl a was extracted in 95% ethanol and quantified by spectrophotometry. AFDW was determined by drying the samples for 24 hours at 80 degree C followed by combustion in a muffle furnace at 500 degree C for 2 hours. Samples for diatom analysis were cleaned using a mixture of 1:1 sulphuric and nitric acid at 60 degree C and mounted on slides with Naphrax mounting medium. Diatoms were then identified and counted with a Zeiss Axiovert 10 inverted microscope at 1000X magnification. A minimum of 400 valves were enumerated for each sample. The presence of other algal constituents was also determined. Statistical analyses included Pearson correlations, stepwise regression analysis and analysis of variance. The chemical and physical properties of the river fluctuated substantially during the sampling season. Nutrient levels were consistently high with total N in the range of 1.2-7.2 mg/L and total P in the range of 0.07-0.37 mg/L, confirming the strong agricultural enrichment of the Boyer River. All measured nitrogen components (total-N, NH sub(3)-N, NO sub(3)-N) showed a decreasing trend during the sampling season while soluble reactive phosphorus (SRP), total dissolved P, conductivity, pH and temperature showed a general increase over the season. Oxygen levels were often wel