Effect of discharge on the temporal dynamics of periphyton in an agriculturally influenced river
Periphyton in an agriculturally enriched river (Boyer River, Québec, 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 Québec 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 Québec 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 80oC followed by combustion in a muffle furnace at 500oC for 2 hours. Samples for diatom analysis were cleaned using a mixture of 1:1 sulphuric and nitric acid at 60°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, NH3-N, NO3-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 well above or below saturation (29-176% of air equilibrium) indicating a strong biological influence on oxygen dynamics. Periphytic biomass (Chl a and AFDW) and diatom community structure showed major fluctuations over time. The temporal changes in biomass were most strongly correlated (negatively) with peak discharge during the preceding week (AFDW : r=-0,748, p < 0,05; and Chl a : r=-0,649, p < 0,05). This discharge variable was strongly correlated (positively) with several physico-chemical variables (total-N, NH3-N, NO3-N, total-P, turbidity). The negative correlation between biomass and total phosphorus suggested that algae in the Boyer River were not nutrient-limited. However, stepwise regressions showed that variations in diatom-specific composition over time were regulated by various physico-chemical variables linked to environmental preferences and tolerance of each species. Navicula seminulum, Navicula cf. subminusculus and Navicula saprophila were strongly influenced by the peak discharge during the week preceding the sampling (R2 =0,76, F(1,8) =25,7, p 0,001; R2 =0,66, F(1,8) =15,82, p < 0,05 and R2=0,55, F(1,8) =9,85, p < 0,05, respectively). Cymbella silesiaca, Cocconeis placentula, Cyclotella meneghiniana and Navicula saprophila were strongly correlated with temperature (R2 =0,58, F(1,8) =11,08, p < 0,05; R2 =0,61, F(1,8) =28,15, p 0,001; R2 =0,41, F(1,8) =5,65, p < 0,05 and R2 =0,59, F(1,8) =11,38, p < 0,005, respectively). Diatoma vulgaris was mostly influenced by suspended solids plus discharge (R2 =0,4, F(1,8) =5,38, p < 0,05; and R2 =0,86, F(1,7) =20,72, p < 0,005, respectively). The abundance of Navicula lanceolata was strongly correlated with conductivity (R2 =45, F(1,8) =6,55, p < 0,05) while that of Nitzschia spp. correlated with total dissolved phosphorus (R2 =49, F(1,8) =7,63, p < 0,05). No significant influence of the physico-chemical environment was observed on Navicula cryptocephala or Surirella brebissonii. Benthic algae in this nutrient-rich ecosystem responded to environmental variations within 2 weeks since major changes in community composition were observed between all sampling dates. Although diatom community structure changed markedly during the sampling season, most of the observed species are indicative of nutrient-enriched rivers and streams. The results of this study show the dynamic nature of periphyton communities in nutrient-enriched rivers and streams and underscore the importance of discharge as a regulator of biomass. The rapid shifts in community structure also imply that benthic algae can be used as a sensitive measure of environmental conditions in agriculturally impacted ecosystems.