Nutrient and light limitation of periphyton in selected streams in Ireland

This study was undertaken to improve knowledge on the role of both phosphorus and nitrogen in eutrophication of riverine systems. The potential for nutrient limitation of algal biomass was examined during the early growth season in 4 good water quality Irish streams. Four treatments using nutrient diffusing substrata were employed in each river to test algal growth responses to addition of nitrogen (N), phosphorus (P), and a combination of N and P at 3 concentrations (0.05, 0.1, and 0.5 mol/L) as well as a control (agar only) in open and closed canopy over a 21 d period. The ambient streamwater nutrient concentrations from all streams remained at low levels with high N: P ratios. Results indicated that algal biomass as chlorophyll a (Chl-a) was significantly greater at all sites located in open canopy than at the closed canopy sites, implicating light as the first limiting factor in the studied streams. Periphyton response to the nutrient treatments was varied: one stream responded significantly to N addition, indicating N limitation; 2 streams were co-limited by both N and P; and a fourth stream did not show significant responses to any treatment. Phosphorus did not seem to be the sole limiting nutrient for algal biofilm in 4 examined Irish streams at this time of year (May-Jun). There was no significant difference in Chl-a concentrations in response to the applied nutrient concentrations for most sites in the open canopy; there was some response in the closed-canopy sites.