Influence of nitrogen to phosphorus supply ratios and physicochemical conditions on cyanobacteria and phytoplankton species composition in the Experimental Lakes Area, Canada

To test the hypothesis that N:P supply ratios influence phytoplankton species composition, and particularly that cyanobacteria are favored by a low ratio, mesocosms at one pelagic and two littoral sites within the Experimental Lakes Area, Ontario, were fertilized for 10-12 weeks with similar amounts of P but different amounts of N. Total N:P supply (L N :L P ) ratios (fertilizer plus natural inputs) ranged from 8:1 to 50:1. Nitrogen deficiency was detected in all mesocosms with L N :L P ratios <17:1, but N 2 -fixing Anabaena gained dominance only in the low-N:P pelagic mesocosms, and only in late summer. Cryptophytes and (or) chlorophytes dominated littoral mesocosms at all N:P ratios, while Pseudoanabaena catenata, a nonheterocystous cyanobacterium, was the late-summer dominant in pelagic mesocosms with L N :L P ratios >17:1. Canonical correlation analysis related cyanobacterial dominance to high P and low CO 2 availability. Low light intensities and low N:P ratios also favored heterocystous (but not nonheterocystous) cyanobacteria. Total phytoplankton biomass and productivity increased with L N :L P ratio, while periphyton growth was maximal at low N:P ratios. Nitrogen limitation of phytoplankton may encourage blooms of N 2 fixers or drive productivity down to the sediment surface where N is more available.