Effects of different NP ratios on competitive interactions between a nitrogen fixer (Anabaena flos aquae) and a non nitrogen fixer (Scenedesmus quadricauda).

DE TEZANOS PINTO, PAULA; LITCHMAN, ELENA

Ramot

Workshop; 15th Workshop of the International Association of Phytoplankton Taxonomy and Ecology (IAP).; 2008

IAP

We experimentally assessed the effects of different nitrogen to phosphorus (NP) ratios on population dynamics and community responses of a nitrogen fixer (Anabaena flos-aquae) and a non nitrogen fixer (Scenedesmus quadricauda). The population dynamics was explored in monocultures under twelve different NP ratios (from 0 to 224 NP by moles) by changing the N concentrations (from N limitation to saturation) at a fixed P. Anabaena behaved as a N fixer from 0 to 32 (high density of heterocysts and increased total N), whereas above 32NP it behaved as a non fixer (no heterocysts and total N equaled supplied N). Competitive outcomes were assessed at four NP ratios (4, 16, 32 and 224 NP). At day 60, competitive hierarchies varied: Anabaena dominated at low NP (4NP=98% of total biomass), coexisted at intermediate NP (16 NP=83% and 32 NP=39% of total biomass) but was almost excluded by Scenedesmus at the highest NP (224NP=2% of total biomass). To determine whether there is stable coexistence in this system, we used mutual invasibility experiments. At each of the twelve NP ratios we invaded steady state monocultures of Anabaena with small densities (100 ind ml-1) of Scenedesmus, and vice versa. After 20 days results showed that a) at 0NP only Anabaena was capable of invading Scenedesmus, b) from 4 to 16 NP both species invaded each other, suggesting stable coexistence, c) from 24 to 128 NP only Scenedesmus invaded Anabaena, and d) at 256 and 512 NP neither species were able to invade, indicating a founder control. This latter result (d) is intriguing, suggesting that blooms of nitrogen fixers may occur at very high N values, depending on the initial densities. This has been observed in nature. Moreover, even though in this study Scenedesmus displaced Anabaena at 224NP, in two previous pilot experiments we found the reversed trend.