Interactive effects of N:P ratios and light on phytoplankton community structure.

DE TEZANOS PINTO, PAULA; LITCHMAN, ELENA

Ramot

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

We experimentally examined the interactive effects of nitrogen to phosphorous (N:P) supply ratios and light levels in determining phytoplankton community structure. We built an eleven multispecies assemblage of freshwater phytoplankton, including a N fixing cyanobacteria (Anabaena flos-aquae). We aimed to better understand the connection between low N:P ratios and cyanobacterial blooms, as N:P ratios are declining in many lakes worldwide. As N fixation is an energetically costly process that depends on light, we further explored how light levels may mediate the effects of N:P ratios on N-fixers. The four treatments included two light levels (15 and 100 ìmol quanta m-2 s-1) and two N:P ratios (4 and 32 by moles) in a factorial design, by duplicate. Results on day 50 showed that both N:P supply ratios and light levels significantly affected total biomass, community composition and the relative abundances of major groups of phytoplankton, including the eco-physiological responses of the N-fixer dominance. The high light and low N:P ratio scenario lead to the dominance of the N-fixer (93% of total biomass), that reached bloom densities. Conversely, in the high light and high N:P situation, the assemblage evolved to the dominance of green algae (Anabaena represented 1% of total biomass). Under low light, both N:P treatments evolved similarly rendering high relative abundance of the N fixer (73 to 84% of total biomass at low and high NP ratio, respectively). The N:P-light interaction also affected the physiological response of the N fixer, as a consequence of differential  nutrient availability: at high light Anabaena behaved as a fixer, whereas at low light Anabaena only behaved as a fixer at low NP). At day 50 we estimated the amount of N fixed in each treatment (total N subtracted by the supplied N). Results show that N fixation occurred in both low N:P treatments and was 2.6 times higher at  increased light. The abundance of heterocysts (N-fixing cells) agreed well with the amount of fixed N.