The role of absolute and relative nutrient levels in determining phytoplankton productivity and community composition

DE TEZANOS PINTO, PAULA; LITCHMAN, ELENA

Amsterdam

Workshop; Predictability in Plankton communities; 2010

There is an ongoing debate on whether the relative or absolute nutrient values should be used to predict phytoplankton productivity and community composition. To test this, we subjected a diverse natural phytoplankton community (zooplankton-free) to two contrasting N:P ratios (low and high) by keeping the N concentrations fixed but changing P and three N levels in a factorial design. Within a N level, contrasting community composition developed at different N:P ratios. Likewise, within a given N:P ratio, different absolute N and P rendered contrasting productivities and compositions. N-fixers reached significant densities only at low N:P and only when N was low or intermediate; at high N green algae dominated. At high N:P, low N rendered low biomass with diatoms dominating, but at intermediate and high N, non N-fixing filamentous cyanobacteria prevailed. Thus both the absolute nutrient levels and their ratios determined community composition. The experiment was run for 40 days, then N:P supply ratios were switched within a given N level for 30 days (total =70 days). After the switch, the communities gradually reverted to the composition corresponding to the treatment, as described above. The reversal was faster when switching from high to low P, than vice versa. These results suggest predictability in the responses of different groups of algae to the absolute and relative availabilities of N and P.