Contrasting effects of different nitrogen to phosphorus stoichiometry in consumer–resource dynamics

Background/Question/Methods

The effects of nutrient stoichiometry on primary producers have been extensively studied at the level of consumer physiology, but much less frequently at higher levels such as population dynamics. At the physiological level, the “growth rate hypothesis” has been largely accepted as a framework in which a nutrient limiting RNA synthesis (which enables protein synthesis) has a marked effect on consumer growth. However, contradictory effects of stoichiometric regimes can appear if we look at either consumer somatic or population growth rates. Here, we studied the influence of N:P stoichiometry of a microalga, Scenedesmus obliquus, on its quality as food for microzooplankton (the rotifer Brachionus calicyflorus) in terms of rotifer population dynamics. We cultured the microalga in continuous conditions at two different N:P regimes, producing either strong nitrogen or phosphorus limitation, and estimated the numerical (growth rate) responses of rotifers on each food source supplied at a range of densities from 0 to food saturation. Finally, we grew the rotifers in continuous cultures of S. obliquus at the high and low N:P stoichiometric regimes and determined their long-term population dynamics, and constructed a predictive model.

Results/Conclusions

Phosphorus limitation caused markedly lower population growth rate than did nitrogen limitation at saturating food levels. Fitted Holling Type-II numerical responses gave estimates of P-limited growth saturating at r = 0.3 day^-1, whereas N-limited growth saturated at r = 0.8 day^-1. This effect was apparently caused by lower egg hatching success (but not lower clutch size) with P-limited food. In continuous rotifer-alga cultures, long-term population dynamics showed that despite the same biomass in both N- and P-limited food, rotifer populations went extinct under P-limitation but not with N-limitation. A mathematical model of population dynamics in continuous culture developed for our system predicts this experimental outcome, primarily as a result of differences in the empirical numerical responses: under P-limitation, the rotifer population is predicted only to be maintained within a narrow range of low dilution rates; whereas under N-limitation, rotifer and microalgal populations showed out-of-phase cycles of decreasing period and amplitude (damped) as dilution rate increases. Our results suggest that the nature of nutrient limitation can have distinct effects on plankton dynamics through indirect influences on algal stoichiometric food quality.