PHYTOPLANKTON NUTRIENT LIMITATION AND FOOD QUALITY FOR DAPHNIA

The influence of nutrient limitation on the quality of Scenedesmus acutus as food for Daphnia obtusa is examined. The nature and degree of nutrient limitation greatly influences the rate at which Daphnia converts Scenedesmus biomass into herbivore biomass. From high to low quality, Scenedesmus food is ranked moderately N limited, severely N limited, and severely P limited. Even a very high concentration (3 mg DW. liter-1) of low quality food yields slow Daphnia growth, and it appears that no amount of low quality food would support rapid Daphnia growth. Food-limited animals display lowered intercepts of length-weight regressions (prereproductive females), reduced rates of biomass gain (both males and females), increased ages at first reproduction, lowered clutch sizes, increased mortality, and lowered reproductive rate. The N and P contents and the N : P ratio of Scenedesmus all vary considerably under N and P limitation, while the N content (and possibly P content) of Daphnia is less variable and the N : P ratio of Daphnia is essentially constant. Clearance and feeding rates are lower on severely P-limited cells than on severely N-limited cells. These results indicate that the mineral nutrition of their algal food may influence the demographics of zooplankton herbivores to a degree not before realized.