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-I) 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. Herbivores face great nutritional challenges because plant matter, relative to animal tissue, is low in nutritional content (Begon et al. 1990). This statement is true from the standpoint of energetics (available calories per gram) as well as for specific elements (e.g. C : N or C : P ratios) or biomolecules (e.g. grams of protein per gram of total biomass). In addition, the biochemical composition of plants varies as growth conditions (mineral nutrition, presence of physical stresses, etc.) vary (Chapin 1980). Algae share these traits with other photosynthetic organisms. Like higher plants, algae have relatively high ratios of C : P and C : N, and under N or P limitation, the C : N : P ratio of phytoplankton varies considerably (Goldman et al. 1979; Moal et al. 1987). Such biochemical responses of algae to resource limitation should affect their quality as food for upper trophic levels. Thus, it appears that the food of herbivorous zooplankton could be characterized as generally poor, but also highly variable, in quality. A substantive body of research describing the growth of Daphnia in defined laboratory conditions has led to the construction of deAcknowledgments We thank T. Chrzanowski for measurements of algal carbon and P. Hebert for taxonomic confirmation of Daphnia obtusa. Financial support was provided by NSF grant BSR 88 17786. tailed models of food intake and assimilation and of subsequent commitments of energy to maintenance, growth, and reproduction (Paloheimo et al. 1982; Lynch et al. 1986; Lynch 1989; Hallam et al. 1990; Gurney et al. 1990). These models forge an important link between Daphnia’s physiology and its demography. A common simplifying assumption to models such as ‘;hese is to equate food resources to a single parameter, either carbon or mass. For example, “. . . we regard food as a homogeneous assemblage within the water, describable by a single density, namely carbon content (or energy or dry mass) per unit volume” (McCauley et al. 1990, p. 713). The evidence in the opening paragraph, though, indicates that this assumption must be carefully scrutinized. Perhaps, additional parameters will be necessary to describe “food” more realisticallY* Daphnia’s natural food base is very heterogeneous. Algae may be the most biochemically diverse guild in nature. Differences in cell walls, storage products, and pigment types among major algal taxa are large (see any introductory phycology text). In addition, the cell contents of nitrogen, proteins, lipids, and carbohydrates vary interspecifically (Moal et al. 1987). Furthermore, the biochemical composition of individual algal taxa varies widely as growth rate varies: the cellular makeup of N-, P-, or lightlimited cells generally all differ from each other as well as from unlimited cells (Caperon and