PARENT SESSION
Oral Session # 82: Aquatic Ecology: Freshwater.
Presiding: A Soja
Thursday, August 7. 1:30 PM to 5:00 PM, SITCC Meeting Room 200.

Competition between grazers along light:nutrient gradients.

Hall, Spencer ^*,1, ¹ University of Chicago, Chicago, IL

ABSTRACT- Ecologists remain challenged to mechanistically explain changes in species composition along environmental gradients. Recent developments in stoichiometric theory suggest that elemental body composition of grazers may be a key trait explaining responses of grazer community composition to changes in elemental food quality (driven by the relative supply of light and nutrients to ecosystems). In a semi-natural pond mesocosm experiment, the phosphorus (P)-rich, planktonic grazer Daphnia responded as predicted by verbal stoichiometric theory: it dominated grazer communities in environments with P-rich food (at high nutrients), and was virtually absent in environments with P-poor food (at low nutrients). Other P-rich species, however, did not respond similarly to the light:nutrient-driven gradient in food quality. In order to understand this result, I considered theoretical competition between two grazers for a single plant resource using analysis of nullclines. The plant varies in P-content, and its production depends upon nutrients and light. Grazers can be carbon- or P-limited, and feed according to linear functional responses. When elemental food quality is good for both grazers, the competitive dominant at equilibrium maintains zero growth rate at a lower level of plant carbon density, N* (a familiar result of resource competition theory). When food quality is bad for both grazers, the winner of competition maintains zero growth rate at a lower level of plant nutrient density, QN* (a new result). This QN* rule can explain response of Daphnia in the experiment, but it also reveals why other P-rich grazers can respond oppositely: grazer body composition is only one of several grazer traits comprising QN*. Therefore, this theory emphasizes that body composition should not be considered separately from a grazer's trait suite. Furthermore, a N*-QN* tradeoff among grazers can permit species coexistence at intermediate nutrient supply, but it also determines shifts in grazer composition along nutrient and light gradients.

Key words: competition, light:nutrient gradients, stoichiometry, species coexistence