Nonlinear effects of aqueous calcium concentration on antipredator response in Daphnia

Abstract

Calcium decline is an emerging environmental concern for thousands of soft-water lakes. Daphnia, an important freshwater herbivore, has reduced reproduction, a weaker, less-rigid carapace, and decreased survival under low aqueous calcium concentrations, but there has been little attention to study how calcium decline will influence predator–prey relationships. We conducted a mesocosm experiment to determine the joint effects of calcium concentration (0.3, 1.2, and 2.3 mg/l) and kairomones from Chaoborus americanus, a ubiquitous invertebrate predator, on Daphnia minnehaha populations. In the presence of Chaoborus kairomones, chemical signals of Chaoborus presence, Daphnia populations had delayed peak abundance and grew antipredator neck spine defenses. However, aqueous calcium concentration influenced the strength of the response. Daphnia in both 0.3 and 2.3 mg Ca/l had stronger neck spine responses and larger reproductive delays than Daphnia in 1.2 mg Ca/l. Our results suggest that as lake calcium concentrations continue to fall, Daphnia populations will respond in complex, nonlinear ways.