PARENT SESSION
Poster Session 3: Aquatic Ecology
Monday, August 8, 5:00 PM - 6:30 PM, Exhibit Hall 210 D, Level 2, Palais des congrès de Montréal

Fitness consequences of declining ambient calcium best described by the intrinsic rate of increase and not juvenile growth rate for Daphnia.

Ashforth, Dawn^*,1, Yan, Norman^{*,1, 2}, ¹ York University, Toronto, ON, Canada² Ministry of the Environment, Dorset, ON, Canada

ABSTRACT- The fitness of daphnids under certain environmental conditions is typically determined through life table experiments and the analysis of the intrinsic rate of population increase (r). However, life table experiments can be time-consuming and labour-intensive. To reduce the amount of experimental effort required to assess fitness, juvenile growth rates have been used successfully as a substitute for r when food concentration is varied (Lampert and Trubetskova 1996, Functional Ecology 10: 631). It would be advantageous if this relationship could be extended to ambient calcium (Ca) as a variable, since Ca is declining in Shield lakes and is required by daphnids for growth. We calculated the juvenile growth rate, g, of Daphnia pulex for fixed time periods of 2, 4, and 6 days (g_f) and for the time to primiparity (g_j) along a Ca gradient of 0.2 to 20 mg Ca l^-1. We found a significant relationship between r and g for g_j (r²=0.495). The fixed time periods produced non-significant, moderate to weak relationships between r and g. The r² values for days 2, 4, and 6 were 0.003, 0.380, and 0.147 respectively. Thus, it is best to conduct life table analyses to determine the fitness consequences of a decline in ambient Ca; the shortcuts, useful for detecting impacts of changes in food level, should not be applied.

Key words: juvenile growth rate, ambient calcium, fitness, Daphnia pulex