Implications of element deficits for zooplankton growth

Abstract

Daily measurements of mass and phosphorus (P) were made for Daphnia magna feeding on a high concentration (1 mg C l−1) of P-sufficient (molar C:P = 70–80, `P+') or P-deficient (C:P = 1000, `P−') green algae (Scenedesmus obliquus or Ankistrodesmus falcatus). Animals feeding on P+ algae for 12 d showed a sharp decline in growth during the last juvenile instar (mean ± SE from 0.57 ± 0.02 to 0.25 ± 0.03 d−1) and a modest decline in P-content at the first adult instar (from 1.55 ± 0.01 to 1.46 ± 0.02% mass). Animals feeding on P− algae showed declines in P-content to as low as 0.84% after 5 d, with the sharpest decline in the first day. Growth of animals feeding on P− algae declined sharply over time and showed a linear relationship with the grazer's P-content. Growth during the first day of exposure to P− algae was relatively high (mean 0.39 ± 0.03 d−1) but approached zero as P-content declined to <0.90% mass. For animals feeding on P+ algae, the ontogenetic decline in P-content resulted in only a small difference between growth calculated for mass and growth calculated for Daphnia P. In contrast, for animals feeding for 4 d on P− algae, growth calculated for mass was 2–5 times higher than growth based on P. Thus, this study shows that short-term growth assays may substantially underestimate the negative impact of P-deficient resources, unless the decline in the grazer's P-content is taken into account. In this situation, measurement of growth in terms of nutrient content (e.g. grazer's P-content) should provide a better indication of the potential for sustained growth than calculations based on change in mass. This study also supports the `integration of growth' hypothesis (Sterner & Schwalbach, 2001), in which animals undergoing diel migration between water layers with P-deficient and P-sufficient resources experience a reduced growth penalty.