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Phytoplankton, not allochthonous carbon, sustains herbivorous zooplankton production

Brett, Michael T., Kainz, Martin J., Taipale, Sami J., Seshan, Hari
Proceedings of the National Academy of Sciences of the United States of America 2009 v.106 no.50 pp. 21197-21201
fatty acids, diet, phytoplankton, reproduction, lakes, aquatic food webs, Daphnia magna, Cryptomonas, zooplankton, lipid composition, food quality, herbivores, particulate organic carbon, leaves, stable isotopes, vegetation, Bacillariophyceae, Scenedesmus obliquus
Terrestrial organic matter inputs have long been thought to play an important role in aquatic food web dynamics. Results from recent whole lake ¹³C addition experiments suggest terrestrial particulate organic carbon (t-POC) inputs account for a disproportionate portion of zooplankton production. For example, several studies concluded that although t-POC only represented [almost equal to]20% of the flux of particulate carbon available to herbivorous zooplankton, this food source accounted for [almost equal to]50% of the C incorporated by zooplankton. We tested the direct dietary impact of t-POC (from the leaves of riparian vegetation) and various phytoplankton on Daphnia magna somatic growth, reproduction, growth efficiency, and lipid composition. By itself, t-POC was a very poor quality resource compared to cryptophytes, diatoms, and chlorophytes, but t-POC had similar food quality compared to cyanobacteria. Small additions of high quality Cryptomonas ozolinii to t-POC-dominated diets greatly increased Daphnia growth and reproduction. When offered alone, t-POC resulted in a Daphnia growth efficiency of 5 ± 1%, whereas 100% Cryptomonas and Scenedesmus obliquus diets resulted in growth efficiencies of 46 ± 8% (± SD) and 36 ± 3%, respectively. When offered in a 50:50 mixed diet with Cryptomonas or Scenedesmus, the t-POC fraction resulted in a partial growth efficiency of 22 ± 9% and 15 ± 6%, respectively. Daphnia that obtained 80% of their available food from t-POC assimilated 84% of their fatty acids from the phytoplankton component of their diet. Overall, our results suggest Daphnia selectively allocate phytoplankton-derived POC and lipids to enhance somatic growth and reproduction, while t-POC makes a minor contribution to zooplankton production.