Terrestrial dissolved organic matter supports growth and reproduction of Daphnia magna when algae are limiting

We experimentally show that Daphnia magna can use terrestrial-derived dissolved organic matter (t-DOM) to support growth and reproduction when alternative food sources are limiting or absent. Unlike previous studies, we restricted available food to limiting algae (0.1 mg C L-1 of Scenedesmus obliquus, provided to all treatments) and bacteria (conditions were not sterile) by excluding heterotrophic protists and running our experiment in darkness to prevent algal growth. Daphnia receiving 10 mg t-DOC L-1 leached from either beech (Fagus sylvatica, t-DOMbeech) or hazel leaves (Corylus maxima, t-DOMhazel) had significantly higher juvenile growth rates and deposited larger clutch sizes across a range of realistic temperatures (15, 20, 25 degrees C) than Daphnia receiving no t-DOM, which failed to deposit eggs. Growth rates of t-DOM-supplied Daphnia were similar (0.10 +/- 0.01 d(-1)) and clutch sizes were low (<2 eggs female(-1)) across temperatures. Our experimental leachate additions mimic fresh t-DOM inputs in natural systems (e.g. during runoff or precipitation) and suggest that t-DOM and t-DOM-supported bacteria can supplement Daphnia growth and reproduction across a range of temperatures even when algae are strongly limiting. Low growth and reproduction rates, however, indicate that t-DOM based resources are unlikely to sustain Daphnia populations independently from sufficient algal contributions.