Phytoplankton, not allochthonous carbon, sustains herbivorous zooplankton production

Abstract

Terrestrial organic matter inputs have long been thought to play an important role in aquatic food web dynamics. Results from recent whole lake 13 C 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 Ϸ20% of the flux of particulate carbon available to herbivorous zooplankton, this food source accounted for Ϸ50% 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.Daphnia ͉ fatty acids ͉ nutritional ecology ͉ planktonic food web

Strong evidence for terrestrial support of zooplankton in small lakes based on stable isotopes of carbon, nitrogen, and hydrogen

“…31 and Table S1). The 13 C addition experiments have two potential biases that could result in overestimates of allochthony: deep feeding by vertically migrating zooplankton and consumption of detritus of algal origin that was produced before the addition of the label (15,21). However, the ambient isotope approach used here did not give substantially lower estimates of ϕT than the 13 C additions, indicating that neither of these potential biases were of major importance for zooplankton in these lakes.…”

“…Demonstrating a terrestrial influence on pelagic food webs in lakes had been both more difficult and more controversial. Terrestrial organic matter could become available to pelagic organisms by several mechanisms: microbial uptake of terrestrial dissolved organic carbon (DOC) followed by consumption of these microbes by protozoans or zooplankton (17,18), direct consumption of terrestrial DOC by zooplankton via osmotrophy (19), or by consumption of terrestrially derived particles by zooplankton (20,21). Terrestrial contributions to zooplankton have been estimated with different methods, predominantly by using ambient 13 C and 15 N (11, 17, 20, 22-34).…”

Periphyton support for littoral secondary production in a highly humic boreal lake

“…Use of allochthonous DOM by bacteria and grazing of bacteria by protozoa (Tulonen et al 1992), which are abundant in the littoral zone in Mekkojärvi (Kairesalo et al 1992), is a possible pathway for allochthonous energy to reach littoral invertebrates. However, considering the generally higher nutritional quality of algae than allochthonous OM (Brett et al 2009, Taipale et al 2014, the very high primary production by periphyton in Mekkojärvi (Vesterinen et al 2016), and the temporally dynamic isotope data that complicate the mixing models (Fry 2006a), the supposition that the highly accessible and higher-quality periphyton could be an even more important basal food resource for most littoral invertebrates than estimated by the SIAR model is reasonable.…”

Allochthonous dissolved organic carbon in river, lake and coastal systems: transport, function and ecological role

“…The changes in microbial food structure as a result of allochthonous carbon inputs are suggested to influence higher trophic levels with studies showing substantial subsidies from terrestrial sources in lakes (Tanentzap et al 2014;Karlsson et al 2015) whereas others argue that autochthonous sources of carbon are the dominant form (Brett et al 2009). Roach (2013) in a review and meta-analysis of food sources for large rivers using stable isotope data found that algal sources dominated but also found that many consumers did assimilate material from C 3 plants in large rivers with high sediment load and low transparency during high-flow pulses.…”