Effects of omnivory and predator-prey elemental stoichiometry on planktonic trophic interactions

Siuda, Amy N. S., and Hans G. Dam

Limnol. Oceanogr., 55(5), 2010, 2107-2116 | DOI: 10.4319/lo.2010.55.5.2107

ABSTRACT: We used laboratory food chains to experimentally explore the independent and combined effects of omnivory and food quality on planktonic trophic interactions. Omnivory is predicted to dampen indirect (trophic cascade) interactions, whereas availability of a low-quality diet, here defined as predator-prey elemental stoichiometric imbalance, should strengthen both direct (grazing and predation) and indirect interactions. Food chains were constructed with a copepod, Acartia tonsa, an aloricate ciliate, Strombidinopsis sp., and two centric diatoms of different size, Thalassiosira weissflogii (larger) and Thalassiosira pseudonana (smaller). Comparing results between food chains with two (copepod and diatoms or ciliate and diatoms) and three (copepod, ciliate, and diatoms) trophic levels tested omnivory effects. Offering grazers phytoplankton grown under nitrogen-replete and nitrogenlimited conditions tested stoichiometric imbalance effects. Carbon ingestion increased by 25% for the copepod and 160% for the ciliate on the nitrogen-limited relative to the nitrogen-replete diatom, demonstrating considerable compensatory feeding by both grazers. On a mixed diet comprised of the nitrogen-limited large diatom and the nitrogen-replete small diatom, the copepod continued to engage in compensatory feeding, but the ciliate did not. Cascading indirect effects of the copepod on phytoplankton via the ciliate were minimal. The decrease in phytoplankton due to direct grazing by the copepod was much greater than any indirect increase resulting from reduced ciliate grazing pressure. Omnivory and compensatory feeding were stronger than positive prey selection for the N-replete intermediate grazer, thus weakening cascading indirect effects of the top predator on phytoplankton.

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