Effects of copepods on natural microplankton communities: do they exert top-down control?
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Top-down effects in the pelagic realm are quite well known in freshwater ecosystems. However, our knowledge of these effects in the ocean remains scant. It is known that copepods prefer to prey on ciliates and heterotrophic dinoflagellates, and their high or low abundances can change the structure of microplankton communities. Field studies in subtropical waters have shown parallel increases of mesozooplankton and phytoplankton without a lag, suggesting a top-down effect of mesozooplankton preying upon microzooplankton and releasing primary producers from predation. In the present work, we added copepods at increasing densities to natural plankton in 24 h experiments. A decrease in aloricated ciliates abundance of nearly 50% and increases in the abundances of picoeukaryotes, Synechococcus, Prochlorococcus, diatoms, and chlorophyll a were observed. No effect of nutrient additions was observed in parallel grazing experiments. Thus, a top-down effect of copepods upon microzooplankton explains the observed changes in the abundance of the different phytoplankton groups. Copepods promote important changes down the food web, structuring the community by predation upon microzooplankton. There are biogeochemical consequences of zooplankton variability over short time scales in the ocean.
We wish to thank Sabrina Sánchez, Almudena Valenciano, Lidia Nieves, Valeria Anabalón, and Claire Schmoker for their help onboard. We also wish to thank David Morales for the time spent in carrying out the experimental work. This study was financially supported by projects Lucifer (CTM2008-03538), Malaspina (CSD2008-00077), Mafia (CTM2012-39587), and Bathypelagic (CTM2016-78853-R). Funding was provided by Ministerio de Economía y Competitividad.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.
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