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Title

Effects of light availability on mixotrophy and microzooplankton grazing in an oligotrophic plankton food web: Evidences from a mesocosm study in Eastern Mediterranean waters

AuthorsCalbet, Albert ; Martínez, Rodrigo Andrés ; Isari, Stamatina ; Zervoudaki, Soultana; Nejstgaard, J.C.; Pitta, Paraskevi; Sazhin, Andrey F.; Sousoni, Despoina; Gomes, Ana ; Berger, Stella A.; Tsagaraki, Tatiana M.; Ptacnik, Robert
KeywordsDilution experiments
Eastern Mediterranean
Light
Mesocosm
Microzooplankton
Mixotrophy
Issue DateAug-2012
PublisherElsevier
CitationJournal of Experimental Marine Biology and Ecology 424-425: 66-77 (2012)
AbstractPlankton biomass and composition in the pelagic zone of oceans is exposed to changes in availability of light and nutrients due to large-scale ocean circulation and water column stratification. We hypothesized that displacement of plankton from surface to deeper darker waters would not only favor heterotrophy over time, as previously suggested, but also first rapidly affect the level of mixotrophy and, consequently, overall microbial grazing in plankton food webs. To test this in an oligotrophic marine system we incubated Eastern Mediterranean water (from 10 m depth north of Crete in September 2010) in 2.8 m3 mesocosms simulating two different light intensities at the sampling station, surface waters (ca. 10 m; mesocosms L1) and deeper layers (ca. 50–60 m; mesocosms L4). The biomass and abundance of the main planktonic groups were monitored either daily or every second day, depending on the group. Microzooplankton grazing rates and the contribution of mixotrophic feeding were estimated by a combination of dilution experiments and incubations with live fluorescently labeled algae (LFLA). Although no nutrients were added to the mesocosms the chlorophyll a increased during the first 2 days of the experiment in both treatments. This increase resulted from phytoplankton growth in the light L1-mesocosm (autotrophic biomass was ca. doubled in L1 compared to L4), but was mostly due to photoadaptation of the algae in the L4-mesocosm, as indicated by lower carbon to chlorophyll a ratios. By the end of the experiment, the total biomass of protozoan and metazoan grazers in L1 was ca. twofold higher than in L4. The microzooplankton responded within the first 24 h, showing different grazing activity in L1 than in L4. Microzooplankton grazing rates on total Chl a were similar in both treatments; however, phytoplankton instantaneous growth rates were higher in the more illuminated mesocosm. This resulted in a closer coupling between both rates in L4, where all production was grazed daily, than in L1. Nevertheless, the overall flux of carbon through the microzooplankton was 33–60% higher in L1 compared to L4 throughout the experiment. The fraction of mixotrophy in the ciliate community varied in L1 (20–50%), but decreased over time in L4 from 50% to 30%. Our results do not support studies from freshwater, postulating that reduced light and nutrient limitation may increase herbivore production due to stoichiometric effects. Finally, we discuss how mixotrophy may bias rate estimates in dilution experiments
Description12 pages, 9 figures, 2 tables
Publisher version (URL)http://dx.doi.org/10.1016/j.jembe.2012.05.005
URIhttp://hdl.handle.net/10261/56936
DOI10.1016/j.jembe.2012.05.005
ISSN0022-0981
Appears in Collections:(ICM) Artículos
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