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

Abstract

Plankton 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 m 3 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

The role of mixotrophic protists in the biological carbon pump

“…In consequence, field measurements of mixotrophy need to deploy mixed tracers (i.e. not solely C tracers) and use a variety of approaches (Zubkov and Tarran, 2008;Calbet et al, 2012). Furthermore, significant changes in photosynthesis may or may not be expected (depending on the fate of the prey C).…”

Effect of CO2, nutrients and light on coastal plankton. III. Trophic cascade, size structure and composition

“…The increasing abundance of cells of sizes > 6 µm can be explained by alleviation of top-down control, as the most important grazers in this size range, cladocerans and copepods (Bautista & Harris 1992, Liu et al 2005, Souza et al 2005, Mercado et al 2007) were removed at the beginning of the experiment. Furthermore, mesozooplankton also prey on microzooplankton (Calbet & Landry 1999, Calbet et al 2012. The proportion of microzooplankton in the copepod diet is highly variable (Halvorsen et al 2001) and depends mainly on the phytoplankton and zooplankton size structure.…”

Effect of CO2, nutrients and light on coastal plankton. I. Abiotic conditions and biological responses

“…Generally, manipulation of bottom-up factors changed microbial growth rates, but the presence or absence of micrograzers was the most important determinant of carbon biomass. Calbet et al (2012) studied the effects of light availability on mixotrophy and microzooplankton grazing in an oligotrophic plankton food web in a mesocosm study in the Eastern Mediterranean waters, and observed that the decrease in nutritional quality of phytoplankton in mesocosms at high irradiance (higher C:chlorophyll ratios) was not reflected in a lower final abundance of grazers (both micro-and mesozooplankton). The authors pointed out that further work is needed in order to better understand the effect of light availability on Mediterranean aquatic food webs.…”