Community structure and trophic role of ciliates and heterotrophic nanoflagellates in Rhone River diluted mesoscale structures (NW Mediterranean Sea)

Abstract

Studies of the Rhone River plume, and in particular its diluted mesoscale structures, have shown over recent years that low salinity water (LSW) is characterised by both high primary and high secondary productivity. Here we compare the structure and grazing activity of the hetorotrophic community (nanoflagellates, ciliates) in the LSW and the surrounding marine water (MW) of the northwestern Mediterranean Sea. Furthermore, we report on the temporal variability of these characteristics all along a Lagrangian track in the LSW. In May 2006, salinity of the LSW ranged from 31.41 to 38.25 down to a depth of 35 m, and surface chlorophyll a concentration reached 15.5 µg l -1 . The ciliate community structure was similar in both the LSW and the MW, characterised by the dominance of the mixotrophic ciliates Laboea strobila and Myrionecta rubra. The ciliate biomass was approximately 6 times greater in the LSW, compared to the MW. This difference was due to the mixotrophic ciliates, which were about 4-to 5-fold more important in terms of abundance and biomass in the LSW, while heterotrophic ciliates only increased by a maximum of 2-fold. M. rubra was the most abundant species reaching 4600 cells l -1 , while L. strobila was dominant in terms of biomass, accounting for 47% of total biomass in the LSW. Ciliates, in particular mixotrophs, were very active in the LSW where they grazed on phytoplankton at rates near their maximum clearance capacities. Mixotrophic and heterotrophic ciliates in LSW consumed about 50 and 10% of the nanophytoplankton stock per day, respectively. The mixotrophic ciliate L. strobila was found to be an efficient grazer of nanophytoplankton. In contrast to the ciliates, the biomass of heterotrophic nanoflagellates was similar in both the LSW and the MW; thus, we suggest that the transfer of bacterial biomass in the LSW towards higher trophic levels was moderate.

Functioning of the planktonic ecosystem on the Gulf of Lions shelf (NW Mediterranean) during spring and its impact on the carbon deposition: a field data and 3-D modelling combined approach

“…A decrease in bacterial production at the surface all along the trajectory (not shown) finally suggests a strong top-down control by ciliates and heterotrophic flagellates on bacteria and small size phytoplankton. Such control mainly exerted by mixotrophs, as shown by Christaki et al (2009), may also explain why a decrease of NO 3 :NH 4 is everywhere observed in the lens and whatever the measured salinity (not shown). Indeed, the top-down control on bacteria and small size phytoplankton may favour the development of large phytoplankton, explaining an important consumption of nitrate along the trajectory.…”

“…These in situ phytoplankton biomasses can then be compared to the three aforementioned phytoplankton size classes (Phy 1 , Phy 2 , and Phy 3 ) represented by the model. In the same way, an analysis of heterotrophic flagellates and ciliates activity and biomass observed during the cruise is presented by Christaki et al (2009), and integrated biomass of copepods were measured by D. Bonnet (personal communication, 2009), allowing the comparison of zooplankton model outputs (resp. Zoo 1 , Zoo 2 , and Zoo 3 ) with in situ measurements.…”

“…Microzooplankton maximum grazing rate was set to a relatively high value in agreement with high ingestion rates observed in the Rhone River plume (Christaki et al, 2009). On the contrary, the maximum grazing rate of mesozooplankton was fixed to a very low value according to in situ measurements from Gaudy et al (1990).…”

Relative importance of phototrophic, heterotrophic, and mixotrophic nanoflagellates in the microbial food web of a river-influenced coastal upwelling area

“…Similarly, high abundances of picophytoplankton, picoeukaryotes (Liu et al 2004), and nanophytoplankton (Jochem 2003) have been observed in river plumes. Recently, Christaki et al (2009) showed that the PNF abundance was 3 to 5 fold higher in the Rhone River plume waters compared to marine waters. Abundances of PNF and HNF were within the ranges reported in other studies of oligotrophic, mesotrophic, and eutrophic marine ecosystems, mostly from 0.1 to 30 cells ml −1 × 10 3 (Table 4 and references in there).…”

Microbial food webs and metabolic state across oligotrophic waters of the Mediterranean Sea during summer

“…4c, 90 % of ciliate biomass). In a recent study in the western Mediterranean Sea, mixotrophs were found to be efficient grazers of nanophytoplankton forming dense populations related to high Chl-a concentrations (Christaki et al, 2009). Tintinnids are also known to form patchy distributions related to their food resources (e.g., Stoecker et al, 1984;Christaki et al, 2008).…”