Impact of micro- and nanograzers on phytoplankton assessed by standard and size-fractionated dilution grazing experiments

Abstract

Grazing by microzooplankton is typically assessed by dilution experiments of the whole natural community. However, in many ecosystems these experiments actually include not only micrograzers but also nanograzers. We discerned the relevance of micro-and nanograzers under contrasting trophic situations in the coastal NW Mediterranean throughout a seasonal cycle. We measured the grazing upon total, <10 µm, and >10 µm chlorophyll a in 11 standard dilution experiments. We also conducted simultaneous dilution experiments with the <10 µm planktonic community, to assess the potential impact of <10 µm grazers when released of predatory pressure by larger consumers. From September 2005 to May 2006 the microbial grazers consumed less than half of the total phytoplankton production. From June 2006 and for the whole summer period, the grazing on total phytoplankton increased, ranging from 76 to 104% of the primary production consumed per day. On annual average, microbial grazers consumed 56% of the total primary production. Grazing on <10 µm phytoplankton was very variable, from not significant (January and March) to >100% of the primary production consumed daily in July and August (the average impact for the whole study period was 58%). Grazing impact on >10 µm cells was very low, only significant in 5 out of 11 experiments (average impact of 23% of the >10 µm primary production consumed daily, range 23 to 71%). When the entire microbial community was size-fractioned by 10 µm, the potential impact of <10 µm nanograzers was evident for most of the year, although during the spring the differences between the impact on phytoplankton <10 µm measured in these experiments and in standard (unfiltered) dilutions were higher. During the warmer months (July and August) the size distribution of the grazers' community slightly shifted towards <10 µm organisms (72 to 88% of the biomass of grazers were <10 µm cells). Heterotrophic flagellates stood out as very relevant grazers in this system. In summary, the data suggest that the coastal NW Mediterranean is a system in which microzooplankton (>10 µm organisms) weakly control the primary producers during the cold season (winter and most of the autumn), switch to nano-sized heterotrophic prey during spring, partially suppressing the impact of this group on phytoplankton, and finally are replaced by nanograzers during the warmer months (end of the summer period), heavily impacting the dominant small primary producers.KEY WORDS: Size-fractionated dilutions · Microzooplankton · Nanograzers · Microbial grazers · Phytoplankton · NW Mediterranean Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 50: [145][146][147][148][149][150][151][152][153][154][155][156] 2008 based on sequential dilutions of natural communities, is not free of artifacts (e.g. Gallegos 1989, Dolan et al. 2000, Dolan & McKeon 2005, Agis et al. 2007). In general, these artifacts can be overcome by precise execution of the method, and in certain situations by the use of...

Copepod feeding in the ocean: scaling patterns, composition of their diet and the bias of estimates due to microzooplankton grazing during incubations

“…Furthermore, the use of the dilution method to estimate microzooplankton grazing is not free from criticisms and uncertainties (Dolan et al, 2000;Agis et al, 2007;Modigh & Franze, 2009) and actually encompasses not only strictly defined microzooplankters (e.g. ciliates and dinoflagellates), but also grazing by heterotrophic nanoflagellates and multi-level trophic cascade effects (Calbet, 2008;Calbet et al, 2008;First et al, 2009). This targeting of the wrong trophic guilds further complicates any attempt to make such corrections.…”

Seasonal and size-dependent variations in the phytoplankton growth and microzooplankton grazing in the southern South China Sea under the influence of the East Asian monsoon

“…Many studies have found pronounced seasonality in the phytoplankton growth and microzooplankton grazing in temperate and high-latitude waters (e.g. Calbet et al, 2008;Kim et al, 2007;Lawrence and Menden-Deuer, 2012;Loebl and Beusekon, 2008). Nevertheless, there are few studies on the seasonality of the microzooplankton grazing in oligotrophic tropical waters.…”

Variability in Protist Grazing and Growth on Different Marine Synechococcus Isolates

“…Synechococcus spp. are an important trophic resource and hold a key position at the base of marine food webs, where grazing mortality is dominated by heterotrophic protists (9,43). However, there is little consensus regarding the most important consumers of Synechococcus.…”