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

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 mu m, and > 10 mu m chlorophyll a in 11 standard dilution experiments. We also conducted simultaneous dilution experiments with the < 10 mu m planktonic community, to assess the potential impact of <10 mu m 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 mu m 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 mu m cells was very low, only significant in 5 out of 11 experiments (average impact of 23 % of the > 10 mu m primary production consumed daily, range 23 to 71 %). When the entire microbial community was size-fractioned by 10 pm, the potential impact of < 10 mu m nanograzers was evident for most of the year, although during the spring the differences between the impact on phytoplankton < 10 mu m 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 mu m organisms (72 to 88% of the biomass of grazers were < 10 mu m 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 mu m 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.