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...

Molecular studies of marine plankton have shown that ecological and/or environmental barriers play an important role in separating populations. Calanoid copepods are central in marine ecosystems, and dramatic biogeographical shifts in copepod assemblages associated with recent climate warming have been reported. We examined spatial population structuring in European waters of the Atlantic Ocean and Mediterranean Sea of Calanus helgolandicus and its sister species, C. euxi nus, from the Black Sea based on genetic and morphometric characters. The aims were to identify barriers to dispersal, relate these to hydrographic characteristics and infer historical patterns of distribution and demography. We analysed a 408 bp fragment of the mitochondrial 16S gene (316 individuals), prosome to urosome length relationships (212 individuals) and sea surface temperatures obtained from 19 European sites. Estimates of genetic differentiation between samples and hierarchical analyses of molecular variance indicated strong spatial population structuring between, as well as within, basins. We identified 7 phylogeographic groups: Fjords, Oceanic inflow, NE Atlantic/ Tyrrhenian, Adriatic, Mljet Island, Aegean, and Black Sea, which explained 39.7% of the total genetic variation. Based on genetic data, C. euxinus is considered to be a differentiated population within the C. helgolandicus distribution range because the most important genetic barrier separates western and eastern Mediterranean populations. Morphometric barriers largely reflect sea surface temperature barriers and are not congruent with the main genetic barriers. Contrary to recent findings for C. finmarchicus, we conclude that C. helgolandicus/C. euxinus populations are not connected by high levels of dispersal and have been vulnerable to past climatic changes.

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Oithona spp. are probably the most abundant and ubiquitous copepods in the world's oceans. However, knowledge of their development and growth rates is scarce compared to that of calanoid copepods. In the present laboratory study, we determined the survival, development and growth rates of the naupliar stages of Oithona davisae under different temperature regimes and food concentrations. Naupliar survival was reduced to approximately 60% at the lowest food concentration tested (11 µg C l -1 after 7 d at 20°C). The development of O. davisae nauplii was equiproportional, but not isochronal. Food concentrations required for maximum development and growth rates were 56 and 87 µg C l -1 , respectively. The Q 10 values for development and growth depended on the temperature range. O. davisae nauplii showed similar developmental times, but lower growth rates and food requirements than values reported in the literature for calanoid copepods. We suggest that these differences may help to explain the ubiquity of Oithona spp. in oceanic environments.KEY WORDS: Nauplii · Oithona davisae · Growth · Development · Survival · Food · TemperatureResale or republication not permitted without written consent of the publisher