Search

Gelatinous zooplankton (GZ) comprise a taxonomically and functionally diverse group of marine organisms which includes ctenophores, cnidarians and pelagic tunicates, sharing a soft, mostly transparent body texture, a high body water content and a lack of exoskeleton. They range in size from less than a millimetre to nearly 2 m for the cnidarian jellyfish Nemopilema nomurai, and comprise some of the fastest growing metazoans on Earth (Hopcroft et al., 1998), sometimes surpassing crustacean zooplankton in their contribution to secondary production (i.e. in subtropical waters; Jaspers et al., 2009). They feed on a wide range of prey sizes, with predator–prey ratios comparable in some cases to those of baleen whales and krill (Deibel and Lee, 1992), and with prey removal rates which are similar to those of their non-gelatinous competitors (Acuña et al., 2011). In spite of early work pointing to gelatinous zooplankton as a trophic dead end (Verity and Smetacek, 1996), evidence is rapidly accumulating which shows that they may potentially channel energy from the picoplankton-sized, microbial loop organisms up to the higher trophic levels, including fish (Llopiz et al., 2010). However, this pathway is still largely neglected in most food web investigations even though it is now becoming clear that GZ represent a major fraction of the diet of several commercially important fish species such as bluefin tuna (Thunnus thynnus) (Cardona et al., 2012).

To investigate the combined effect of temperature and light availability on organic matter production and degradation during a winter/spring phytoplankton bloom in Kiel Bight, we conducted a mesocosm study applying two temperature regimes, ambient (T + 0) and plus 6°C (T + 6) and three irradiance levels. Rising temperature accelerated the onset of the phytoplankton bloom, while light intensity played only a minor role for the timing and bloom development. Maximum build-up of chlorophyll a and particulate organic carbon were ∼20% lower at T + 6 compared with T + 0, probably caused by a combination of elevated heterotrophic processes and enhanced sedimentation during the bloom. The latter is supported by increased TEP concentrations at T + 6 (TEP/POC 0.18 mol C/mol C) compared with T + 0 (0.11 mol C/mol C) during bloom conditions, which may have promoted cell aggregation and sinking. Dissolved organic carbon concentrations increased more rapidly at elevated temperature. For a warmer future ocean, we can hence expect two counteracting mechanisms controlling organic matter flow during phytoplankton blooms: (1) enhanced processing of organic matter via the microbial loop resulting in a faster recycling and (2) depending on the dominating phytoplankton species, enhanced TEP formation resulting in increased particle aggregation and thus export of carbon and nutrients.

Since its first reported appearance in the Baltic Sea in 2007, there has been a little research on the role of the Arctic ctenophore Mertensia ovum in this ecosystem. We provide results from the first feeding experiments of M. ovum from the northern Baltic Sea. Experiments were conducted with various prey types; picocyanobacteria Synechococcus bacillaris, ciliates Mesodinium rubrum, nauplii of mixed copepod species and copepodites of Eurytemora affinis. Molecular gut content analyses were also used to measure in situ feeding of M. ovum on the picocyanobacteria. The observed clearance rates on M. rubrum (< 9.0 mL predator(-1) h(-1)), and S. bacillaris (

We analysed the structure of the microbial plankton communities of different types of freshwater environments from the southernmost region of South America (Tierra del Fuego). Water bodies were grouped in four categories: humic lakes, clear oligotrophic lakes, beaver ponds and steppe shallow lakes, which differed in their nutrient and dissolved organic carbon (DOC) contents. We tested if microbial planktonic communities were different among lakes with dissimilar nutrient and DOC concentrations, analysing to what extent the known large-scale patterns of lake trophic structure applies to a diverse but localized set of lakes. We found that mixotrophs dominated over strict autotrophs in both humic and clear oligotrophic systems, whereas in eutrophic lakes autotrophy was a successful strategy. The functional phytoplankton approach also allowed the separation between oligotrophic (clear and humic) and eutrophic systems, with different functional groups. The lowest abundances of picoplankton were found in oligotrophic lakes, picoeukaryotes being more abundant than picocyanobacteria in beaver ponds and humic lakes. Our results show that in low nutrient environments, mixotrophic strategies thrive over strict autotrophs suggesting the paramount importance of the microbial loop when compared with high trophic status systems where the prevalence of autotrophy indicates that the energy flux depends on phytoplankton. Fil: Saad, Juan Francisco. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Ecología, Genética y Evolución de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Ecología, Genética y Evolución de Buenos Aires; Argentina Fil: Schiaffino, María Romina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Ecología, Genética y Evolución de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Ecología, Genética y Evolución de Buenos Aires; Argentina Fil: Vinocur, Alicia Liliana. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Biodiversidad y Biología Experimental; Argentina Fil: O'farrell, Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Ecología, Genética y Evolución de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Ecología, Genética y Evolución de Buenos Aires; Argentina Fil: Tell, Hector Guillermo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Ecología, Genética y Evolución de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Ecología, Genética y Evolución de Buenos Aires; Argentina Fil: Izaguirre, Irina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Ecología, Genética y Evolución de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Ecología, Genética y Evolución de Buenos Aires; Argentina

on microbial plankton isolated from the Senegal River Estuary. Experiments performed in microcosms were based on size-class fractionation (3, 12, 20 and 60 m size classes) of the microbial community in order to obtain different assemblages, including bacteria, heterotrophic nanoflagellates (HNF), ciliates, pico- and nanophytoplankton. Removal of bacterial predators >3 m increased the net growth rate of heterotrophic bacteria from 0.437 day � 1 in the presence of all predators (fraction 12 m caused an increased net growth rate of flagellates from 0.102 day � 1 (fraction 12 m by this copepod, allowing an increase in HNF growth rates through indirect effect. The highest ingestion rate over 60 h of experiment was recorded for the copepod Temora (61.1 m 3 ngC � 1 h � 1 ) and the lowest value for barnacle larvae (25.6 m 3 ngC � 1 h � 1 ). An intermediate value (47.5m 3 ngC � 1 h � 1 )was recorded for thecopepodAcartia clausi.Alltheseingestion ratesweremainlychanneled in the size classes between

Metazooplankton grazing on bacteria and on the phytoplankton of various sizes was estimated in shallow eutrophic lakes Kaiavere and Vortsjarv (Estonia) by in situ feeding experiments with fluorescent microspheres (diameters 0.5 μm for bacteria and 3, 6 and 24 μm for phytoplankton). Zooplankton community composition, abundance and food density were important factors determining grazing rates in these lakes. Cladocerans and rotifers filtering rates (FR) and ingestion rates (IR) on bacteria and phytoplankton were several times higher in Lake Kaiavere where bacterivorous rotifers and Daphnia contributed more to zooplankton assemblage. While cladocerans were generally the main phytoplankton consumers, both lakes differed with respect to the groups of bacterivores. Based on consumption of fluorescent microspheres, the metazooplankton grazing rates were relatively low and had low impact on production and standing stock of bacteria and ingestible phytoplankton (

This study examined the relative contributions of bacterial and phytoplankton production to the pelagic carbon flow of Lake Okeechobee, a large and shallow subtropical lake. Due to the predominance of cyanobacteria in this lake, we hypothesized that bacterial carbon flow to grazers would be larger than phytoplankton carbon flow to grazers. Using epifluorescent and light microscopy and radiotracer techniques, we measured the carbon biomass of planktonic functional groups and carbon flow between these groups. The functional groups that we used in this study included: picophytoplankton, autotrophic nanoflagellates (ANAN), microphytoplankton, bacteria, heterotrophic nanoflagellates (HNAN), ciliates, microzooplankton (rotifers and copepod nauplii), and macrozooplankton (cladocerans, copepodites, and adult copepods). Microphytoplankton dominated the carbon biomass of all plankton, whereas the calanoid copepod, Diaptomus, dominated the carbon biomass of the grazers. Phytoplankton carbon flow often was higher than bacterial carbon flow to grazers; however, bacterial carbon constituted a large percentage of the total carbon flow to grazers (33.7 ± 22.4%). Bacterial carbon provided roughly one quarter of the carbon flow to macrozooplankton (27.1 ± 25.4%), whereas it provided half of the carbon flow to microzooplankton (57.4 ± 20.3%) and to protozoans (47.2 ± 25.8%). These results suggest that microbial pathways play an important role in the energetics of subtropical lake plankton communities. Although microbial loop pathways are important in many systems, direct bacterial carbon flow to macrozooplankton also may be important in copepod- and cyanobacteriadominated lakes.

Gel-like transparent exopolymer particles (TEP) formed from polysaccharides exuded copiously by natural phytoplankton and bacteria occur ubiquitously in sea water, but the utilization of these particles as a food source by zooplankton is poorly known. The common marine calanoid copepod Calanus pacificus consumed TEP and produced abundant fecal pellets when TEP were offered both alone and in the presence of diatom cells. TEP consumption increased with TEP concentration and the presence of TEP did not inhibit ingestion of cells. However, because of their low carbon content, TEP contributed only 15% to total carbon consumption in the presence of phytoplankton food. While copepods consumed pure TEP in this study, sticky TEP in nature contain abundant inclusions of bacteria and microorganisms too small for copepods to capture individually. Thus, consumption of TEP may be most significant as a mechanism by which carbon may be shunted from the microbial loop to higher trophic levels.

The development of the Daphnia longispina (0. F. Muller) population in a highly humic boreal lake was followed throughout one growing season, and the amount of secondary production was estimated in relation to primary production and available food resources. The growth rate method was applied in the secondary production measurements, Daphnia longispina did not appear in the water column until 16 May, after which the animals were present throughout the growing season. The population showed three density peaks; the first appeared in early June, and the second and third in mid-July and at the beginning of September, respectively. Somatic production followed a seasonal pattern, with highest production rates in midsummer. The maximum value of 127 mg C m -2 2 day 1 was recorded at the beginning of July. The total annual net production of D. longispina was 7.9 g C m 2 . During most of the growing season, the primary productivity in the lake was well below 100 mg C m -2 day -1 and the total annual productivity of photosynthetic algae was 5.0 g C m -2 . We conclude that in this lake the zooplankton population did not rely on phytoplankton primary production as a sole carbon source, but that most of the carbon must have originated from bacterial production either directly or through a microbial loop.

It was recently proposed that there is, in the pelagic environment, a continuum of trophic pathways ranging from the herbivorous food web to the multivorous food web, the microbial food web and the microbial loop. It was also suggested that combining specific ecological ratios could provide a way of assessing the dominance of planktonic food webs by specific trophic pathways. Three large data sets, collected in different but adjacent marine systems (the Scotian Shelf, off eastern Canada; the Gulf of St Lawrence; and nearshore waters of the Baie des Chaleurs, in the northwestern Gulf), are used to compute two ratios: small-sized ( 5 μm) particulate phytoplankton production, and phaeopigments in small- to phaeopigments in large-sized particles. By combining the two ecological ratios it is possible to delineate successfully, in each studied system, coherent periods during the year; to interpret the combined ratios in terms of dominance by specific trophic pathways; and to show that the interpretation is consistent with additional information collected in the three systems.

The hypothesis that ubiquitous, sticky transparent exopolymer particles (TEP) formed from phytoplankton exudates will adhere to and coat the feeding structures of marine zooplankton grazers, and thus depress feeding on phytoplankton, was tested using the euphausiid, Euphausia pacifica, as a model organism. During two feeding experiments, E.pacifica were offered cells of the diatom Thalassiosira weissflogii, TEP, or both TEP and T.weissflogii cells. Ingestion rates on cells were lower in the presence of TEP. However, contrary to the hypothesis, grazing on cells was not inhibited by TEP. Rather, TEP-clusters, aggregates which formed from TEP and nano-sized particles normally too small for the filtering apparatus of E.pacifica to retain, served as an alternative food source for E.pacifica, reducing their ingestion of cells. These clusters were very similar in form to the TEP actually available to marine grazers in nature. TEP-clusters were similar to cells in size and food quality, and were grazed at similar rates. When feeding on TEP-clusters, euphausiids short circuit the food web by feeding on nano- and picoplankton directly, bypassing the microbial loop. Thus, the presence of TEP appears to enhance, rather than depress, macrozooplankton grazing.

Data are synthesized on biomass and fluxes of components of the pelagic food web, downward particulate flux, and associated physical and chemical oceanographic parameters in austral winter and spring in the Subtropical Front (STF) region east of New Zealand. All four food web types (sensu Legendre and Rassoulzadegan, Ophelia, 41, 153-172, 1995) are represented in this region. The STF food web is classified as 'multivorous' in spring and 'herbivorous'/'multivorous' in winter, even though grazing was dominated by microzooplankton. Based on relatively few data over each season, STF community biomass was dominated by large phytoplankton cells (>20 µm) in both seasons. Post- bloom conditions were sampled in spring and highest fucoxanthin vertical fluxes were recorded at this time. A phytoplankton sedimentation event may have occurred just after our mid-October sampling. Subtropical (ST) waters exhibited pronounced seasonality in all plankton compartments. The ST food web is classified as 'multivorous' in both winter and spring. Organic flux in sediment traps was highest in ST water relative to subantarctic (SA) and STF waters. Although the ST data coverage was limited, high spring algal specific growth rates, the large change in total community biomass between winter and spring, and small seasonal differences in early winter and spring macronutrient concentrations suggest that the structure and functioning of this ecosystem depended on the macronutrients entrained during deep winter mixing. Algal populations in SA high-nitrate-low-chlorophyll waters (HNLC) were dominated by small cells. Heterotrophic bacteria dominated community biomass in SA waters in winter and equalled phytoplankton biomass in spring. In contrast to Legendre and Rassoulzadegan's classification of the subarctic Pacific HNLC food web as 'multivorous', the SA food web is classified as 'microbial' tending towards 'microbial loop' in winter (i.e. there was greater carbon uptake by bacteria than phytoplankton) and 'microbial' in spring. Chlorophyll (Chl) a levels in spring were similar to those in winter, but because of a doubling of the C:Chl ratio, algal biomass (as carbon) was 2-fold higher in SA waters in spring, compared with winter. This seasonal increase in algal carbon biomass in spring occurred in spite of a >5-fold increase from winter to spring in microzooplankton biomass, challenging the notion of grazer control of algal biomass. Our findings are compared with Russian data collected in the South Pacific STF region in summer 1985. Aspects of ecosystem func- tioning in the STF region east of New Zealand may hold for other longitudes in the Southern Ocean, although it is likely that some characteristics are regional in nature.

The elemental composition and growth rate of Rhodomonas and heterotrophic bacteria were studied in batch cultures in the presence and absence of Daphnia and at two different levels of phosphorus li ...

17 pages, 4 figures, 1 table, Many protist plankton are mixotrophs, combining phototrophy and phagotrophy. Their role in freshwater and marine ecology has emerged as a major developing feature of plankton research over recent decades. To better aid discussions, we suggest these organisms are termed “mixoplankton”, as “planktonic protist organisms that express, or have potential to express, phototrophy and phagotrophy”. The term “phytoplankton” then describes phototrophic organisms incapable of phagotrophy. “Protozooplankton” describes phagotrophic protists that do not engage in acquired phototrophy. The complexity of the changes to the conceptual base of the plankton trophic web caused by inclusion of mixoplanktonic activities are such that we suggest that the restructured description is termed the “mixoplankton paradigm”. Implications and opportunities for revision of survey and fieldwork, of laboratory experiments and of simulation modelling are considered. The main challenges are not only with taxonomic and functional identifications, and with measuring rates of potentially competing processes within single cells, but with decades of inertia built around the traditional paradigm that assumes a separation of trophic processes between different organisms. In keeping with the synergistic nature of cooperative photo- and phagotrophy in mixoplankton, a comprehensive multidisciplinary approach will be required to tackle the task ahead, This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 766327