We present an analysis of seasonal variations in the trophic pathways of carbon in a highly productive coastal upwelling region in the Humboldt current system off Chile. Seasonal changes in phytoplankton, protozooplankton, and bacteria biomass, along with rates of primary production (PP), bacterial growth, secondary production, vertical particle fluxes, and feeding by protozooplankton, omnivorous mesozooplankton, and carnivorous gelatinous zooplankton were determined from July 2004 to June 2005. Phytoplankton biomass and PP were maximal during spring/summer months, associated with upwelling episodes. Heterotrophic nanoflagellates (HNF) were the principal consumers of bacteria, removing .100% of their biomass daily. During autumn/winter, the protozooplankton grazed down a large fraction of HNF production (56% to 96% d 21 ). The mesozooplankton consumed 1-6% of the PP d 21 ; the different size fractions of copepods were omnivorous mostly during autumn/winter months, and ctenophores preyed most strongly on small copepods (0.5% to 5% d 21 ). A large part of the PP was channeled through the microbial food web, and only a small part AcknowledgmentsWe thank the captains and crew of the RV Kay Kay (Universidad de Concepció n, Chile) and the many undergraduate and graduate students who participated in our cruises (L. Lizá rraga, V. Aguilera, C. Aparicio, E. Menschel, and A. Araneda). We also thank José Luis Acuñ a and Albert Calbet for their valuable suggestions that substantially improved an earlier version of the manuscript and two anonymous reviewers for their critical and helpful comments.

), respectively, from winter to spring. In addition, the bacterial secondary production to primary production (BSP:PP) ratio decreased from 3.7 to 0.2 in Reloncaví Fjord, suggesting a transition from microbial to classical pelagic food webs. The higher solar radiation and extended photoperiod of springtime promoted the growth of diatoms in a nutrient-replete water column. Allochthonous (river discharge) and autochthonous (phytoplankton exudates) organic matter maintained high year-round bacteria biomass and secondary production. In spring, grazing pressure from zooplankton on the microplankton (largely diatoms) resulted in the relative dominance of the classical food web, with increased export production of zooplankton faecal pellets and ungrazed diatoms. Conversely, in winter, zooplankton grazing, mainly on nanoplankton, resulted in a relative dominance of the microbial loop with lower export production than found in spring. Carbon fluxes and fjord-system functioning are highly variable on a seasonal basis, and both the multivorous trophic webs and the carbon export were more uncoupled from local PP than coastal areas.

We investigated the effects of an increase in dissolved CO2 on the microbial communities of the Mediterranean Sea during two mesocosm experiments in two contrasting seasons: winter, at the peak of the annual phytoplankton bloom, and summer, under low nutrient conditions. The experiments included treatments with acidification and nutrient addition, and combinations of the two. We followed the effects of ocean acidification (OA) on the abundance of the main groups of microorganisms (diatoms, dinoflagellates, nanoeukaryotes, picoeukaryotes, cyanobacteria, and heterotrophic bacteria) and on bacterial activity, leucine incorporation, and extracellular enzyme activity. Our results showed a clear stimulation effect of OA on the abundance of small phytoplankton (pico- and nanoeukaryotes), independently of the season and nutrient availability. A large number of the measured variables showed significant positive effects of acidification in summer compared with winter, when the effects were sometimes negative. Effects of OA were more conspicuous when nutrient concentrations were low. Our results therefore suggest that microbial communities in oligotrophic waters are considerably affected by OA, whereas microbes in more productive waters are less affected. The overall enhancing effect of acidification on eukaryotic pico- and nanophytoplankton, in comparison with the non-significant or even negative response to nutrient-rich conditions of larger groups and autotrophic prokaryotes, suggests a shift towards medium-sized producers in a future acidified ocean.

Mixotrophs combine photosynthesis with phagotrophy to cover their demands in energy and essential nutrients. This gives them a competitive advantage under oligotropihc conditions, where nutrients and bacteria concentrations are low. As the advantage for the mixotroph depends on light, the competition between mixo- and heterotrophic bacterivores should be regulated by light. To test this hypothesis, we incubated natural plankton from the ultra-oligotrophic Eastern Mediterranean in a set of mesocosms maintained at 4 light levels spanning a 10-fold light gradient. Picoplankton (heterotrophic bacteria (HB), pico-sized cyanobacteria, and small-sized flagellates) showed the fastest and most marked response to light, with pronounced predator-prey cycles, in the high-light treatments. Albeit cell specific activity of heterotrophic bacteria was constant across the light gradient, bacterial abundances exhibited an inverse relationship with light. This pattern was explained by light-induced top-down control of HB by bacterivorous phototrophic eukaryotes (PE), which was evidenced by a significant inverse relationship between HB net growth rate and PE abundances. Our results show that light mediates the impact of mixotrophic bacterivores. As mixo- and heterotrophs differ in the way they remineralize nutrients, these results have far-reaching implications for how nutrient cycling is affected by light.

Marine planktonic organisms endure fluctuations in food abundance and quality during their life. The degree of resource variability in each specific environment may have forced adaptive survival responses on the organisms inhabiting them. We studied the adaptations to feast and famine of 2 strains of the heterotrophic dinoflagellates Gyrodinium dominans (GYR-DK from Denmark; GYR-BCN from Barcelona) and Oxyrrhis marina (OXY-BCN from Barcelona; OXY-CRB from the Caribbean). Overall, the OXY strains showed contrasting results in terms of feeding, metabolism, and biochemical composition, whereas both GYR strains presented similar responses to the variables measured. OXY-BCN exhibited higher maximum ingestion rates, better capacity to exploit a pulse of food, higher carbon assimilation efficiency and lipid storage capacity, and longer survival time to starvation. When feeding on a fatty acid-rich alga (Rhodomonas salina, RHO), OXY-BCN displayed very high (75%) gross growth efficiencies (GGE), but showed no growth when conditioned to one that was more fatty acid deficient (Dunaliella tertiolecta, DUN). In contrast, both GYR strains had higher GGE when feeding on DUN (> 50%) compared to a diet of RHO (16 to 22%). OXY-CRB showed low GGE (< 2 0%), despite feeding actively on both prey. All strains maintained their carbon and nitrogen stoichiometry after 5 d starvation, but lost some fatty acids, especially OXY. Additionally, when starving, respiration rates decreased by 70% in OXY-BCN, 50% in GYR-DK, and by 25% in OXY-CRB. Our results demonstrate that OXY-BCN is a more opportunistic organism, perfectly adapted to heterogeneous or unstable environments; although it requires a suitable biochemical composition in its prey. On the other hand, GYR seems better conditioned to more stable habitats, such as coastal and open waters. This study also stresses the phenotypic differences between strains (especially of OXY) originating from different ecosystems.

We studied the effects of future climate change scenarios on plankton communities of a Norwegian fjord using a mesocosm approach. After the spring bloom, natural plankton were enclosed and treated in duplicates with inorganic nutrients elevated to pre-bloom conditions (N, P, Si; eutrophication), lowering of 0.4 pH units (acidification), and rising 3°C temperature (warming). All nutrient-amended treatments resulted in phytoplankton blooms dominated by chain-forming diatoms, and reached 13–16 μg chlorophyll (chl) a l−1. In the control mesocosms, chl a remained below 1 μg l−1. Acidification and warming had contrasting effects on the phenology and bloom-dynamics of autotrophic and heterotrophic microplankton. Bacillariophyceae, prymnesiophyceae, cryptophyta, and Protoperidinium spp. peaked earlier at higher temperature and lower pH. Chlorophyta showed lower peak abundances with acidification, but higher peak abundances with increased temperature. The peak magnitude of autotrophic dinophyceae and ciliates was, on the other hand, lowered with combined warming and acidification. Over time, the plankton communities shifted from autotrophic phytoplankton blooms to a more heterotrophic system in all mesocosms, especially in the control unaltered mesocosms. The development of mass balance and proportion of heterotrophic/autotrophic biomass predict a shift towards a more autotrophic community and less-efficient food web transfer when temperature, nutrients and acidification are combined in a future climate-change scenario. We suggest that this result may be related to a lower food quality for microzooplankton under acidification and warming scenarios and to an increase of catabolic processes compared to anabolic ones at higher temperatures.

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Plankton biomass and composition in the pelagic zone of oceans is exposed to changes in availability of light and nutrients due to large-scale ocean circulation and water column stratification. We hypothesized that displacement of plankton from surface to deeper darker waters would not only favor heterotrophy over time, as previously suggested, but also first rapidly affect the level of mixotrophy and, consequently, overall microbial grazing in plankton food webs. To test this in an oligotrophic marine system we incubated Eastern Mediterranean water (from 10 m depth north of Crete in September 2010) in 2.8 m 3 mesocosms simulating two different light intensities at the sampling station, surface waters (ca. 10 m; mesocosms L1) and deeper layers (ca. 50-60 m; mesocosms L4). The biomass and abundance of the main planktonic groups were monitored either daily or every second day, depending on the group. Microzooplankton grazing rates and the contribution of mixotrophic feeding were estimated by a combination of dilution experiments and incubations with live fluorescently labeled algae (LFLA). Although no nutrients were added to the mesocosms the chlorophyll a increased during the first 2 days of the experiment in both treatments. This increase resulted from phytoplankton growth in the light L1-mesocosm (autotrophic biomass was ca. doubled in L1 compared to L4), but was mostly due to photoadaptation of the algae in the L4-mesocosm, as indicated by lower carbon to chlorophyll a ratios. By the end of the experiment, the total biomass of protozoan and metazoan grazers in L1 was ca. twofold higher than in L4. The microzooplankton responded within the first 24 h, showing different grazing activity in L1 than in L4. Microzooplankton grazing rates on total Chl a were similar in both treatments; however, phytoplankton instantaneous growth rates were higher in the more illuminated mesocosm. This resulted in a closer coupling between both rates