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Coexistence of two organisms competing for the same nutrient is possible if one is an ÔuptakeÕ, and the other a Ôpredation defenceÕ specialist. In pelagic food webs this principle has been linked to cell size. Small osmotroph cells, with their high surface : volume ratio, have been argued to be uptake specialists, while larger osmotrophs avoiding the intense grazing pressure from small protozoan predators might represent Ôpredation defenceÕ specialists. This may seem like an obligatory trade-off situation that necessitates a choice of either being small or being large, and thus being potentially dominant in oligotrophic or in eutrophic environments, respectively. However, in a more precise form, the theory for nutrient diffusion states that it is the Ôsurface : cell requirement of limiting elementÕ ratio, rather than the Ôsurface : volumeÕ ratio, that is important. The distinction is crucial, since it opens up the possibility of there being life strategies that use a non-limiting element to increase size. Hypothesized to maximize uptake and predator defence simultaneously, such strategies should be particularly successful. We suggest that this strategy is exploited by osmotrophs with different size and physiology, such as heterotrophic bacteria, unicellular cyanobacteria and diatoms. Since the strategy implies a shift in organism stoichiometry, the biogeochemical implications are strong, illustrating the tight relationships between physical micro-scale processes, organism life strategies, biodiversity, food web structure, and biogeochemistry.

Predicting the ocean's role in the global carbon cycle requires an understanding of the stoichiometric coupling between carbon and growth-limiting elements in biogeochemical processes. A recent addition to such knowledge is that the carbon/nitrogen ratio of inorganic consumption and release of dissolved organic matter may increase in a high-CO(2) world. This will, however, yield a negative feedback on atmospheric CO(2) only if the extra organic material escapes mineralization within the photic zone. Here we show, in the context of an Arctic pelagic ecosystem, how the fate and effects of added degradable organic carbon depend critically on the state of the microbial food web. When bacterial growth rate was limited by mineral nutrients, extra organic carbon accumulated in the system. When bacteria were limited by organic carbon, however, addition of labile dissolved organic carbon reduced phytoplankton biomass and activity and also the rate at which total organic carbon accumulated, explained as the result of stimulated bacterial competition for mineral nutrients. This counterintuitive 'more organic carbon gives less organic carbon' effect was particularly pronounced in diatom-dominated systems where the carbon/mineral nutrient ratio in phytoplankton production was high. Our results highlight how descriptions of present and future states of the oceanic carbon cycle require detailed understanding of the stoichiometric coupling between carbon and growth-limiting mineral nutrients in both autotrophic and heterotrophic processes.

Abstract. Increasing atmospheric carbon dioxide (CO 2 ) concentrations due to anthropogenic fossil fuel combustion are currently changing the ocean's chemistry. Increasing oceanic [CO 2 ] and consequently decreasing seawater pH have the potential to significantly impact marine life. Here we describe and analyze the build-up and decline of a natural phytoplankton bloom initiated during the 2005 mesocosm Pelagic Ecosystem CO 2 Enrichment study (PeECE III). The draw-down of inorganic nutrients in the upper surface layer of the mesocosms was reflected by a concomitant increase of organic matter until day t 11 , the peak of the bloom. From then on, biomass standing stocks steadily decreased as more and more particulate organic matter was lost into the deeper layer of the mesocosms. We show that organic carbon export to the deeper layer was significantly enhanced at elevated CO 2 . This phenomenon might have impacted organic matter remineralization leading to decreased oxygen concentrations in the deeper layer of the high CO 2 mesocosms as indicated by deep water ammonium concentrations. This would have important implications for our understanding of pelagic ecosystem functioning and future carbon cycling.

Abstract.A CO 2 enrichment experiment (PeECE III) was carried out in 9 mesocosms in which the seawater carbonate system was manipulated to achieve three different levels of pCO 2 . At the onset of the experimental period, nutrients were added to all mesocosms in order to initiate phytoplankton blooms. Primary production rates were measured by in-vitro incubations based on 14 C-incorporation and oxygen production/consumption. Size fractionated particulate primary production was also determined by 14 C incubation and is discussed in relation to phytoplankton composition. Primary production rates increased in response to nutrient addition and a net autotrophic phase with 14 C-fixation rates up to 4 times higher than initial was observed midway through the 24 days experiment before net community production (NCP) returned to near-zero and 14 C-fixation rates dropped below initial values. No clear heterotrophic phase was observed during the experiment. Based on the 14 C-measurements we found higher cumulative primary production at higher pCO 2 towards the end of the experiment. CO 2 related differences were also found in size fractionated primary production. The most noticeable responses to CO 2 treatments with respect to primary production rates occurred in the second half of the experiment when phytoplankton growth had become nutrient limited, and the phytoplankton community changed from diatom to flagellate dominance. This opens for two alternative hypotheses that the effects are either associated with mineral nutrient limited growth, and/or with a change in phytoplankton species composition. The lack of a clear net heterotrophic phase in the last part of the experiment supports the idea that

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Abstract. We report the transient population dynamic response of the osmotrophic community initiated by a nutrient pulse in mesocosms exposed to different pCO2 levels as well as quantitative variations in phytoplankton and heterotrophic bacteria created by the difference in CO2 exposure. Coastal seawater was enclosed in floating mesocosms (27 m3) and nutrients were supplied initially in order to stimulate growth of microbial organisms, including the coccolitophorid Emiliania huxleyi. The mesocosms were modified to achieve 350 μatm (1×CO2), 700 μatm (2×CO2) and 1050 µatm (3×CO2) CO2 pressure. The temporal dynamics was related to the nutrient conditions in the enclosures. Numerically small osmotrophs (picoeukaryotes and Synechoccocus sp.) dominated initially and towards the end of the experiment, whereas intermediate sized osmotrophs bloomed as the initial bloom of small sized osmotrophs ceased. Maximum concentrations of E. huxleyi were approximately 4.6×103 cells ml−1 whereas other intermediate sized osmotrophs reached approximately twice as high concentrations. Osmotrophic succession pattern did not change, and we were not able to detect differences with regard to presence or absence of specific osmotrophic taxa as a consequence of altered atmospheric CO2 concentration. Quantitative effects on the microbial communities associated with the CO2 treatment were, however, observed towards the end of the experiment.

A minimum mathematical model of the marine pelagic microbial food web has previously shown to be able to reproduce central aspects of observed system response to different bottom-up manipulations in a mesocosm experiment Microbial Ecosystem Dynamics (MEDEA) in Danish waters. In this study, we apply this model to two mesocosm experiments (Polar Aquatic Microbial Ecology (PAME)-I and PAME-II) conducted at the Arctic location Kongsfjorden, Svalbard. The different responses of the microbial community to similar nutrient manipulation in the three mesocosm experiments may be described as diatom-dominated (MEDEA), bacteria-dominated (PAME-I), and flagellated-dominated (PAME-II). When allowing ciliates to be able to feed on small diatoms, the model describing the diatom-dominated MEDEA experiment give a bacteria-dominated response as observed in PAME I in which the diatom community comprised almost exclusively small-sized cells. Introducing a high initial mesozooplankton stock as observed in PAME-II, the model gives a flagellate-dominated response in accordance with the observed response also of this experiment. The ability of the model originally developed for temperate waters to reproduce population dynamics in a 10°C colder Arctic fjord, does not support the existence of important shifts in population balances over this temperature range. Rather, it suggests a quite resilient microbial food web when adapted to in situ temperature. The sensitivity of the model response to its mesozooplankton component suggests, however, that the seasonal vertical migration of Arctic copepods may be a strong forcing factor on Arctic microbial food webs.