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    Helgoland, Heligoland Airport (EDXH)

    hgh gmbh helgoland Ocean acidification is considered as a crucial stressor for marine communities. The focus of this study was on microzooplankton and its interactions with phytoplankton and mesozooplankton. The microzooplankton community was dominated by ciliates, especially small Strombidium sp. We did not observe any effects of high CO hgh gmbh helgoland on the community composition and diversity of microzooplankton. Helgolan ciliate abundance, biomass and growth rate were not affected by elevated CO 2we observed hgh gmbh helgoland positive effect of elevated CO 2 on dinoflagellate abundances. Additionally, growth rates of dinoflagellates were significantly higher in the high CO 2 treatments. Hgh gmbh helgoland, the results from the present study covering the most important part of the growing season sintesi degli ormoni steroidei that coastal microzooplankton communities are rather robust towards realistic acidification scenarios.

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    Ocean acidification is considered as a crucial stressor for marine communities. The focus of this study was on microzooplankton and its interactions with phytoplankton and mesozooplankton.

    The microzooplankton community was dominated by ciliates, especially small Strombidium sp. We did not observe any effects of high CO 2 on the community composition and diversity of microzooplankton. While ciliate abundance, biomass and growth rate were not affected by elevated CO 2 , we observed a positive effect of elevated CO 2 on dinoflagellate abundances. Additionally, growth rates of dinoflagellates were significantly higher in the high CO 2 treatments. Overall, the results from the present study covering the most important part of the growing season indicate that coastal microzooplankton communities are rather robust towards realistic acidification scenarios.

    Evidence from a Long-Term Mesocosm Study. April 18, ; Accepted: October 18, ; Published: This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

    The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The authors have declared that no competing interests exist. This drop in pH is referred to as ocean acidification OA. While there are differences in CO 2 uptake depending on the region [ 3 ], an overall pH decrease of about 0.

    Accordingly, MZP can have a strong impact on biomass and species composition of the phytoplankton community and even play an important role in suppressing phytoplankton blooms, especially at times when mesozooplankton grazing is low [ 7 ].

    Moreover, MZP can also be strongly top-down controlled itself, as it is a preferred food source for mesozooplankton such as copepods [ 5 , 8 ].

    OA is predicted to affect different marine biological and biogeochemical processes, potentially resulting in adverse effects not only on the species level but also on the community and ecosystem level [ 9 ].

    Others, however, were not directly affected by a change in pH [ 13 ]. Nonetheless, CO 2 effects can also be transmitted indirectly, via changes in phytoplankton availability, community composition, or food quality [ 14 , 15 ]. Based on the enhanced growth of especially small-sized phytoplankton species which benefit from the higher carbon availability under OA [ 16 — 19 ], OA has the potential to lead to an increase in MZP productivity as well.

    Despite of the pivotal role of MZP, small-scale laboratory experiments providing information about the impacts of OA on single species or simplified food webs are comparatively rare in contrast to the number of studies available for phytoplankton or mesozooplankton e. Additionally, there is a lack of information about OA impacts on community level.

    Mesocosm studies are useful to fill this gap as they allow us to gain insight into the effects of OA on the plankton community and whether biotic interactions could dampen or amplify known responses in MZP [ 15 , 22 ].

    The studies available so far indicate that MZP communities, especially in coastal areas, are rather tolerant to OA at incubation times up to weeks [ 22 — 25 ].

    Yet, to study evolutionary adaptations to OA which are likely to occur due to the short generation times of planktonic organisms, long-term experiments would be required [ 26 ]. Experiments with longer durations are also necessary to allow observing possible numerical responses of mesozooplankton as it reacts time-delayed to CO 2 -induced changes on phytoplankton or MZP level. Consequently, an in-situ mesocosm approach using a natural plankton assemblage and a sufficiently large incubation volume to allow for a long self-sustained runtime is a step towards the understanding of effects on ecosystem level [ 27 ].

    We investigated the impacts of high CO 2 levels on natural plankton communities during a long-term mesocosm study in the Gullmar Fjord, Skagerrak. Starting before the onset of the spring bloom in March, the long runtime until end of June made it possible to follow the natural succession of a plankton community during the transition from spring to summer. In the following, we will present the analysis of MZP succession patterns focusing on ciliates and heterotrophic dinoflagellates and their interactions with phytoplankton.

    Moreover, grazing experiments should provide additional information both regarding the grazing impact of MZP and indirect effects of a high CO 2 level which are more likely to be detected when MZP is released from grazing pressure. The setup is described in detail by Bach et al. Each mesocosm made of polyurethane foil had a diameter of 2 m and was 19 m long, with a conical sediment trap at the lower end.

    The experiment ran from 7 March till 26 June Upon closing of the mesocosms, a net with 1 mm mesh size was passed through the enclosed pre-bloom seawater to remove large organisms. Five mesocosms served as control with ambient CO 2 levels while CO 2 -enriched seawater was added to the other five. In order to compensate for outgassing, CO 2 -enriched water was added to the high CO 2 treatments at five time points days 17, 46, 48, 68 and Herring larvae Clupea harengus and sea urchin larvae Strongylocentrotus droebrachiensis were added to the mesocosms on day 48 and day 56 of the experiment, respectively.

    Regular sampling every 2 nd day included CTD casts, water column sampling, and sediment sampling. Plankton net hauls were performed every eight days. Plankton samples were taken from the carboys as soon as they were back on shore. MZP samples were taken once per week. Depending on the plankton abundance, 50 or mL of each sample were transferred into a sedimentation chamber.

    After 24 h of sedimentation, the whole surface of the chamber was counted at fold magnification to reduce the counting bias against rare species and to assure comparability of the counts both at high and low abundances. MZP was identified to the lowest possible taxonomic level species or genus level according to Carey [ 30 ], Montagnes et al.

    Most dinoflagellates are capable of heterotrophic feeding modes and can be considered as mixotrophic MZP. Only few taxa such as Ceratium sp. Based on the digitally measured dimensions of 20 random cells per species or size class distinguished AxioVision 4. MZP carbon biomass was estimated from the biovolumes using the conversion factors provided by Putt and Stoecker [ 34 ] and Menden-Deuer and Lessard [ 35 ] for ciliates and dinoflagellates, respectively.

    The cells were counted either on half or total area of the chamber at fold magnification or on 2 to 4 stripes at or fold magnification. Plankton were identified following Tomas et al. Diversity H', log e was calculated after Shannon and Weaver [ 38 ] on a sample day basis. The bloom timing D max was defined as the experimental day when phytoplankton abundance or MZP biomass reached its peak in each mesocosm max.

    D max , maximum and net growth rate data were first tested for normality and homogeneity using a Shapiro-Wilk test and Levene's Test for Homogeneity of Variance and log transformed if necessary. Biomass, abundance and diversity data were log transformed if it improved the outcome of the GAMM as indicated by the R 2 value.

    R Studio was used for all analyses with the additional packages mgcv , vegan and car Version 0. In order to further investigate the impact of microzooplankton grazing on phytoplankton, a dilution experiment after Landry and Hassett [ 39 ] was conducted.

    The experiment took place on day 34 during the 1 st phytoplankton bloom peak to ensure high phytoplankton densities in the samples. By releasing the MZP from copepod grazing pressure, indirect effects of CO 2 on MZP based on changes in phytoplankton abundance or composition are more likely to become visible.

    Additionally, this grazing setup allows the determination of the natural taxon-specific phytoplankton growth rates despite of a separation of phytoplankton and micrograzers not being possible due to their similar size. For setting up the dilutions, filtered mesocosm water was obtained by using 0.

    In triplicates, the dilutions were gently transferred into 0. Animals were picked under a stereomicroscope and were kept in 1 L bottles filled with filtered sea water in the climate room overnight prior to addition to the incubation bottles.

    Another three incubation bottles per mesocosm were set up without the addition of nutrients to serve as control. After 24 h, samples were taken from every incubation bottle. MZP was analyzed as described in the previous section. For phytoplankton counts, in principle the same method was used as previously described but with a sample volume between 10 and 50 mL depending on the phytoplankton abundance.

    At least cells per abundant taxon were counted in tracks of the sedimentation slide using a Zeiss Axiovert inverted microscope. Phytoplankton was identified after Tomas et al.

    For the estimation of phytoplankton biovolume 20 pictures per species or size class distinguished were taken AxioVision 4. Phytoplankton biovolumes were calculated from the measurements according to Hillebrand et al. The MZP grazing rate g is the negative phytoplankton mortality. All negative grazing rates were set to zero. Tukey's HSD test was used as post hoc test.

    In addition to the dilution experiment, community grazing experiments were performed twice during the mesocosm study. Moreover, they allow the calculation of MZP growth rates. The time points chosen were day 37 and 53, after the 1 st and the 2 nd phytoplankton bloom peaks. From each mesocosm, 5 L of seawater was sampled with an integrating water sampler.

    It has to be noted that use of integrated water samplers led to an underestimation of the mesozooplankton grazing impact as copepods were partly able to escape from the sampler. Control of the copepod abundances revealed an abundance reduction by half compared to the mesocosms. Nutrients were not added to the incubation bottles. Samples from every incubation bottle were taken after 24 h of incubation at ambient conditions using a plankton wheel. MZP and phytoplankton were counted microscopically as described for the dilution experiment.

    Phytoplankton and MZP growth rates were calculated with Eq 1 as previously described. The experiment ran from 7 March day -2 until 26 June day MZP sampling took place from 10 March until 20 June. Within this time, temperature increased from 1. Despite of CO 2 fluctuations in the mesocosms due to outgassing and subsequent addition of CO 2 -enriched water, the treatments did not overlap at any time point.

    Mean CO 2 concentrations in the low blue line and high CO 2 treatments red line are shown from day 1 to day as well as mean temperature black line. Error bars indicate the standard deviation.

    Starting conditions on 10 March were Chlorophyll a in the following: Chl a concentrations of 0. Based on the Chl a development, the experiment was divided in four phases: Error bars represent the standard deviation.

    Vertical black lines and Latin numbers indicate the experimental phases I-IV. B Phytoplankton abundances in log ind. Total phytoplankton abundances at the beginning of the experiment day -1 were at 1. Still, microscopic analysis of this size class revealed a dominance of the diatom Arcocellulus sp.

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