Availability of phosphate for phytoplankton and bacteria and of labile organic carbon for bacteria at different pCO2 levels in a mesocosm study

Availability of phosphate for phytoplankton and bacteria and of glucose for bacteria at different pCO2 levels were studied in a mesocosm experiment (PeECE III). Using nutrient-depleted SW Norwegian fjord waters, three different levels of pCO2 (350 μatm: 1×CO2; 700 μatm: 2×CO2; 1050 μatm: 3×CO2) were set up, and nitrate and phosphate were added at the start of the experiment in order to induce a phytoplankton bloom. Despite similar responses of total particulate P concentration and phosphate turnover time at the three different pCO2 levels, the size distribution of particulate P and 33PO4 uptake suggested that phosphate transferred to the >10 μm fraction was greater in the 3×CO2 mesocosm during the first 6–10 days when phosphate concentration was high. During the period of phosphate depletion (after Day 12), specific phosphate affinity and specific alkaline phosphatase activity (APA) suggested a P-deficiency (i.e. suboptimal phosphate supply) rather than a P-limitation for the phytoplankton and bacterial community at the three different pCO2 levels. Specific phosphate affinity and specific APA tended to be higher in the 3×CO2 than in the 2×CO2 and 1×CO2 mesocosms during the phosphate depletion period, although no statistical differences were found. Glucose turnover time was correlated significantly and negatively with bacterial abundance and production but not with the bulk DOC concentration. This suggests that even though constituting a small fraction of the bulk DOC, glucose was an important component of labile DOC for bacteria. Specific glucose affinity of bacteria behaved similarly at the three different pCO2 levels with measured specific glucose affinities being consistently much lower than the theoretical maximum predicted from the diffusion-limited model. This suggests that bacterial growth was not severely limited by the glucose availability. Hence, it seems that the lower availability of inorganic nutrients after the phytoplankton bloom reduced the bacterial capacity to consume labile DOC in the upper mixed layer of the stratified mesocosms

Effects of inorganic and organic nutrient addition on a coastal microbial community (Isefjord, Denmark)

International audienc

Arctic ocean acidification: pelagic ecosystem and biogeochemical responses during a mesocosm study

The growing evidence of potential biological impacts of ocean acidification affirms that this global change phenomenon may pose a serious threat to marine organisms and ecosystems. Whilst ocean acidification will occur everywhere, it will happen more rapidly in some regions than in others. Due to the high CO2 solubility in the cold surface waters of high-latitude seas, these areas are expected to experience the strongest changes in seawater chemistry due to ocean acidification. This will be most pronounced in the Arctic Ocean. If atmospheric pCO2 levels continue to rise at current rates, about 10% of the Arctic surface waters will be corrosive for aragonite by 2018 (Steinacher et al., 2009). By 2050 one-half of the Arctic Ocean will be sub-saturated with respect to aragonite. By the end of this century corrosive conditions are projected to have spread over the entire Arctic Ocean (Steinacher et al., 2009). In view of these rapid changes in seawater chemistry, marine organisms and ecosystems in the Arctic are considered particularly vulnerable to ocean acidification. With this in mind, the European Project on Ocean Acidification (EPOCA) chose the Arctic Ocean as one of its focal areas of research

Effects of migrating mesopelagic fishes on the biological carbon pump

Mesopelagic fishes and invertebrates contribute to the biological carbon pump (BCP) through direct and indirect effects on the gravitational, diffusive, and migrant (active) fluxes. Here, we analyzed the effect of these organisms on total carbon export and sequestration using an idealized depth-resolved food-web model. We constrained a baseline scenario with observations from the Red Sea where mesopelagic fishes perform extensive diel vertical migration (DVM), presumably making them particularly important in carbon export. Our results are consistent with previous studies suggesting that mesopelagic fishes fuel a hotspot of heterotrophic activity in the mesopelagic zone of the Red Sea. While the supply of new nutrients to the euphotic zone is of first order in framing total carbon export and sequestration, we found 2 modulating effects of fish DVM. First, DVM enhances carbon sequestration because active carbon flux attenuates less than the gravitational and diffusive fluxes, and also because migrators fuel the gravitational flux deep into the mesopelagic zone. Secondly, through microbial food-web cascades, fish predation affects the relative contribution of the active and the passive fluxes with non-linear, less intuitive consequences for carbon export and sequestration. These food-web cascades involve a surprisingly large variation in the depth of the euphotic zone that is associated with a ‘diatom nutrient trap’ at the bottom of the euphotic zone.This study received funding from the European Union’s Horizon 2020 under grant agreement no. 817806 (‘Sustainable management of mesopelagic re sources’, SUMMER) and under grant agreement no. 326896 from the Norwegian Research Council (‘How can fisheries contribute to a sustainable future?’)

Ecological investigations in the Barents Sea, August 1985. Report from Pro Mare-cruise no. 5

Between 29 July and 19 August 1985, the R/V "G.O.Sars" carried out investigations in the northern Barents Sea. The cruise was organized by the Institute of Marine Research (IMR). The main purpose was to carry out investigations on the physical and biological environment and the feeding conditions of capelin, an IMR project under PRO MARE. Several scientist working on other PRO MARE projects participated on this cruise in a coordinated study program. This report presents the first results of the different project