Abstract. The impact of ocean acidification and increased water temperature on marine ecosystems, in particular those involving calcifying organisms, has been gradually recognised. We examined the individual and combined effects of increased pCO 2 (180 ppmV CO 2 , 380 ppmV CO 2 and 750 ppmV CO 2 corresponding to past, present and future CO 2 conditions, respectively) and temperature (13 • C and 18 • C) during the exponential growth phase of the coccolithophore E. huxleyi using batch culture experiments. We showed that cellular production rate of Particulate Organic Carbon (POC) increased from the present to the future CO 2 treatments at 13 • C. A significant effect of pCO 2 and of temperature on calcification was found, manifesting itself in a lower cellular production rate of Particulate Inorganic Carbon (PIC) as well as a lower PIC:POC ratio at future CO 2 levels and at 18 • C. Coccosphere-sized particles showed a size reduction with both increasing temperature and CO 2 concentration. The influence of the different treatments on coccolith morphology was studied by categorizing SEM coccolith micrographs. The number of well-formed coccoliths decreased with increasing pCO 2 while temperature did not have a significant impact on coccolith morphology. No interacting effects of pCO 2 and temperature were observed on calcite production, coccolith morphology or on coccosphere size. Finally, our results suggest that ocean acidification might have a larger adverse impact on coccolithophorid calcification than surface water warming.

Summary1. Positive effects of species diversity on ecosystem functioning have often been demonstrated in 'macrobial' communities. This relation and the responsible mechanisms are far less clear for microbial communities. Most experimental studies on microorganisms have used randomly assembled communities that do not resemble natural communities. It is therefore difficult to predict the consequences of realistic, non-random diversity loss. 2. In this study, we used naturally co-occurring diatom species from intertidal mudflats to assemble communities with realistically decreasing diversity and analysed the effect of non-random species loss on biomass production. 3. Our results demonstrate a highly positive biodiversity effect on production, with mixtures outperforming the most productive component species in more than half of the combinations. These strong positive diversity effects could largely be attributed to positive complementarity effects (including both niche complementarity and facilitation), despite the occurrence of negative selection effects which partly counteracted the positive complementarity effects at higher diversities. 4. Facilitative interactions were, at least in part, responsible for the higher biomass production. For one of the species, Cylindrotheca closterium, we show its ability to significantly increase its biomass production in response to substances leaked into the culture medium by other diatom species. In these conditions, the species drastically reduced its pigment concentration, which is typical for mixotrophic growth. 5. Synthesis. We show that both species richness and identity have strong effects on the biomass production of benthic diatom biofilms and that transgressive overyielding is common in these communities. In addition, we show mechanistic evidence for facilitation which is partly responsible for enhanced production. Understanding the mechanisms by which diversity enhances the performance of ecosystems is crucial for predicting the consequences of species loss for ecosystem functioning.

Nitrification, the oxidation of ammonium to nitrite and then nitrate, is often discounted as a source of nitrate in euphotic zone waters due to photoinhibition of nitrifying microorganisms and/or competition for ammonium with phytoplankton. However, there have also been counterarguments that nitrification represents a significant “regenerated” nitrate source to phytoplankton, augmenting the “new” nitrate supplied via physical processes. If nitrification is an appreciable nitrate source in the euphotic zone, then the assumption of a balance between nitrate uptake and organic matter export will overestimate export production. We investigated the relative importance of nitrification and nitrate uptake in the subarctic North Atlantic in late spring and late summer. The rates and vertical distributions of primary production, nitrogen uptake, and ammonium and nitrite oxidation were determined through isotope tracer experiments and the distributions of the nitrogen and oxygen isotopes of nitrate. In surface waters, ammonium and nitrite oxidation rates were low, representing an average of 5.2% and 2.5% of total euphotic zone nitrate uptake, respectively. The nitrogen and oxygen isotopes of nitrate confirmed that nitrification was not significant within the euphotic zone. Comparison of the rates of nitrogen uptake and primary production showed that while springtime phytoplankton growth could be fully supported by new nitrate and recycled ammonium, up to 50% of summertime productivity was likely fueled by dissolved organic nitrogen (DON). Uptake of DON implies that the fraction of primary production exported from surface waters in the late summer was significantly lower than the measured nitrate and ammonium uptake rates suggest.

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