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Abstract. The ongoing oceanic uptake of anthropogenic carbon dioxide (CO2) is significantly altering the carbonate chemistry of seawater, a phenomenon referred to as ocean acidification. Experimental manipulations have been increasingly used to gauge how continued ocean acidification will potentially impact marine ecosystems and their associated biogeochemical cycles in the future; however, results amongst studies, particularly when performed on natural communities, are highly variable, which in part likely reflects inconsistencies in experimental approach. To investigate the potential for identification of more generic responses and greater experimentally reproducibility, we devised and implemented a series of highly replicated (n = 8), short term (2–4 days) multi-level (&amp;geq; 4 conditions) carbonate chemistry/nutrient manipulation experiments on a range of natural microbial communities sampled in Northwest European shelf seas. Carbonate chemistry manipulations and resulting biological responses were found to be highly reproducible within individual experiments and to a lesser extent between geographically different experiments. Statistically robust reproducible physiological responses of phytoplankton to increasing pCO2, characterized by a suppression of net growth for small sized cells (< 10 μm), were observed in the majority of the experiments, irrespective of nutrient status. Remaining between-experiment variability was potentially linked to initial community structure and/or other site-specific environmental factors. Analysis of carbon cycling within the experiments revealed the expected increased sensitivity of carbonate chemistry to biological processes at higher pCO2 and hence lower buffer capacity. The results thus emphasize how biological-chemical feedbacks may be altered in the future ocean.

Abstract. Coccolithophores are a key functional group in terms of the pelagic production of calcium carbonate (calcite), although their contribution to shelf sea biogeochemistry, and how this relates to environmental conditions, is poorly constrained. Measurements of calcite production (CP) and coccolithophore abundance were made on the northwest European shelf to examine trends in coccolithophore calcification along natural gradients of carbonate chemistry, macronutrient availability and plankton composition. Similar measurements were also made in three bioassay experiments where nutrient (nitrate, phosphate) and pCO 2 levels were manipulated. Nanoflagellates (< 10 µm) dominated chlorophyll biomass and primary production (PP) at all but one sampling site, with CP ranging from 0.6 to 9.6 mmol C m −2 d −1 . High CP and coccolithophore abundance occurred in a diatom bloom in fully mixed waters off Heligoland, but not in two distinct coccolithophore blooms in the central North Sea and Western English Channel. Coccolithophore abundance and CP showed no correlation with nutrient concentrations or ratios, while significant (p < 0.01) correlations between CP, cell-specific calcification (cell-CF) and irradiance in the water column highlighted how light availability exerts a strong control on pelagic CP. In the experimental bioassays, Emiliania-huxleyi-dominated coccolithophore communities in shelf waters (northern North Sea, Norwegian Trench) showed a strong response in terms of CP to combined nitrate and phosphate addition, mediated by changes in cell-CF and growth rates. In contrast, an offshore diverse coccolithophore community (Bay of Biscay) showed no response to nutrient addition, while light availability or mortality may have been more important in controlling this community. Sharp decreases in pH and a rough halving of calcite saturation states in the bioassay experiments led to decreased CP in the Bay of Biscay and northern North Sea, but not the Norwegian Trench. These decreases in CP were related to slowed growth rates in the bioassays at elevated pCO 2 (750 µatm) relative to those in the ambient treatments. The combined results from our study highlight the variable coccolithophore responses to irradiance, nutrients and carbonate chemistry in north-west European shelf waters, which are mediated by changes in growth rates, cell-CF and species composition.

Phytoplankton photosynthesis under the rapidly fluctuating irradiance which results from turbulent mixing through the vertical light gradient is poorly understood. Ship-based measurements often apply the fast repetition rate fluorescence (FRRF) technique in situ or in vivo to gauge the physiological state of the phytoplankton community and infer some of the physical properties of the water column (such as mixing time scales). We describe the development and validation of a model of photosynthetic electron turnover at photosystem II with consideration of downstream limitation, based on the redox state of photosystem II. We also include empirical formulations for slower processes such as photo-protection (from nonphotochemical quenching) and photo-inhibition. By confronting the simple model with laboratory data for Dunaliella tertiolecta, we were able to refine the model so that it faithfully produced rates of photosynthetic electron transfer determined by FRR fluorescence. Further, we were able to validate the model estimates of linear photosynthetic electron transfer rates against completely independent measurements obtained using 14 C-bicarbonate assimilation in photosynthesis-light curves.The light dependence of phytoplankton photosynthesis is one of the most intensively studied aspects of phytoplankton physiology (Jassby and Platt 1976;Cullen 1990;Falkowski and Raven 2007). Nonetheless, most commonly used incubation procedures (e.g., Knap et al. [1996] p. 159) do not resolve photosynthesis rates on the time scales of variability in photon flux density (PFD) that are experienced by the phytoplankton in situ. It has long been recognized that variability in the light environment due to vertical mixing can affect the accuracy of estimates of in situ photosynthesis (Marra 1980;MacIntyre et al. 2000). Higher frequency variability, such as that caused by wavefocusing in the upper euphotic zone (Dera and Gordon 1970), does not appear to affect photosynthetic physiology in those eukaryotes studied (Stramski et al. 1993; Mouget et al. 1995a, b) Empirical approaches may fail to match the variability (both magnitude and frequency) imposed by the experimenter in the incubation to the variability that is experienced by phytoplankton in the natural environment. Accurately reproducing the natural light regime requires a priori knowledge of both the dynamic range and rate of change of irradiance. These can be derived from three parameters (the attenuation coefficient, the depth of the mixed layer, and the vertical diffusivity) for deck incubations and two (mixed-layer depth and diffusivity) for in situ incubations. Each of these input variables can vary within the duration of an hours-long incubation. Methods that have been tested or proposed for providing a match between the natural variability and the imposed regime include analysis of dye diffusion (Mallin and Paerl 1992), incorporation of motion sensors on a submersible incubator (Kirkpatrick et al. 1990) and parallel estimation of diffusivity, using an acousti...