Phytoplankton responses and associated carbon cycling during shipboard carbonate chemistry manipulation experiments conducted around Northwest European shelf seas

The ongoing oceanic uptake of anthropogenic carbon dioxide (CO2) is significantly altering the carbon- ate chemistry of seawater, a phenomenon referred to as ocean acidification. Experimental manipulations have been increasingly used to gauge how continued ocean acidifica- tion will potentially impact marine ecosystems and their as- sociated biogeochemical cycles in the future; however, re- sults amongst studies, particularly when performed on nat- ural communities, are highly variable, which may reflect community/environment-specific responses or inconsisten- cies in experimental approach. To investigate the potential for identification of more generic responses and greater ex- perimentally reproducibility, we devised and implemented a series (n = 8) of short-term (2-4 days) multi-level ( 4 con- ditions) carbonate chemistry/nutrient manipulation experi- ments on a range of natural microbial communities sam- pled in Northwest European shelf seas. Carbonate chem- istry manipulations and resulting biological responses were found to be highly reproducible within individual experi- ments and to a lesser extent between geographically sep- arated experiments. Statistically robust reproducible phys- iological responses of phytoplankton to increasing pCO2, characterised by a suppression of net growth for small-sized cells (< 10 µm), were observed in the majority of the ex- periments, irrespective of natural or manipulated nutrient status. Remaining between-experiment variability was po- tentially linked to initial community structure and/or other site-specific environmental factors. Analysis of carbon cy- cling 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 emphasise how biogeochemical feedbacks may be al- tered in the future ocean.