Fluxes of NH4+ (acid) and HCO3- (base), and whole body calcium content were measured in European lobster (Homarus gammarus) during intermoult (megalopae stage), and during the first 24 h for postmoult juveniles under control (similar to 2000 mu eq/L) and low seawater alkalinity (similar to 830 mu eq/L). Immediately after moulting, animals lost 45% of the total body calcium via the shed exoskeleton (exuvia), and only 11% was retained in the uncalcified body. At 24 h postmoult, exoskeleton calcium increased to similar to 46% of the intermoult stage. Ammonia excretion was not affected by seawater alkalinity. After moulting, bicarbonate excretion was immediately reversed from excretion to uptake (similar to 4-6 fold higher rates than intermoult) over the whole 24 h postmoult period, peaking at 3-6 h. These data suggest that exoskeleton calcification is not completed by 24 h postmoult Low seawater alkalinity reduced postmoult bicarbonate uptake by 29% on average. Net acid-base flux (equivalent to net base uptake) followed the same pattern as HCO3- fluxes, and was 22% lower in low alkalinity seawater over the whole 24 h postmoult period. The common occurrence of low alkalinity in intensive aquaculture systems may slow postmoult calcification in juvenile H. gammarus, increasing the risk of mortalities through cannibalism. (C) 2015 Elsevier Inc All rights reserved.