Mechanisms for inorganic carbon acquisition in macroalgal assemblages today could indicate how coastal ecosystems will respond to predicted changes in ocean chemistry due to elevated carbon dioxide (CO 2 ). We identified the proportion of noncalcifying macroalgae with particular carbon use strategies using the natural abundance of carbon isotopes and pH drift experiments in a kelp forest. We also identified all calcifying macroalgae in this system; these were the dominant component of the benthos (by % cover) at all depths and seasons while cover of noncalcareous macroalgae increased at shallower depths and during summer. All large canopy-forming macroalgae had attributes suggestive of active uptake of inorganic carbon and the presence of a CO 2 concentration mechanism (CCM). CCM species covered, on average, 15-45% of the benthos and were most common at shallow depths and during summer. There was a high level of variability in carbon isotope discrimination within CCM species, probably a result of energetic constraints on active carbon uptake in a low light environment. Over 50% of red noncalcifying species exhibited values below À30% suggesting a reliance on diffusive CO 2 uptake and no functional CCM. Non-CCM macroalgae covered on average 0-8.9% of rock surfaces and were most common in deep, low light habitats. Elevated CO 2 has the potential to influence competition between dominant coralline species (that will be negatively affected by increased CO 2 ) and noncalcareous CCM macroalgae (neutral or positive effects) and relatively rare (on a % cover basis) non-CCM species (positive effects). Responses of macroalgae to elevated CO 2 will be strongly modified by light and any responses are likely to be different at times or locations where energy constrains photosynthesis. Increased growth and competitive ability of noncalcareous macroalgae alongside negative impacts of acidification on calcifying species could have major implications for the functioning of coastal reef systems at elevated CO 2 concentrations.

Coastal ecosystems that are characterized by kelp forests encounter daily pH fluctuations, driven by photosynthesis and respiration, which are larger than pH changes owing to ocean acidification (OA) projected for surface ocean waters by 2100. We investigated whether mimicry of biologically mediated diurnal shifts in pH-based for the first time on pH time-series measurements within a kelp forest-would offset or amplify the negative effects of OA on calcifiers. In a 40-day laboratory experiment, the calcifying coralline macroalga, Arthrocardia corymbosa, was exposed to two mean pH treatments (8.05 or 7.65). For each mean, two experimental pH manipulations were applied. In one treatment, pH was held constant. In the second treatment, pH was manipulated around the mean (as a step-function), 0.4 pH units higher during daylight and 0.4 units lower during darkness to approximate diurnal fluctuations in a kelp forest. In all cases, growth rates were lower at a reduced mean pH, and fluctuations in pH acted additively to further reduce growth. Photosynthesis, recruitment and elemental composition did not change with pH, but d 13 C increased at lower mean pH. Including environmental heterogeneity in experimental design will assist with a more accurate assessment of the responses of calcifiers to OA.

A changing climate is altering many ocean properties that consequently will modify marine productivity. Previous phytoplankton manipulation studies have focused on individual or subsets of these properties. Here, we investigate the cumulative e ects of multi-faceted change on a subantarctic diatom Pseudonitzschia multiseries by concurrently manipulating five stressors (light/nutrients/CO 2 /temperature/iron) that primarily control its physiology, and explore underlying reasons for altered physiological performance. Climate change enhances diatom growth mainly owing to warming and iron enrichment, and both properties decrease cellular nutrient quotas, partially o setting any e ects of decreased nutrient supply by 2100. Physiological diagnostics and comparative proteomics demonstrate the joint importance of individual and interactive e ects of temperature and iron, and reveal biased future predictions from experimental outcomes when only a subset of multi-stressors is considered. Our findings for subantarctic waters illustrate how composite regional studies are needed to provide accurate global projections of future shifts in productivity and distinguish underlying species-specific physiological mechanisms.A n ongoing major challenge is to grasp how climate-changemediated alteration of environmental conditions will influence biota across different scales, from organismal health to community structure 1,2 . Oceanographers have employed climate-change models 3,4 , time-series observations 5 and manipulation experiments 6 to understand the biological ramifications of global change. Phytoplankton manipulation studies reveal how alteration of individual properties, such as CO 2 , affects physiology 2,6,7 . However, the validity of such singleparameter findings 6,8,9 , in the context of complex ocean change 1,2 , is challenged by research that reveals interactive effects between multi-stressors on phytoplankton physiology 10,11 . We need to diagnose and understand the physiological mechanisms that underpin interconnected responses to multi-stressors, which together set the cumulative response of phytoplankton species to changing conditions 4,6,8 .Understanding the combined effects, across the global ocean, of complex change on phytoplankton physiology requires a gradualist approach 12,13 . Individual provinces will encounter different permutations of multi-faceted change 14 , and each is characterized by a range of resident phytoplankton groups (termed biomes 5 ). Earth System models provide a framework of projections of regional change 14 that stimulate improved experimental design to understand the biological effects of oceanic change. In return, a new generation of manipulation studies must deliver estimates of the combined effects of complex change on many phytoplankton species, and distinguish the underlying mechanisms that underpin these physiological outcomes.Here, we target subantarctic diatoms, which are ubiquitous and bloom-formers 15 . We experimentally manipulate a representative species 6,15 (Pseudonitzschi...

Coralline algae provide important ecosystem services but are susceptible to the impacts of ocean acidification. However, the mechanisms are uncertain, and the magnitude is species specific. Here, we assess whether species-specific responses to ocean acidification of coralline algae are related to differences in pH at the site of calcification within the calcifying fluid/medium (pH ) using δ B as a proxy. Declines in δ B for all three species are consistent with shifts in δ B expected if B(OH) was incorporated during precipitation. In particular, the δ B ratio in Amphiroa anceps was too low to allow for reasonable pH values if B(OH) rather than B(OH) was directly incorporated from the calcifying fluid. This points towards δ B being a reliable proxy for pH for coralline algal calcite and that if B(OH) is present in detectable proportions, it can be attributed to secondary postincorporation transformation of B(OH) . We thus show that pH is elevated during calcification and that the extent is species specific. The net calcification of two species of coralline algae (Sporolithon durum, and Amphiroa anceps) declined under elevated CO , as did their pH . Neogoniolithon sp. had the highest pH , and most constant calcification rates, with the decrease in pH being ¼ that of seawater pH in the treatments, demonstrating a control of coralline algae on carbonate chemistry at their site of calcification. The discovery that coralline algae upregulate pH under ocean acidification is physiologically important and should be included in future models involving calcification.