Marine life is controlled by multiple physical and chemical drivers and by diverse ecological processes. Many of these oceanic properties are being altered by climate change and other anthropogenic pressures. Hence, identifying the influences of multifaceted ocean change, from local to global scales, is a complex task. To guide policy-making and make projections of the future of the marine biosphere, it is essential to understand biological responses at physiological, evolutionary and ecological levels. Here, we contrast and compare different approaches to multiple driver experiments that aim to elucidate biological responses to a complex matrix of ocean

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...