The effect of Ocean Acidification (OA) on marine biota is quasi-predictable at best. While perturbation studies, in the form of incubations under elevated pCO2, reveal sensitivities and responses of individual species, one missing link in the OA story results from a chronic lack of pH data specific to a given species' natural habitat. Here, we present a compilation of continuous, high-resolution time series of upper ocean pH, collected using autonomous sensors, over a variety of ecosystems ranging from polar to tropical, open-ocean to coastal, kelp forest to coral reef. These observations reveal a continuum of month-long pH variability with standard deviations from 0.004 to 0.277 and ranges spanning 0.024 to 1.430 pH units. The nature of the observed variability was also highly site-dependent, with characteristic diel, semi-diurnal, and stochastic patterns of varying amplitudes. These biome-specific pH signatures disclose current levels of exposure to both high and low dissolved CO2, often demonstrating that resident organisms are already experiencing pH regimes that are not predicted until 2100. Our data provide a first step toward crystallizing the biophysical link between environmental history of pH exposure and physiological resilience of marine organisms to fluctuations in seawater CO2. Knowledge of this spatial and temporal variation in seawater chemistry allows us to improve the design of OA experiments: we can test organisms with a priori expectations of their tolerance guardrails, based on their natural range of exposure. Such hypothesis-testing will provide a deeper understanding of the effects of OA. Both intuitively simple to understand and powerfully informative, these and similar comparative time series can help guide management efforts to identify areas of marine habitat that can serve as refugia to acidification as well as areas that are particularly vulnerable to future ocean change.

Community structure and assembly are determined in part by environmental heterogeneity. While reef-building corals respond negatively to warming (i.e. bleaching events) and ocean acidification (OA), the extent of present-day natural variability in pH on shallow reefs and ecological consequences for benthic assemblages is unknown. We documented high resolution temporal patterns in temperature and pH from three reefs in the central Pacific and examined how these data relate to community development and net accretion rates of early successional benthic organisms. These reefs experienced substantial diel fluctuations in temperature (0.78°C) and pH (>0.2) similar to the magnitude of ‘warming’ and ‘acidification’ expected over the next century. Where daily pH within the benthic boundary layer failed to exceed pelagic climatological seasonal lows, net accretion was slower and fleshy, non-calcifying benthic organisms dominated space. Thus, key aspects of coral reef ecosystem structure and function are presently related to natural diurnal variability in pH.

Rising anthropogenic CO 2 in the atmosphere is accompanied by an increase in oceanic CO 2 and a concomitant decline in seawater pH (ref. 1). This phenomenon, known as ocean acidification (OA), has been experimentally shown to impact the biology and ecology of numerous animals and plants 2 , most notably those that precipitate calcium carbonate skeletons, such as reef-building corals 3 . Volcanically acidified water at Maug, Commonwealth of the Northern Mariana Islands (CNMI) is equivalent to near-future predictions for what coral reef ecosystems will experience worldwide due to OA. We provide the first chemical and ecological assessment of this unique site and show that acidification-related stress significantly influences the abundance and diversity of coral reef taxa, leading to the often-predicted shift from a coral to an algae-dominated state 4,5 . This study provides field evidence that acidification can lead to macroalgae dominance on reefs.Coral reefs contain the highest concentration of biodiversity in the marine realm, with abundant flora and fauna that form the backbone of complex and dynamic ecosystems 6 . From an anthropocentric standpoint, coral reefs provide valuable goods and services, supporting fisheries and tourism, and protect shorelines from storms 7 . Recently, widespread coral mortality has led to the flattening of reef frameworks and the loss of essential habitat 4 . This trend will be accelerated by ocean acidification (OA), as calcification is impaired, and dissolution is accelerated 8,9 . Furthermore, experimental evidence suggests that OA could enhance the growth 10 and competitive ability of fleshy macroalgae 11 . This OA-induced shift in the competitive balance between corals and algae could exacerbate direct effects of OA on calcifying reef species 12 and lead to ecosystem shifts favouring non-reef-forming algae over coral 4,5 . Understanding the individual responses of taxa to OA, as well as alteration of multi-species assemblages, is therefore critical to predicting ecosystem persistence and managing reef health in an era of global change.At present, much of what is known concerning the impacts of OA on coral reef biota has been laboratory-based experimental work focused on the responses of select taxa 2 . This has been expanded to mesocosm-based studies, allowing manipulation of groups of organisms and investigation of community responses 13 .Although these multi-species experimental studies are vital, they cannot recreate the variability (physical, chemical, biological) of real-world reef systems 14 . In an effort to overcome the limitations of laboratory studies, real-world low-saturation-state (Ω) sites have been investigated. In the eastern Pacific, nutrient and CO 2 -enriched upwelled waters impact coral calcification and the precipitation of carbonate cements, influencing the distribution of reefs 15 . In Mexico, freshwater springs depress Ω, influencing coral calcification and species distributions 16 . In Palau, restricted circulation and biological activity contribute to ...