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.

As the surface ocean equilibrates with rising atmospheric CO 2 , the pH of surface seawater is decreasing with potentially negative impacts on coral calcification. A critical question is whether corals will be able to adapt or acclimate to these changes in seawater chemistry. We use high precision CT scanning of skeletal cores of Porites astreoides, an important Caribbean reef-building coral, to show that calcification rates decrease significantly along a natural gradient in pH and aragonite saturation (Ω arag ). This decrease is accompanied by an increase in skeletal erosion and predation by boring organisms. The degree of sensitivity to reduced Ω arag measured on our field corals is consistent with that exhibited by the same species in laboratory CO 2 manipulation experiments. We conclude that the Porites corals at our field site were not able to acclimatize enough to prevent the impacts of local ocean acidification on their skeletal growth and development, despite spending their entire lifespan in low pH, low Ω arag seawater.reef framework | caribbean corals acidic springs S cleractinian corals, whose calcium carbonate (CaCO 3 ) skeletons provide the structural framework of coral reef ecosystems, are subject to numerous direct and indirect stressors and are facing steep global decline (1-3). As the ocean absorbs anthropogenic CO 2 , surface ocean pH and the availability of carbonate ions to corals and other reef calcifiers are decreasing (1-5). Global climate models predict a drop of 0.3 pH units, from 8.1 to 7.8 by the end of the 21st century (6-8), resulting in a 50% reduction in carbonate ion concentration (9). Consequently, it is predicted that ocean acidification will result in a widespread reduction in coral calcification by the year 2065 (10), causing large-scale reef degradation and loss (11).The predicted response of coral reef calcification to decreasing aragonite-saturation (Ω arag ) state is based primarily on model calculations of future Ω arag (6,7,9,12) and the observed response of coral calcification to low Ω arag in short-term laboratory-based or mesocosm carbonate chemistry manipulation experiments (11, 13-16). Additionally, field-based observations of net coral reef ecosystem calcification responses to changes in Ω arag state in situ also suggest declines in calcification (17-21). However, key questions remain regarding the acclimation and adaptation potential of coral calcification to ocean acidification. Acclimatization, or the potential for an organism to adjust to changes in an environment via physical modifications, is distinguished from adaptation, or permanent evolutionary modifications made by an organism in response to repeated stressors. Specifically, an outstanding question is whether corals will be able to acclimate or adapt to maintain sufficient rates of calcification to sustain the reef structure (17,22,23). To address these questions, field-based studies where corals have been naturally exposed to chronic low pH conditions for extended periods could provide important new ins...

Rising atmospheric CO 2 and its equilibration with surface ocean seawater is lowering both the pH and carbonate saturation state (X) of the oceans. Numerous calcifying organisms, including reef-building corals, may be severely impacted by declining aragonite and calcite saturation, but the fate of coral reef ecosystems in response to ocean acidification remains largely unexplored. Naturally low saturation (X * 0.5) low pH (6.70-7.30) groundwater has been discharging for millennia at localized submarine springs (called ''ojos'') at Puerto Morelos, México near the Mesoamerican Reef. This ecosystem provides insights into potential long term responses of coral ecosystems to low saturation conditions. In-situ chemical and biological data indicate that both coral species richness and coral colony size decline with increasing proximity to low-saturation, low-pH waters at the ojo centers. Only three scleractinian coral species (Porites astreoides, Porites divaricata, and Siderastrea radians) occur in undersaturated waters at all ojos examined. Because these three species are rarely major contributors to Caribbean reef framework, these data may indicate that today's more complex frame-building species may be replaced by smaller, possibly patchy, colonies of only a few species along the Mesoamerican Barrier Reef. The growth of these scleractinian coral species at undersaturated conditions illustrates that the response to ocean acidification is likely to vary across species and environments; thus, our data emphasize the need to better understand the mechanisms of calcification to more accurately predict future impacts of ocean acidification.

a b s t r a c tSubmarine groundwater discharge (SGD) to the coastal environment along the eastern Yucatan Peninsula, Quintana Roo, Mexico was investigated using a combination of tracer mass balances and analytical solutions. Two distinct submarine groundwater sources including water from the unconfined surficial aquifer discharging at the beach face and water from a deeper aquifer discharging nearshore through submarine springs (ojos) were identified. The groundwater of nearshore ojos was saline and significantly enriched in short-lived radium isotopes ( 223 Ra, 224 Ra) relative to the unconfined aquifer beach face groundwater. We estimated SGD from ojos using 223 Ra and used a salinity mass balance to estimate the freshwater discharge at the beach face. Analytical calculations were also used to estimate wave set-up and tidally driven saline seepage into the surf zone and were compared to the salinity-based freshwater discharge estimates. ). Discharge at the beach face was in the range of 3.3-8.5 m 3 d À 1 m À 1 for freshwater and 2.7 m 3 d À 1 m À 1 for saline water based on the salinity mass balance and wave-and tidally-driven discharge, respectively. Although discharge from the ojos was larger in volume than discharge from the unconfined aquifer at the beach face, dissolved inorganic nitrogen (DIN) was significantly higher in beach groundwater; thus, discharge of this unconfined beach aquifer groundwater contributed significantly to total DIN loading to the coast. DIN fluxes were up to 9.9 mol d À 1 m À 1 from ojos and 2.1 mol d À 1 m À 1 from beach discharge and varied regionally along the 500 km coastline sampled. These results demonstrate the importance of considering the beach zone as a significant nutrient source to coastal waters for future management strategies regarding nutrient loading to reef environments and coastal development. This study also identifies the importance of understanding the connectivity of submarine spring discharge to the nearshore coastal environment and the impact of inland anthropogenic activities may have on coastal health.

The lakes that form via ice‐rich permafrost thaw emit CH4 and CO2 to the atmosphere from previously frozen ancient permafrost sources. Despite this potential to positively feedback to climate change, lake carbon emission sources are not well understood on whole‐lake scales, complicating upscaling. In this study, we used observations of radiocarbon (14C) and stable carbon (13C) isotopes in the summer and winter dissolved CH4 and CO2 pools, ebullition‐CH4, and multiple independent mass balance approaches to characterize whole‐lake emission sources and apportion annual emission pathways. Observations focused on five lakes with variable thermokarst in interior Alaska. The 14C age of discrete ebullition‐CH4 seeps ranged from 395 ± 15 to 28,240 ± 150 YBP across all study lakes; however, dissolved 14CH4 was younger than 4,730 YBP. In the primary study lake, Goldstream L., the integrated whole‐lake 14C age of ebullition‐CH4, as determined by three different approaches, ranged from 3,290 to 6,740 YBP. A new dissolved‐14C‐CH4‐based approach to estimating ebullition 14C age and flux showed close agreement to previous ice‐bubble surveys and bubble‐trap flux estimates. Differences in open water versus ice‐covered dissolved gas concentrations and their 14C and 13C isotopes revealed the influence of winter ice trapping and forcing ebullition‐CH4 into the underlying water column, where it comprised 50% of the total dissolved CH4 pool by the end of winter. Across the study lakes, we found a relationship between the whole‐lake 14C age of dissolved CH4 and CO2 and the extent of active thermokarst, representing a positive feedback system that is sensitive to climate warming.