High-Frequency Dynamics of Ocean pH: A Multi-Ecosystem Comparison

Abstract

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.

How will Ocean Acidification Affect Baltic Sea Ecosystems? An Assessment of Plausible Impacts on Key Functional Groups

“…0.7 pH units seasonally (Thomsen et al 2010), and diurnal fluctuations of ±0.15 pH units are common in shallow bays of the Skagerrak (personal observation). Similar observations have been made in a variety of open ocean and coastal locations (Wootton et al 2008;Hofmann et al 2011). These diurnal changes-and a substantial fraction of the seasonal changes-in pH are driven by direct effects of photosynthesis and respiration.…”

Interactive network configuration maintains bacterioplankton community structure under elevated CO<sub>2</sub> in a eutrophic coastal mesocosm experiment

“…It has previously been proposed that the observed insignificant effects of ocean acidification on coastal bacterioplankton may be due to their adaptation to strong natural variability in pH in coastal ecosystems, where amplitudes of > 0.3 units from diel fluctuations and seasonal dynamics are commonly seen (Hofmann et al, 2011). The comparative ecological network analysis in this study to some extent explains the resilience of the bacterioplankton community to elevated CO 2 levels.…”

Species-specific responses to ocean acidification should account for local adaptation and adaptive plasticity

“…by a process of equilibration with the atmosphere 16,17 . This diversity of causative drivers and the high degree of variability in coastal environments, when compared with open ocean systems, limits the direct extrapolation of ocean acidification concepts and scenarios between these different environments 18 .…”

“…To be able to project how future ocean acidification will affect marine organisms, populations and ecosystems, it is therefore necessary to monitor present day pH/p CO2 conditions 16,22,23 and design relevant ocean acidification experimental scenarios accordingly. Surprisingly, many scientists continue to expose coastal species to ocean acidification scenarios derived for the open ocean 24 .…”