Article pubs.acs.org/est
Bubble Stripping as a Tool To Reduce High Dissolved CO2 in Coastal Marine Ecosystems David A. Koweek,*,† David A. Mucciarone,† and Robert B. Dunbar† †
Department of Earth System Science, Stanford University, Stanford, California 94305, United States S Supporting Information *
ABSTRACT: High dissolved CO2 concentrations in coastal ecosystems are a common occurrence due to a combination of large ecosystem metabolism, shallow water, and long residence times. Many important coastal species may have adapted to this natural variability over time, but eutrophication and ocean acidification may be perturbing the water chemistry beyond the bounds of tolerance for these organisms. We are currently limited in our ability to deal with the geochemical changes unfolding in our coastal ocean. This study helps to address this deficit of solutions by introducing bubble stripping as a novel geochemical engineering approach to reducing high CO2 in coastal marine ecosystems. We use a process-based model to find that air/sea gas exchange rates within a bubbled system are 1−2 orders of magnitude higher than within a nonbubbled system. By coupling bubbling-enhanced ventilation to a coastal ecosystem metabolism model, we demonstrate that strategically timed bubble plumes can mitigate exposure to high CO2 under present-day conditions and that exposure mitigation is enhanced in the more acidic conditions predicted by the end of the century. We argue that shallow water CO2 bubble stripping should be considered among the growing list of engineering approaches intended to increase coastal resilience in a changing ocean.
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INTRODUCTION Shallow coastal marine ecosystems, such as coral reefs, seagrass meadows, mangroves, and estuaries, experience large diel variability in pH, pCO2, and carbonate mineral saturation state (Ω) due to high rates of community metabolism (production/respiration and calcification/dissolution) coupled with shallow water depths and long residence times.1,2 Localized human impacts such as eutrophication have further altered the coastal CO2 system chemistry. The combination of natural variability and local human stressors frequently leads to diel pH variability exceeding 0.6−1 units,2,3 with potentially negative impacts across a broad range of marine taxa.4,5 Compounding the natural variability and local human stressors, global ocean acidification (OA) is forecast to lower open ocean pH by ∼0.3 units from its current value of ∼8.1 to a value of ∼7.8 by the end of the century,6 which will contribute to even greater pH variability in coastal ecosystems due to the reduced seawater buffering capacity at lower pH.7 A wide range of marine organisms exhibit a negative response to experimental acidic conditions.4 However, questions remain regarding how well many of these results can be extrapolated to the coastal ocean, where carbonate chemistry, temperature, and other environmental variables are far more dynamic.8 Some coastal organisms may have adapted or show the potential to adapt to this environmental variability.9,10 However, the increased carbonate system variability brought on by both local human activities and OA may be driving many other organisms beyond their adaptive thresholds or forcing © XXXX American Chemical Society
organisms to make energetic trade-offs to survive under increasingly acidic conditions.8 The anticipated impacts of local and global anthropogenic stressors on coastal ecosystem goods and services necessitate careful examination of intervention strategies. These approaches have thus far been limited in scope. Regulatory approaches focus on trying to control runoff and point source acidification, primarily through existing legislation such as the Clean Water Act in the United States and the Water Framework Directive in the European Union.11−13 International effort to limit coastal acidification has focused on controlling land-based sources of pollution within the United Nations Environment Programme (UNEP) regional seas frameworks.13 Engineering approaches to increase coastal ecosystem resilience to climate change and OA include planting seagrass meadows upstream of coastal calcifiers,14,15 boosting alkalinity through dissolution of lime and calcium carbonate,16 and accelerating adaptation through assisted evolution of corals.17 Each of these techniques may be effective at increasing ecosystem resilience via either mitigation or adaptation to pH and/or thermal variability, however each of these techniques may also potentially have unintended and unanticipated ecological consequences. Received: September 27, 2015 Revised: March 3, 2016 Accepted: March 7, 2016
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DOI: 10.1021/acs.est.5b04733 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology We present a new geochemical engineering approach: shallow water bubble plumes of compressed air to enhance nighttime degassing as a means to locally modulate aqueous pH, pCO2, and Ω. We explore the efficacy of this approach through a series of numerical models of increasing complexity. Our model results demonstrate that carefully targeted applications of bubble plumes may prove to be an effective means to manipulate coastal ocean chemistry. Techniques to control CO2 levels in shore-based, closed-loop aquaculture systems have been in existence for some time,18,19 and use various technologies to promote CO2 removal in addition to bubble stripping.18,20,21 While useful for aquaculture, most of these techniques are not optimized for CO2 concentration ranges in natural waters, or for systems with complex bottom structure, occasional wave swell, strong tidal and thermal variability, large mobile animals, and sometimes intermittent sources of electrical power. We consider CO2 bubble stripping in an oceanographic context by evaluating air/ sea gas exchange rates, and by coupling to a coastal ecosystem model to explore potential biogeochemical feedbacks of bubbling. In contrast to some of the equipment used in aquaculture operations, the equipment considered here, bubble diffusers (airstones), plastic tubing, and high-volume, lowpressure air blowers, are widely available and can be deployed across a range of coastal ecosystems.
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MATERIALS AND METHODS Bubble Model Description. Early applications of discrete bubble mass transfer models primarily focused on oxygenation of deep hypolimnetic lakes and only modeled fluxes of O2 and N2.22 Later models built upon this foundation to simulate plumes of CH423,24 and CO225 emitting from natural seeps. To model CO2 bubble stripping in shallow environments we modified the discrete bubble mass transfer model presented in ref 26 by adding fluxes of CO2 and seawater−carbonate chemistry, along with seawater-based gas-specific Henry’s law constants and gas transfer coefficients. The discrete bubble mass transfer model presented in this study consists of three ordinary differential equations that resolve the depth-dependent gaseous fluxes G of O2, N2, and CO2 into and out of a bubble plume as bubbles rise through a shallow water column (Figure 1): dGO2 dz
dG N2 dz dGCO2 dz
=
=
=
−4πr 2N k O2(mO2 − O2,aq ) wb
(1)
−4πr 2N k N2(m N2 − N2,aq) wb
(2)
−4πr 2N k CO2(mCO2 − CO2,aq ) wb
Figure 1. Conceptual model of CO2 bubble stripping in a high CO2, low O2 shallow water environment. A source of compressed gas passed through bubble diffusers generates small (
