Potential acidification impacts on zooplankton in CCS leakage scenarios

Carbon capture and storage (CCS) technologies involve localized acidification of significant volumes of seawater, inhabited mainly by planktonic species. Knowledge on potential impacts of these techniques on the survival and physiology of zooplankton, and subsequent consequences for ecosystem health in targeted areas, is scarce. The recent literature has a focus on anthropogenic greenhouse gas emissions into the atmosphere, leading to enhanced absorption of CO₂ by the oceans and a lowered seawater pH, termed ocean acidification. These studies explore the effects of changes in seawater chemistry, as predicted by climate models for the end of this century, on marine biota. Early studies have used unrealistically severe CO₂/pH values in this context, but are relevant for CCS leakage scenarios. Little studied meso- and bathypelagic species of the deep sea may be especially vulnerable, as well as vertically migrating zooplankton, which require significant residence times at great depths as part of their life cycle.

Biological impacts of enhanced alkalinity in Carcinus maenas

Further steps are needed to establish feasible alleviation strategies that are able to reduce the impacts of ocean acidification, whilst ensuring minimal biological side-effects in the process. Whilst there is a growing body of literature on the biological impacts of many other carbon dioxide reduction techniques, seemingly little is known about enhanced alkalinity. For this reason, we investigated the potential physiological impacts of using chemical sequestration as an alleviation strategy. In a controlled experiment, Carcinus maenas were acutely exposed to concentrations of Ca(OH)₂ that would be required to reverse the decline in ocean surface pH and return it to pre-industrial levels. Acute exposure significantly affected all individuals’ acid–base balance resulting in slight respiratory alkalosis and hyperkalemia, which was strongest in mature females. Although the trigger for both of these responses is currently unclear, this study has shown that alkalinity addition does alter acid–base balance in this comparatively robust crustacean species.

Addressing ocean acidification as part of sustainable ocean development

Many of the declarations and outcome documents from prior United Nations international meetings address ocean issues such as fishing, pollution, and climate change, but they do not address ocean acidification. This progressive alteration of seawater chemistry caused by uptake of atmospheric carbon dioxide (CO2) is an emerging issue of concern that has potential consequences for marine ecosystems and the humans that depend on them. Addressing ocean acidification will require mitigation of global CO2 emissions at the international level accompanied by regional marine resource use adaptations that reduce the integrated pressure on marine ecosystems while the global community works towards implementing permanent CO2 emissions reductions. Addressing ocean acidification head-on is necessary because it poses a direct challenge to sustainable development targets such as the Millennium Development Goals, and it cannot be addressed adequately with accords or geoengineering plans that do not specifically decrease atmospheric carbon dioxide levels. Here, we will briefly review the current state of ocean acidification knowledge and identify several mitigation and adaptation strategies that should be considered along with reductions in CO2 emissions to reduce the near-term impacts of ocean acidification. Our goal is to present potential options while identifying some of their inherent weaknesses to inform decisionmaking discussions, rather than to recommend adoption of specific policies. While the reduction of CO2 emissions should be the number one goal of the international community, it is unlikely that the widespread changes and infrastructure redevelopment necessary to accomplish this will be achieved soon, before ocean acidification’s short-term impacts become significant. Therefore, a multi-faceted approach must be employed to address this growing problem.

The response of abyssal organisms to low pH conditions during a series of CO2-release experiments simulating deep-sea carbon sequestration

The effects of low-pH, high-pCO₂ conditions on deep-sea organisms were examined during four deep-sea CO₂ release experiments simulating deep-ocean C sequestration by the direct injection of CO₂ into the deep sea. We examined the survival of common deep-sea, benthic organisms (microbes; macrofauna, dominated by Polychaeta, Nematoda, Crustacea, Mollusca; megafauna, Echinodermata, Mollusca, Pisces) exposed to low-pH waters emanating as a dissolution plume from pools of liquid carbon dioxide released on the seabed during four abyssal CO₂-release experiments. Microbial abundance in deep-sea sediments was unchanged in one experiment, but increased under environmental hypercapnia during another, where the microbial assemblage may have benefited indirectly from the negative impact of low-pH conditions on other taxa. Lower abyssal metazoans exhibited low survival rates near CO₂ pools. No urchins or holothurians survived during 30–42 days of exposure to episodic, but severe environmental hypercapnia during one experiment (E1; pH reduced by as much as ca. 1.4 units). These large pH reductions also caused 75% mortality for the deep-sea amphipod, Haploops lodo, near CO₂ pools. Survival under smaller pH reductions (ΔpH<0.4 units) in other experiments (E2, E3, E5) was higher for all taxa, including echinoderms. Cephalopods, gastropods, and fish were more tolerant than most other taxa. The gastropod Mohnia vernalis and octopus Benthoctopus sp. survived exposure to pH reductions that episodically reached −0.3 pH units. Ninety percent of abyssal zoarcids (Pachycara bulbiceps) survived exposure to pH changes reaching ca. −0.3 pH units during 30–42 day-long experiments.

Tropical coral reef habitat in a geoengineered, high-CO2 world

Continued anthropogenic CO₂ emissions are expected to impact tropical coral reefs by further raising sea surface temperatures (SST) and intensifying ocean acidification (OA). Although geoengineering by means of Solar Radiation Management (SRM) may mitigate temperature increases, OA will persist, raising important questions regarding the impact of different stressor combinations. We apply statistical Bioclimatic Envelope Models to project changes in shallow-water tropical coral reef habitat as a single niche (without resolving biodiversity or community composition) under various Representative Concentration Pathway and SRM scenarios, until 2070. We predict substantial reductions in habitat suitability centered on the Indo-Pacific Warm Pool under net anthropogenic radiative forcing of ≥3.0 W/m². The near-term dominant risk to coral reefs is increasing SSTs; below 3 W/m² reasonably favorable conditions are maintained, even when achieved by SRM with persisting OA. ‘Optimal’ mitigation occurs at 1.5 W/m² because tropical SSTs over-cool in a fully-geoengineered (i.e. pre-industrial global mean temperature) world.

Effects of CO2 on benthic biota: an in situ benthic chamber experiment in Storfjorden (Norway)

Carbon capture and storage (CCS) methods, either sub-seabed or in ocean depths, introduces risk of CO₂ leakage and subsequent interaction with the ecosystem. It is therefore important to obtain information on possible effects of CO₂. In situ CO₂ exposure experiments were carried out twice for 10 days during 2005 using a Benthic Chamber system at 400 m depth in Storfjorden, Norway. pCO₂ in the water above the sediment in the chambers was controlled at approximately 500, 5000 and 20,000 μatm, respectively. This article describes the experiment and the results from measured the biological responses within the chamber sediments. The results show effects of elevated CO₂ concentrations on biological processes such as increased nanobenthos density. Methane production and sulphate reduction was enhanced in the approximately 5000 μatm chamber.

Enhanced chemical weathering as a geoengineering strategy to reduce atmospheric carbon dioxide, a nutrient source and to mitigate ocean acidification

[1] Chemical weathering is an integral part of both the rock and carbon cycles and is being affected by changes in land use, particularly as a result of agricultural practices such as tilling, mineral fertilization, or liming to adjust soil pH. These human activities have already altered the chemical terrestrial cycles and land-ocean flux of major elements, although the extent remains difficult to quantify. When deployed on a grand scale, Enhanced Weathering (a form of mineral fertilization), the application of finely ground minerals over the land surface, could be used to remove CO₂ from the atmosphere. The release of cations during the dissolution of such silicate minerals would convert dissolved CO₂ to bicarbonate, increasing the alkalinity and pH of natural waters. Some products of mineral dissolution would precipitate in soils or taken up by ecosystems, but a significant portion would be transported to the coastal zone and the open ocean, where the increase in alkalinity would partially counteract “ocean acidification” associated with the current marked increase in atmospheric CO₂. Other elements released during this mineral dissolution, like Si, P or K, could stimulate biological productivity, further helping to remove CO₂ from the atmosphere. On land, the terrestrial carbon-pool would likely increase in response to Enhanced Weathering in areas where ecosystem growth rates are currently limited by one of the nutrients that would be released during mineral dissolution.In the ocean, the biological carbon pumps (which export organic matter and CaCO₃ to the deep ocean) may be altered by the resulting influx of nutrients and alkalinity to the ocean.

[2] This review merges current interdisciplinary knowledge about Enhanced Weathering, the processes involved, and the applicability as well as some of the consequences and risks of applying the method.

Geoengineering impact of open ocean dissolution of olivine on atmospheric CO2, surface ocean pH and marine biology

Ongoing global warming induced by anthropogenic emissions has opened the debate as to whether geoengineering is a ‘quick fix’ option. Here we analyse the intended and unintended effects of one specific geoengineering approach, which is enhanced weathering via the open ocean dissolution of the silicate-containing mineral olivine. This approach would not only reduce atmospheric CO₂ and oppose surface ocean acidification, but would also impact on marine biology. If dissolved in the surface ocean, olivine sequesters 0.28 g carbon per g of olivine dissolved, similar to land-based enhanced weathering. Silicic acid input, a byproduct of the olivine dissolution, alters marine biology because silicate is in certain areas the limiting nutrient for diatoms. As a consequence, our model predicts a shift in phytoplankton species composition towards diatoms, altering the biological carbon pumps. Enhanced olivine dissolution, both on land and in the ocean, therefore needs to be considered as ocean fertilization. From dissolution kinetics we calculate that only olivine particles with a grain size of the order of 1 μm sink slowly enough to enable a nearly complete dissolution. The energy consumption for grinding to this small size might reduce the carbon sequestration efficiency by ~30%.

Disrupting the effects of synergies between stressors: improved water quality dampens the effects of future CO2 on a marine habitat

Summary

1. Synergies among stressors drive unanticipated changes to alternative states, yet little has been done to assess whether alleviating one or more contributing stressors may disrupt these interactions. It would be particularly useful to understand whether the synergistic effects of global and local stressors could be alleviated, leading to slower change or faster recovery, if conditions under the control of local management alone were managed (i.e. nutrient pollution).
2. We utilized field-based mesocosms to manipulate CO₂ (i.e. forecasted global concentrations) and nutrients (i.e. local pollution) to test the hypothesis that, where synergies exist, reducing one contributing stressor would limit the effect of the other. In testing this hypothesis, we considered the response of turfing algae, which can displace kelp forests on urbanized coastlines.
3. Initial manipulations of CO₂ and nutrient enrichment produced an anticipated synergistic effect on the biomass of turfing algae.
4. Following exposure of algal turfs to a combination of enriched nutrients and CO₂, a subsequent reduction in nutrients was able to substantially slow further increases in turf growth. Despite this substantial effect, the historical legacy of previous nutrient enrichment was evident as greater turf was maintained relative to ambient conditions (i.e. ambient CO₂ and nutrients). Such legacies of past stressors may be stubborn (e.g. persist as intergenerational change) where the alternative state (i.e. turf algae) has substantial resilience to restorative actions.
5. Synthesis and applications. As stressors accumulate across global to local scales, some combine to produce synergistic effects which cause changes of disproportionate ecological magnitude. While strong synergies attract heavy scrutiny, there remains substantial merit in assessing whether their influence can be ameliorated by managing a contributing stressor. Of note, we show that by reducing a locally determined stressor (nutrients), its synergistic effects with a globally determined stressor (CO₂ enrichment) on a key taxon (turf algae) may be substantially reduced. These results suggest that in the face of changing climate (e.g. ocean acidification), the management of local stressors (e.g. water pollution) may have a greater contribution in determining the dominant state than current thinking allows.

Preparing to manage coral reefs for ocean acidification: lessons from coral bleaching

Ocean acidification is a direct consequence of increasing atmospheric carbon dioxide concentrations and is predicted to compromise the structure and function of coral reefs within this century. Research into the effects of ocean acidification on coral reefs has focused primarily on measuring and predicting changes in seawater carbon (C) chemistry and the biological and geochemical responses of reef organisms to such changes. To date, few ocean acidification studies have been designed to address conservation planning and management priorities. Here, we discuss how existing marine protected area design principles developed to address coral bleaching may be modified to address ocean acidification. We also identify five research priorities needed to incorporate ocean acidification into conservation planning and management: (1) establishing an ocean C chemistry baseline, (2) establishing ecological baselines, (3) determining species/habitat/community sensitivity to ocean acidification, (4) projecting changes in seawater carbonate chemistry, and (5) identifying potentially synergistic effects of multiple stressors.