Posts Tagged 'dissolution'

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Futureproofing the green-lipped mussel aquaculture industry against ocean acidification

Published 25 December 2020 Newsletters and reports , Science Closed
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Two mitigation strategies – waste shell and aeration – were tested in field experiments to see how effective they are at mitigating acidification around mussel farms. This report outlines the results and recommendations from this research. 


Primary results:

  • The inner Firth of Thames currently experiences the lowest seasonal pH of the sites monitored, with a daily minimum of 7.84 (7.79–7.96) in autumn, with short-term (15-minute) pH minima as low as 7.2. Time-series data in the inner and outer Firth of Thames, and also on a mussel farm in the western Firth, show episodic declines in carbonate saturation to the critical carbonate saturation state ΩAR = 1.0 at which solid aragonite (the form of carbonate in mussel shells) will start to dissolve. Consequently, mussels in the Firth of Thames experience episodic corrosive conditions.
  • The mean pH in the Marlborough Sounds region is projected to decrease by 0.15–0.4 by 2100 depending on future emission scenario. The corresponding decline of 0.5–1.25 in the saturation state of aragonite (ΩAR), results in the critical threshold of ΩAR =1 being reached by 2100 under the worst-case scenario. These projections are based only on future CO2 emission scenarios and do not consider other coastal sources of acidity in coastal waters which may also alter in the future.

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Ocean warming and acidification uncouple calcification from calcifier biomass which accelerates coral reef decline

Published 4 December 2020 Science Closed
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Global climate change will drive declines in coral reefs over coming decades. Yet, the relative role of temperature versus acidification, and the ability of resultant ecosystems to retain core services such as coastal protection, are less clear. Here, we investigate changes to the net chemical balances of calcium carbonate within complex experimental coral reefs over 18 months under conditions projected for 2100 if CO2 emissions continue unmitigated. We reveal a decoupling of calcifier biomass and calcification under the synergistic impact of warming and acidification, that combined with increased night-time dissolution, leads to an accelerated loss of carbonate frameworks. Climate change induced degradation will limit the ability of coral reefs to keep-up with sea level rise, possibly for thousands of years. We conclude that instead of simply transitioning to alternate states that are capable of buffering coastlines, reefs are at risk of drowning leading to critical losses in ecosystem functions.

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Ocean acidification and short‐term organic matter enrichment alter coral reef sediment metabolism through different pathways

Published 30 November 2020 Science Closed
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Ocean acidification (OA) and organic matter (OM) enrichment (due to coastal eutrophication) could act in concert to shift coral reef carbonate sediments from a present state of net calcification to a future state of net dissolution, but no studies have examined the combined effect of these stressors on sediment metabolism and dissolution. This study used 22‐hour incubations in flume aquaria with captive sediment communities to measure the combined effect of elevated pCO2 (representing Ocean Acidification) and particulate organic carbon (representing coastal eutrophication) on coral reef sediment gross primary productivity (GPP), respiration (R), and net calcification (Gnet). Relative to control sediment communities, both OA (pCO2 ~ 1000 μatm) and OM enrichment (~ + 40 μmol C L‐1) significantly decreased rates of sediment Gnet by 1.16 and 0.18 mmol CaCO3 m‐2 h‐1, respectively, but the mechanism behind this decrease differed. The OA‐mediated transition to net dissolution was physiochemical, as rates of GPP and R remained unaffected and dissolution was solely enhanced by a decline in the aragonite saturation state (Ωarg) of the overlying water column and the physical factors governing the porewater exchange rate with this overlying water column. In contrast, the OM‐mediated decline in Gnet was due to a decline in the overlying seawater Ωarg due to the increased respiratory addition of CO2. The decrease in Gnet in response to a combination of both stressors was additive (‐ 0.09 mmol CaCO3 m‐2 h‐1 relative to OA alone) but this decrease did not significantly differ from the individual effect of either stressor. In this study OA was the primary driver of future carbonate sediment dissolution, but longer‐term experiments with chronic organic matter enrichment are required.

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Predicting potential impacts of ocean acidification on marine calcifiers from the Southern Ocean

Published 18 November 2020 Science Closed
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Understanding the vulnerability of marine calcifiers to ocean acidification is a critical issue, especially in the Southern Ocean (SO), which is likely to be the one of the first, and most severely affected regions. Since the industrial revolution, ~30% of anthropogenic CO2 has been absorbed by the oceans. Seawater pH levels have already decreased by 0.1 and are predicted to decline by ~ 0.3 by the year 2100. This process, known as ocean acidification (OA), is shallowing the saturation horizon, which is the depth below which calcium carbonate (CaCO3) dissolves, likely increasing the vulnerability of many marine calcifiers to dissolution. The negative impact of OA may be seen first in species depositing more soluble CaCO3 mineral phases such as aragonite and high-Mg calcite (HMC). These negative effects may become even exacerbated by increasing sea temperatures. Here we combine a review and a quantitative meta-analysis to provide an overview of the current state of knowledge about skeletal mineralogy of major taxonomic groups of SO marine calcifiers and to make predictions about how OA might affect different taxa. We consider their geographic range, skeletal mineralogy, biological traits and potential strategies to overcome OA. The meta-analysis of studies investigating the effects of the OA on a range of biological responses such as shell state, development and growth rate shows response variation depending on mineralogical composition. Species-specific responses due to mineralogical composition suggest taxa with calcitic, aragonitic and HMC skeletons may be more vulnerable to the expected carbonate chemistry alterations, and low magnesium calcite (LMC) species may be mostly resilient. Environmental and biological control on the calcification process and/or Mg content in calcite, biological traits and physiological processes are also expected to influence species specific responses.

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Irradiance, photosynthesis and elevated pCO2 effects on net calcification in tropical reef macroalgae

Published 16 November 2020 Science Closed
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Highlights

  • Most species from high-light environments are not able to calcifying under OA at night
  • Low-light species may be more susceptible to OA compared to high-light
  • Some species exhibit light-triggered calcification independent of photosystem II
  • Photosystem II independent calcification not sustained under OA

Abstract


Calcifying tropical macroalgae produce sediment, build three-dimensional habitats, and provide substrate for invertebrate larvae on reefs. Thus, lower calcification rates under declining pH and increasing ocean pCO2, or ocean acidification, is a concern. In the present study, calcification rates were examined experimentally under predicted end-of-the-century seawater pCO2 (1116 μatm) and pH (7.67) compared to ambient controls (pCO2 409 μatm; pH 8.04). Nine reef macroalgae with diverse calcification locations, calcium carbonate structure, photophysiology, and site-specific irradiance were examined under light and dark conditions. Species included five from a high light patch reef on the Florida Keys Reef Tract (FKRT) and four species from low light reef walls on Little Cayman Island (LCI). Experiments on FKRT and LCI species were conducted at 500 and 50 μmol photons m−2 s−1 in situ irradiance, respectively. Calcification rates independent of photosystem-II (PSII) were also investigated for FKRT species. The most consistent negative effect of elevated pCO2 on calcification rates in the tropical macroalgae examined occurred in the dark. Most species (89%) had net calcification rates of zero or net dissolution in the dark at low pH. Species from the FKRT that sustained positive net calcification rates in the light at low pH also maintained ~30% of their net calcification rates without PSII at ambient pH. However, calcification rates in the light independent of PSII were not sustained at low pH. Regardless of these low pH effects, most FKRT species daily net calcification rates, integrating light/dark rates over a 24h period, were not significantly different between low and ambient pH. This was due to a 10-fold lower dark, compared to light, calcification rate, and a strong correspondence between calcification and photosynthetic rates. Interestingly, low-light species sustained calcification rates on par with high-light species without high rates of photosynthesis. Low-light species’ morphology and physiology that promote high calcification rates at ambient pH, may increase their vulnerability to low pH. Our data indicate that the negative effect of elevated pCO2 and low pH on tropical macroalgae at the organismal level is their impact on dark net calcification, probably enhanced dissolution. However, elevated pCO2 and low pH effects on macroalgae daily calcification rates are greatest in species with lower net calcification rates in the light. Thus, macroalgae able to maintain high calcification rates in the light (high and low irradiance) at low pH, and/or sustain strong biotic control with high [H+] in the bulk seawater, are expected to dominate under global change.

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Differential sensitivity of a symbiont‐bearing foraminifer to seawater carbonate chemistry in a decoupled DIC‐pH experiment

Published 28 October 2020 Science Closed
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Larger benthic foraminifera (LBF) are unicellular eukaryotic calcifying organisms and an important component of tropical and subtropical modern and ancient oceanic ecosystems. They are major calcium carbonate producers and important contributors to primary production due to the photosynthetic activity of their symbiotic algae. Studies investigating the response of LBF to seawater carbonate chemistry changes are therefore essential for understanding the impact of climate changes and ocean acidification (OA) on shallow marine ecosystems. In this study, calcification, respiration, and photosynthesis of the widespread diatom‐bearing LBF Operculina ammonoides were measured in laboratory experiments that included manipulation of carbonate chemistry parameters. pH was altered while keeping dissolved inorganic carbon (DIC) constant, and DIC was altered while keeping pH constant. The results show clear vulnerability of O. ammonoides to low pH and CO32− under constant DIC conditions, and no increased photosynthesis or calcification under high DIC concentrations. Our results call into question previous hypotheses, suggesting that mechanisms such as the degree of cellular control on calcification site pH/DIC and/or enhanced symbiont photosynthesis in response to OA may render the hyaline (perforate and calcitic‐radial) LBF to be less responsive to OA than porcelaneous LBF. In addition, manipulating DIC did not affect calcification when pH was close to present seawater levels in a model encompassing the total population size range. In contrast, larger individuals (>1,200 μm, >1 mg) were sensitive to changes in DIC, a phenomenon we attribute to their physiological requirement to concentrate large quantities of DIC for their calcification process.

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Effects of low pH and low salinity induced by meltwater inflow on the behavior and physical condition of the Antarctic limpet, Nacella concinna

Published 27 October 2020 Science Closed
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Seawater acidification and freshening in the intertidal zone of Marian Cove, Antarctica, which occurs by the freshwater inflow from snow fields and glaciers, could affect the physiology and behavior of intertidal marine organisms. In this study, we exposed Antarctic limpets, Nacella concinna, to two different pH (8.00 and 7.55) and salinity (34.0 and 27.0 psu) levels and measured their righting ability after being flipped over, mortality, condition factor, and shell dissolution. During the 35-day exposure, there was no significant difference in behavior and mortality between different treatments. However, the condition factor was negatively affected by low salinity. Both low pH and low salinity negatively influenced shell formation by decreasing the aragonite saturation state (Ωarg) and enhancing shell dissolution. Our results suggest that, though limpets can tolerate short-term low pH and salinity conditions, intrusions of meltwater accompanied by the glacial retreat may act as a serious threat to the population of N. concinna.

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Coral reef sediment dissolution in a changing ocean: insights from a temporal field study

Published 7 October 2020 Science Closed
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Calcium carbonate sediments form an essential part of coral reefs yet have often been overlooked when studying the effects of future ocean acidification (OA). This original field-based research aims to assess the temporal variability of organic and inorganic sediment metabolism under ambient and elevated pCO2. OA caused a shift from net precipitation to net dissolution, but the sensitivity to OA varied seasonally, depending on interactions with temperature and benthic productivity. A slack-water approach of net ecosystem calcification revealed that sediments can play an important role in carbonate budgets, particularly at night, and become increasingly important as the oceans continue acidifying.

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Crumbling reefs and cold-water coral habitat loss in a future ocean: evidence of “coralporosis” as an indicator of habitat integrity

Published 21 September 2020 Science Closed
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Ocean acidification is a threat to the net growth of tropical and deep-sea coral reefs, due to gradual changes in the balance between reef growth and loss processes. Here we go beyond identification of coral dissolution induced by ocean acidification and identify a mechanism that will lead to a loss of habitat in cold-water coral reef habitats on an ecosystem-scale. To quantify this, we present in situ and year-long laboratory evidence detailing the type of habitat shift that can be expected (in situ evidence), the mechanisms underlying this (in situ and laboratory evidence), and the timescale within which the process begins (laboratory evidence). Through application of engineering principals, we detail how increased porosity in structurally critical sections of coral framework will lead to crumbling of load-bearing material, and a potential collapse and loss of complexity of the larger habitat. Importantly, in situ evidence highlights that cold-water corals can survive beneath the aragonite saturation horizon, but in a fundamentally different way to what is currently considered a biogenic cold-water coral reef, with a loss of the majority of reef habitat. The shift from a habitat with high 3-dimensional complexity provided by both live and dead coral framework, to a habitat restricted primarily to live coral colonies with lower 3-dimensional complexity represents the main threat to cold-water coral reefs of the future and the biodiversity they support. Ocean acidification can cause ecosystem-scale habitat loss for the majority of cold-water coral reefs.

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Evidence for stage-based larval vulnerability and resilience to acidification in Crassostrea virginica

Published 18 September 2020 Science Closed
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Using image analysis of scanning electron micrographs (SEMs), we compared differences in growth of D-stage veligers [i.e. prodissoconch I and II (PI and PII) larvae] of eastern oysters Crassostrea virginica grown in mesohaline water under high- and low-CO2 conditions. We found SEMs to reveal no evidence of dissolution or shell structure deformity for larval shells in either of the CO2 treatments but detected prominent growth lines in the PII regions of larval shells. The number of growth lines closely approximated the duration of the experiment, suggesting that growth lines are generated daily. Mean growth line interval widths were 20% greater for larval shells cultured in low- vs high-CO2 conditions. Crassostrea virginica veliger larvae were shown to tolerate high CO2 levels and aragonite saturation states (Ωarag) < 1.0, but larval growth was slowed substantially under these conditions. Differences in growth line interval width translate into substantial changes in shell area and account for previously observed differences in total shell area between the treatments, as determined by light microscopy and image analysis. Other studies have documented high mortality and malformation of D-stage larvae in bivalves when pre-veliger life stages (i.e. eggs, gastrula and trochophores) were exposed to elevated CO2. Our experiments revealed statistical differences in rates of larval survival, settlement and subsequent early-stage spat mortality for veligers reared in high- and low-CO2 conditions. Although each of these rates was measurably affected by high CO2, the magnitude of these differences was small (range across categories = 0.7–6.3%) suggesting that the impacts may not be catastrophic, as implied by several previous studies. We believe the apparent disparity among experimental results may be best explained by differential vulnerability of pre-veliger stage larvae and veligers, whereby PI and PII larvae have greater physiological capacity to withstand environmental conditions that may be thermodynamically unfavourable to calcification (i.e. Ωarag < 1.0).

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Ocean acidification effects on calcification and dissolution in tropical reef macroalgae

Published 31 August 2020 Science Closed
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Net calcification rates for coral reef and other calcifiers have been shown to decline as ocean acidification (OA) occurs. However, the role of calcium carbonate dissolution in lowering net calcification rates is unclear. The objective of this study was to distinguish OA effects on calcification and dissolution rates in dominant calcifying macroalgae of the Florida Reef Tract, including two rhodophytes (Neogoniolithon strictum, Jania adhaerens) and two chlorophytes (Halimeda scabra, Udotea luna). Two experiments were conducted: (1) to assess the difference in gross (45Ca uptake) versus net (total alkalinity anomaly) calcification rates in the light/dark and (2) to determine dark dissolution (45CaCO3), using pH levels predicted for the year 2100 and ambient pH. At low pH in the light, all species maintained gross calcification rates and most sustained net calcification rates relative to controls. Net calcification rates in the dark were ~84% lower than in the light. In contrast to the light, all species had lower net calcification rates in the dark at low pH with chlorophytes exhibiting net dissolution. These data are supported by the relationship (R2 = 0.82) between increasing total alkalinity and loss of 45Ca from pre-labelled 45CaCO3 thalli at low pH in the dark. Dark dissolution of 45CaCO3-labelled thalli was ~18% higher in chlorophytes than rhodophytes at ambient pH, and ~ twofold higher at low pH. Only Udotea, which exhibited dissolution in the light, also had lower daily calcification rates integrated over 24 h. Thus, if tropical macroalgae can maintain high calcification rates in the light, lower net calcification rates in the dark from dissolution may not compromise daily calcification rates. However, if organismal dissolution in the dark is additive to sedimentary carbonate losses, reef dissolution may be amplified under OA and contribute to erosion of the Florida Reef Tract and other reefs that exhibit net dissolution.

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The role of gastropod shell composition and microstructure in resisting dissolution caused by ocean acidification

Published 18 August 2020 Science Closed
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Highlights

• Two gastropods with different shell microstructure were exposed to low pH (six months).

• Micro-CT scans indicate decreased densities on exterior-most shell in both gastropods.

• Fibrous calcite layers experience more dissolution than homogeneous calcite layers.

• Microstructural crystal arrangement likely determines susceptibility to dissolution.

• Tegula funebralis shells are critically vulnerable to changes in ocean chemistry.

Abstract

Organisms, such as molluscs, that produce their hard parts from calcium carbonate are expected to show increased difficulties growing and maintaining their skeletons under ocean acidification (OA). Any loss of shell integrity increases vulnerability, as shells provide protection against predation, desiccation, and disease. Not all species show the same responses to OA, which may be due to the composition and microstructural arrangement of their shells. We explore the role of shell composition and microstructure in resisting dissolution caused by decreases in seawater pH using a combination of microCT scans, XRD analysis, and SEM imaging. Two gastropods with different shell compositions and microstructure, Tegula funebralis and Nucella ostrina, were exposed to simulated ocean acidification conditions for six months. Both species showed signs of dissolution on the exterior of their shells, but changes in density were significantly more pronounced in T. funebralis. XRD analysis indicated that the exterior layer of both shell types was made of calcite. T. funebralis may be more prone to dissolution because their outer fibrous calcite layer has more crystal edges and faces exposed, potentially increasing the surface area on which dissolution can occur. These results support a previous study where T. funebralis showed significant decreases in both shell growth and strength, but N. ostrina only showed slight reductions in shell strength, and unaffected growth. We suggest that microstructural arrangement of shell layers in molluscs, more so than their composition alone, is critical for determining the vulnerability of mollusc shells to OA.

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Geochemical reconstructions of Southern Ocean pH and temperature over the last glacial cycle

Published 13 August 2020 Science Closed
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The Southern Ocean is widely thought to play an important role in atmospheric CO₂ change over glacial-interglacial cycles. It has been suggested that as the region that ventilates the majority of the world’s carbon-rich deep waters today, reduced exchange between deep waters and the atmosphere in the Southern Ocean acted to draw down CO₂ over glacial timescales. However, direct evidence of the Southern Ocean’s role in glacial CO₂ drawdown has been lacking thus far. Here I apply the boron-isotope pH-proxy to foraminifera from the Antarctic Zone sediment core PS1506 over the last glacial cycle. The low boron concentrations in these polar foraminifera makes these samples particularly sensitive to boron blank and so a close examination of the sources of blank, and an assessment of the precision of blank measurements, has been made. The ratios of trace elements to calcium in foraminiferal shells are widely applied as proxies for palaeoenvironmental parameters such as temperature. As Southern Ocean carbonate sediments are particularly prone to dissolution, which can affect trace element concentrations, an assessment of dissolution has been made. Firstly, dissolution experiments were conducted to constrain the impact of dissolution in a controlled setting, and secondly, shell mass and trace elements were evaluated for the downcore record. Imaging reveals similar etching textures in both experimentally dissolved samples and deglacial intervals, when shell mass is also low and several trace elements exhibit an excursion to lower values. Boron isotope data for PS1506 show that during the penultimate interglacial, surface water pH was low. At the onset of atmospheric CO₂ drawdown, pH increased, indicating low CO₂ surface waters. This is consistent with the signature predicted for a more stratified Southern Ocean, and is evidence that stratification in the Antarctic Zone acted to contribute to CO₂ drawdown early in the transition to a glacial state.

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Biogenic carbonate dissolution in shallow marine environments

Published 8 July 2020 Science 1 Comment
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Ocean acidification (OA), the decrease in surface ocean pH and seawater saturation state with respect to carbonate minerals (Ω), is expected to increase carbonate mineral dissolution. However, the influence of OA on carbonate dissolution has been largely neglected despite evidence that it is more sensitive to OA than calcification. Increases in the rate of carbonate dissolution could have severe impacts for ecosystems such as coral reefs, which rely on the accumulation of carbonate structures and substrates to exist. At present, dissolution rates of bulk shallow biogenic carbonate sediments are largely unknown and laboratory dissolution rates exceed in situ rates by orders of magnitude. The goal of this study was to develop a better understanding of the drivers and controls of bulk carbonate sediment dissolution in coral reef environments. Based on results from in situ benthic chambers and laboratory free-drift experiments of bulk biogenic carbonate sediments from global locations, dissolution rates were found to be primarily controlled by organic matter decomposition, but significantly influenced by the overlying seawater carbonate chemistry and the solubility of the most soluble mineral phase in the sediments. Shallow carbonate dissolution will therefore be enhanced via ocean acidification, increased respiration, or a combination of these processes. The sensitivity of bulk sediment dissolution rates to changes in Ω was not related to median grain size or mineralogy, but may be attributed to organic coatings on sediment grains. Dissolution rates in bulk sediments increased ~2-3-fold when these coatings were removed, suggesting that they act as a protective barrier that limits direct interaction of seawater with the mineral surface, thus inhibiting dissolution. On the ecosystem scale, carbonate dissolution was inferred from calcium anomalies measured using a novel spectrophotometric titration system and confirms seasonal and inter-annual trends in reef biogeochemical processes based on parallel alkalinity measurements. However, calcium measurements may be best employed in environments where multiple processes significantly influence alkalinity or Mg-calcites are precipitating and dissolving. Although many questions remain, this work has elucidated certain key drivers and controls of shallow carbonate sediment dissolution and how they may respond to a rapidly changing ocean.

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Interactive effects of pH and temperature on native and alien mussels from the west coast of South Africa

Published 26 June 2020 Science Closed
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Global warming and ocean acidification influence marine calcifying organisms, particularly those with external shells. Among these, mussels may compensate for environmental changes by phenotypic plasticity, but this may entail trade-offs between shell deposition, growth and reproduction. We assessed main and interactive effects of pH and temperature on four mussel species on the west coast of South Africa (33°48′ S, 18°27′ E) in October 2012 by comparing shell dissolution, shell growth, shell breaking force and condition index of two native species, the ribbed mussel Aulacomya atra and the black mussel Choromytilus meridionalis, and two aliens, the Mediterranean mussel Mytilus galloprovincialis and the bisexual mussel Semimytilus algosus. Live mussels and dead shells were exposed for 42 days to seawater of pH 7.5 or 8.0, at 14 °C or 20 °C. Low pH, high temperature and their combination increased shell dissolution of the two aliens but their growth rates and condition indices remained unchanged. Aulacomya atra also experienced greater shell dissolution at a low pH and high temperature, but grew faster in low-pH treatments. For C. meridionalis, shell dissolution was unaffected by pH or temperature; it also grew faster in low-pH treatments, but had a lower condition index in the higher temperature treatment. Shell strength was not determined by thickness alone. In most respects, all four species proved to be robust to short-term reduction of pH and elevation of temperature, but the native species compensated for greater shell dissolution at low pH by increasing growth rate, whereas the aliens did not, so their invasive success cannot be ascribed to benefits accruing from climate change.

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Calcification of planktonic foraminifer Pulleniatina obliquiloculata controlled by seawater temperature rather than ocean acidification

Published 19 June 2020 Science Closed
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Highlights

• A method is provided to correct the dissolution effect on foraminiferal SNW

• Core-top ISNWP. obli is positively correlated with calcification temperature

• ISNWP. obli linked to seawater temperature, but not atmospheric pCO2, since 250 ka

• Temperature is the dominant factor controlling P. obliquiloculata calcification

Abstract

Planktonic foraminifera represent a major component of global marine carbonate production, and understanding environmental influences on their calcification is critical to predicting marine carbon cycle responses to modern climate change. The present study investigated the effects of different environmental influences on calcification of the planktonic foraminifer Pulleniatina obliquiloculata. By correcting the dissolution effect on the size-normalized weight (SNW) of P. obliquiloculata from deep-sea sediments, we provide a means of estimating initial size-normalized weight (ISNW) from which to assess secular changes in the degree of calcification of P. obliquiloculata. Core-top ISNW in P. obliquiloculata from the global tropical oceans is significantly positively correlated with calcification temperature, suggesting that temperature is the dominant control on calcification. Using Neogloboquadrina dutertrei SNW as an independent deep-water Δ[CO32−] proxy, we present an ISNW record for P. obliquiloculata from the western tropical Pacific since 250 ka. The response of ISNW to past seawater temperature variations further confirms the dominant influence of temperature on P. obliquiloculata calcification. A potential increase in calcification as a result of ocean warming may have reduced oceanic uptake of CO2 from the atmosphere and increased atmospheric pCO2, generating a positive feedback for global warming.

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Influence of water masses on the biodiversity and biogeography of deep-sea benthic ecosystems in the North Atlantic

Published 6 May 2020 Science Closed
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Circulation patterns in the North Atlantic Ocean have changed and re-organized multiple times over millions of years, influencing the biodiversity, distribution, and connectivity patterns of deep-sea species and ecosystems. In this study, we review the effects of the water mass properties (temperature, salinity, food supply, carbonate chemistry, and oxygen) on deep-sea benthic megafauna (from species to community level) and discussed in future scenarios of climate change. We focus on the key oceanic controls on deep-sea megafauna biodiversity and biogeography patterns. We place particular attention on cold-water corals and sponges, as these are ecosystem-engineering organisms that constitute vulnerable marine ecosystems (VME) with high associated biodiversity. Besides documenting the current state of the knowledge on this topic, a future scenario for water mass properties in the deep North Atlantic basin was predicted. The pace and severity of climate change in the deep-sea will vary across regions. However, predicted water mass properties showed that all regions in the North Atlantic will be exposed to multiple stressors by 2100, experiencing at least one critical change in water temperature (+2°C), organic carbon fluxes (reduced up to 50%), ocean acidification (pH reduced up to 0.3), aragonite saturation horizon (shoaling above 1000 m) and/or reduction in dissolved oxygen (>5%). The northernmost regions of the North Atlantic will suffer the greatest impacts. Warmer and more acidic oceans will drastically reduce the suitable habitat for ecosystem-engineers, with severe consequences such as declines in population densities, even compromising their long-term survival, loss of biodiversity and reduced biogeographic distribution that might compromise connectivity at large scales. These effects can be aggravated by reductions in carbon fluxes, particularly in areas where food availability is already limited. Declines in benthic biomass and biodiversity will diminish ecosystem services such as habitat provision, nutrient cycling, etc. This study shows that the deep-sea VME affected by contemporary anthropogenic impacts and with the ongoing climate change impacts are unlikely to withstand additional pressures from more intrusive human activities. This study serves also as a warning to protect these ecosystems through regulations and by tempering the ongoing socio-political drivers for increasing exploitation of marine resources.

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Acidification and dissolution in marine sediments of bays

Published 17 April 2020 Science Closed
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Besides sea level rise, climate change main consequences are ocean warming and its evil twin acidification. Ocean acidification has been identified as ‘a future global climate change impact concern’ because it has been slowly affecting entire ecosystems, and it is a threat to local economies, especially shellfish and fisheries productions. The scientific community has limited understanding of ocean acidification impacts, yet local and other researchers continue to monitor and evaluate them in many parts of the world. Marine bay environments are some of the richest and most biodiverse areas in the world. Ocean current circulation and upwelling of deep cold waters brings nutrient rich waters to the surface at certain times of the year, increasing productivity. Bays act as a carbon sink. Global oceans absorb twenty-five percent of our carbon dioxide emissions. So, when there is an excess of carbon dioxide emissions in the atmosphere, this excess is absorbed in seawater and marine sediment interstitial water, and a series of chemical reactions lowers pH increasing ocean acidity. These “ocean acidification” changes have serious implications for our coastal ecosystems, altering marine life behavior and development yet this question remains addressed. Increased acidity interferes with the process by which calcifying organisms such as crab, oysters, mussels, and certain plankton and benthic Foraminifera build their shells. Foraminifera are the base of the ocean food chain as first consumers in the ocean.

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Determining how biotic and abiotic variables affect the shell condition and parameters of Heliconoides inflatus pteropods from a sediment trap in the Cariaco Basin

Published 16 April 2020 Science Closed
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Pteropods have been nicknamed the “canary in the coal mine” for ocean acidification because they are predicted to be among the first organisms to be affected by changing ocean chemistry. This is due to their fragile, aragonitic shells and high abundances in polar and subpolar regions where the impacts of ocean acidification are most pronounced. For pteropods to be used most effectively as indicators of ocean acidification, the biotic and abiotic factors influencing their shell formation and dissolution in the modern ocean need to be quantified and understood. Here, we measured the shell condition (i.e., the degree to which a shell has dissolved) and shell characteristics, including size, number of whorls, shell thickness, and shell volume (i.e., amount of shell material) of nearly 50 specimens of the pteropod species Heliconoides inflatus sampled from a sediment trap in the Cariaco Basin, Venezuela, over an 11-month period. The shell condition of pteropods from sediment traps has the potential to be altered at three stages: (1) when the organisms are live in the water column associated with ocean acidification, (2) when organisms are dead in the water column associated with biotic decay of organic matter and/or abiotic dissolution associated with ocean acidification, and (3) when organisms are in the closed sediment trap cup associated with abiotic alteration by the preservation solution. Shell condition was assessed using two methods: the Limacina Dissolution Index (LDX) and the opacity method. The opacity method was found to capture changes in shell condition only in the early stages of dissolution, whereas the LDX recorded dissolution changes over a much larger range. Because the water in the Cariaco Basin is supersaturated with respect to aragonite year-round, we assume no dissolution occurred during life, and there is no evidence that shell condition deteriorated with the length of time in the sediment trap. Light microscope and scanning electron microscope (SEM) images show the majority of alteration happened to dead pteropods while in the water column associated with the decay of organic matter. The most altered shells occurred in samples collected in September and October when water temperatures were warmest and when the amount of organic matter degradation, both within the shells of dead specimens and in the water column, was likely to have been the greatest.

Continue reading ‘Determining how biotic and abiotic variables affect the shell condition and parameters of Heliconoides inflatus pteropods from a sediment trap in the Cariaco Basin’

Pore water conditions driving calcium carbonate dissolution in reef sands

Published 16 April 2020 Science Closed
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Due to decreases in seawater pH resulting from ocean acidification, permeable calcium carbonate reef sands are predicted to be net dissolving by 2050. However, the rate of dissolution and factors that control this rate remain poorly understood. Experiments performed in benthic chambers predict that reefs will become net dissolving when the aragonite saturation state (Ωa) in sea water falls below ∼ 3, as underlying reef sediments start net dissolution due to lower saturation states in the pore water. We used flow-through reactors to investigate the rate of dissolution at various Ωa at the pore scale. The sediment became net dissolving at Ωa = 1.68 – 2.25, which is significantly greater than 1. This indicates that the bulk pore water does not represent conditions at the site of dissolution, and dissolution probably occurs in microniches inside porous sand grains. Measured dissolution rates were much higher under oxic conditions than anoxic conditions, but were not affected by the addition of carbonic anhydrase. Analysis of δ13C-CO2 produced in the flow-through reactors revealed a bias in the conventional alkalinity anomaly method under anoxic conditions, showing that some of the CO2 attributed to metabolism by may actually be derived from carbonate dissolution. This deviation likely originates from alkalinity consumption by fermentation, which masks the alkalinity generated by dissolution. Therefore, dissolution rates determined by alkalinity changes in reef sands with anaerobic metabolisms may underestimate actual values.

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