Posts Tagged 'dissolution'

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

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.

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

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

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).

Continue reading ‘Evidence for stage-based larval vulnerability and resilience to acidification in Crassostrea virginica’

Ocean acidification effects on calcification and dissolution in tropical reef macroalgae

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.

Continue reading ‘Ocean acidification effects on calcification and dissolution in tropical reef macroalgae’

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

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.

Continue reading ‘The role of gastropod shell composition and microstructure in resisting dissolution caused by ocean acidification’

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

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.

Continue reading ‘Geochemical reconstructions of Southern Ocean pH and temperature over the last glacial cycle’

Biogenic carbonate dissolution in shallow marine environments

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.

Continue reading ‘Biogenic carbonate dissolution in shallow marine environments’

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

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.

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

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

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.

Continue reading ‘Calcification of planktonic foraminifer Pulleniatina obliquiloculata controlled by seawater temperature rather than ocean acidification’

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

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.

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

Acidification and dissolution in marine sediments of bays

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.

Continue reading ‘Acidification and dissolution in marine sediments of bays’


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