Posts Tagged 'dissolution'

Changes in coral reef community structure in response to year-long incubations under contrasting pCO2 regimes

Coral reefs are threatened by ocean acidification (OA), which depresses net calcification of corals, calcified algae, and coral reef communities. These effects have been quantified for many organisms, but most experiments last weeks-to-months, and do not test for effects on community structure. Here, the effects of OA on back reef communities from Mo’orea, French Polynesia (17.492 S, 149.826 W), were tested from 12 November 2015 to 16 November 2016 in outdoor flumes maintained at mean pCO2 levels of 364 µatm, 564 µatm, 761 µatm, and 1067 µatm. The communities consisted of four corals and two calcified algae, with change in mass (Gnet, a combination of gross accretion and dissolution) and percent cover recorded monthly. For massive Porites and Montipora spp., Gnet differed among treatments, and at 1067 µatm (relative to ambient) was reduced and still positive; for Porolithon onkodes, all of which died, Gnet was negative at high pCO2, revealing dissolution (sample sizes were too small for analysis of Gnet for other taxa). Growth rates (% cover month−1) were unaffected by pCO2 for Montipora spp., P. rus, Pocillopora verrucosa, and Lithophyllum kotschyanum, but were depressed for massive Porites at 564 µatm. Multivariate community structure changed among seasons, and the variation under all elevated pCO2 treatments differed from that recorded at 364 µatm, and was greatest under 564 µatm and 761 µatm pCO2. Temporal variation in multivariate community structure could not be attributed solely to the effects of OA on the chemical and physical properties of seawater. Together, these results suggest that coral reef community structure may be more resilient to OA than suggested by the negative effects of high pCO2 on Gnet of their component organisms.

Continue reading ‘Changes in coral reef community structure in response to year-long incubations under contrasting pCO2 regimes’

Ocean acidification affects calcareous tube growth in adult stage and reared offspring of serpulid polychaetes

The energetically costly transition from free-swimming larvae to benthic life stage and maintenance of a calcareous structure can make calcifying marine invertebrates vulnerable to ocean acidification. The first goal of this study was to evaluate the impacts of ocean acidification on calcified tube growth for two Serpulidae polychaete worms. Spirorbis sp. and Spirobranchus triqueter were collected at 11 m depth from the Northwest Mediterranean Sea and maintained for 30 and 90 d, at three mean pHT levels (total scale) of 8.1 (ambient), 7.7, and 7.4. Moderately decreased tube elongation rates were observed in both species at a pHT of 7.7 while severe reductions occurred at pHT 7.4. There was visual evidence of dissolution and tubes were more fragile at lower pH but, fragility was not attributed to changes in fracture toughness. Instead, it appeared to be due to the presence of larger alveoli covered in a thinner calcareous layer. The second objective of the study was to test for effects in offspring development of the species S. triqueter. Spawning was induced, and offspring were reared in the same pH conditions the parents experienced. Trochophore size was reduced at the lowest pH level but settlement success was similar across pH conditions. Post-settlement tube growth was most affected. At 38 d post-settlement, juvenile tubes at pHT of 7.7 and 7.4 were half the size of those at pHT 8.1. Results suggest future carbonate chemistry will negatively affect initiation and persistence of both biofouling and epiphytic polychaete tube worms.

Continue reading ‘Ocean acidification affects calcareous tube growth in adult stage and reared offspring of serpulid polychaetes’

Wasting away in the intertidal: the fate of chiton valves in an acidifying ocean

Chitons are locally common in New Zealand, and several studies have suggested that their valves are resistant to dissolution, so it seems contradictory that they are under-represented in the sediment and fossil records of New Zealand. Indeed, special resistance to dissolution seems counterintuitive since the valves are primarily made of aragonite. Here we examine the resistance of chiton skeletal material to dissolution in order to expand our understanding of how taphonomic forces affect chitons and to provide insight into the preservation potential of chiton valves. Live individuals of eight species of chitons were collected from Otago Peninsula, South Island, New Zealand. The valves were subjected to one of two pH treatments: ambient pH of 8.10 and reduced pH of 7.70. Notoplax violacea, Sypharochiton pelliserpentis, and S. sinclairi were the most resistant to dissolution while Acanthochitona zelandica, Chiton glaucus, Onithochiton neglectus, and Ischnochiton maorianus were more vulnerable to dissolution. Leptochiton inquinatus lost the most mass in both treatments, but did not show a significant difference between them. SEM images of the dorsal and ventral surfaces on each valve revealed low-pH damage to crystal structures in the articulamentum, while the tegmentum showed no significant damage. Chiton skeletal material in general does not appear to resist dissolution any better than other examined mollusks, but the resistant tegmentum confers considerable resilience to lowered pH. Chiton valves can last up to an estimated 45 years before becoming unrecognizable, which is much shorter than the normal temperate shallow-water exposure time of hundreds to thousands of years.

Continue reading ‘Wasting away in the intertidal: the fate of chiton valves in an acidifying ocean’

Quantifying susceptibility of marine invertebrate biocomposites to dissolution in reduced pH

Ocean acidification threatens many ecologically and economically important marine calcifiers. The increase in shell dissolution under the resulting reduced pH is an important and increasingly recognized threat. The biocomposites that make up calcified hardparts have a range of taxon-specific compositions and microstructures, and it is evident that these may influence susceptibilities to dissolution. Here, we show how dissolution (thickness loss), under both ambient and predicted end-century pH (approx. 7.6), varies between seven different bivalve molluscs and one crustacean biocomposite and investigate how this relates to details of their microstructure and composition. Over 100 days, the dissolution of all microstructures was greater under the lower pH in the end-century conditions. Dissolution of lobster cuticle was greater than that of any bivalve microstructure, despite its calcite mineralogy, showing the importance of other microstructural characteristics besides carbonate polymorph. Organic content had the strongest positive correlation with dissolution when all microstructures were considered, and together with Mg/Ca ratio, explained 80–90% of the variance in dissolution. Organic content, Mg/Ca ratio, crystal density and mineralogy were all required to explain the maximum variance in dissolution within only bivalve microstructures, but still only explained 50–60% of the variation in dissolution.

Continue reading ‘Quantifying susceptibility of marine invertebrate biocomposites to dissolution in reduced pH’

Experimental validation of planktic foraminifera fragmentation index as a proxy for end-cretaceous ocean acidification

The final ~50 ky of the Maastrichtian leading up to the Cretaceous-Tertiary boundary mass extinction at Bidart (France) show records of poor carbonate preservation, the final ~25 ky being critical. This event has been proposed as evidence for ocean acidification immediately preceding the mass extinction. High planktic foraminifera test fragmentation index, anomalously low bulk-rock magnetic susceptibility and peak mercury content in this same interval link this crisis interval to peak Deccan volcanism in India. New results provide experimental validation for fragmentation index as an authentic proxy of end-Cretaceous ocean acidification event. Pristine Cretaceous planktic foraminifera morphotypes were exposed to buffers of pH 8.0, 7.5, 7.0 and 6.5 for 15 days each and their preservation state was quantified as a function of time. The critical variables affecting test vulnerability and taphonomy are morphology, pH and time of exposure. Thin-walled fragile biserial species(60%) such as Heterohelix globulosa and H. planata are the most susceptible to dissolution, followed by simple coiled forms such as Rugoglobigerina (19%) sp. and Hedbergella sp(6.4%). The globotruncanids(12%) appear to be least susceptible to chemical and physical damage. Tests exposed to low pH conditions clearly show a higher vulnerability to fragmentation. These results indicate a strong influence of chemical and physical taphonomy on planktic foraminifera census data with serious palaeoenvironmental implications. Results also indicate that an overestimation of the abundance of environmentally sensitive Cretaceous species (e.g. globotruncanids) due to taphonomic preservation bias could result in underestimation of the degree/nature of faunal crisis and tempo of extinctions in the pre-extinction acidification interval.

Continue reading ‘Experimental validation of planktic foraminifera fragmentation index as a proxy for end-cretaceous ocean acidification’

Degradation of internal organic matter is the main control on pteropod shell dissolution after death

The potential for preservation of thecosome pteropods is thought to be largely governed by the chemical stability of their delicate aragonitic shells in seawater. However, sediment trap studies have found that significant carbonate dissolution can occur above the carbonate saturation horizon. Here we present the results from experiments conducted on two cruises to the Scotia Sea to directly test whether the breakdown of the organic pteropod body influences shell dissolution. We find that, on the timescales of three to thirteen days, the oxidation of organic matter within the shells of dead pteropods is a stronger driver of shell dissolution than the saturation state of seawater. Three to four days after death, shells became milky white and nano‐SEM images reveal smoothing of internal surface features and increased shell porosity, both indicative of aragonite dissolution. These findings have implications for the interpretation of the condition of pteropod shells from sediment traps and the fossil record, as well as for understanding the processes controlling particulate carbonate export from the surface ocean.

Continue reading ‘Degradation of internal organic matter is the main control on pteropod shell dissolution after death’

Sponge bioerosion versus aqueous pCO2: morphometric assessment of chips and etching fissures

Bioeroding sponges are important macroborers that chemically cut out substrate particles (chips) and mechanically remove them, thereby contributing to reef-associated sediment. These chemical and mechanical proportions vary with elevated levels of partial pressure of carbon dioxide (pCO2). To assess related impacts, the morphometric parameters “chip diameter” and “etching fissure width” were analyzed for Cliona orientalis Thiele, 1900, hypothesizing that their dimensions would differ with different pCO2 exposures (72 h at ca. 400, 750 and 1700 μatm). Under ambient conditions, we obtained a mean chip diameter of 21.6 ± 0.7 μm and a mean fissure width of 0.29 ± 0.01 μm. Chips were evenly distributed across the medium and coarse silt fractions regardless of treatment. We could not find a reliable pCO2 treatment effect for chip diameter and fissure width, but we observed strong data variability not related to our key questions. A hierarchical data design further reduced the test power. Fissure width was the more sensitive, but also more variable parameter. Sample size analyses nevertheless indicated that we had processed enough data. Thus, we reject our scenario of an increase in fissure width and consequent reduction in chip size to explain why chemical sponge bioerosion increases more strongly than the mechanical counterpart. Instead, we propose that a lowered ambient pH may favor respiratory acid build-up in the sponge tissue, possibly leading to a less localized bioerosion, causing bias towards more chemical bioerosion. Overall, this does not seem to affect the morphometry of sponge chips and the quality of sponge-generated sediment.

Continue reading ‘Sponge bioerosion versus aqueous pCO2: morphometric assessment of chips and etching fissures’


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