Posts Tagged 'calcification'

Net heterotrophy and carbonate dissolution in two subtropical seagrass meadows

The net ecosystem productivity (NEP) of two seagrass meadows within one of the largest seagrass ecosystems in the world, Florida Bay, was assessed using direct measurements over consecutive diel cycles during a short study in the fall of 2018. We report significant differences between NEP determined by dissolved inorganic carbon (NEPDIC) and by dissolved oxygen (NEPDO), likely driven by differences in air–water gas exchange and contrasting responses to variations in light intensity. We also acknowledge the impact of advective exchange on metabolic calculations of NEP and net ecosystem calcification (NEC) using the “open-water” approach and attempt to quantify this effect. In this first direct determination of NEPDIC in seagrass, we found that both seagrass ecosystems were net heterotrophic, on average, despite large differences in seagrass net above-ground primary productivity. NEC was also negative, indicating that both sites were net dissolving carbonate minerals. We suggest that a combination of carbonate dissolution and respiration in sediments exceeded seagrass primary production and calcification, supporting our negative NEP and NEC measurements. However, given the limited spatial (two sites) and temporal (8 d) extent of this study, our results may not be representative of Florida Bay as a whole and may be season-specific. The results of this study highlight the need for better temporal resolution, accurate carbonate chemistry accounting, and an improved understanding of physical mixing processes in future seagrass metabolism studies.

Continue reading ‘Net heterotrophy and carbonate dissolution in two subtropical seagrass meadows’

Late Quaternary coccolith weight variations in the northern South China Sea and their environmental controls

Highlights

• High-resolution records of coccolith weights in the South China Sea.

• Different coccolithophore species show different variations in calcification.

•Coccolithophore calcification in nature is supposed to be affected by multiple factors.

Abstract

Coccolithophores are one of the most abundant and widespread groups of calcifying plankton and have attracted extensive study in terms of their likely response to ocean acidification. Conflicting results concerning coccolithophore calcification have been reported from both experimental and field studies. Due to their minute size, it is difficult to estimate the amount of calcite in coccoliths. Here we apply the SYRACO system to analyzing the weights and lengths of coccoliths produced by the dominant coccolithophore family Noëlaerhabdaceae. We obtain high-resolution coccolith weight and length records of GEO (Gephyrocapsa oceanica) and SPC (Emiliania huxleyi and small Gephyrocapsa spp.) groups from sediment core MD05-2904 in the northern South China Sea (SCS) over the past 200 kyr. A calcification index (CI) based on the coccolith weight and length is applied to evaluate the changes in coccolithophore calcification. The two groups of coccolith weights / CIs show different patterns on long term variations and during the last two terminations. We compare the coccolith weight and CI records with the environmental variables and carbonate chemistry parameters calculated in the same core. Our data reveals that sea surface temperature and insolation have weak correlations to coccolith weight and CI on long-term variations. The SPC weight / CI are correlated with the seawater pH and pCO2 variations while the GEO weight/ CI are more related to the nutrient variations. This imply a more significant role of ocean carbonate chemistry in the calcification of less calcified coccolithophores and nutrient concentration in the heavier calcifying coccolighophores.

Continue reading ‘Late Quaternary coccolith weight variations in the northern South China Sea and their environmental controls’

Independent effects of ocean warming versus acidification on the growth, survivorship and physiology of two Acropora corals

Climate change is the greatest threat to coral reef ecosystems. Importantly, gradual changes in seawater chemistry compounds upon increasing temperatures leading to declines in calcification and survivorship of reef-building corals. To assess relative versus synergistic effects of warming versus ocean acidification, Acropora muricata and Acropora hyacinthus were subjected to three temperature treatments (26 °C, 28.5 °C, 31 °C) crossed with three levels of pCO2 (410 μatm, 652 μatm, 934 μatm), representing current, mid and end-of-century scenarios for 12 weeks. Temperature increased gradually in the tanks from 26 °C to target temperatures over 5 weeks. Once stress was evident in the 31 °C (+ 2.5 °C above historical summer max) tanks, water temperature was decreased to normal summertime levels (29 °C) to assess recovery. pCO2 was gradually changed from control values (410 μatm) to target values over a 3 week period where they remained constant until the end of the experiment at 12 weeks. Temperature stress (31 °C) significantly impacted survivorship (90–95% decline), and over the long-term, there was a 50–90% decline in calcification across both coral species. Negative effects of mid and end-of-century pCO2 were largely independent of temperature and caused moderate reductions (36–74%) in calcification rates compared to temperature, over the long-term. Corals that survived temperature stress had higher lipid and protein content, showing that enhanced physiological condition provides an increased capacity to tolerate adverse temperatures. This study demonstrates that given the mortality rates in response to + 2.5 °C temperature stress, warming oceans (as opposed to ocean acidification) throughout the remainder of this century poses the greatest threat to reef-building corals.

Continue reading ‘Independent effects of ocean warming versus acidification on the growth, survivorship and physiology of two Acropora corals’

Intercomparison of four methods to estimate coral calcification under various environmental conditions

Coral reefs are constructed by calcifiers that precipitate calcium carbonate to build their shells or skeletons through the process of calcification. Accurately assessing coral calcification rates is crucial to determine the health of these ecosystems and their response to major environmental changes such as ocean warming and acidification. Several approaches have been used to assess rates of coral calcification but there is a real need to compare these approaches in order to ascertain that high quality and intercomparable results can be produced. Here, we assessed four methods (total alkalinity anomaly, calcium anomaly, 45Ca incorporation and 13C incorporation) to determine coral calcification of the reef-building coral Stylophora pistillata. Given the importance of environmental conditions on this process, the study was performed under two pH (ambient and low level) and two light (light and dark) conditions. Under all conditions, calcification rates estimated using the alkalinity and calcium anomaly techniques as well as 45Ca incorporation were highly correlated. Such a strong correlation between the alkalinity anomaly and 45Ca incorporation techniques has not been observed in previous studies and most probably results from improvements described in the present paper. The only method which provided calcification rates significantly different from the other three techniques was 13C incorporation. Calcification rates based on this method were consistently higher than those measured using the other techniques. Although reasons for these discrepancies remain unclear, the use of this technique for assessing calcification rates in corals is not recommended without further investigations.

Continue reading ‘Intercomparison of four methods to estimate coral calcification under various environmental conditions’

In situ growth and bioerosion rates of Lophelia pertusa in a Norwegian fjord and open shelf cold-water coral habitat

Coral reef resilience depends on the balance between carbonate precipitation, leading to reef growth, and carbonate degradation, for example, through bioerosion. Changes in environmental conditions are likely to affect the two processes differently, thereby shifting the balance between reef growth and degradation. In cold-water corals estimates of accretion-erosion processes in their natural habitat are scarce and solely live coral growth rates were studied with regard to future environmental changes in the laboratory so far, limiting our ability to assess the potential of cold-water coral reef ecosystems to cope with environmental changes. In the present study, growth rates of the two predominant colour morphotypes of live Lophelia pertusa as well as bioerosion rates of dead coral framework were assessed in different environmental settings in Norwegian cold-water coral reefs in a 1-year in situ experiment. Net growth (in weight gain and linear extension) of live L. pertusa was in the lower range of previous estimates and did not significantly differ between inshore (fjord) and offshore (open shelf) habitats. However, slightly higher net growth rates were obtained inshore. Bioerosion rates were significantly higher on-reef in the fjord compared to off-reef deployments in- and offshore. Besides, on-reef coral fragments yielded a broader range of individual growth and bioerosion rates, indicating higher turnover in live reef structures than off-reef with regard to accretion-bioerosion processes. Moreover, if the higher variation in growth rates represents a greater variance in (genetic) adaptations to natural environmental variability in the fjord, inshore reefs could possibly benefit under future ocean change compared to offshore reefs. Although not significantly different due to high variances between replicates, growth rates of orange branches were consistently higher at all sites, while mortality was statistically significantly lower, potentially indicating higher stress-resistance than the less pigmented white phenotype. Comparing the here measured rates of net accretion of live corals (regardless of colour morphotype) with net erosion of dead coral framework gives a first estimate of the dimensions of both processes in natural cold-water coral habitats, indicating that calcium carbonate loss through bioerosion amounts to one fifth to one sixth of the production rates by coral calcification (disregarding accretion processes of other organisms and proportion of live and dead coral framework in a reef). With regard to likely accelerating bioerosion and diminishing growth rates of corals under ocean acidification, the balance of reef accretion and degradation may be shifted towards higher biogenic dissolution in the future.

Continue reading ‘In situ growth and bioerosion rates of Lophelia pertusa in a Norwegian fjord and open shelf cold-water coral habitat’

Reef dissolution : rates and mechanisms of coral dissolution by bioeroding sponges and reef communities

For coral reefs to persist, the rate of CaCO3 production must be greater than the rate of erosion to enable positive growth. Negative impacts of global change (ocean acidification and warming) and local stressors (eutrophication, overfishing) on accretion co-occur with positive effects of these changes on bioerosion capacity and chemical dissolution by excavating euendolithic organisms. This is especially relevant for reefs characterised with low calcifying rates as they will tip faster into net loss. The Caribbean reefs suffered from a decrease by up to 80% in scleractinian coral cover in the past 50 years, their configuration bears very little resemblance with reefs pre1980s, in terms of benthic composition, coral cover and structural complexity. Specifically, excavating sponges can contribute up to 90% of the total macroborer activity on coral reefs and their rates of bioerosion are positively affected by pCO2. The overarching aim of this thesis was to quantify and understand the accretion and loss terms of coral reef communities with a focus on the interactions of anthropogenic ocean acidification and eutrophication with bioerosion by coral-excavating sponges.The use of incubations was central in this piece of work. Changes in the chemical composition of the water overlying sponges and reef communities indicate the relative contribution of metabolic processes such as net calcification/dissolution and net respiration/production. However, we first used fluorescence microscopy to investigate the underlying mechanisms of CaCO3 dissolution by excavating sponges. It revealed that they promote CaCO3 dissolution by decreasing pH at the sponge/coral interface. The high [H+] at this site is achieved through delivery of low-pH vesicles by the etching cells. The enzyme carbonic anhydrase, which is responsible for significantly increasing the speed of the reversible reaction H2O+CO2↔H++HCO3−, has been shown to be associated to the sponge’s etching processes and is therefore thought to play a role in the dissolution of CaCO3. By blocking its activity whilst incubating sponges and analysing the rate of dissolution, CA was found to play an important role in speeding up protonation of HCO3− ions at the dissolution site, enabling CO2 to diffuse out of the etching area. When exposed to different ranges of ocean acidification and eutrophication, bioerosion rates increased with both variables but no synergistic relation was revealed. Incubations performed at the community level around Saba and Curacao yielded net community calcification (NCC) rates which were lower than those reported for reef flats worldwide. Still, Saba coral reefs are considered relatively pristine sites compared to the average within the wider Caribbean. Around Curaçao, incubations on reef assemblages dominated by coral yielded even lower NCC rates. Incubations of other benthic assemblages that currently characterized shallow Caribbean reef substrate (such as bioeroding sponges, benthic cyanobacterial mats and sand) all resulted in net dissolution. For both Saba and Curaçao, results suggest that reef calcification on these sites is barely able to compensate the CaCO3 losses due to dissolution from other opportunistic benthic residents. With the ongoing global and local pressures, the delicate balance between CaCO3 accretion and loss is likely to tip.

Continue reading ‘Reef dissolution : rates and mechanisms of coral dissolution by bioeroding sponges and reef communities’

Low CO2 evasion rate from the mangrove surrounding waters of Sundarban

Globally, water bodies adjacent to mangroves are considered sources of atmospheric CO2. We directly measured the partial pressure of CO2 in water, pCO2(water), and other related biogeochemical parameters with very high (1-min) temporal resolution at Dhanchi Island in India’s Sundarbans during the post-monsoon season. We used elemental, stable isotopic, and optical signatures to investigate the sources of dissolved inorganic carbon (DIC) and organic matter (OM) in these waters. Diel mean pCO2(water) was marginally oversaturated in creeks (efflux, 69 ± 180 µmol m−2 h−1) and undersaturated along the island boundary and in the main river (influx, −17 ± 53 and −31 ± 73 µmol m−2 h−1, respectively) compared to the atmospheric CO2 concentration. The possibility in earlier studies of over- or underestimating the CO2 flux because of an inability to capture tidal minima and maxima was minimized in the present study, which confirmed that the waters surrounding mangroves in this region can act as a sink or a very weak source of atmospheric CO2. δ13C values for DIC suggest a mixed DIC source, and a three-end-member stable isotope mixing model and optical signatures of OM suggest negligible riverine contribution of freshwater to OM. We conclude that the CO2 sink or weak source character was due to a reduced input of riverine freshwater [which usually has high pCO2(water)] and the predominance of pCO2-lean water from the coastal sea, which eventually increases the buffering capacity of the water as evidenced by the Revelle factor. Up-scaling the CO2 flux data for all seasons and the entire estuary, we propose that the CO2 evasion rate observed in this study is much lower than the recently estimated world average. Mangrove areas having such low emissions should be given due emphasis when up-scaling the global mangrove carbon budget from regional observations.

Continue reading ‘Low CO2 evasion rate from the mangrove surrounding waters of Sundarban’


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Ocean acidification in the IPCC AR5 WG II

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