Posts Tagged 'corals'



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’

Influence of the seagrass Thalassia hemprichii on coral reef mesocosms exposed to ocean acidification and experimentally elevated temperatures

Highlights

• The combined effect of OA and rising temperatures stimulated the growth of macroalgae.

• OA resulted in higher coral calcification rates when corals were co-incubated with seagrass.

• Macroalgal growth was lower in seagrass-containing mesocosms.

• Coral and macroalgal, but not seagrass, growth suffered at 31°C under OA conditions.

• Seagrass helped to stabilize the system’s metabolism in response to projected climate change stressors.

Abstract

Ocean acidification (OA) and warming currently threaten coastal ecosystems across the globe. However, it is possible that the former process could actually benefit marine plants, such as seagrasses. The purpose of this study was to examine whether the effects of the seagrass Thalassia hemprichii can increase the resilience of OA-challenged coral reef mesocosms whose temperatures were gradually elevated. It was found that shoot density, photosynthetic efficiency, and leaf growth rate of the seagrass actually increased with rising temperatures under OA. Macroalgal growth rates were higher in the seagrass-free mesocosms, but the calcification rate of the model reef coral Pocillopora damicornis was higher in coral reef mesocosms featuring seagrasses under OA condition at 25 and 28°C. Both the macroalgal growth rate and the coral calcification rate decreased in all mesocosms when the temperature was raised to 31°C under OA conditions. However, the variation in gross primary production, ecosystem respiration, and net ecosystem production in the seagrass mesocosms was lower than in seagrass-free controls, suggesting that the presence of seagrass in the mesocosms helped to stabilize the metabolism of the system in response to simulated climate change.

Continue reading ‘Influence of the seagrass Thalassia hemprichii on coral reef mesocosms exposed to ocean acidification and experimentally elevated temperatures’

Coral reef monitoring, reef assessment technologies, and ecosystem-based management

Coral reefs are exceptionally biodiverse and human dependence on their ecosystem services is high. Reefs experience significant direct and indirect anthropogenic pressures, and provide a sensitive indicator of coastal ocean health, climate change, and ocean acidification, with associated implications for society. Monitoring coral reef status and trends is essential to better inform science, management and policy, but the projected collapse of reef systems within a few decades makes the provision of accurate and actionable monitoring data urgent. The Global Coral Reef Monitoring Network has been the foundation for global reporting on coral reefs for two decades, and is entering into a new phase with improved operational and data standards incorporating the Essential Ocean Variables (EOVs) (www.goosocean.org/eov) and Framework for Ocean Observing developed by the Global Ocean Observing System. Three EOVs provide a robust description of reef health: hard coral cover and composition, macro-algal canopy cover, and fish diversity and abundance. A data quality model based on comprehensive metadata has been designed to facilitate maximum global coverage of coral reef data, and tangible steps to track capacity building. Improved monitoring of events such as mass bleaching and disease outbreaks, citizen science, and socio-economic monitoring have the potential to greatly improve the relevance of monitoring to managers and stakeholders, and to address the complex and multi- dimensional interactions between reefs and people. A new generation of autonomous vehicles (underwater, surface, and aerial) and satellites are set to revolutionize and vastly expand our understanding of coral reefs. Promising approaches include Structure from Motion image processing, and acoustic techniques. Across all systems, curation of data in linked and open online databases, with an open data culture to maximize benefits from data integration, and empowering users to take action, are priorities. Action in the next decade will be essential to mitigate the impacts on coral reefs from warming temperatures, through local management and informing national and international obligations, particularly in the context of the Sustainable Development Goals, climate action, and the role of coral reefs as a global indicator. Mobilizing data to help drive the needed behavior change is a top priority for coral reef observing systems.

Continue reading ‘Coral reef monitoring, reef assessment technologies, and ecosystem-based management’

Living coral tissue slows skeletal dissolution related to ocean acidification

Climate change is causing major changes to marine ecosystems globally, with ocean acidification of particular concern for coral reefs. Using a 200 d in situ carbon dioxide enrichment study on Heron Island, Australia, we simulated future ocean acidification conditions, and found reduced pH led to a drastic decline in net calcification of living corals to no net growth, and accelerated disintegration of dead corals. Net calcification declined more severely than in previous studies due to exposure to the natural community of bioeroding organisms in this in situ study and to a longer experimental duration. Our data suggest that reef flat corals reach net dissolution at an aragonite saturation state (ΩAR) of 2.3 (95% confidence interval: 1.8–2.8) with 100% living coral cover and at ΩAR > 3.5 with 30% living coral cover. This model suggests that areas of the reef with relatively low coral mortality, where living coral cover is high, are likely to be resistant to carbon dioxide-induced reef dissolution.

Continue reading ‘Living coral tissue slows skeletal dissolution related to ocean acidification’

Environmental and biological controls on Na∕Ca ratios in scleractinian cold-water corals (update)

Here we present a comprehensive attempt to correlate aragonitic Na∕Ca ratios from Desmophyllum pertusum (formerly known as Lophelia pertusa), Madrepora oculata and a caryophylliid cold-water coral (CWC) species with different seawater parameters such as temperature, salinity and pH. Living CWC specimens were collected from 16 different locations and analyzed for their Na∕Ca ratios using solution-based inductively coupled plasma-optical emission spectrometry (ICP-OES) measurements.

The results reveal no apparent correlation with salinity (30.1–40.57 g kg−1) but a significant inverse correlation with temperature (0.31±0.04mmolmol1C1). Other marine aragonitic organisms such as Mytilus edulis (inner aragonitic shell portion) and Porites sp. exhibit similar results highlighting the consistency of the calculated CWC regressions. Corresponding Na∕Mg ratios show a similar temperature sensitivity to Na∕Ca ratios, but the combination of two ratios appears to reduce the impact of vital effects and domain-dependent geochemical variation. The high degree of scatter and elemental heterogeneities between the different skeletal features in both Na∕Ca and Na∕Mg, however, limit the use of these ratios as a proxy and/or make a high number of samples necessary. Additionally, we explore two models to explain the observed temperature sensitivity of Na∕Ca ratios for an open and semi-enclosed calcifying space based on temperature-sensitive Na- and Ca-pumping enzymes and transport proteins that change the composition of the calcifying fluid and consequently the skeletal Na∕Ca ratio.

Continue reading ‘Environmental and biological controls on Na∕Ca ratios in scleractinian cold-water corals (update)’

Flow-driven micro-scale pH variability affects the physiology of corals and coralline algae under ocean acidification

Natural variability in pH in the diffusive boundary layer (DBL), the discrete layer of seawater between bulk seawater and the outer surface of organisms, could be an important factor determining the response of corals and coralline algae to ocean acidification (OA). Here, two corals with different morphologies and one coralline alga were maintained under two different regimes of flow velocities, pH, and light intensities in a 12 flumes experimental system for a period of 27 weeks. We used a combination of geochemical proxies, physiological and micro-probe measurements to assess how these treatments affected the conditions in the DBL and the response of organisms to OA. Overall, low flow velocity did not ameliorate the negative effect of low pH and therefore did not provide a refugia from OA. Flow velocity had species-specific effects with positive effects on calcification for two species. pH in the calcifying fluid (pHcf) was reduced by low flow in both corals at low light only. pHcf was significantly impacted by pH in the DBL for the two species capable of significantly modifying pH in the DBL. The dissolved inorganic carbon in the calcifying fluid (DICcf) was highest under low pH for the corals and low flow for the coralline, while the saturation state in the calcifying fluid and its proxy (FWHM) were generally not affected by the treatments. This study therefore demonstrates that the effects of OA will manifest most severely in a combination of lower light and lower flow habitats for sub-tropical coralline algae. These effects will also be greatest in lower flow habitats for some corals. Together with existing literature, these findings reinforce that the effects of OA are highly context dependent, and will differ greatly between habitats, and depending on species composition.

Continue reading ‘Flow-driven micro-scale pH variability affects the physiology of corals and coralline algae under ocean acidification’


Subscribe to the RSS feed

Powered by FeedBurner

Follow AnneMarin on Twitter

Blog Stats

  • 1,306,930 hits

OA-ICC HIGHLIGHTS

Ocean acidification in the IPCC AR5 WG II

OUP book