Conservation Measures and using the Fates of Past Reefs to project Future Scenarios

by Dawn Barlow

Kennedy et al. (2013) examine the structural and ecological human-caused damage on coral reefs in the Caribbean since the 1960s and then generate modeled predictions for what coral reefs in the Caribbean might look like between now and the year 2080 if practices of local conservation and global action were to be implemented. They examine driving ecological factors of Caribbean reefs, reconstructing past disturbances to address the important roles that all of the many pieces play in contributing to overall health of the reefs. Some of these disturbances between the 1960s and the 2000s include depletion of the reef due to overfishing, loss of branching corals because of disease, bleaching, and bioerosion, hyperabundance of urchins because their predators were lost to overfishing, loss of urchins because of disease, poor watershed management that has led to changes in nutrient abundance, and ongoing climate change since the 1960s. In order for a coral reef to maintain its structure, the rate at which carbonate is produced must be greater than the rate of erosion—the carbonate budget must be positive. In order to maintain a carbonate budget that is either positive or at equilibrium, Kennedy et al. suggest that local management for the protection of grazers such as parrotfishes is important and can create a positive carbonate budget for reefs starting out with higher coral cover and keep reefs with lower coral cover to begin with near equilibrium, at least in the short term. In the long term however, aggressive mitigations will need to take effect on a global scale for there to be hope of maintaining a carbonate budget near equilibrium. Continue reading

Can Corals Acclimate to Large Temperature Changes?

by Dawn Barlow

Over just the past few decades ocean temperature has contributed to significant losses in global coral cover, and the extent to which corals can undergo physiological acclimatization or genetic adaptation to thermal changes remains uncertain. However, this information will be crucial for the effectiveness of conservation strategies and accuracy of projections of reef futures. This study conducted by Howells et al. (2013) investigates the potential for corals to acclimatize to temperatures that exceed historical thermal regimes. This is done by investigating several parameters—bleaching, mortality, Symbiodinium type fidelity, and reproductive timing—in coral colonies that have been transplanted between warm central regions and cool southern regions of the Great Barrier Reef for a period of 14 months. Continue reading

Comparing the Near-Future Effect of Temperature and Acidification on Early Life History Stages of Corals

by Dawn Barlow

Both ocean temperatures and pH are projected to increase due to climate change in the near future—it is predicted that temperatures will be raised by 2°C and that acidity will increase by ~0.2 pH units by the end of the century. While much investigation has been done on the effects of temperature and acidity on the ability of adult corals to form the structure necessary to maintain the integrity of the reef, Chua et al. (2013) investigated the direct effects of increased temperature and acidity on the early life history stages of corals. They looked at fertilization, development, survivorship, and metamorphosis of coral larvae under control conditions as well as under elevated temperature and acidity, both separately and in combination. When the two factors were combined, the results were inconsistent. Overall, the conclusion drawn from this study was that acidification alone is unlikely to be a direct threat to early life history stages of corals, at least in the near future. Increasing temperature, on the other hand, was found to increase the rate of larval development and thereby affect coral population dynamics by changing patterns of connectivity. Continue reading

Seawater Acidity Reduces the Physiological Ability of Corals to Calcify

by Dawn Barlow

This study by Venn et al. (2013) addresses how ocean acidification reduces the calcification rate of corals by reducing internal pH at the calcifying tissue-skeleton interface. They aim to predict how corals will respond and potentially acclimate to ocean acidification by looking at how acidification impacts the physiological mechanisms that drive calcification itself. Coral skeletons are formed from calcium carbonate crystals (aragonite), produced in the fluid-filled subcalcioblastic medium (SCM), which underlies the calcifying tissue. The calcifying tissue elevates pH the SCM relative to the pH of the exterior seawater, favoring the conversion of bicarbonate to carbonate, and enhancing precipitation at the site of calcification. This ability of corals to regulate internal pH is anticipated to be critical in their resilience to ocean acidification, and overall, findings from this study suggest that reef corals may be able to mitigate the effects of seawater acidification by regulating pH in the SCM. Continue reading

The Other Ocean Acidification Problem: CO2 as a Resource

by Dawn Barlow

This study addresses the effects of enhanced CO2 levels in the ocean by looking at how increased acidity might indirectly cause phase shifts in community structure of coral reef and kelp forest ecosystems in temperate and tropical waters. Under elevated acidity and temperature conditions, productivity of certain photosynthetic organisms such as mat-forming algae (low-profile ground-covering macroalgal and turf communities) can increase, making CO2 not only a direct stressor but also an indirect stressor by being a resource for certain competitive organisms, creating enormous potential for shifts in species dominance. Additionally, ocean acidification acts together with other environmental stressors and primary consumers, and these factors also influence community response to acidic conditions. Connell et al. (2013) investigate the prevalence of mat-forming algae in three different scenarios where CO2 levels were either ambient or elevated: in the laboratory, in mesocosms in the field, and at naturally occurring CO2 vents that locally alter the seawater chemistry. They find that in all the scenarios, the algae mats respond positively to the elevated conditions, increasing growth rate and cover to so that the algae became a majority space holder regardless of any herbivory. This is likely because the new environmental conditions favor species with fast growth and colonization rates and short generation times, and these are the species that are capable of completely… Continue reading

Heavy Metal Contamination from Domestic Wastewater in Tropical Lagoon Sediments

 

This study by Fujita et al. (2014) investigates the contamination of heavy metals in the sediments of a tropical lagoon in Funafuti Atoll, Tuvalu. Sediment samples were analyzed from densely populated, sparsely populated, open dumping, and undisturbed sites along the islet. The highest concentrations of heavy metals and acid-volatile sulfates were found at the densely populated sites, where poorly constructed sanitary facilities are the cause of marine pollution due to domestic wastewater leakage. The currents were examined in order to see if they might have a role in the dispersal of heavy metals in the lagoon. Sediment samples were analyzed to measure acid-volatile sulfides, contamination factor, and pollution load index, and were analyzed using principal components analysis. Results showed that various types of grey wastewater were the primary source of contamination by heavy metals such as chromium, zinc, copper, lead, and cadmium. Manganese and nickel were found in higher concentrations at the open dumping site, supporting the idea that these contaminants are likely a result of disposed batteries. The prevalence of sanitary facilities and dumping sites such as the ones found on Funafuti across atolls in the Central and South Pacific poses significant ecological threats to the coastal sediment and coral reef environments that they support, as well as threats to human health and the availability of food resources.–Submitted by Dawn Barlow

Fujita, M., Ide, Y., Sato, D., Kench, P.S., Kuwahara, Y., Yokoki, H., Kayanne, H., 2014. Heavy metal contamination of coastal lagoon sediments: Fongafale Islet, Funafuti Atoll, Tuvalu. Chemosphere 95, 628 –634. Continue reading

Recovery of an Isolated Coral Reef System Following Severe Disturbance

This study looks at the potential for isolated coral reef ecosystems to recover after severe disturbance, in this case ocean warming events. The study system that is examined by Gilmour et al. is Scott Reef, an oceanic reef at the edge of Western Australia’s continental shelf. Because Scott reef is so far removed, it means that there is a lack of connectivity that makes it far more susceptible to disturbances because recruits cannot easily come from elsewhere. Additionally, this isolation means that there is negligible human activity that could stress the system and inhibit its recovery. In 1998 there was a catastrophic bleaching event caused by a sudden extreme warming in sea surface temperatures, which led to a mass coral die-off in regions across the Pacific. It was not expected that the Scott Reef system would be able to return to pre-disturbance conditions, and if it did recover the projections were that it would be decades before it reached anything comparable because all recruits would have to be locally produced—none could come from outside. However, since the area is not fished there are large populations of herbivorous fish to keep down the algae that would otherwise take over a disturbed reef and inhibit recruitment. Within twelve years of the bleaching event, Scott Reef was able to recover to a point where reproductive output and recruitment were comparable to pre-disturbance levels.  Dawn Barlow

Gilmour, J.P., Smith, L.D., Heyward, A.J., Baird, A.H., Pratchett, M.S., 2013. Recovery of an isolated coral reef system following severe disturbance. Science 340, 69-71. 

                  In this study, Gilmour et al. did not manipulate pieces of the ecosystem. Instead, they used an existing system for which they had data for coral abundance and recruitment rates as well as ocean temperatures, and then monitored these same things during and after the mass bleaching event. They surveyed permanent transect lines on the reef, which they categorized as either reef crest (~3m), reef slope (~9m), and upper reef slope (~6m), depending on the depth of the coral. Recruitment was monitored using settlement tiles, which were deployed at regular intervals along the transect line. Thus they were able to observe the correlation of these factors and influences as the reef recovered after severe disturbance.
                  In February of 1998, seawater temperatures at Scott Reef rose to an unheard of 13.3°C and remained at extreme high temperatures for the next two months. In the next six months, 80-90% of live coral cover was lost from the reef crest and reef slope, and nearly 70% was lost from the upper reef slope. Recruit numbers decreased to zero from the previous years’ 2600—5600, and in the following six years recruitment rates were less than 6% of years prior to the disturbance event.
                  In other systems that have been studied following severe disturbance, what usually takes place is a phase shift to macroalgae which outcompetes the corals so that there is no longer a substrate on which recruits can settle. But what happened at Scott Reef was that the substrata that were made available by the dead corals were colonized by fine turfing and crustose coralline algae. There was also an observed increase in the densities of herbivorous fishes following the event, and the inability of macroalgae to colonize the dead coral was very likely due to the dramatic increase in grazing capacity by large herbivorous fishes. This surplus in grazing capacity in the Scott Reef system was able to assist subsequent coral recruitment, allowing for more coral to be locally produced as well as giving them a place to settle and therefore increasing overall survival. On reefs experiencing chronic pressures from human impact, the mean survival of recruits tends to average around 50%. On Scott Reef during this recovery period however, mean survival of recruits ranged from 83—93%. Within a decade of the bleaching, reproductive output and recruitment were similar to the levels prior to the bleaching, and after 12 years the overall health of the reef was determined to be comparable to the time before the disturbance.

                  This study shows that anthropogenic pressures, particularly fishing, are dramatically inhibiting coral reef recovery after disturbance if a system as isolated as Scott Reef was able to recover from 90% coral loss without recruits from other populations within a decade, simply because of the absence of human activity. This observational study demonstrated the importance of having an intact ecosystem—the fact that the herbivorous fish had not all been caught—and the importance of limiting anthropogenic stressors if we aim to protect the reefs from further degradation. The authors of this study suggest that creating marine protected areas could be a key to promoting coral reef resilience through protecting healthy, intact ecosystems.

Ocean Acidification and Warming Scenarios Increase Microbioerosion of Coral Skeletons

Microbioerosion in corals is a process that is caused by chemical dissolution and driven by metabolic activities of internal microborers within coral skeletons. This study hypothesizes that the increase in temperature and acidity of seawater that is projected to take place in the coming decades is will reduce calcification as well as increase dissolution in corals and crustose coralline algae because it will alter microbioerosion processes on coral skeletons. Reyes-Nivia et al. compared current ocean conditions with two elevated CO2-temperature scenarios to explore how skeletons of branching and encrusting corals respond combined acidification and warming, and to see if these elevated scenarios would alter the microbioerosion processes. They measured the rate of calcification under these varying conditions, and found that dissolution of coral skeletons was driven predominantly by photosynthetic emdolithic algae microborers, and that this was increased with combined acidity and warming. They also found that that the projected acidification and warming scenarios appear to favor the accumulation of endolithic algae, which would lead to significant bioerosion within coral reef frameworks.—Dawn Barlow
ReyesNivia, C., DiazPulido, G., Kline, D., Guldberg, O. H., & Dove, S., 2013. Ocean acidification and warming scenarios increase microbioerosion of coral skeletons. Global change biology 19, 1919-1921

                  Reyes-Nivia et al. conducted this experiment in the lab using a system of custom software that allowed for CO2 and temperature to follow seasonally appropriate fluctuations measured at a reference field site, providing a near-perfect match to the control conditions. In the lab, three different acidity-temperature scenarios were created by bubbling different amounts of CO2 into tanks that were maintained at different temperatures using industrial heater-chillers. As the aim of this study was to examine the activity and abundance of microborers on coral skeletons in these different scenarios, samples of a branching (Porites cylindrica) and an encrusting (Isopora cuneata) coral harboring high amounts of endolithic algae were collected and the coral tissue was removed from the skeletons to simulate recently dead coral substrates before they were placed in the different tanks. A subset of these samples was kept in the dark so as to inhibit the reestablishment of these endolithic algae which require sunlight to thrive. The pH was measured in the water as well as within the coral skeleton through drilled holes throughout the study. Total calcification and bioerosion was measured as change in the amount of calcium carbonate using the buoyant weight method, which was also used to measure monthly microbioerosion rates normalized to the total surface area. The loss-on-ignition method was used to calculate the biomass of the endolithic algae at the end of the duration of the experiment.
                  What Reyes-Nivia et al. found was that, under natural day-night cycles, all of the coral skeletons with endolithic microalgae lost calcium carbonate. Under elevated CO-temperature conditions, the total microbioerosion increased in both types of coral. For the coral skeletons under full dark conditions—those without photosynthetic endolithic algae—there was no sign of dissolution; in fact there was a positive increase in buoyant weight.
                  This study showed that the dissolution of coral skeletons, which is driven predominantly by photosynthetic microborers, increased under combined ocean acidification-warming scenarios, and that this dissolution varied with the specific scenario and the coral substrate. This variance shows that the skeletal structure of the coral—microskeletal architecture, porosity, density, minerology—plays an important role in determining the magnitude of how the acidity-warming scenario will affect coral skeletal dissolution. The projected acidification and warming scenarios appear to heavily favor biomass accumulation of photosynthetic microalgae, which in turn would mean significant bioerosion of coral reef frameworks.

                  The implications of this study are that the conditions within projected future oceans appear to influence both the biological and ecological responses of endolithic microborers, which lead to the dissolution of coral skeletons. This is demonstrated by the enhanced biomass, shifts in community structure, and increased respiration rates of endolithic algae under elevated CO2-temperature conditions. This implies that if the oceans continue to acidify as predicted, not only will the corals not be able to calcify as well in more acidic waters, but the increased abundance of photosynthetic endolithic algae will contribute simultaneously to their dissolution

Coral Reef Calcifiers Buffer their Responses to Ocean Acidification Using Both Bicarbonate and Carbonate

The highest known marine biodiversity is hosted by the calcium carbonate framework of corals and calcifying algae that form rich and dynamic coral reefs. Coral reefs are threatened by ocean acidification caused by an increase in dissolved carbon dioxide in the water, and this in turn decreases carbonate ion concentrations and increases bicarbonate ion concentrations. In this study, Comeau et al. evaluated the roles of carbonate and bicarbonate in the calcification of coral and crustose coralline algae in both light and dark conditions. They found that coral can maintain present-day calcification rates in the light if the decrease in carbonate concentrations is compensated for by an increase in bicarbonate concentration but this was not sufficient for calcification in the dark. Though crustose coralline algae were able to calcify using bicarbonate, it was not sufficient to compensate for the decrease in carbonate that is a result of ocean acidification. The results of this study show that the response of tropical coral reef communities to ocean acidification might be less dramatic than previously predicted due to the ability of calcifying organisms to utilize bicarbonate while it is light, but the decrease in the ability to calcify in the dark will undoubtedly result in an overall reduction in the calcifying ability of coral reefs in increasingly acidic oceans.—Dawn Barlow
                  Comeau, S., Carpenter, R., Edmunds, P., 2013. Coral reef calcifiers buffer their response to ocean acidification using both bicarbonate and carbonate. Proceedings of the Royal Society B: Biological Sciences 280, 20122374

                 
This experiment conducted by Comeau et al. was unique in that instead of simply increasing the acidity of the water that the calcifying organisms were in, they addressed the effects of varying concentrations of carbonate and bicarbonate ions. The carbonate chemistry of the water used in this lab experiment was manipulated using carbon dioxide-equilibrated air, and varying carbon dioxide treatments were created by bubbling ambient air, carbon dioxide-enriched air, and carbon dioxide-depleted air into tanks of seawater. One coral species, Porites rus, and one crustose coralline algae species, Hydrolithon onkodes, were used as study species of calcifying organisms for this experiment. Buoyant weight measurements were used to evaluate calcification over the two-week period in which this study was conducted, and the alkalinity anomaly technique was used to examine short-term calcification in light and dark conditions.
The methods used by Comeau et al. allowed them to examine the relative concentrations of carbonate and bicarbonate and the calcifying ability of coral and crustose coralline algae in both light and dark. They found that for coral, calcification in the light was affected by both carbonate and bicarbonate concentrations, and in the dark by carbonate but not bicarbonate. For crustose coralline algae, light and dark calcification were affected by both carbonate and bicarbonate concentrations. The implications of the fact that the coral was better able to calcify in the presence of bicarbonate in the light are consistent with other studies, which couple calcification with photosynthesis and demonstrate that photosynthesis can, under some conditions, be stimulated by additions of bicarbonate. Carbonate is important for calcification, and bicarbonate is important for both calcification and photosynthesis. However, the way that ocean acidification works is that increasing bicarbonate means decreasing carbonate, and so the fact that bicarbonate increases calcification in the light is offset by the fact that overall calcification is decreasing due to the declining amount of carbonate ions in the water.

These results provide more insight into the rate at which reef-building organisms can calcify in increasingly acidic ocean waters. The fact that there appears to be a positive correlation between bicarbonate and calcification in the light shows that the future ability of coral reefs to calcify may not be quite as dire as previously thought. But ultimately the decreasing concentrations of carbonate that come with increased acidity mean that the increased ability to calcify in the light will be offset and the reef-building organisms will struggle to calcify as ocean acidification continues to be an issue in the future.