Posts Tagged 'communityMF'

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’

The combined effects of ocean acidification with fleshy macroalgae and filamentous turfs on tropical crustose coralline algae

Global climate change induces multiple stressors on tropical coral reefs that threaten their persistence. Ocean acidification decreases calcification in most dominant reef builders, such as crustose coralline algae (CCA). Climate change also has the potential to increase the biomass of fleshy macroalgae and filamentous turf in coral reef ecosystems. While fleshy macroalgae and turf may shade, abrade, and have otherwise negative consequences on CCA metabolism, their high rates of photosynthesis may mitigate OA locally through carbon uptake, resulting in a local increase in pH. This thesis explored the effects of OA, combined with the presence of either fleshy macroalgae or algal turfs, on Lithophyllum kotschyanum, an abundant species of CCA in Moorea, French Polynesia. In a mesocosm study, three canopy types, clear mimics, dark mimics, and S. pacificum, were crossed with two CO2 levels, ambient (400 μatm) and elevated (1000 μatm). The clear, dark, and S. pacificum canopies resulted in stepwise decreases in calcification of L. kotschyanum. This response suggests that shading and likely flow moderation decrease CCA calcification. To separate the effects of fleshy macroalgal metabolism from the effects of its physical structure, a subsequent mesocosm and field experiment were performed. In the mesocosm study, a header tank that provided S. pacificum-treated seawater to treatment tanks was used to determine the metabolic effect of S. pacificum on L. kotschyanum. In the field, S. pacificum canopies were attached to 20  30 cm grids, upstream from CCA samples. Data from the mesocosm study support a positive effect of carbon uptake by S. pacificum, but the metabolic effect did not translate into the field. Because S. pacificum was placed in closer proximity to CCA samples in the field than in lab, the difference in L. kotschyanum calcification between the mesocosm and field experiment may be due to physical effects of the canopy in the field, such as shading. The combined results of these two studies suggest that upstream macroalgal communities have the potential to mitigate the negative effects of OA to downstream calcifiers, but will not benefit understory calcifiers. Finally, a mesocosm experiment was conducted to address the combined effects of OA and the presence of epiphytic turf algae on host CCA. In a factorial experiment, L. kotschyanum samples with and without epiphytic turf algae were placed in flow through tanks where pCO2 was ambient (400 μatm) or elevated (1000 μatm). Results indicated a significant effect of elevated pCO2 on CCA calcification and a negative effect of turf presence, despite a higher pH in the presence of turf during light incubations. This indicates that any benefit of higher daytime pH within the DBL of L. kotschyanum was outweighed by the negative effects, such as shading, nighttime anoxia and low pH. Overall, these studies indicate that fleshy macroalgae and filamentous turfs can raise seawater pH locally, but any benefit of this effect is outweighed by the negative effects of fleshy macroalgae and turf presence. The only instance during which CCA may incur a net benefit from fleshy macroalgae occurs when calcifiers are situated downstream of a dense macroalgal community, entirely unaffected by its physical structure. Ultimately, fleshy macroalgae and turf affect CCA negatively, regardless of OA treatment.

Continue reading ‘The combined effects of ocean acidification with fleshy macroalgae and filamentous turfs on tropical crustose coralline algae’

Functional loss in herbivores drives runaway expansion of weedy algae in a near-future ocean

Highlights

• Elevated CO2 and warming increased productivity of turf algae.

• Elevated CO2 increased per capita feeding rates of gastropods.

• Ocean warming reduced grazer diversity, density, and biomass.

• As a result, ocean warming drove a fourfold expansion of weedy algal species.

Abstract

The ability of a community to absorb environmental change without undergoing structural modification is a hallmark of ecological resistance. The recognition that species interactions can stabilize community processes has led to the idea that the effects of climate change may be less than what most considerations currently allow. We tested whether herbivory can compensate for the expansion of weedy algae triggered by CO2 enrichment and warming. Using a six-month mesocosm experiment, we show that increasing per capita herbivory by gastropods absorbs the boosted effects of CO2 enrichment on algal production in temperate systems of weak to moderate herbivory. However, under the combined effects of acidification and warming this compensatory effect was eroded by reducing the diversity, density and biomass of herbivores. This loss of functionality combined with boosted primary productivity drove a fourfold expansion of weedy algal species. Our results demonstrate capacity to buffer ecosystems against CO2 enrichment, but loss of this capacity through ocean warming either in isolation or combined with CO2, driving significant algal turf expansion. Identifying compensatory processes and the circumstances under which they prevail could potentially help manage the impacts of ocean warming and acidification, which are further amplified by local disturbances such as habitat loss and herbivore over-exploitation.

Continue reading ‘Functional loss in herbivores drives runaway expansion of weedy algae in a near-future ocean’

The harmful algae, Cochlodinium polykrikoides and Aureococcus anophagefferens, elicit stronger transcriptomic and mortality response in larval bivalves (Argopecten irradians) than climate change stressors

Global ocean change threatens marine life, yet a mechanistic understanding of how organisms are affected by specific stressors is poorly understood. Here, we identify and compare the unique and common transcriptomic responses of an organism experiencing widespread fisheries declines, Argopecten irradians (bay scallop) exposed to multiple stressors including high pCO2, elevated temperature, and two species of harmful algae, Cochlodinium (aka Margalefidinium) polykrikoides and Aureococcus anophagefferens using high‐throughput sequencing (RNA‐seq). After 48 hr of exposure, scallop transcriptomes revealed distinct expression profiles with larvae exposed to harmful algae (C. polykrikoides and A. anophagefferens) displaying broader responses in terms of significantly and differentially expressed (DE) transcripts (44,922 and 4,973; respectively) than larvae exposed to low pH or elevated temperature (559 and 467; respectively). Patterns of expression between larvae exposed to each harmful algal treatment were, however, strikingly different with larvae exposed to A. anophagefferens displaying large, significant declines in the expression of transcripts (n = 3,615; 87% of DE transcripts) whereas exposure to C. polykrikoides increased the abundance of transcripts, more than all other treatments combined (n = 43,668; 97% of DE transcripts). Larvae exposed to each stressor up‐regulated a common set of 21 genes associated with protein synthesis, cellular metabolism, shell growth, and membrane transport. Larvae exposed to C. polykrikoides displayed large increases in antioxidant‐associated transcripts, whereas acidification‐exposed larvae increased abundance of transcripts associated with shell formation. After 10 days of exposure, each harmful algae caused declines in survival that were significantly greater than all other treatments. Collectively, this study reveals the common and unique transcriptional responses of bivalve larvae to stressors that promote population declines within coastal zones, providing insight into the means by which they promote mortality as well as traits possessed by bay scallops that enable potential resistance.

Continue reading ‘The harmful algae, Cochlodinium polykrikoides and Aureococcus anophagefferens, elicit stronger transcriptomic and mortality response in larval bivalves (Argopecten irradians) than climate change stressors’

Individual and population level effects of ocean acidification on a predator−prey system with inducible defenses: bryozoan−nudibranch interactions in the Salish Sea

Ocean acidification (OA) from in creased oceanic CO2 concentrations imposes significant physiological stresses on many calcifying organisms. OA effects on individual organisms may be synergistically amplified or reduced by inter- and intraspecies interactions as they propagate up to population and community
levels, altering predictions by studies of calcifier responses in isolation. The calcifying colonial bryozoan Membranipora membranacea and the predatory nudibranch Corambe steinbergae comprise a trophic system strongly regulated by predator induced defensive responses and space limitation, presenting a unique system to investigate OA effects on these regulatory mechanisms at individual and population levels. We experimentally quantified OA effects across a range of pH from 7.0 to 7.9 on growth, calcification, senescence and predator-induced spine formation in Membranipora, with or without waterborne predator cue, and on zooid consumption rates in Corambe at Friday Harbor Laboratories, San Juan Island, WA. Membranipora exhibited maximum growth and calcification at moderately low pH (7.6), and continued spine formation in all pH treatments.
Spines reduced Corambe zooid consumption rates, with lower pH weakening this effect. Using a spatially explicit model of colony growth, where colony area
serves as a proxy for colony fitness, we assessed the population-level impacts of these experimentally determined individual-level effects in the context of
space limitation. The area-based fitness costs associated with defense measured at the individual level led to amplified effects predicted for the population level due to competition. Our coupled experimental and modeling results demonstrate the need to consider population-level processes when assessing ecological responses to stresses from changing environments.

Continue reading ‘Individual and population level effects of ocean acidification on a predator−prey system with inducible defenses: bryozoan−nudibranch interactions in the Salish Sea’

The ability of macroalgae to mitigate the negative effects of ocean acidification on four species of North Atlantic bivalve (updated)

Coastal ecosystems can experience acidification via upwelling, eutrophication, riverine discharge, and climate change. While the resulting increases in pCO2 can have deleterious effects on calcifying animals, this change in carbonate chemistry may benefit some marine autotrophs. Here, we report on experiments performed with North Atlantic populations of hard clams (Mercenaria mercenaria), eastern oysters (Crassostrea virginica), bay scallops (Argopecten irradians), and blue mussels (Mytilus edulis) grown with and without North Atlantic populations of the green macroalgae, Ulva. In six of seven experiments, exposure to elevated pCO2 levels ( ∼ 1700µatm) resulted in depressed shell- and/or tissue-based growth rates of bivalves compared to control conditions, whereas rates were significantly higher in the presence of Ulva in all experiments. In many cases, the co-exposure to elevated pCO2 levels and Ulva had an antagonistic effect on bivalve growth rates whereby the presence of Ulva under elevated pCO2 levels significantly improved their performance compared to the acidification-only treatment. Saturation states for calcium carbonate (Ω) were significantly higher in the presence of Ulva under both ambient and elevated CO2 delivery rates, and growth rates of bivalves were significantly correlated with Ω in six of seven experiments. Collectively, the results suggest that photosynthesis and/or nitrate assimilation by Ulva increased alkalinity, fostering a carbonate chemistry regime more suitable for optimal growth of calcifying bivalves. This suggests that large natural and/or aquacultured collections of macroalgae in acidified environments could serve as a refuge for calcifying animals that may otherwise be negatively impacted by elevated pCO2 levels and depressed Ω.

Continue reading ‘The ability of macroalgae to mitigate the negative effects of ocean acidification on four species of North Atlantic bivalve (updated)’


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Ocean acidification in the IPCC AR5 WG II

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