Ocean acidification has the potential to significantly impact both aquaculture and wild-caught mollusk fisheries around the world. In this work, we build upon a previously published integrated assessment model of the US Atlantic Sea Scallop (Placopecten magellanicus) fishery to determine the possible future of the fishery under a suite of climate, economic, biological, and management scenarios. We developed a 4x4x4x4 hypercube scenario framework that resulted in 256 possible combinations of future scenarios. The study highlights the potential impacts of ocean acidification and management for a subset of future climate scenarios, with a high CO2 emissions case (RCP8.5) and lower CO2 emissions and climate mitigation case (RCP4.5). Under RCP4.5 and the highest impact and management scenario, ocean acidification has the potential to reduce sea scallop biomass by approximately 13% by the end of century; however, the lesser impact scenarios cause very little change. Under RCP8.5, sea scallop biomass may decline by more than 50% by the end of century, leading to subsequent declines in industry landings and revenue. Management-set catch limits improve the outcomes of the fishery under both climate scenarios, and the addition of a 10% area closure increases future biomass by more than 25% under the highest ocean acidification impacts. However, increased management still does not stop the projected long-term decline of the fishery under ocean acidification scenarios. Given our incomplete understanding of acidification impacts on P. magellanicus, these declines, along with the high value of the industry, suggest population-level effects of acidification should be a clear research priority. Projections described in this manuscript illustrate both the potential impacts of ocean acidification under a business-as-usual and a moderately strong climate-policy scenario. We also illustrate the importance of fisheries management targets in improving the long-term outcome of the P. magellanicus fishery under potential global change.
Posts Tagged 'individualmodeling'
Projected impacts of future climate change, ocean acidification, and management on the US Atlantic sea scallop (Placopecten magellanicus) fishery
Published 25 September 2018 Science ClosedTags: fisheries, individualmodeling, mollusks, socio-economy
Quantitative interpretation of vertical profiles of calcium and pH in the coral coelenteron
Published 11 July 2018 Science ClosedTags: biological response, calcification, corals, individualmodeling, laboratory, modeling
Highlights
- • In this study, pH and Ca2+ microsensors were reported together with a theoretical analysis by a reaction-diffusion model to study the dynamics of pH and Ca2+ in the coelenteron of the reef corals Turbinaria reniformis and Acropora millepora.
- • Our study showed that Ca2+ concentrations linearly decreased from the mouth to the base of the coelenteron due to calcification.
- • The estimated H+ gradient between the coelenteron cavity and the calcification site was >10 times higher than previously predicted between outside seawater and the calcification site.
- • Our numerical simulation reveals that OA reduces the internal pH at the base of the coelenteron, and this pH decline is greatly amplified in corals with a deeper coelenteron.
Abstract
Scleratinian corals (hard corals) and their symbiotic algae are the ecological engineers of biodiverse and geological important coral reef habitats. The complex, linked physiological processes that enable the holobiont (coral + algae) to calcify and generate reef structures are consequently of great interest. However, the mechanism of calcification is difficult to study for several reasons including the small spatial scales of the processes and the close coupling between the symbiont and host. In this study, we explore the utility of pH and Ca2+ microelectrodes for constraining the rates and spatial distribution of photosynthesis, respiration, and calcification. The work focuses on vertical profiles of pH and Ca2+ through the coelenteron cavity, a semi-isolated compartment of modified seawater amenable to quantitative interpretation. In two studied species, Turbinaria reniformis and Acropora millepora, Ca2+ concentrations decreased in a roughly linear manner from the mouth to the base of the coelenteron, indicating the primary physiological process affecting Ca2+ concentration is removal for calcification below the coelenteron. In contrast, the H+ concentration remained relatively constant over much of the coelenteron cavity before it increased sharply toward the base of the coelenteron, indicative of proton-pumping from the calcification fluid below. The estimated H+ gradient between the coelenteron cavity and the calcification site was >10 times higher than previously predicted. Consequently, the energy required to export protons from the calcifying fluid was estimated to be ~3 times higher than previously calculated. A one-dimensional reaction-diffusion model was used to interpret the pH profile considering the effects of photosynthesis, respiration, and calcification. This model provided a good fit to the observed pH profile and helped to constrain the rates and spatial distribution of these processes. Our modeling results also suggested that adult corals with deeper polyps may be more sensitive to ocean acidification (OA) because of enhanced difficulty to transport H+ out of the coelenteron and into the surrounding seawater.
Inorganic carbon and pH dependency of Trichodesmium’s photosynthetic rates
Published 18 April 2018 Science ClosedTags: biological response, individualmodeling, laboratory, modeling, photosynthesis, prokaryotes
We established the relationship between photosynthetic carbon fixation rates and pH, CO2 and HCO3– concentrations in the diazotroph Trichodesmium erythraeum IMS101. Inorganic 14C-assimilation was measured in TRIS-buffered ASW medium where the absolute and relative concentrations of CO2, pH and HCO3– were manipulated. First, we varied the total dissolved inorganic carbon concentration (TIC) (< 0 to ~ 5 mM) at constant pH, so ratios of CO2 and HCO3– remained relatively constant. Second, we varied pH (~ 8.54 to 7.52) at constant TIC, so CO2 increased whilst HCO3– declined. We found that 14C-assimilation could be described by the same function of CO2 for both approaches but showed different dependencies on HCO3– when pH was varied at constant TIC than when TIC was varied at constant pH. A numerical model of Trichodesmium’s CCM showed carboxylation rates are modulated by HCO3– and pH. The decrease in Ci assimilation at low CO2, when TIC was varied, is due to HCO3– uptake limitation of the carboxylation rate. Conversely, when pH was varied, Ci assimilation declined due to a high-pH mediated increase in HCO3– and CO2 leakage rates, potentially coupled to other processes (uncharacterised within the CCM model) that restrict Ci assimilation rates under high-pH conditions.
Continue reading ‘Inorganic carbon and pH dependency of Trichodesmium’s photosynthetic rates’
Probabilistic risk assessment of the effect of acidified seawater on development stages of sea urchin (Strongylocentrotus droebachiensis)
Published 2 March 2018 Science ClosedTags: biological response, chemistry, echinoderms, individualmodeling, modeling, morphology, mortality, reproduction
Growing evidence indicates that ocean acidification has a significant impact on calcifying marine organisms. However, there is a lack of exposure risk assessments for aquatic organisms under future environmentally relevant ocean acidification scenarios. The objective of this study was to investigate the probabilistic effects of acidified seawater on the life-stage response dynamics of fertilization, larvae growth, and larvae mortality of the green sea urchin (Strongylocentrotus droebachiensis). We incorporated the regulation of primary body cavity (PBC) pH in response to seawater pH into the assessment by constructing an explicit model to assess effective life-stage response dynamics to seawater or PBC pH levels. The likelihood of exposure to ocean acidification was also evaluated by addressing the uncertainties of the risk characterization. For unsuccessful fertilization, the estimated 50% effect level of seawater acidification (EC50 SW ) was 0.55 ± 0.014 (mean ± SE) pH units. This life stage was more sensitive than growth inhibition and mortality, for which the EC50 values were 1.13 and 1.03 pH units, respectively. The estimated 50% effect levels of PBC pH (EC50 PBC ) were 0.99 ± 0.05 and 0.88 ± 0.006 pH units for growth inhibition and mortality, respectively. We also predicted the probability distributions for seawater and PBC pH levels in 2100. The level of unsuccessful fertilization had 50 and 90% probability risks of 5.07–24.51 (95% CI) and 0–6.95%, respectively. We conclude that this probabilistic risk analysis model is parsimonious enough to quantify the multiple vulnerabilities of the green sea urchin while addressing the systemic effects of ocean acidification. This study found a high potential risk of acidification affecting the fertilization of the green sea urchin, whereas there was no evidence for adverse effects on growth and mortality resulting from exposure to the predicted acidified environment.
Numerical modelling of physiological and ecological impacts of ocean acidification on coccolithophores
Published 26 February 2018 Science ClosedTags: biological response, calcification, communitymodeling, dissolution, individualmodeling, modeling, photosynthesis, phytoplankton
Highlights
• Intracellular biochemical-kinetic processes of coccolithophore were modelled.
• The model showed that the calcification is promoted under an acidified environment.
• Calcite remains at saturation levels in a coccolith even when it is below saturation.
• A coccolith can dissolve even in water where calcite saturation exceeds 1.
• The behavior of coccolithophores in the ecosystem were also simulated numerically.
• High CO2 concentration could lead to an increase of coccolithophores to diatoms.
Abstract
Ocean surface acidification due to increasing atmospheric CO2 concentration is currently attracting much attention. Coccolithophores distribute widely across the world’s oceans and represent a carbon sink containing about 100 million tonnes of carbon. For this reason, there is concern about dissolution of their shells, which are made of calcium carbonate, due to decreasing pH. In this study, intracellular calcification, photosynthesis, and mass transport through biomembranes of Emiliania huxleyi were modelled numerically for understanding biological response in calcifying organisms. Unknown parameters were optimised by a generic algorithm to match existing experimental results. The model showed that the production of calcium carbonate rather than its dissolution is promoted under an acidified environment. Calcite remains at saturation levels in a coccolith even when it is below saturation levels in the external seawater. Furthermore, a coccolith can dissolve even in water where calcite saturation exceeds 1, because the saturation may be below the threshold level locally around the cell membrane. The present model also showed that the different calcification rates of E. huxleyi with respect to rising CO2 concentrations reported in the literature are due to differences in experimental conditions; in particular, how the CO2 concentration is matched. Lastly, the model was able to reproduce differences in calcification rates among coccolithophore species. The above biochemical-kinetic model was then incorporated into an ecosystem model, and the behaviour of coccolithophores in the ecosystem and the influence of increases in CO2 concentration on water quality were simulated and validated by comparison with existing experimental results. The model also suggests that increased CO2 concentration could lead to an increase in the biomass ratio of coccolithophores to diatoms at high CO2 concentrations, particularly in oligotrophic environments, and to a consequent decrease in pH due to calcium dissolution.
Ocean acidification affects coral growth by reducing skeletal density
Published 30 January 2018 Science ClosedTags: biological response, corals, field, growth, individualmodeling, modeling, morphology, North Pacific, South Pacific
Significance
Ocean acidification (OA) threatens coral reef futures by reducing the concentration of carbonate ions that corals need to construct their skeletons. However, quantitative predictions of reef futures under OA are confounded by mixed responses of corals to OA in experiments and field observations. We modeled the skeletal growth of a dominant reef-building coral, Porites, as a function of seawater chemistry and validated the model against observational data. We show that OA directly and negatively affects one component of the two-step growth process (density) but not the other (linear extension). Combining our growth model with Global Climate Model output, we show that skeletal density of Porites corals could decline by up to 20.3% over the 21st century solely due to OA.
Abstract
Ocean acidification (OA) is considered an important threat to coral reef ecosystems, because it reduces the availability of carbonate ions that reef-building corals need to produce their skeletons. However, while theory predicts that coral calcification rates decline as carbonate ion concentrations decrease, this prediction is not consistently borne out in laboratory manipulation experiments or in studies of corals inhabiting naturally low-pH reefs today. The skeletal growth of corals consists of two distinct processes: extension (upward growth) and densification (lateral thickening). Here, we show that skeletal density is directly sensitive to changes in seawater carbonate ion concentration and thus, to OA, whereas extension is not. We present a numerical model of Porites skeletal growth that links skeletal density with the external seawater environment via its influence on the chemistry of coral calcifying fluid. We validate the model using existing coral skeletal datasets from six Porites species collected across five reef sites and use this framework to project the impact of 21st century OA on Porites skeletal density across the global tropics. Our model predicts that OA alone will drive up to 20.3 ± 5.4% decline in the skeletal density of reef-building Porites corals.
Continue reading ‘Ocean acidification affects coral growth by reducing skeletal density’
Functional spatial contextualisation of the effects of multiple stressors in marine bivalves
Published 23 January 2018 Science ClosedTags: biological response, individualmodeling, laboratory, modeling, mollusks, morphology, multiple factors, oxygen, performance, physiology, reproduction
Many recent studies have revealed that the majority of environmental stressors experienced by marine organisms (ocean acidification, global warming, hypoxia etc.) occur at the same time and place, and that their interaction may complexly affect a number of ecological processes. Here, we experimentally investigated the effects of pH and hypoxia on the functional and behavioural traits of the mussel Mytilus galloprovincialis, we then simulated the potential effects on growth and reproduction dynamics trough a Dynamic Energy Budget (DEB) model under a multiple stressor scenario. Our simulations showed that hypercapnia had a remarkable effect by reducing the maximal habitat size and reproductive output differentially as a function of the trophic conditions, where modelling was spatially contextualized. This study showed the major threat represented by the hypercapnia and hypoxia phenomena for the growth, reproduction and fitness of mussels under the current climate change context, and that a mechanistic approach based on DEB modelling can illustrate complex and site-specific effects of environmental change, producing that kind of information useful for management purposes, at larger temporal and spatial scales.
Coccolithophore growth and calcification in a changing ocean
Published 26 October 2017 Science ClosedTags: biological response, calcification, growth, individualmodeling, modeling, morphology, phytoplankton
Coccolithophores are the most abundant calcifying phytoplankton in the ocean. These tiny primary producers have an important role in the global carbon cycle, substantially contributing to global ocean calcification, ballasting organic matter to the deep sea, forming part of the marine food web base, and influencing ocean-atmosphere CO2 exchange. Despite these important impacts, coccolithophores are not explicitly simulated in most marine ecosystem models and, therefore, their impacts on carbon cycling are not represented in most Earth system models. Here, we compile field and laboratory data to synthesize overarching, across-species relationships between environmental conditions and coccolithophore growth rates and relative calcification (reported as a ratio of particulate inorganic carbon to particulate organic carbon in coccolithophore biomass, PIC/POC). We apply our relationships in a generalized coccolithophore model, estimating current surface ocean coccolithophore growth rates and relative calcification, and projecting how these may change over the 21st century using output from the Community Earth System Model large ensemble. We find that average increases in sea surface temperature of ∼2-3 °C leads to faster coccolithophore growth rates globally ( >10% increase) and increased calcification at high latitudes. Roughly an ubiquitous doubling of surface ocean pCO2 by the end of the century has the potential to moderately stimulate coccolithophore growth rates, but leads to reduced calcification ( ∼25% decrease). Decreasing nutrient availability (from warming-induced increases in stratification) produces increases in relative calcification, but leads to ∼25% slower growth rates. With all drivers combined, we observe decreases in calcification and growth in most low and mid latitude regions, with possible increases in both of these responses in most high latitude regions. Major limitations of our coccolithophore model stem from a lack of conclusive physiological responses to changes in irradiance (we do not include light limitation in our model), and a lack of physiological data for major coccolithophore species. Species within the Umbellosphaera genus, for example, are dominant in mid to low latitude regions where we predict some of the largest decreases in coccolithophore growth rate and calcification.
Continue reading ‘Coccolithophore growth and calcification in a changing ocean’
Numerical modeling of fish mortality at high CO2 concentrations representing acclimation
Published 21 August 2017 Science ClosedTags: adaptation, biological response, fish, individualmodeling, modeling, mortality, otherprocess, physiology, respiration
A mortality model was developed to predict the impact of high CO2 concentrations on marine fish. To allow for acclimation to a gradual increase of PCO2, the model includes acid–base regulation processes, such as respiration and ion-exchange at the gills. Because the physiological mechanism connecting a decrease in blood pH and mortality is not known, a statistical model was adopted. The mortality calculated for Japanese sillago experiencing a transient change of PCO2 compares moderately well with observations.
Moderate increase in TCO2 enhances photosynthesis of seagrass Zostera japonica, but not Zostera marina: implications for acidification mitigation
Published 7 August 2017 Science ClosedTags: biological response, individualmodeling, laboratory, light, modeling, multiple factors, North Pacific, phanerogams, photosynthesis
Photosynthesis and respiration are vital biological processes that shape the diurnal variability of carbonate chemistry in nearshore waters, presumably ameliorating (daytime) or exacerbating (nighttime) short-term acidification events, which are expected to increase in severity with ocean acidification (OA). Biogenic habitats such as seagrass beds have the capacity to reduce CO2 concentration and potentially provide refugia from OA. Further, some seagrasses have been shown to increase their photosynthetic rate in response to enriched total CO2 (TCO2). Therefore, the ability of seagrass to mitigate OA may increase as concentrations of TCO2 increase. In this study, we exposed native Zostera marina and non-native Zostera japonica seagrasses from Padilla Bay, WA (USA) to various levels of irradiance and TCO2. Our results indicate that the average maximum net photosynthetic rate (Pmax) for Z. japonica as a function of irradiance and TCO2 was 3x greater than Z. marina when standardized to chlorophyll (360 ± 33 μmol TCO2 mg chl−1 h−1 and 113 ± 10 μmol TCO2 mg chl−1 h−1, respectively). Additionally, Z. japonica increased its Pmax ~50% when TCO2 increased from ~1,770 to 2,051 μmol TCO2 kg−1. In contrast, Z. marina did not display an increase in Pmax with higher TCO2, possibly due to the variance of photosynthetic rates at saturating irradiance within TCO2 treatments (coefficient of variation: 30–60%) relative to the range of TCO2 tested. Our results suggest that Z. japonica can affect the OA mitigation potential of seagrass beds, and its contribution may increase relative to Z. marina as oceanic TCO2 rises. Further, we extended our empirical results to incorporate various biomass to water volume ratios in order to conceptualize how these additional attributes affect changes in carbonate chemistry. Estimates show that the change in TCO2 via photosynthetic carbon uptake as modeled in this study can produce positive diurnal changes in pH and aragonite saturation state that are on the same order of magnitude as those estimated for whole seagrass systems. Based on our results, we predict that seagrasses Z. marina and Z. japonica both have the potential to produce short-term changes in carbonate chemistry, thus offsetting anthropogenic acidification when irradiance is saturating.
Forecasting future recruitment success for Atlantic cod in the warming and acidifying Barents Sea
Published 7 August 2017 Science ClosedTags: biological response, fish, individualmodeling, morphology, mortality, multiple factors, North Atlantic, reproduction, temperature
Productivity of marine fish stocks is known to be affected by environmental and ecological drivers, and global climate change is anticipated to alter recruitment success of many stocks. While the direct effects of environmental drivers on fish early life stage survival can be quantified experimentally, indirect effects in marine ecosystems and the role of adaptation are still highly uncertain.
We developed an integrative model for the effects of ocean warming and acidification on the early life stages of Atlantic cod in the Barents Sea, termed SCREI (Simulator of Cod Recruitment under Environmental Influences). Experimental results on temperature and CO2 effects on egg fertilization, egg and larval survival and development times are incorporated. Calibration using empirical time series of egg production, temperature, food and predator abundance reproduces age-0 recruitment over three decades. We project trajectories of recruitment success under different scenarios and quantify confidence limits based on variation in experiments. A publicly accessible web version of the SCREI model can be run under www.oceanchange.uni-bremen.de/SCREI.
Severe reductions in average age-0 recruitment success of Barents Sea cod are projected under uncompensated warming and acidification towards the middle to end of this century. Although high population stochasticity was found, considerable rates of evolutionary adaptation to acidification and shifts in organismal thermal windows would be needed to buffer impacts on recruitment. While increases in food availability may mitigate short-term impacts, an increase in egg production achieved by stock management could provide more long-term safety for cod recruitment success.
The SCREI model provides a novel integration of multiple driver effects in different life stages and enables an estimation of uncertainty associated with inter-individual and ecological variation. The model thus helps to advance towards an improved empirical foundation for quantifying climate change impacts on marine fish recruitment, relevant for ecosystem-based assessments of marine systems under climate change.
Ocean acidification hampers sperm-egg collisions, gamete fusion, and generation of Ca2+ oscillations of a broadcast spawning bivalve, Tegillarca granosa
Published 27 July 2017 Science ClosedTags: biological response, individualmodeling, laboratory, modeling, mollusks, physiology, reproduction
Although the effect of ocean acidification on fertilization success of marine organisms is increasingly well documented, the underlying mechanisms are not completely understood. The fertilization success of broadcast spawning invertebrates depends on successful sperm-egg collisions, gamete fusion, and standard generation of Ca2+oscillations. Therefore, the realistic effects of future ocean pCO2 levels on these specific aspects of fertilization of Tegillarca granosa were investigated in the present study through sperm velocity trials, fertilization kinetics model analysis, and intracellular Ca2+assays, respectively. Results obtained indicated that ocean acidification significantly reduced the fertilization success of T. granosa, which could be accountable by (i) decreased sperm velocity hence reducing the probability for sperm-egg collisions; (ii) lowered probability of gamete fusion for each gamete collision event; and (iii) disrupted intracellular Ca2+ oscillations.
Spatial patterns of Anchoveta (Engraulis ringens) eggs and larvae in relation to pCO2 in the Peruvian upwelling system
Published 30 May 2017 Science ClosedTags: abundance, biological response, chemistry, field, fish, individualmodeling, modeling, otherprocess, reproduction, South Pacific, zooplankton
Large and productive fisheries occur in regions experiencing or projected to experience ocean acidification. Anchoveta (Engraulis ringens) constitute the world’s largest single-species fishery and live in one of the ocean’s highest pCO2 regions. We investigated the relationship of the distribution and abundance of Anchoveta eggs and larvae to natural gradients in pCO2 in the Peruvian upwelling system. Eggs and larvae, zooplankton, and data on temperature, salinity, chlorophyll a and pCO2 were collected during a cruise off Peru in 2013. pCO2 ranged from 167–1392 µatm and explained variability in egg presence, an index of spawning habitat. Zooplankton abundance explained variability in the abundance of small larvae. Within the main spawning and larva habitats (6–10°S), eggs were found in cool, low-salinity, and both extremely low (less than 200 µatm) and high (more than 900 µatm) pCO2 waters, and larvae were collected in warmer, higher salinity, and moderate (400–600 µatm) pCO2 waters. Our data support the hypothesis that Anchoveta preferentially spawned at high pCO2 and these eggs had lower survival. Enhanced understanding of the influence of pCO2 on Anchoveta spawning and larva mortality, together with pCO2 measurements, may enable predictions of ocean acidification effects on Anchoveta and inform adaptive fisheries management.
The combined effects of increased temperature and ocean acidification on the early life history stages of Caribbean coral and its implication for the recovery potential of Florida reefs
Published 23 May 2017 Science ClosedTags: biological response, corals, individualmodeling, laboratory, modeling, morphology, mortality, multiple factors, North Atlantic, reproduction, respiration, temperature
The early life history stages of coral are an essential component determining the recovery potential of coral reefs through sexual reproduction and recruitment. The pelagic larval phase is inherent in all coral species regardless of differing reproductive strategies and is the only time in coral life history where large scale movement is possible allowing for the repopulation of reef areas both within and outside the natal reef habitat. In the face of climate change, the larval dispersal and recruitment phase will take place in a warmer more acidic ocean if we continue on the path of unabated fossil fuel emissions. While much research has focused on how increased temperature or ocean acidification affect coral larvae independently, our understanding of how these factors interact to shape larval response is limited, especially in regards to Caribbean coral species.
To gain a better understanding of how the early life history stages of Caribbean coral may be affected by climate change, this dissertation investigates the effects of increased temperature (2.5 °C above historical averages in the Florida Keys) and carbon dioxide levels (900-1000 parts per million CO2) on corals from the Florida Reef tract by investigating the effects on larval metabolism, survivorship, settlement, and post-settlement growth and survival. Additionally, a coupled biophysical model was developed to determine the potential changes in connectivity that may result from the biological effects of increased temperature and ocean acidification on the larval phase. The larval respiratory response of three Caribbean coral species revealed Orbicella faveolata as the most environmentally responsive with significant increases in respiration after 1 day exposure to increased temperature (68% greater than control conditions) with a counteracting effect of ocean acidification significantly decreasing respiration. The changes in metabolism over time correlated with decreased time to competency under elevated temperature in O. faveolata larvae, resulting in a greater number of settlers (76% greater than control) and a relative increase in local retention and self-recruitment rates as revealed by the biophysical model (5 and 7% greater than control respectively). However, when increased temperature occurred in combination with elevated CO2 levels, respiration was not significantly increased relative to control conditions and development of competency is minimally impacted. This resulted in a smaller increase in settlers (13% greater than control) and no significant changes in connectivity patterns. The post-settlement phase was similarly impacted with counteracting effects of increased temperature and ocean acidification on recruit growth.
Overall, this dissertation reveals the potential for adaptation to increased temperature in at least one important coral species (Orbicella faveolata) that is greatly diminished when encountered in combination with ocean acidification. These results encourage the reduction of carbon emissions to give coral species the chance to adapt to elevated temperatures through the recruitment of more resilient individuals without the additional stress of ocean acidification.
Climate change and tropical sponges: The effect of elevated pCO₂ and temperature on the sponge holobiont
Published 16 May 2017 Science ClosedTags: biological response, growth, individualmodeling, laboratory, mortality, multiple factors, performance, photosynthesis, physiology, porifera, respiration, South Pacific, temperature
As atmospheric CO₂ concentrations rise, associated ocean warming (OW) and ocean acidification (OA) are predicted to cause declines in reef-building corals globally, shifting reefs from coral-dominated systems to those dominated by less sensitive species. Sponges are important structural and functional components of coral reef ecosystems, but despite increasing field-based evidence that sponges may be ‘winners’ in response to environmental degradation, our understanding of how they respond to the combined effects of OW and OA is limited. This PhD thesis explores the response of four abundant Great Barrier Reef species – the phototrophic Carteriospongia foliascens and Cymbastela coralliophila and the heterotrophic Stylissa flabelliformis and Rhopaloeides odorabile to OW and OA levels predicted for 2100, under two CO₂ Representative Concentration Pathways (RCPs). The overall aim of this research is to bridge gaps in our understanding of how these important coral reef organisms will respond to projected climate change, to begin to explore whether a sponge dominated state is a possible future trajectory for coral reefs.
To determine the tolerance of adult sponges to climate change, these four species were exposed to OW and OA in the Australian Institute of Marine Science’s (AIMS) National Sea Simulator (SeaSim) in a 3-month experimental study. The first data chapter explores the physiological responses of these sponges to OW and OA to gain a broad understanding of sponge holobiont survival and functioning under these conditions. In this chapter I also address the hypothesis that phototrophic and heterotrophic sponges will exhibit differential responses to climate change. In the second and third data chapters I explore the cellular lipid and fatty acid composition of sponges, and how these biochemical constituents vary with OW and OA. Lipids and fatty acids are not only vital energy stores, they form the major components of cell membranes, and the structure and composition of these biochemical constituents ultimately determines the integrity and physiological competency of a cell. Therefore through these analyses I aimed to determine how OW and OA affects the metabolic balance of sponges, and to understand mechanisms underpinning observed systemic sponge responses. Finally, to provide greater insight into the population level impacts of climate change on tropical sponges, in the last data chapter I explore the response of the phototrophic species Carteriospongia foliascens to OW/OA throughout its developmental stages.
I found that while sponges can generally tolerate climate change scenarios predicted under the RCP6.0 conditions for 2100 (30ºC/ pH 7.8), environmental projections for the end of this century under the RCP8.5 (31.5ºC/ pH 7.6) will have significant implications for their survival. Temperature effects were much stronger than OA effects for all species; however, phototrophic and heterotrophic species responded differently to OA. Elevated pCO₂ exacerbated temperature stress in heterotrophic sponges but somewhat ameliorated thermal stress in phototrophic species. Furthermore, sponges with siliceous spiculated skeletons resisted the RCP 8.5 conditions for longer than the aspiculate species. Biochemical analysis revealed that spiculated species also have greater cell membrane support features, which is likely to contribute to the observed stress tolerance. I also found that the additional energy available to phototrophic sponges under OA conditions may be used for investment into cell membrane support, providing protection against thermal stress. Finally, larval survival and settlement success of C. foliascens was unaffected by OW and OA treatments, and juvenile sponges exhibited greater tolerance than their adult counterparts, again with evidence that OA reduces OW stress for some of these life stages.
Based on the species studied here, this thesis confirms that sponges are better able to deal with OW and OA levels predicted for 2100 under RCP6.0, compared to many corals for which survival in a high CO₂ world requires OW to remain below 1.5°C. This suggests sponges may be future ‘winners’ on coral reefs under global climate change. However, if CO₂ atm concentrations reach levels predicted under RCP8.5, the prognosis for sponge survival by the end of this century changes as inter-species sponge tolerances to OW and OA differ. Under this projection it is likely we will also start to see a shift in sponge populations to those dominated by phototrophic sponges with siliceous spiculated skeletons. Overall, this thesis gives a holistic view of OW and OA impacts on tropical sponges and provides the basis from which to explore the potential for a sponge-coral regime shift in a high CO₂ world.
Independence of nutrient limitation and carbon dioxide impacts on the Southern Ocean coccolithophore Emiliania huxleyi
Published 3 May 2017 Science ClosedTags: abundance, biological response, calcification, individualmodeling, laboratory, morphology, multiple factors, nutrients, otherprocess, photosynthesis, phytoplankton, primary production
Future oceanic conditions induced by anthropogenic greenhouse gas emissions include warming, acidification and reduced nutrient supply due to increased stratification. Some parts of the Southern Ocean are expected to show rapid changes, especially for carbonate mineral saturation. Here we compare the physiological response of the model coccolithophore Emiliania huxleyi (strain EHSO 5.14, originating from 50oS, 149oE) with pH/CO2 gradients (mimicking ocean acidification ranging from 1 to 4 × current pCO2 levels) under nutrient-limited (nitrogen and phosphorus) and -replete conditions. Both nutrient limitations decreased per cell photosynthesis (particulate organic carbon (POC) production) and calcification (particulate inorganic carbon (PIC) production) rates for all pCO2 levels, with more than 50% reductions under nitrogen limitation. These impacts, however, became indistinguishable from nutrient-replete conditions when normalized to cell volume. Calcification decreased three-fold and linearly with increasing pCO2 under all nutrient conditions, and was accompanied by a smaller ~30% nonlinear reduction in POC production, manifested mainly above 3 × current pCO2. Our results suggest that normalization to cell volume allows the major impacts of nutrient limitation (changed cell sizes and reduced PIC and POC production rates) to be treated independently of the major impacts of increasing pCO2 and, additionally, stresses the importance of including cell volume measurements to the toolbox of standard physiological analysis of coccolithophores in field and laboratory studies.
Factors affecting coral recruitment and calcium carbonate accretion rates on a Central Pacific coral reef
Published 22 March 2017 Science ClosedTags: biological response, calcification, chemistry, corals, field, individualmodeling, modeling, North Pacific, reproduction
Coral recruitment and calcium carbonate (CaCO3) accretion are fundamental processes that help maintain coral reefs. Many reefs worldwide have experienced degradation, including a decrease in coral cover and biodiversity. Successful coral recruitment helps degraded reefs to recover, while CaCO3 accretion by early successional benthic organisms maintains the topographic complexity of a coral reef system. It is therefore important to understand the processes that affect coral recruitment and CaCO3 accretion rates in order to understand how coral reefs recover from disturbances.
The aim of this thesis was to determine how biophysical forcing factors affect coral recruitment, calcification and bioerosion on a pristine coral reef. I used artificial settlement tiles to measure coral recruitment and CaCO3 accretion at ten sites (four on the fore reef, four on the Western Reef Terrace and two at the Entrance Channel) at Palmyra Atoll. Fungia skeletons and pieces of dead coral rock were used to measure bioerosion rates, which were combined with the CaCO3 accretion rates to obtain a net CaCO3 budget of the reef substratum. Interactions between coral recruits and other benthic organisms on the settlement tiles were recorded to determine the settlement preferences and competitive strength of coral recruits. The settlement preference of Pocillopora damicornis for divots shaped like steephead and bumphead parrotfish bites marks was determined by adding P. damicornis larvae to a container with a settlement tile with the aforementioned divots.
I found that coral recruitment and CaCO3 accretion are influenced by biophysical forcing factors. Most pocilloporids likely recruit close to their parents while the origin of poritid larvae is much more distant. Pocilloporid recruitment rates were also significantly correlated with the successional stage of the benthic community on the settlement tiles, especially the cover of biofilm and bryozoa. Biofilm and crustose coralline algae (CCA) were preferred as settlement substrata by coral larvae, however both pocilloporids and poritids settled on a large number of different benthic substrata. P. damicornis larvae showed a significant settlement preference for divots shaped like parrotfish bite marks over a flat settlement surface. Coral recruits were good competitors against encrusting algae but were often outcompeted by filamentous and upright algae. Settlement tiles were almost entirely colonised by benthic organisms within three to twelve months of deployment. The mass of CaCO3 deposited onto the settlement tiles negatively correlated with herbivore grazing pressure on the benthic community. Bioerosion rates within pieces of coral (internal bioerosion) increased over time but overall bioerosion rates (internal and external) rarely exceeded CaCO3 deposition by CCA.
My results show how variability in biophysical forcing factors leads to natural variation in coral recruitment and CaCO3 accretion. This thesis highlights the importance of measuring herbivore grazing, CCA and turf algae cover to gain a better understanding of reef resilience. I conclude that models constructed for Caribbean reefs may not be suited to predict resilience in Pacific reefs and that within the Pacific, two different kinds of resilience models need to be constructed, one for human-inhabited coral reefs and one for uninhabited coral reefs.
Mediterranean versus Red sea corals facing climate change, a transcriptome analysis
Published 2 March 2017 Science ClosedTags: algae, biological response, corals, individualmodeling, laboratory, Mediterranean, molecular biology, multiple factors, photosynthesis, physiology, Red Sea, temperature
The anthropogenic increase in atmospheric CO2 that drives global warming and ocean acidification raises serious concerns regarding the future of corals, the main carbonate biomineralizers. Here we used transcriptome analysis to study the effect of long-term gradual temperature increase (annual rate), combined with lowered pH values, on a sub-tropical Red Sea coral, Stylophora pistillata, and on a temperate Mediterranean symbiotic coral Balanophyllia europaea. The gene expression profiles revealed a strong effect of both temperature increase and pH decrease implying for synergism response. The temperate coral, exposed to a twice as high range of seasonal temperature fluctuations than the Red Sea species, faced stress more effectively. The compensatory strategy for coping apparently involves deviating cellular resources into a massive up-regulation of genes in general, and specifically of genes involved in the generation of metabolic energy. Our results imply that sub-lethal, prolonged exposure to stress can stimulate evolutionary increase in stress resilience.
Integrated regulatory and metabolic networks of the marine diatom Phaeodactylum tricornutum predict the response to rising CO2 levels
Published 21 February 2017 Science ClosedTags: biological response, individualmodeling, molecular biology, physiology, phytoplankton
Diatoms are eukaryotic microalgae that are responsible for up to 40% of the ocean’s primary productivity. How diatoms respond to environmental perturbations such as elevated carbon concentrations in the atmosphere is currently poorly understood. We developed a transcriptional regulatory network based on various transcriptome sequencing expression libraries for different environmental responses to gain insight into the marine diatom’s metabolic and regulatory interactions and provide a comprehensive framework of responses to increasing atmospheric carbon levels. This transcriptional regulatory network was integrated with a recently published genome-scale metabolic model of Phaeodactylum tricornutum to explore the connectivity of the regulatory network and shared metabolites. The integrated regulatory and metabolic model revealed highly connected modules within carbon and nitrogen metabolism. P. tricornutum’s response to rising carbon levels was analyzed by using the recent genome-scale metabolic model with cross comparison to experimental manipulations of carbon dioxide.
IMPORTANCE: Using a systems biology approach, we studied the response of the marine diatom Phaeodactylum tricornutum to changing atmospheric carbon concentrations on an ocean-wide scale. By integrating an available genome-scale metabolic model and a newly developed transcriptional regulatory network inferred from transcriptome sequencing expression data, we demonstrate that carbon metabolism and nitrogen metabolism are strongly connected and the genes involved are coregulated in this model diatom. These tight regulatory constraints could play a major role during the adaptation of P. tricornutum to increasing carbon levels. The transcriptional regulatory network developed can be further used to study the effects of different environmental perturbations on P. tricornutum’s metabolism.
Sensitivity of sea urchin fertilization to pH varies across a natural pH mosaic
Published 20 February 2017 Science ClosedTags: biological response, chemistry, echinoderms, field, individualmodeling, North Pacific, reproduction
In the coastal ocean, temporal fluctuations in pH vary dramatically across biogeographic ranges. How such spatial differences in pH variability regimes might shape ocean acidification resistance in marine species remains unknown. We assessed the pH sensitivity of the sea urchin Strongylocentrotus purpuratus in the context of ocean pH variability. Using unique male–female pairs, originating from three sites with similar mean pH but different variability and frequency of low pH (pHT ≤ 7.8) exposures, fertilization was tested across a range of pH (pHT 7.61–8.03) and sperm concentrations. High fertilization success was maintained at low pH via a slight right shift in the fertilization function across sperm concentration. This pH effect differed by site. Urchins from the site with the narrowest pH variability regime exhibited the greatest pH sensitivity. At this site, mechanistic fertilization dynamics models support a decrease in sperm–egg interaction rate with decreasing pH. The site differences in pH sensitivity build upon recent evidence of local pH adaptation in S. purpuratus and highlight the need to incorporate environmental variability in the study of global change biology.
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