Under future increased CO2 concentrations, seagrasses are predicted to perform better as a result of increased photosynthesis, but the effects in carbon balance and growth are unclear and remain unexplored for early life stages such as seedlings, which allow plant dispersal and provide the potential for adaptation under changing environmental conditions. Furthermore, the outcome of the concomitant biochemical changes in plant-herbivore interactions has been poorly studied, yet may have important implications in plant communities. In this study we determined the effects of experimental exposure to current and future predicted CO2 concentrations on the physiology, size and defense strategies against herbivory in the earliest life stage of the Mediterranean seagrass Posidonia oceanica. The photosynthetic performance of seedlings, assessed by fluorescence, improved under increased pCO2 conditions after 60 days, although these differences disappeared after 90 days. Furthermore, these plants exhibited bigger seeds and higher carbon storage in belowground tissues, having thus more resources to tolerate and recover from stressors. Of the several herbivory resistance traits measured, plants under high pCO2 conditions had a lower leaf N content but higher sucrose. These seedlings were preferred by herbivorous sea urchins in feeding trials, which could potentially counteract some of the positive effects observed.
Posts Tagged 'phanerogams'
Seagrass (Posidonia oceanica) seedlings in a high-CO2 world: from physiology to herbivory
Published 6 December 2016 Science ClosedTags: abundance, biological response, laboratory, Mediterranean, morphology, mortality, otherprocess, performance, phanerogams, photosynthesis, physiology
Seagrasses (Zostera marina) and (Zostera japonica) display a differential photosynthetic response to TCO2: implications for acidification mitigation
Published 17 August 2016 Science ClosedTags: biological response, individualmodeling, laboratory, light, modeling, multiple factors, North Pacific, phanerogams, photosynthesis
Excess atmospheric CO2 is being absorbed at an unprecedented rate by the global and coastal oceans, shifting the baseline pCO2 and altering seawater carbonate chemistry in a process known as ocean acidification (OA). Recent attention has been given to near-shore vegetated habitats, such as seagrass beds, which may have the potential to mitigate the effects of acidification on vulnerable calcifying organisms via photosynthesis. Seagrasses are capable of raising seawater pH and calcium carbonate saturation state during times of high photosynthetic activity. To better understand the photosynthetic potential of seagrass OA mitigation, we exposed Pacific Northwest populations of native Zostera marina and non-native Zostera japonica seagrasses from Padilla Bay, WA, to various irradiance and total CO2 (TCO2) concentrations ranging from ~1770 – 2100 μmol TCO2 kg-1.
Our results indicate that the 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 ± 74 μmol TCO2 mgchl-1 hr-1 and 113 ± 21 μmol TCO2 mgchl-1 hr-1, respectively). In addition, Z. japonica increased its Pmax 77% (± 56%) when TCO2 increased from ~1770 to 2050 μmol TCO2 kg-1, whereas Z. marina did not display an increase in Pmax with higher TCO2. The lack of response by Z. marina to TCO2 is a departure from previous findings; however, it is likely that the variance within our treatments (coefficient of variation: 30 – 60%) obscured any positive effect of TCO2 on Z. marina given the range of concentrations tested. Because previous findings have shown that Z. marina is saturated with respect to HCO3- at low pH (≥ 7.5) we, therefore, suggest that the unequivocal positive response of Z. japonica to TCO2 is a result of increased HCO3- utilization in addition to increased CO2 uptake.
Considering that Z. japonica displays a greater photosynthetic rate than Z. marina when normalized to chlorophyll, particularly under enhanced TCO2 conditions, the ability of Z. japonica to mitigate OA may also increase relative to Z. marina in the future ocean. Higher photosynthetic rates by Z. japonica result in a greater potential, on a per chlorophyll basis, to increase pH and calcium carbonate saturation state—both of which affect acid-base regulation and calcification of calcifying organisms vulnerable to acidification. While it is important to consider genotypic differences throughout Z. marina and Z. japonica’s biogeographical distribution, our findings help elucidate the potential contribution both seagrasses have on variations in carbonate chemistry. Further, our results could be applied to ecosystem service models aimed at determining how specific seagrass species can be grown in a controlled setting to help mitigate OA hotspots that affect commercial shellfish aquaculture.
Effects of elevated CO2 and nutrients on the community metabolism of a Cymodocea nodosa bed
Published 10 May 2016 Science ClosedTags: biological response, BRcommunity, mesocosms, multiple factors, North Atlantic, nutrients, phanerogams, primary production
We assessed the combined effects of elevated CO2 and nutrients on the metabolism of a benthic community dominated by the seagrass Cymodocea nodosa (Ucria) Ascherson in a mesocosm experiment. C. nodosa plants and their associated community were exposed to two CO2 levels simulating future (700 ppm, pH 7.84) and current (360 ppm, pH 8.12) conditions, and two nutrient levels (enriched and ambient concentration) in a total of four treatments (-C-N, -C+N, +C-N, +C+N). Net community production (NCP) was estimated from changes in the concentration of dissolved inorganic carbon in the seawater in light incubations using benthic chambers. The variation pattern of NCP with the ordinance was consistent for all treatments. Although differences among treatments were not statistically significant, average NCP values were lowest under CO2 enrichment conditions. NCP was lower at a high CO2 level and ambient nitrogen concentration compared to when nutrient availability was higher, suggesting that the low nutrient availability may modulate the community response to CO2 enrichment. The results obtained suggest that the stimulation of the net community production of C. nodosa by elevated CO2 concentrations may be curtailed by low nutrient availability.
Effects of in situ CO2 enrichment on structural characteristics, photosynthesis, and growth of the Mediterranean seagrass Posidonia oceanica (update)
Published 14 April 2016 Science ClosedTags: abundance, biological response, field, growth, Mediterranean, morphology, otherprocess, phanerogams, photosynthesis, respiration
Seagrass is expected to benefit from increased carbon availability under future ocean acidification. This hypothesis has been little tested by in situ manipulation. To test for ocean acidification effects on seagrass meadows under controlled CO2/pH conditions, we used a Free Ocean Carbon Dioxide Enrichment (FOCE) system which allows for the manipulation of pH as continuous offset from ambient. It was deployed in a Posidonia oceanica meadow at 11 m depth in the Northwestern Mediterranean Sea. It consisted of two benthic enclosures, an experimental and a control unit both 1.7 m3, and an additional reference plot in the ambient environment (2 m2) to account for structural artifacts. The meadow was monitored from April to November 2014. The pH of the experimental enclosure was lowered by 0.26 pH units for the second half of the 8-month study. The greatest magnitude of change in P. oceanica leaf biometrics, photosynthesis, and leaf growth accompanied seasonal changes recorded in the environment and values were similar between the two enclosures. Leaf thickness may change in response to lower pH but this requires further testing. Results are congruent with other short-term and natural studies that have investigated the response of P. oceanica over a wide range of pH. They suggest any benefit from ocean acidification, over the next century (at a pH of ∼ 7.7 on the total scale), on Posidonia physiology and growth may be minimal and difficult to detect without increased replication or longer experimental duration. The limited stimulation, which did not surpass any enclosure or seasonal effect, casts doubts on speculations that elevated CO2 would confer resistance to thermal stress and increase the buffering capacity of meadows.
Nitrate fertilisation does not enhance CO2 responses in two tropical seagrass species
Published 23 March 2016 Science ClosedTags: biological response, growth, laboratory, multiple factors, nitrogen fixation, nutrients, phanerogams, photosynthesis, physiology, primary production, respiration, South Pacific
Seagrasses are often considered “winners” of ocean acidification (OA); however, seagrass productivity responses to OA could be limited by nitrogen availability, since nitrogen-derived metabolites are required for carbon assimilation. We tested nitrogen uptake and assimilation, photosynthesis, growth, and carbon allocation responses of the tropical seagrasses Halodule uninervis and Thalassia hemprichii to OA scenarios (428, 734 and 1213 μatm pCO2) under two nutrients levels (0.3 and 1.9 μM NO3−). Net primary production (measured as oxygen production) and growth in H. uninervis increased with pCO2 enrichment, but were not affected by nitrate enrichment. However, nitrate enrichment reduced whole plant respiration in H. uninervis. Net primary production and growth did not show significant changes with pCO2 or nitrate by the end of the experiment (24 d) in T. hemprichii. However, nitrate incorporation in T. hemprichii was higher with nitrate enrichment. There was no evidence that nitrogen demand increased with pCO2 enrichment in either species. Contrary to our initial hypothesis, nutrient increases to levels approximating present day flood plumes only had small effects on metabolism. This study highlights that the paradigm of increased productivity of seagrasses under ocean acidification may not be valid for all species under all environmental conditions.
Nighttime dissolution in a temperate coastal ocean ecosystem increases under acidification
Published 21 March 2016 Science ClosedTags: algae, biological response, BRcommunity, calcification, chemistry, field, mollusks, North Pacific, phanerogams
Anthropogenic emissions of carbon dioxide (CO2) are causing ocean acidification, lowering seawater aragonite (CaCO3) saturation state (Ωarag), with potentially substantial impacts on marine ecosystems over the 21st Century. Calcifying organisms have exhibited reduced calcification under lower saturation state conditions in aquaria. However, the in situ sensitivity of calcifying ecosystems to future ocean acidification remains unknown. Here we assess the community level sensitivity of calcification to local CO2-induced acidification caused by natural respiration in an unperturbed, biodiverse, temperate intertidal ecosystem. We find that on hourly timescales nighttime community calcification is strongly influenced by Ωarag, with greater net calcium carbonate dissolution under more acidic conditions. Daytime calcification however, is not detectably affected by Ωarag. If the short-term sensitivity of community calcification to Ωarag is representative of the long-term sensitivity to ocean acidification, nighttime dissolution in these intertidal ecosystems could more than double by 2050, with significant ecological and economic consequences.
Light levels affect carbon utilisation in tropical seagrass under ocean acidification
Published 8 March 2016 Science ClosedTags: biological response, growth, laboratory, light, multiple factors, phanerogams, photosynthesis, primary production, respiration, South Pacific
Under future ocean acidification (OA), increased availability of dissolved inorganic carbon (DIC) in seawater may enhance seagrass productivity. However, the ability to utilise additional DIC could be regulated by light availability, often reduced through land runoff. To test this, two tropical seagrass species, Cymodocea serrulata and Halodule uninervis were exposed to two DIC concentrations (447 μatm and 1077 μatm pCO2), and three light treatments (35, 100, 380 μmol m-2 s-1) for two weeks. DIC uptake mechanisms were separately examined by measuring net photosynthetic rates while subjecting C. serrulata and H. uninervis to changes in light and addition of bicarbonate (HCO3-) use inhibitors (carbonic anhydrase inhibitor, acetazolamide) and TRIS buffer (pH 8.0). We observed a strong dependence on energy driven H+-HCO3- co-transport (TRIS, which disrupts H+ extrusion) in C. serrulata under all light levels, indicating greater CO2 dependence in low light. This was confirmed when, after two weeks exposure, DIC enrichment stimulated maximum photosynthetic rates (Pmax) and efficiency (α) more in C. serrulata grown under lower light levels (36–60% increase) than for those in high light (4% increase). However, C. serrulata growth increased with both DIC enrichment and light levels. Growth, NPP and photosynthetic responses in H. uninervis increased with higher light treatments and were independent of DIC availability. Furthermore, H. uninervis was found to be more flexible in HCO3- uptake pathways. Here, light availability influenced productivity responses to DIC enrichment, via both carbon fixation and acquisition processes, highlighting the role of water quality in future responses to OA.
Large-scale prediction of seagrass distribution integrating landscape metrics and environmental factors: The case of Cymodocea nodosa (Mediterranean–Atlantic)
Published 12 February 2016 Science ClosedTags: biological response, communitymodeling, Mediterranean, modeling, North Atlantic, phanerogams
Understanding the factors that affect seagrass meadows encompassing their entire range of distribution is challenging yet important for their conservation. Here, we predict the realized and potential distribution for the species Cymodocea nodosa modelling its environmental niche in the Mediterranean and adjacent Atlantic coastlines. We use a combination of environmental variables and landscape metrics to perform a suite of predictive algorithms which enables examination of the niche and find suitable habitats for the species. The most relevant environmental variables defining the distribution of C. nodosa were sea surface temperature (SST) and salinity. We found suitable habitats at SST from 5.8 °C to 26.4 °C and salinity ranging from 17.5 to 39.3. Optimal values of mean winter wave height ranged between 1.2 and 1.5 m, while waves higher than 2.5 m seemed to limit the presence of the species. The influence of nutrients and pH, despite having weight on the models, was not so clear in terms of ranges that confine the distribution of the species. Landscape metrics able to capture variation in the coastline enhanced significantly the accuracy of the models, despite the limitations caused by the scale of the study. We found potential suitable areas not occupied by the seagrass mainly in coastal regions of North Africa and the Adriatic coast of Italy. The present study describes the realized and potential distribution of a seagrass species, providing the first global model of the factors that can be shaping the environmental niche of C. nodosa throughout its range. We identified the variables constraining its distribution as well as thresholds delineating its environmental niche. Landscape metrics showed promising prospects for the prediction of coastal species dependent on the shape of the coast. By contrasting predictive approaches, we defined the variables affecting the distributional areas that seem unsuitable for C. nodosa as well as those suitable habitats not occupied by the species. These findings are encouraging for its use in future studies on climate-related marine range shifts and meadow restoration projects of these fragile ecosystems.
Chefaoui R. M., Assis J., Duarte C. M. & Serrão E. A., 2016. Large-scale prediction of seagrass distribution integrating landscape metrics and environmental factors: The case of Cymodocea nodosa (Mediterranean–Atlantic). Estuaries and Coasts 39(1):123-137. Article (subscription required)
Responses of seagrass to anthropogenic and natural disturbances do not equally translate to its consumers
Published 28 January 2016 Science ClosedTags: biological response, communityMF, crustaceans, echinoderms, laboratory, mesocosms, morphology, multiple factors, North Atlantic, nutrients, performance, phanerogams
Coastal communities are under threat from many and often co-occurring local (e.g., pollution, eutrophication) and global stressors (e.g., climate change), yet understanding the interactive and cumulative impacts of multiple stressors in ecosystem function is far from being accomplished. Ecological redundancy may be key for ecosystem resilience, but there are still many gaps in our understanding of interspecific differences within a functional group, particularly regarding response diversity, that is, whether members of a functional group respond equally or differently to anthropogenic stressors. Herbivores are critical in determining plant community structure and the transfer of energy up the food web. Human disturbances may alter the ecological role of herbivory by modifying the defense strategies of plants and thus the feeding patterns and performance of herbivores. We conducted a suite of experiments to examine the independent and interactive effects of anthropogenic (nutrient and CO2 additions) and natural (simulated herbivory) disturbances on a seagrass and its interaction with two common generalist consumers to understand how multiple disturbances can impact both a foundation species and a key ecological function (herbivory) and to assess the potential existence of response diversity to anthropogenic and natural changes in these systems. While all three disturbances modified seagrass defense traits, there were contrasting responses of herbivores to such plant changes. Both CO2 and nutrient additions influenced herbivore feeding behavior, yet while sea urchins preferred nutrient-enriched seagrass tissue (regardless of other experimental treatments), isopods were deterred by these same plant tissues. In contrast, carbon enrichment deterred sea urchins and attracted isopods, while simulated herbivory only influenced isopod feeding choice. These contrasting responses of herbivores to disturbance-induced changes in seagrass help to better understand the ecological functioning of seagrass ecosystems in the face of human disturbances and may have important implications regarding the resilience and conservation of these threatened ecosystems.
Effects of in situ CO2 enrichment on structural characteristics, photosynthesis, and growth of the Mediterranean seagrass Posidonia oceanica
Published 19 January 2016 Science ClosedTags: abundance, biological response, field, growth, Mediterranean, morphology, otherprocess, phanerogams, photosynthesis, respiration
Seagrass are expected to benefit from increased carbon availability under future ocean acidification. This hypothesis has been little tested by in situ manipulation. To test for ocean acidification effects on seagrass meadows under controlled CO2/pH conditions, we used a Free Ocean Carbon Dioxide Enrichment (FOCE) system which allows for the precise manipulation of pH as an offset from the ambient. This system was deployed in a Posidonia oceanica meadow at 11 m depth in the Northwestern Mediterranean Sea. It consisted of two benthic enclosures, an experimental and a control unit both 1.7 m3, and an additional reference plot in the ambient (2 m2) to account for structural artifacts. The meadow was monitored from April to November 2014. The pH of the experimental enclosure was lowered by 0.26 pH units for the second half of the eight-month study. Changes in P. oceanica leaf biometrics, photosynthesis, and leaf growth accompanied seasonal changes recorded in the environment and values were similar between the two enclosures. Leaf thickness may change in response to lower pH but this requires further testing. Results suggest any benefit from ocean acidification, over the next century, on Posidonia physiology and growth may be minimal. The limited stimulation casts doubts on speculations that elevated CO2 would confer resistance to thermal stress and increase buffering capacity of meadows.
Community metabolism in shallow coral reef and seagrass ecosystems, lower Florida Keys
Published 19 January 2016 Science ClosedTags: biological response, BRcommunity, calcification, chemistry, corals, field, North Atlantic, phanerogams, primary production
Diurnal variation of net community production (NEP) and net community calcification (NEC) were measured in coral reef and seagrass biomes during October 2012 in the lower Florida Keys using a mesocosm enclosure and the oxygen gradient flux technique. Seagrass and coral reef sites showed diurnal variations of NEP and NEC, with positive values at near-seafloor light levels >100–300 µEinstein m-2 s-1. During daylight hours, we detected an average NEP of 12.3 and 8.6 mmol O2 m-2 h-1 at the seagrass and coral reef site, respectively. At night, NEP at the seagrass site was relatively constant, while on the coral reef, net respiration was highest immediately after dusk and decreased during the rest of the night. At the seagrass site, NEC values ranged from 0.20 g CaCO3 m-2 h-1 during daylight to -0.15 g CaCO3 m-2 h-1 at night, and from 0.17 to -0.10 g CaCO3 m-2 h-1 at the coral reef site. There were no significant differences in pH and aragonite saturation states (Ωar) between the seagrass and coral reef sites. Decrease in light levels during thunderstorms significantly decreased NEP, transforming the system from net autotrophic to net heterotrophic.
Variability in carbon availability and eelgrass (Zostera marina) biometrics along an estuarine gradient in Willapa Bay, WA, USA
Published 4 November 2015 Science ClosedTags: abundance, biological response, laboratory, otherprocess, phanerogams, primary production
Because photosynthesis requires CO2, carbon limitation in aquatic environments could restrict primary production and provide signals in tissue chemistry. We took advantage of spatial variability of aqueous [CO2] in estuaries to examine within-estuary variation in biometrics of intertidal eelgrass (Zostera marina) during peak summer production. As expected from the sensitivity of carbonate equilibria to pH, aqueous [CO2] increased along an ocean-to-river gradient in Willapa Bay, WA, USA. The scale of pH variability also changed, reflecting weather-driven upwelling near the ocean, tidal advection near rivers, and reduced diel fluctuation up-estuary. Z. marina studied at eight sites in the bay integrated across these different temporal fluctuations in water chemistry to exhibit increased tissue carbon and depleted δ 13C up-estuary. However, seagrass production did not change as expected from aqueous [CO2]. Instead, small standing biomass occurred at sites with organic-rich sediments or high wave energy, investment in branching showed trends along the estuarine gradient that changed seasonally, and specific growth rates based on leaf extension did not shift with the estuarine gradient or with standing biomass. These results reinforce that estuarine seagrasses are likely to experience modified mean pH and variability due not only to ocean acidification in the strict sense (anthropogenic CO2 absorbed from the atmosphere) but also from land use, upwelling, and feedbacks from biological processes. However, responses via productivity may be less evident than in tissue chemistry.
Evaluating community impacts of ocean acidification using qualitative network models
Published 14 October 2015 Science ClosedTags: abundance, biological response, BRcommunity, communitymodeling, laboratory, methods, modeling, mollusks, North Atlantic, otherprocess, performance, phanerogams, phytoplankton, primary production
We applied Qualitative Network Models (QNMs) to evaluate the potential community effects of ocean acidification (OA) in a major shellfish-producing estuary (Willapa Bay, Washington). QNMs are well-suited to data-limited systems and only require information on the sign (+, -, 0) of the interactions between species. We examined qualitative predictions of community responses to 13 different OA scenarios that corresponded to 3 broad categories of hypothesized OA effects: (1) increased primary productivity, (2) reductions in bivalve populations, and (3) enhanced predation interactions between bivalves and their crab and gastropod predators. The cultivated bivalve Manila clam tended to respond negatively across scenarios, while primary producers (phytoplankton and eelgrasses) and Chinook salmon tended to respond positively. Tradeoffs between species were also assessed: Manila clam and Pacific oyster were predicted to decrease and increase, respectively, when direct OA effects were limited to eelgrasses and the reverse occurred when phytoplankton alone was stimulated by OA. We analyzed the QNMs to identify key linkages that influenced the sign outcome of community members and might therefore warrant future quantitative study. QNMs may be particularly relevant to researchers as a simple method for identifying conditions under which the sign response of species to OA, as inferred from single-species OA experiments, will likely hold in the wild. Given data limitations in most systems, QNMs are a practical alternative or complement to data-intensive quantitative approaches and may help accelerate our understanding of the community-wide effects of OA in marine systems.
The effects of long-term in situ CO2 enrichment on tropical seagrass communities at volcanic vents
Published 15 September 2015 Science ClosedTags: abundance, biological response, BRcommunity, community composition, field, morphology, otherprocess, phanerogams, primary production, South Pacific
The effects of long-term exposure to elevated levels of carbon dioxide (CO2) on seagrass communities are still poorly understood. This study investigates the tropical subtidal seagrass communities at three shallow volcanic CO2 vents in Papua New Guinea. Seagrass cover and biomass increased threefold and fivefold, respectively, from control to medium and high pCO2 sites (average pH = 7.9, 7.7, and 7.5, respectively). The seagrass community composition differed significantly between the pCO2 sites: Cymodocea serrulata, Cymodocea rotundata, and Halodule uninervis were more abundant at high pCO2 sites, whereas Halophila ovalis, Thalassia hemprichii, and Syringodium isoetifolium occurred only at low and mid pCO2 sites. Cymodocea rotundata was the only species common among all pCO2 sites and locations. The δ13C in its leaves significantly declined with increasing pCO2, indicating that additional CO2 influenced seagrass carbon uptake, and specifically, that there was discrimination against the heavier isotope (13C) when carbon was more abundant. Size-specific leaf growth rates (i.e. leaf turnover) also significantly declined with increasing pCO2; however, leaf growth rates showed no consistent difference in response to elevated pCO2 in two of four surveys. Our study suggests that progressive ocean acidification may lead to higher cover and above- and below-ground biomass, but lower size-specific growth and altered species composition in tropical seagrass communities. The effects of co-limiting factors, such as light and nutrient availability, on early-responding parameters, such as growth rates, require further attention to improve projections.
Effects of ocean acidification on Posidonia oceanica epiphytic community and shoot productivity
Published 10 September 2015 Science ClosedTags: abundance, algae, biological response, BRcommunity, calcification, community composition, laboratory, Mediterranean, morphology, otherprocess, phanerogams, photosynthesis, primary production, respiration
1.Biological interactions can alter predictions that are based on single species physiological response. It is known that leaf segments of the seagrass Posidonia oceanica will increase photosynthesis with lowered pH but, it is not clear whether the outcome will be altered when the whole plant and its epiphyte community, with different respiratory and photosynthetic demands, are included. In addition, the effects on the Posidonia epiphyte community has rarely been tested under controlled conditions, at near future pH levels.
2.In order to better evaluate the effects of pH levels as projected for the upcoming decades on seagrass meadows, shoots of P. oceanica with their associated epiphytes were exposed in the laboratory to three pH levels (ambient: 8.1, 7.7, and 7.3, on the total scale) for four weeks. Net productivity, respiration, net calcification, and leaf fluorescence were measured on several occasions. At the end of the study, epiphyte community abundance and composition, calcareous mass and crustose coralline algae growth were determined. Finally, photosynthesis versus irradiance curves (PE) were produced from segments of secondary leaves cleaned of epiphytes and pigments extracted.
3.Posidonia leaf fluorescence and chlorophyll concentrations did not differ between pH treatments. Net productivity of entire shoots and epiphyte-free secondary leaves increased significantly at the lowest pH level yet limited or no stimulation in productivity was observed at the intermediate pH treatment. Under both pH treatments, significant decreases in epiphytic cover were observed, mostly due to the reduction of crustose coralline algae. The loss of the dominant epiphyte producer yet similar photosynthetic response for epiphyte-free secondary leaves and shoots, suggests a minimal contribution of epiphytes to shoot productivity under experimental conditions.
4.Synthesis Observed responses indicate that, under future ocean acidification conditions foreseen in the next century, an increase in Posidonia productivity is not likely despite the partial loss of epiphytic coralline algae which are competitors for light. A decline in epiphytic cover could, however, reduce the feeding capacity of the meadow for invertebrates. In situ long-term experiments that consider both acidification and warming scenarios are needed to improve ecosystem-level predictions.
Chemoreception of the seagrass Posidonia Oceanica by benthic invertebrates is altered by seawater acidification
Published 9 September 2015 Science ClosedTags: annelids, biological response, chemistry, crustaceans, laboratory, Mediterranean, mollusks, performance, phanerogams
Several plants and invertebrates interact and communicate by means of volatile organic compounds (VOCs). These compounds may play the role of infochemicals, being able to carry complex information to selected species, thus mediating inter- or intra-specific communications. Volatile organic compounds derived from the wounding of marine diatoms, for example, carry information for several benthic and planktonic invertebrates. Although the ecological importance of VOCs has been demonstrated, both in terrestrial plants and in marine microalgae, their role as infochemicals has not been demonstrated in seagrasses. In addition, benthic communities, even the most complex and resilient, as those associated to seagrass meadows, are affected by ocean acidification at various levels. Therefore, the acidification of oceans could produce interference in the way seagrass-associated invertebrates recognize and choose their specific environments. We simulated the wounding of Posidonia oceanica leaves collected at two sites (a control site at normal pH, and a naturally acidified site) off the Island of Ischia (Gulf of Naples, Italy). We extracted the VOCs and tested a set of 13 species of associated invertebrates for their specific chemotactic responses in order to determine if: a) seagrasses produce VOCs playing the role of infochemicals, and b) their effects can be altered by seawater pH. Our results indicate that several invertebrates recognize the odor of wounded P. oceanica leaves, especially those strictly associated to the leaf stratum of the seagrass. Their chemotactic reactions may be modulated by the seawater pH, thus impairing the chemical communications in seagrass-associated communities in acidified conditions. In fact, 54 % of the tested species exhibited a changed behavioral response in acidified waters (pH 7.7). Furthermore, the differences observed in the abundance of invertebrates, in natural vs. acidified field conditions, are in agreement with these behavioral changes. Therefore, leaf-produced infochemicals may influence the structure of P. oceanica epifaunal communities, and their effects can be regulated by seawater acidification.
Predicting carbon isotope discrimination in Eelgrass (Zostera marina L.) from the environmental parameters—light, flow, and [DIC]
Published 4 August 2015 Science ClosedTags: biological response, flow, individualmodeling, light, multiple factors, phanerogams, photosynthesis
Isotopic discrimination against 13C during photosynthesis is determined by a combination of environmental conditions and physiological mechanisms that control delivery of CO2 to RUBISCO. This study investigated the effects of light, flow, dissolved inorganic carbon (DIC) concentration, and its speciation, on photosynthetic carbon assimilation of Zostera marina L. (eelgrass) using a combination of laboratory experiments and theoretical calculations leading to a mechanistic understanding of environmental conditions that influence leaf carbon uptake and determine leaf stable carbon isotope signatures (δ13C). Photosynthesis was saturated with respect to flow at low velocity (∼ 3 cm s−1), but was strongly influenced by [DIC], and particularly aqueous CO2 (CO2(aq)) under all flow conditions. The non-linear responses of light- and flow-saturated photosynthesis to [DIC] were used to quantify the maximum physiological capacity for photosynthesis, and to determine the degree of photosynthetic carbon limitation for light-saturated photosynthesis, which provided a mechanistic pathway for modeling regulation of carbon uptake and 13C discrimination. Model predictions of δ13C spanned the typical range of values reported for a variety of seagrass taxa, and were most sensitive to [DIC] (predominantly [CO2(aq)]) and flow, but less sensitive to DIC source [CO2(aq) vs. inline image]. These results provide a predictive understanding of the role of key environmental parameters (light, flow, and DIC availability) can have in driving δ13C of seagrasses, which will become increasingly important for predicting the response of these ecosystem engineers to local processes that affect light availability and flow, as well as global impacts of climate warming and ocean acidification in the Anthropocene.
Predicting effects of ocean warming, acidification, and water quality on Chesapeake region eelgrass
Published 4 August 2015 Science ClosedTags: abundance, biological response, communitymodeling, field, light, modeling, multiple factors, North Atlantic, otherprocess, phanerogams, photosynthesis, temperature
Although environmental requirements of seagrasses have been studied for years, reliable metrics for predicting their response to current or future conditions remain elusive. Eelgrass (Zostera marina L.) populations of the Chesapeake region lie near the southern limit of their range in the Western North Atlantic, exposing them to increasing thermal stress as the climate warms. However, CO2 stimulated photosynthesis may offset some of the negative effects of temperature stress. The combined effects of temperature, CO2, and light availability controlled by water quality and epiphytes were explored using GrassLight, a bio-optical model that provided a predictive environment for evaluating the interaction of multiple stressors on eelgrass distribution and density across the submarine landscape. Model predictions were validated against in situ measures of spectral diffuse attenuation, eelgrass density, and distribution. The potential for photosynthesis stimulated by ocean acidification to mitigate the effects of high temperature on eelgrass populations growing near the southern limit of their distribution was explored. The model accurately reproduced the submarine light environment from measured water quality parameters, and predicted their impacts on eelgrass distribution. It also reproduced the negative effects of warm summer temperatures on eelgrass distributions, and demonstrated that CO2 increases projected for the next century should stimulate photosynthesis sufficiently to offset the negative effects of thermal stress on eelgrass growing in the Chesapeake region, even in the presence of epiphytes. Thus, improved water quality should facilitate the survival of eelgrass populations in Chesapeake region, even in the face of a warming climate.
Responses of seagrass to anthropogenic and natural disturbances do not equally translate to its consumers
Published 22 July 2015 Science ClosedTags: biological response, crustaceans, echinoderms, laboratory, multiple factors, nutrients, performance, phanerogams
Coastal communities are under threat from many and often co-occurring local (e.g. pollution, eutrophication) and global stressors (e.g. climate change), yet understanding the interactive and cumulative impacts of multiple stressors in ecosystem function is far from being accomplished. Ecological redundancy may be key for ecosystem resilience, but there are still many gaps in our understanding of interspecific differences within a functional group, particularly regarding response diversity, i.e. whether members of a functional group respond equally or differently to anthropogenic stressors. Herbivores are critical in determining plant community structure and the transfer of energy up the food web. Human disturbances may alter the ecological role of herbivory by modifying the defense strategies of plants and thus the feeding patterns and performance of herbivores. We conducted a suite of experiments to examine the independent and interactive effects of anthropogenic (nutrient and CO2 additions) and natural (simulated herbivory) disturbances on a seagrass and its interaction with two common generalist consumers in order to understand how multiple disturbances can impact both a foundation species and a key ecological function (herbivory), and to assess the potential existence of response diversity to anthropogenic and natural changes in these systems. While all three disturbances modified seagrass defence traits, there were contrasting responses of herbivores to such plant changes. Both CO2 and nutrient additions influenced herbivore feeding behaviour, yet while sea urchins preferred nutrient enriched seagrass tissue (regardless of other experimental treatments), isopods were deterred by these same plant tissues. In contrast, carbon enrichment deterred sea urchins and attracted isopods, while simulated herbivory only influenced isopod feeding choice.
Response of key stress-related genes of the seagrass Posidonia oceanica in the vicinity of submarine volcanic vents (update)
Published 17 July 2015 Science ClosedTags: biological response, field, Mediterranean, molecular biology, multiple factors, phanerogams, physiology, toxicants
Submarine volcanic vents are being used as natural laboratories to assess the effects of increased ocean acidity and carbon dioxide (CO2) concentration on marine organisms and communities. However, in the vicinity of volcanic vents other factors in addition to CO2, which is the main gaseous component of the emissions, may directly or indirectly confound the biota responses to high CO2. Here we used for the first time the expression of antioxidant and stress-related genes of the seagrass Posidonia oceanica to assess the stress levels of the species. Our hypothesis is that unknown factors are causing metabolic stress that may confound the putative effects attributed to CO2 enrichment only. We analyzed the expression of 35 antioxidant and stress-related genes of P. oceanica in the vicinity of submerged volcanic vents located in the islands of Ischia and Panarea, Italy, and compared them with those from control sites away from the influence of vents. Reverse-transcription quantitative polymerase chain reaction (RT-qPCR) was used to characterize gene expression patterns.
Fifty-one percent of genes analyzed showed significant expression changes. Metal detoxification genes were mostly down-regulated in relation to controls at both Ischia and Panarea, indicating that P. oceanica does not increase the synthesis of heavy metal detoxification proteins in response to the environmental conditions present at the two vents. The up-regulation of genes involved in the free radical detoxification response (e.g., CAPX, SODCP and GR) indicates that, in contrast with Ischia, P. oceanica at the Panarea site faces stressors that result in the production of reactive oxygen species, triggering antioxidant responses. In addition, heat shock proteins were also activated at Panarea and not at Ischia. These proteins are activated to adjust stress-accumulated misfolded proteins and prevent their aggregation as a response to some stressors, not necessarily high temperature.
This is the first study analyzing the expression of target genes in marine plants living near natural CO2 vents. Our results call for contention to the general claim of seagrasses as “winners” in a high-CO2 world, based on observations near volcanic vents. Careful consideration of factors that are at play in natural vents sites other than CO2 and acidification is required. This study also constitutes a first step for using stress-related genes as indicators of environmental pressures in a changing ocean.
