Posts Tagged 'phanerogams'

Ocean acidification alleviates dwarf eelgrass (Zostera noltii) lipid landscape remodeling under warming stress

Simple Summary

Expected climate change scenarios will have inevitable and important impacts on key foundation marine species such as seagrasses. This study was aimed to understand how the dwarf eelgrass Zostera noltii leaf lipid landscapes are altered under predicted ocean warming (+4 °C) and acidification (ΔpH 0.4) conditions. A severe reduction in the leaf total fatty acid (FA) content was observed in seagrasses individually exposed to hypercapnic or warming conditions, and this depletion was ameliorated under combined exposure to ocean warming and acidification conditions. The tested treatments also impacted the FA composition of all lipid classes, with warming exposure leading to decreases in polyunsaturated fatty acids (PUFAs). Galactolipid remodeling seems to have key roles in the physiological changes observed in seagrasses under these tested conditions, highlighting the higher impact of warming and that the proposed stress alleviation effect induced by increased water-dissolved CO2 availability. Neutral lipids were substantially increased under warming conditions, mainly with increases in C18 FA, impairing their use as substrates to maintain the osmotic balance of the cells. Nonetheless, the pace at which ocean warming is occurring can overcome the ameliorative capacity induced by higher CO2 availability, leaving seagrasses under severe heat stress beyond their lipid-remodeling capacity.

Abstract

Coastal seagrass meadows provide a variety of essential ecological and economic services, including nursery grounds, sediment stabilization, nutrient cycling, coastal protection, and blue carbon sequestration. However, these ecosystems are highly threatened by ongoing climatic change. This study was aimed to understand how the dwarf eelgrass Zostera noltii leaf lipid landscapes are altered under predicted ocean warming (+4 °C) and hypercapnic (ΔpH 0.4) conditions. Warming and hypercapnic conditions were found to induce a severe reduction in the leaf total fatty acid, though the combined treatment substantially alleviated this depletion. The lipid discrimination revealed a significant increase in the relative monogalactosyldiacylglycerol (MGDG) content in both hypercapnic and warming conditions, allied to plastidial membrane stabilization mechanisms. Hypercapnia also promoted enhanced phosphatidylglycerol (PG) leaf contents, a mechanism often associated with thylakoid reinvigoration. In addition to changing the proportion of storage, galacto- and phospholipids, the tested treatments also impacted the FA composition of all lipid classes, with warming exposure leading to decreases in polyunsaturated fatty acids (PUFAs); however, the combination of both stress conditions alleviated this effect. The observed galactolipid and phospholipid PUFA decreases are compatible with a homeoviscous adaptation, allowing for the maintenance of membrane stability by counteracting excessive membrane fluidity. Neutral lipid contents were substantially increased under warming conditions, especially in C18 fatty acids (C18), impairing their use as substrates for fatty acylated derivatives essential for maintaining the osmotic balance of cells. An analysis of the phospholipid and galactolipid fatty acid profiles as a whole revealed a higher degree of discrimination, highlighting the higher impact of warming and the proposed stress alleviation effect induced by increased water-dissolved CO2 availability. Still, it is essential to remember that the pace at which the ocean is warming can overcome the ameliorative capacity induced by higher CO2 availability, leaving seagrasses under severe heat stress beyond their lipid remodeling capacity.

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Resilient consumers accelerate the plant decomposition in a naturally acidified seagrass ecosystem

Anthropogenic stressors are predicted to alter biodiversity and ecosystem functioning worldwide. However, scaling up from species to ecosystem responses poses a challenge, as species and functional groups can exhibit different capacities to adapt, acclimate, and compensate under changing environments. We used a naturally acidified seagrass ecosystem (the endemic Mediterranean Posidonia oceanica) as a model system to examine how ocean acidification (OA) modifies the community structure and functioning of plant detritivores, which play vital roles in the coastal nutrient cycling and food web dynamics. In seagrass beds associated with volcanic CO2 vents (Ischia, Italy), we quantified the effects of OA on seagrass decomposition by deploying litterbags in three distinct pH zones (i.e., ambient, low, extreme low pH), which differed in the mean and variability of seawater pH. We replicated the study in two discrete vents for 117 days (litterbags sampled on day 5, 10, 28, 55, and 117). Acidification reduced seagrass detritivore richness and diversity through the loss of less abundant, pH-sensitive species but increased the abundance of the dominant detritivore (amphipod Gammarella fucicola). Such compensatory shifts in species abundance caused more than a three-fold increase in the total detritivore abundance in lower pH zones. These community changes were associated with increased consumption (52-112%) and decay of seagrass detritus (up to 67% faster decomposition rate for the slow-decaying, refractory detrital pool) under acidification. Seagrass detritus deployed in acidified zones showed increased N content and decreased C:N ratio, indicating that altered microbial activities under OA may have affected the decay process. The findings suggest that OA could restructure consumer assemblages and modify plant decomposition in blue carbon ecosystems, which may have important implications for carbon sequestration, nutrient recycling, and trophic transfer. Our study highlights the importance of within-community response variability and compensatory processes in modulating ecosystem functions under extreme global change scenarios.

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Commentary: overstated potential for seagrass meadows to mitigate coastal ocean acidification

A Commentary on
Overstated Potential For Seagrass Meadows To Mitigate Coastal Ocean Acidification

By Van Dam, B., Lopes, C., Zeller, M. A., Ribas-ribas, M., Wang, H., and Thomas, H. (2021). Front. Mar. Sci. 8:729992. doi: 10.3389/fmars.2021.729992

Van Dam et al. (henceforth VD) published an Opinion (Van Dam et al., 2021a) and subsequent Corrigendum (Van Dam et al., 2021b) about our work regarding amelioration of low pH in seagrass ecosystems (Ricart et al., 2021). Below we discuss troubling details in the authors’ approach, an unaddressed error, misrepresentations, and problematic inferences; each contravenes VD’s argument of “overstated potential” for mitigation of low pH.

To start, VD’s original comment was rejected previously by Global Change Biology due to 1425 spurious data points and two invalid graphs. Despite being informed of these mistakes, VD submitted the identical, unchanged critique to Frontiers in Marine Science. The erroneous publication and Corrigendum resulted.

Even following correction, we disagree with VD’s two primary assertions:

1) VD claim that using ΔpH is “mathematically incorrect” because corresponding Δ[H+] values depend on initial pH, a rather strident statement given the relationship is well known (Fassbender et al., 2021; note that in our study, initial pH is that outside seagrass; i.e., Δ=measurement inside minus that outside). VD then confusingly duplicate a single set of measurements in their Figure 1A, plotting it as two separate data clusters. One cluster (their red points) improperly inverts values to show “–ΔpH” instead of “ΔpH” on the y-axis. The other, teal cluster employs within-meadow pH rather than outside-meadow pH as the independent variable, a choice unsuited to assessing whether seagrass ecosystems elevate pH relative to impinging waters, and one that is misleading. The correct relationship (Figure 1A here) demonstrates that although ΔpH and pH indeed covary, the greatest low-pH amelioration (strongest Δ[H+] depression) occurs when outside-meadow pH is low and acidification stress is high. Most importantly, key patterns of Ricart et al. (2021) remain unchanged when Δ[H+] is used instead of ΔpH (Figures 1B–D here). Therefore, our conclusions are robust to either ΔpH or Δ[H+], and pH broadens audience accessibility.

2) VD claim that we overstate the capacity of seagrasses to ameliorate low pH. However, we believe this stance relies too heavily on categorical thinking.

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Potential resilience to ocean acidification of benthic foraminifers living in Posidonia oceanica meadows: the case of the shallow venting site of Panarea

This research shows the results regarding the response to acidic condition of the sediment and Posidonia foraminiferal assemblages collected around the Panarea Island. The Aeolian Archipelago represents a natural laboratory and a much-promising study site for multidisciplinary marine research (carbon capture and storage, geochemistry of hydrothermal fluids and ocean acidification vs. benthic and pelagic organisms). The variability and the complexity of the interaction of the ecological factors characterizing extreme environments such as shallow hydrothermal vents did not allow us to carry out a real pattern of biota responses in situ, differently from those observed under controlled laboratory conditions. However, the study provides new insights into foraminiferal response to increasing ocean acidification (OA) in terms of biodiversity, faunal density, specific composition of the assemblages and morphological variations of the shells. The study highlights how the foraminiferal response to different pH conditions can change depending on different environmental conditions and microhabitats (sediments, Posidonia leaves and rhizomes). Indeed, mineral sediments were more impacted by acidification, whereas Posidonia microhabitats, thanks to their buffer effect, can offer “refugia” and more mitigated acidic environment. At species level, rosalinids and agglutinated group represent the most abundant taxa showing the most specific resilience and capability to face acidic conditions.

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The role of epiphytes in seagrass productivity under ocean acidification

Ocean Acidification (OA), due to rising atmospheric CO2, can affect the seagrass holobiont by changing the plant’s ecophysiology and the composition and functioning of its epiphytic community. However, our knowledge of the role of epiphytes in the productivity of the seagrass holobiont in response to environmental changes is still very limited. CO2 vents off Ischia Island (Italy) naturally reduce seawater pH, allowing to investigate the adaptation of the seagrass Posidonia oceanica L. (Delile) to OA. Here, we analyzed the percent cover of different epiphytic groups and the epiphytic biomass of P. oceanica leaves, collected inside (pH 6.9–7.9) and outside (pH 8.1–8.2) the CO2 vents. We estimated the contribution of epiphytes to net primary production (NPP) and respiration (R) of leaf sections collected from the vent and ambient pH sites in laboratory incubations. Additionally, we quantified net community production (NCP) and community respiration (CR) of seagrass communities in situ at vent and ambient pH sites using benthic chambers. Leaves at ambient pH sites had a 25% higher total epiphytic cover with encrusting red algae (32%) dominating the community, while leaves at vent pH sites were dominated by hydrozoans (21%). Leaf sections with and without epiphytes from the vent pH site produced and respired significantly more oxygen than leaf sections from the ambient pH site, showing an average increase of 47 ± 21% (mean ± SE) in NPP and 50 ± 4% in R, respectively. Epiphytes contributed little to the increase in R; however, their contribution to NPP was important (56 ± 6% of the total flux). The increase in productivity of seagrass leaves adapted to OA was only marginally reflected by the results from the in situ benthic chambers, underlining the complexity of the seagrass community response to naturally occurring OA conditions.

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Eelgrass beds can mitigate local acidification and reduce oyster malformation risk in a subarctic lagoon, Japan: a three-dimensional ecosystem model study

Highlights

  • An ecosystem model representing carbonate systems in a lagoon was developed.
  • The effect of ocean acidification on oyster malformation was evaluated.
  • Simulation under the absence of eelgrass bed was also performed.
  • The model could reproduce the spatiotemporal variations of the observed values.
  • Eelgrass beds mitigate the adverse effects of acidification on oyster growth.

Abstract

It is well known that ocean acidification (OA) inhibits growth of marine calcifying organisms. Therefore, the adverse effects of acidification on marine ecosystems and aquaculture, such as oyster farming, are of concern. Since eelgrass beds in neritic areas have a high potential for carbon assimilation, this study focuses on local scale mitigation of OA effects. Using a three-dimensional lower-trophic system ecosystem model, we modeled nitrogen and carbon cycles, and the dynamics of carbonate parameters in a subarctic shallow lagoon and bay, where nitrogen availability limits the photosynthesis of primary producers. Simulation of the present conditions allowed reproduction of spatiotemporal variations in water quality and, by assuming future environmental changes quantitatively, revealed that the progress of OA significantly elevated the probability of shell malformation in juvenile oysters. The results represent the spatiotemporal variations in carbonate parameters inside and outside eelgrass beds and enable the evaluation of the alleviation effect on local acidification by the presence of a dense eelgrass bed. Our study shows that in the absence of the eelgrass bed scenario, the effect of OA on oysters became more remarkable. The simulations revealed that maintaining eelgrass beds is essential to mitigate the effects of acidification on oysters.

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Response of Cymodocea nodosa to ocean acidification and warming in the Canary Islands: direct and indirect effects

Hightlights

  • Ocean acidification increase growth and gross primary production of Cymodocea nodosa.
  • The rise of temperature limited the net and gross primary production of Cymodocea nodosa.
  • A positive effect of decrased pH on greater vulnerability to consumption by Paracentrotus lividus.
  • A future scenario of climate change will affect metabolic rates of C.nodosa.
  • Different responses to climate change have been observed by C. nodosa from Canary Islands.

Abstract

As detected in warming and ocean acidification, global change can have profound impact on marine life. Its effects on seagrasses are becoming increasingly well-known, since several studies have focused on the responses of these species to global change conditions. However a few studies have assessed the combined effect of temperature and acidification on seagrasses. Overall in this study, the combined effects of increased ocean temperature and pH levels expected at the end of this century (+5 °C and pH 7.5) on Cymodocea nodosa from Canary Islands, were evaluated for one month through manipulative laboratory experiments. Growth, net production, respiration, gross primary production, chlorophyll-a concentration and its vulnerability to herbivory were quantified. Results showed a positive effect of decreased pH on growth and gross primary production, as well as greater vulnerability to consumption by the sea urchin Paracentrotus lividus. In contrast, increased temperature limited net and gross primary production. This study shows than in future scenarios, C. nodosa from the Canary Islands may be a losing species in the global change stakes.

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Temperature effects on leaf and epiphyte photosynthesis, bicarbonate use and diel O2 budgets of the seagrass Zostera marina L.

Ocean warming along with nutrient enrichment are major stressors causing global seagrass decline. While the effects of global warming on metabolic parameters in seagrasses are well described, the effect of increasing temperature on the epiphytic overgrowth of seagrass leaves and the consequences for the seagrass plant are poorly understood. Here, we investigated the effects of elevating temperature on the photosynthetic efficiency of the seagrass species Zostera marina L. and its associated epiphytes, to explore how ocean warming might affect epiphytism in seagrasses. Gas exchange and final pH measurements on bare seagrass leaves, leaves with epiphytes, and epiphytes separated from seagrass leaves were used to quantify photosynthesis and respiration rates, and the inorganic carbon extraction capacity of leaves and epiphytes as a function of photon scalar irradiance and temperature (12, 17, 22, and 27°C). Seagrass without epiphytic biofilm had a high ability to exploit the incoming irradiance regardless of the light intensity and temperature, shown as continuously high light use efficiency and maximum net photosynthesis rates. The presence of epiphytic biofilm on the seagrass leaves impaired plant photosynthesis by increasing light requirements and reducing the photosynthetic efficiency (especially at 27°C). Epiphytes showed the lowest respiration rates in darkness and had the highest oxygen surplus over diel cycles up to 22°C, whereas bare leaves had the highest diel oxygen surplus at 27°C. Both bare leaves and epiphytes lost the ability to utilize bicarbonate at 27°C, and epiphytes also did not show use of bicarbonate at 12°C. Our results indicate a competitive advantage for epiphytes in cold CO2-rich environments, whereas seagrass with bare leaves could be less affected under elevated seawater temperatures.

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Can seagrass modify the effects of ocean acidification on oysters?

Highlights

  • Effect of seagrass on oyster shell growth and extracellular pH at ambient pCO2
  • No effects of seagrass on oyster growth or extracellular pH at elevated pCO2
  • Microbiome of oysters was not affected by elevated pCO2.
  • Microbiome of seagrass was altered by elevated pCO2.
  • Seagrass may not modify the impacts of climate change.

Abstract

Solutions are being sought to ameliorate the impacts of anthropogenic climate change. Seagrass may be a solution to provide refugia from climate change for marine organisms. This study aimed to determine if the seagrass Zostera muelleri sub spp. capricorni benefits the Sydney rock oyster Saccostrea glomerata, and if these benefits can modify any anticipated negative impacts of ocean acidification. Future and ambient ocean acidification conditions were simulated in 52 L mesocosms at control (381 μatm) and elevated (848 μatm) CO2 with and without Z. muelleri. Oyster growth, physiology and microbiomes of oysters and seagrass were measured. Seagrass was beneficial to oyster growth at ambient pCO2, but did not positively modify the impacts of ocean acidification on oysters at elevated pCO2. Oyster microbiomes were altered by the presence of seagrass but not by elevated pCO2. Our results indicate seagrasses may not be a panacea for the impacts of climate change.

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Seagrass meadows under the changing climate: a review of the impacts of climate stressors

Seagrass meadows provide important ecological functions, particularly by serving as carbon sinks and breeding grounds for marine species. Climate change has threatened seagrass communities, causing their replacement, loss and increased vulnerability. This review examined scholarly articles published between 2010-2021 to comprehensively present the impacts of climate change on seagrass meadows. It shows that ocean warming negatively affects seagrass communities by favouring communities of lower structuring capacities, thus reducing the effectiveness of their ecological functions. Ocean warming also promotes the propagation and spread of invasive species, and changes the trophic structures leading to further loss of seagrasses of value. Higher seawater temperature is associated with shoot mortality and retarded growth of certain seagrasses. Sea level rise causes more wave energy to be received by coastal seagrass communities, thus, creating more damage to the communities. Deepening sea limits light penetration and alters distribution of seagrass meadows. Carbon dioxide enrichment of seawater increases photosynthetic rate of seagrasses but ocean warming and acidification counteract this beneficial effect. Carbon dioxide enrichment affects different seagrass species and different parts of a seagrass species differently, and, where beneficial to seagrass communities, could enhance their ecological services. Temperature extremes could kill seagrasses while marine heatwaves and flooding could act synergistically to increase carbon demand of certain seagrasses and unfavourably change their biomass. These impacts are often aggravated by anthropogenic activities. This review calls for more studies and conservation efforts to understand the impacts of climate change on seagrass communities and future-proof them against the changing climate.

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Ocean acidification affects volatile infochemicals production and perception in fauna and flora associated with Posidonia oceanica (L.) delile

Communication among marine organisms are generally based on production, transmission, and interpretation of chemical cues. Volatile organic compounds (VOCs) can act as infochemicals, and ocean acidification can alter their production in the source organisms as well as the interpretation of the information they drive to target organisms. Two diatoms (Cocconeis scutellum var. parva and Diploneis sp.) and a macroalga (Ulva prolifera), all common epiphytes of Posidonia oceanica leaves, were isolated and cultured at two pH conditions (8.2 and 7.7). Their biomass was collected, and the VOCs produced upon wounding were extracted and analyzed using gas chromatography. Chemotactic reactions of invertebrates triggered by VOCs were tested using a static choice experimental arena and a flow-through flume system. Odor choice experiments were performed on several invertebrates associated with P. oceanica meadows to investigate the modification of behavioral responses due to the growth of algae in acidified environments. Complex patterns of behavioral responses were recorded after exposure to algal VOCs. This study demonstrated that a) ocean acidification alters the bouquet of VOCs released by diatoms and macroalgae and b) these compounds act as infochemicals and trigger peculiar behavioral responses in benthic invertebrates. In addition, behavioral responses are species-specific, dose-dependent, and are modified by environmental constraints. In fact, the static diffusion in choice arenas produced different responses as compared to flow-through flume systems. In conclusion, we demonstrate that in future marine environments higher CO2 concentrations (leading to a pH 7.7 by the end of this century) will modify the production of VOCs by micro- and macroalgae as well as the recognition of these infochemicals by marine invertebrates.

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Reviews and syntheses: spatial and temporal patterns in seagrass metabolic fluxes

Seagrass meadow metabolism has been measured for decades to gain insight into ecosystem energy, biomass production, food web dynamics, and, more recently, to inform its potential in ameliorating ocean acidification (OA). This extensive body of literature can be used to infer trends and drivers of seagrass meadow metabolism. Here, we synthesize the results from 56 studies reporting in situ rates of seagrass gross primary productivity, respiration, and/or net community productivity to highlight spatial and temporal variability in oxygen (O2) fluxes. We illustrate that daytime net community production (NCP) is positive overall and similar across seasons and geographies. Full-day NCP rates, which illustrate the potential cumulative effect of seagrass beds on seawater biogeochemistry integrated over day and night, were also positive overall but were higher in summer months in both tropical and temperate ecosystems. Although our analyses suggest seagrass meadows are generally autotrophic, the effects on seawater oxygen are relatively small in magnitude. We also find positive correlations between gross primary production and temperature, although this effect may vary between temperate and tropical geographies and may change under future climate scenarios if seagrasses approach thermal tolerance thresholds. In addition, we illustrate that periods when full-day NCP is highest could be associated with lower nighttime O2 and increased diurnal variability in seawater O2. These results can serve as first-order estimates of when and where OA amelioration by seagrasses may be likely. However, improved understanding of variations in  ratios and increased work directly measuring metabolically driven alterations in seawater pH will further inform the potential for seagrass meadows to serve in this context.

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How does ocean acidification affect the early life history of Zostera marina? A series of experiments find parental carryover can benefit viability or germination

Elevated partial pressure of carbon dioxide (pCO2) as a concomitant of global climate change may facilitate the establishment of future seagrass meadows and subsequently its benefit could be incorporated into techniques to increase restoration success. In five manipulative experiments, we determined how increased CO2 affects the maturation of flowers, and the development of seeds and seedlings for the foundation species Zostera marina. Experiments tested the development from both seeds collected from non-treated flowering shoots (direct) and seeds harvested from flowering shoots after CO2 exposure (parental carryover). Flowering shoots were collected along the western coast of Sweden near the island of Skafto. The seeds produced were used in experiments conducted at Kristineberg, Sweden and Dauphin Island, AL, United States. Experiments varied in temperature (16, 18°C) and salinity (19, 33 ppt), as well as duration and magnitude of elevated CO2 exposure. Flowering maturation, spathe number, seed production, and indicators of seed quality did not appear to be affected by 39–69 days of exposure to CO2 conditions outside of natural variability (pCO2 = 1547.2 ± 267.60 μatm; pHT = 7.53 ± 0.07). Yet, seeds produced from these flowers showed twofold greater germination success. In another experiment, flowering shoots were exposed to an extreme CO2 condition (pCO2 = 5950.7 ± 1,849.82 μatm; pHT = 6.96 ± 0.15). In this case, flowers generated seeds that demonstrated a fivefold increase in an indicator for seed viability (sinking velocity). In the latter experiment, however, germination appeared unaffected. Direct CO2 effects on germination and seedling production were not observed. Our results provide evidence of a parental CO2 effect that can benefit germination or seed viability, but early benefits may not lead to bed establishment if other environmental conditions are not well suited for seedling development. Outcomes have implications for restoration; CO2 can be supplied to flowering shoot holding tanks to bolster success when the purpose is to redistribute seeds to locations where beds are extant and water quality is adequate.

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Spatiotemporal variability in kelp forest and seagrass ecosystems: can local biogeochemical modification combat acidification stress?

Anthropogenic carbon dioxide (CO2) emissions have driven widespread ocean acidification (OA). OA has reduced surface ocean pH by at least 0.1 pH units since the beginning of the industrial era and global models forecast a further decrease of 0.3 to 0.4 pH units by the end of the century. Submerged aquatic vegetation, such as kelp forests and seagrass beds, has the potential to locally ameliorate OA by removing CO2 during photosynthesis and storing it as fixed carbon. Thus, understanding the contribution of these habitats to local biogeochemistry is essential to inform coastal management and policy, especially as the impacts of anthropogenic climate change become more prevalent. The following work describes high resolution spatiotemporal variability in seagrass and kelp forest biogeochemistry (Chapters 1 and 2) and in the surface canopy extent of a giant kelp forest (Chapter 3).

In order to understand the contributions of kelp forest and seagrass metabolism to their respective local biogeochemistry, we must determine the natural variability in these systems and disentangle the physical and biological drivers of local biogeochemical variability. In Chapter 1, I deployed an extensive instrument array in Monterey Bay, CA, inside and outside of a kelp forest to assess the degree to which kelp locally ameliorates present-day acidic conditions, which we expect to be further exacerbated by OA. Interactions between upwelling exposure, internal bores, and biological production shaped the local biogeochemistry inside and outside of the kelp forest. Significantly elevated pH, attributed to kelp canopy productivity, was observed at the surface inside the kelp forest. This modification was largely limited to a narrow band of surface water, implying that while kelp forests have the potential to locally ameliorate ocean acidification stress, this benefit may largely be limited to organisms living in the upper part of the canopy. In Chapter 2, I quantified net community production (NCP) over a mixed seagrass-coral community on Ngeseksau Reef, Ngermid Bay, Republic of Palau. We observed a net heterotrophic diel signal over the deployment, but dissolved oxygen (O2) fluxes during the day were largely positive, illustrating daytime autotrophy. pH, O2, and temperature followed a clear diel pattern with maxima typically occurring in the afternoon. The relationship between tidal regime and time of day drove the magnitude of the signals observed. The case studies described in Chapters 1 and 2 emphasize the importance of high-resolution measurements (high temporal frequency as well as high horizontal and vertical spatial resolution) and consideration of the multiple drivers responsible for shaping the observed biogeochemical variability. In addition to the photosynthetic biomass (kelp and seagrass) at the center of these studies, the physical environment played an important role in dictating the signals observed, in particular water circulation and residence time.

Biogeochemical studies rarely look beyond a few deployment sites, but the ecosystem contributing to the local biogeochemical variability includes influences from beyond those discrete points. Describing the area around these discrete points is important for accurate assessment of factors driving the signals observed at those points. Remote sensing can help us capture and describe the spatial patterns of biomass contributing to changes observed in our chemical records. In Chapter 3, I established a low altitude unmanned aerial vehicle (UAV) record of giant kelp surface areal extent over 18 months on the wave-protected side of Cabrillo Point (Hopkins Marine Station) in Monterey Bay, CA. This was the same canopy responsible for elevating pH in Chapter 1; however, in this case, the kelp canopy mapping did not overlap in time with biogeochemical measurements in the kelp forest. I compared the UAV kelp classification to canopy cover determined from Landsat satellite images obtained over the same period. There was a linear relationship between the drone kelp ratio and Landsat kelp canopy fraction for spatially-matched pixels; a Landsat kelp fraction of 0.64 was equivalent to 100% kelp cover in the drone data. The level of resolution provided by UAV, compared with Landsat images, could allow more detailed mapping of kelp responses to environmental change. Future studies should pair mapping flights with biogeochemical measurements to quantify the relationship between changes in canopy area and the relative surface canopy modification of pH.

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Who wins or loses matters: strongly interacting consumers drive seagrass resistance under ocean acidification

Highlights

  • Ocean acidification (OA) directly facilitated algal epiphyte and seagrass.
  • Co-occurring grazers variably controlled algal overgrowth on seagrass.
  • pH-tolerant, strongly interacting grazers maintained overall grazing pressure.
  • Grazing and OA additively increased seagrass productivity.

Abstract

Global stressors are increasingly altering ecosystem resistance, resilience, and functioning by reorganizing vital species interactions. However, our predictive understanding of these changes is hindered by failures to consider species-specific functional roles and stress responses within communities. Stressor-driven loss or reduced performance of strongly interacting species may generate abrupt shifts in ecosystem states and functions. Yet, empirical support for this prediction is scarce, especially in marine climate change research. Using a marine assemblage comprising a habitat-forming seagrass (Phyllospadix torreyi), its algal competitor, and three consumer species (algal grazers) with potentially different functional roles and pH tolerance, we investigated how ocean acidification (OA) may, directly and indirectly, alter community resistance. In the field and laboratory, hermit crabs (Pagurus granosimanus and P. hirsutiusculus) and snails (Tegula funebralis) displayed distinct microhabitat use, with hermit crabs more frequently grazing in the area of high algal colonization (i.e., surfgrass canopy). In mesocosms, this behavioral difference led to hermit crabs exerting ~2 times greater per capita impact on algal epiphyte biomass than snails. Exposure to OA variably affected the grazers: snails showed reduced feeding and growth under extreme pH (7.3 and 7.5), whereas hermit crabs (P. granosimanus) maintained a similar grazing rate under all pH levels (pH 7.3, 7.5, 7.7, and 7.95). Epiphyte biomass increased more rapidly under extreme OA (pH 7.3 and 7.5), but natural densities of snails and hermit crabs prevented algal overgrowth irrespective of pH treatments. Finally, grazers and acidification additively increased surfgrass productivity and delayed the shoot senescence. Hence, although OA impaired the function of the most abundant consumers (snails), strongly interacting and pH-tolerant species (hermit crabs) largely maintained the top-down pressure to facilitate seagrass dominance. Our study highlights significant within-community variation in species functional and response traits and shows that this variation has important ecosystem consequences under anthropogenic stressors.

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A unique diel pattern in carbonate chemistry in the seagrass meadows of Dongsha island: the enhancement of metabolic carbonate dissolution in a semienclosed lagoon

In contrast to other seagrass meadows where seawater carbonate chemistry generally shows strong diel variations with higher pH but lower partial pressure of CO2 (pCO2) during the daytime and lower pH but higher pCO2 during nighttime due to the alternation in photosynthesis and respiration, the seagrass meadows of the inner lagoon (IL) on Dongsha Island had a unique diel pattern with extremely high pH and low pCO2 across a diel cycle. We suggest that this distinct diel pattern in pH and pCO2 could be associated with the enhancement of total alkalinity (TA) production coupled to carbonate sediment dissolution in a semienclosed lagoon. The confinement of the IL may hamper water exchange and seagrass detritus export to the adjacent open ocean, which may result in higher organic matter loading to the sediments, and longer residence time of the water in the IL, accompanied by microbial respiration (both aerobic and anaerobic) that may reduce carbonate saturation level to drive carbonate dissolution and thus TA elevation, thereby forming such a unique diel pattern in carbonate chemistry. This finding further highlights the importance of considering TA production through metabolic carbonate dissolution when evaluating the potential of coastal blue carbon ecosystems to buffer ocean acidification and to absorb atmospheric CO2, in particular in a semienclosed setting.

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Marine macrophytes as carbon sinks: comparison between seagrasses and the non-native alga Halimeda incrassata in the Western Mediterranean (Mallorca)

Seagrass species play a critical role in the mitigation of climate change by acting as valuable carbon sinks and storage sites. Another important ecosystem service of this coastal vegetation is nutrient removal. However, coastal ecosystems are under increasing pressure of global warming and associated establishment of invasive species. To elucidate the respective contributions of seagrass species Posidonia oceanica and Cymodocea nodosa and the non-native macroalga Halimeda incrassata as primary producers and nutrient sinks in coastal habitats we conducted in-situ incubations in the North-western Mediterranean Sea. Measured metabolic activity and nutrient removal as well as calcification rates in these habitats over a 24 h period in spring and summer confirmed that the endemic seagrass P. oceanica represents a valuable ecosystem with high O2 production and considerable carbon capture. The documented regression of P. oceanica meadows with higher temperatures and decline in autotrophy as measured here causes concern for the continuity of ecosystem services rendered by this habitat throughout the Mediterranean Sea with progressing climate warming. In contrast, the enhanced performance of C. nodosa and the calcifying alga H. incrassata with increasing temperatures, under expected rates of future warming is uncertain to mitigate loss of productivity in case of a potential shift in marine vegetation. This could ultimately lead to a decline in ecosystem services, decreased carbon storage and mitigation of climate change. Furthermore, this study provides a first estimate for the growth rate of H. incrassata in the Mediterranean Sea, supporting evidence for the mechanism of its rapid extension.

Continue reading ‘Marine macrophytes as carbon sinks: comparison between seagrasses and the non-native alga Halimeda incrassata in the Western Mediterranean (Mallorca)’

Overstated potential for seagrass meadows to mitigate coastal ocean acidification

Ricart et al. (2021, R21) explored the potential for seagrass meadows to act as a buffer of coastal ocean acidification (OA). This work relied largely on sensor-based pH measurements during 29 separate instrument deployments along the California coast, arguing that (1) seagrass aerobic metabolism consistently enhances local pH, and that (2) this OA-amelioration can persist for lengths of up to 21 days. While pH was often greater in seagrass meadows than in adjacent unvegetated regions, we are concerned with the extent to which this is attributed to seagrass ecosystem metabolism, and the interpretation that this OA-amelioration is sustained over daily and tidal timescales. By incompletely considering alternative explanations for elevated pH, irrespective of seagrass productivity, R21 offers insufficient support of OA-mitigation as a reliable ecosystem service of seagrasses. We therefore see the authors’ claim that seagrass ecosystems could be “leveraged as local management tools to mitigate the consequences of OA” as worthy of critical inspection.

First, there is a fundamental issue in R21 with the mathematical treatment of ΔpH (ΔpH = pHseagrass – pHnonvegetated), stemming from the fact that pH is on a logarithmic scale. To illustrate this issue, it is a simple fact that a ΔpH of 0.1 involves a change in [H+] that is 10 times larger when starting at pH = 7 than at pH=8. This effect is made abundantly clear by the linear relationship between pH and ΔpH (Figure 1A) present in the R21 dataset. As expected, pH also varies with salinity, with slopes that vary across sites (Figure 1B) and seasons (Figure 5 in R21). Such comparisons of ΔpH across sites, seasons, etc., are mathematically inappropriate, when made without also considering Δ[H+] (Fassbender et al., 2021). We therefore find the interpretation of ΔpH in R21 to be misleading because it does not consider the impact of initial pH on the size of ΔpH.

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In situ experiments on the effect of low pH on the ultrastructure of the seagrasses Cymodocea nodosa and Posidonia oceanica

The present study investigates the impacts of low pH on the cell structure of the seagrasses Posidonia oceanica (L.) Delile and Cymodocea nodosa (Ucria) Ascherson. The study was applied with in situ experiments at the Castello Aragonese of Ischia (Naples, Italy), where shallow submarine vents, lowering the pH, can be used as natural laboratories. Shoots of the seagrasses were transferred from the control area (pH 8.1) to the two venting areas (pH 7.8 and 6.8) for different times. Epidermal cells of young leaves were examined using transmission electron microscopy (TEM) and tubulin immunofluorescence. After one week at pH 7.8, the cell structure of Posidonia oceanica was normal, while in Cymodocea nodosa microtubule (MT) network and cell structure were affected. In addition, in C. nodosa, ultrastructural analysis revealed a gradual degradation of the nuclei, a disorganization of the chloroplasts, and an increase in the number of mitochondria and dictyosomes. The exposure of both plants for 3 weeks at pH 6.8 resulted in the aggregation and finally in the dilation of the endoplasmic reticulum (ER) membranes. Tubulin immunofluorescence revealed that after three weeks, the MT cytoskeleton of both plants was severely affected. All these alterations can be considered as indications of an apoptotic like programmed cell death (AL-PCD) which may be executed in order to regulate stress response.

Continue reading ‘In situ experiments on the effect of low pH on the ultrastructure of the seagrasses Cymodocea nodosa and Posidonia oceanica’

Rapid enhancement of multiple ecosystem services following the restoration of a coastal foundation species

The global decline of marine foundation species (kelp forests, mangroves, salt marshes, and seagrasses) has contributed to the degradation of the coastal zone and threatens the loss of critical ecosystem services and functions. Restoration of marine foundation species has had variable success, especially for seagrasses, where a majority of restoration efforts have failed. While most seagrass restorations track structural attributes over time, rarely do restorations assess the suite of ecological functions that may be affected by restoration. Here we report on the results of two small-scale experimental seagrass restoration efforts in a central California estuary where we transplanted 117 0.25-m2 plots (2,340 shoots) of the seagrass species Zostera marina. We quantified restoration success relative to persistent reference beds, and in comparison to unrestored, unvegetated areas. Within three years, our restored plots expanded ˜8,500%, from a total initial area of 29 to 2,513 m2. The restored beds rapidly began to resemble the reference beds in (1) seagrass structural attributes (canopy height, shoot density, biomass), (2) ecological functions (macrofaunal species richness and abundance, epifaunal species richness, nursery function), and (3) biogeochemical functions (modulation of water quality). We also developed a multifunctionality index to assess cumulative functional performance, which revealed restored plots are intermediate between reference and unvegetated habitats, illustrating how rapidly multiple functions recovered over a short time period. Our comprehensive study is one of few published studies to quantify how seagrass restoration can enhance both biological and biogeochemical functions. Our study serves as a model for quantifying ecosystem services associated with the restoration of a foundation species and demonstrates the potential for rapid functional recovery that can be achieved through targeted restoration of fast-growing foundation species under suitable conditions.

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