Posts Tagged 'phanerogams'

Primary producers may ameliorate impacts of daytime CO2 addition in a coastal marine ecosystem

Predicting the impacts of ocean acidification in coastal habitats is complicated by bio-physical feedbacks between organisms and carbonate chemistry. Daily changes in pH and other carbonate parameters in coastal ecosystems, associated with processes such as photosynthesis and respiration, often greatly exceed global mean predicted changes over the next century. We assessed the strength of these feedbacks under projected elevated CO2 levels by conducting a field experiment in 10 macrophyte-dominated tide pools on the coast of California, USA. We evaluated changes in carbonate parameters over time and found that under ambient conditions, daytime changes in pH, pCO2, net ecosystem calcification (NEC), and O2 concentrations were strongly related to rates of net community production (NCP). CO2 was added to pools during daytime low tides, which should have reduced pH and enhanced pCO2. However, photosynthesis rapidly reduced pCO2 and increased pH, so effects of CO2 addition were not apparent unless we accounted for seaweed and surfgrass abundances. In the absence of macrophytes, CO2 addition caused pH to decline by ∼0.6 units and pCO2 to increase by ∼487 µatm over 6 hr during the daytime low tide. As macrophyte abundances increased, the impacts of CO2 addition declined because more CO2 was absorbed due to photosynthesis. Effects of CO2addition were, therefore, modified by feedbacks between NCP, pH, pCO2, and NEC. Our results underscore the potential importance of coastal macrophytes in ameliorating impacts of ocean acidification.

Continue reading ‘Primary producers may ameliorate impacts of daytime CO2 addition in a coastal marine ecosystem’

The response of seagrass (Posidonia oceanica) meadow metabolism to CO2 levels and hydrodynamic exchange determined with aquatic eddy covariance

We investigated light, water velocity, and CO2 as drivers of primary production in Mediterranean seagrass (Posidonia oceanica) meadows and neighboring bare sands using the aquatic eddy covariance technique. Study locations included an open-water meadow and a nearshore meadow, the nearshore meadow being exposed to greater hydrodynamic exchange. A third meadow was located at a CO2 vent. We found that, despite the oligotrophic environment, the meadows had a remarkably high metabolic activity, up to 20 times higher than the surrounding sands. They were strongly autotrophic, with net production half of gross primary production. Thus, P. oceanica meadows are oases of productivity in an unproductive environment. Secondly, we found that turbulent oxygen fluxes above the meadow can be significantly higher in the afternoon than in the morning at the same light levels. This hysteresis can be explained by the replenishment of nighttime-depleted oxygen within the meadow during the morning. Oxygen depletion and replenishment within the meadow do not contribute to turbulent O2 flux. The hysteresis disappeared when fluxes were corrected for the O2 storage within the meadow and, consequently, accurate metabolic rate measurements require measurements of meadow oxygen content. We further argue that oxygen-depleted waters in the meadow provide a source of CO2 and inorganic nutrients for fixation, especially in the morning. Contrary to expectation, meadow metabolic activity at the CO2 vent was lower than at the other sites, with negligible net primary production.

Continue reading ‘The response of seagrass (Posidonia oceanica) meadow metabolism to CO2 levels and hydrodynamic exchange determined with aquatic eddy covariance’

Global change effects on seagrass ecosystem

Rising carbon dioxide (CO2) concentrations in the atmosphere will increase the average pCO2 level in the world oceans, which will have a knock-on effect on the marine ecosystem. Coastal seagrass communities one of the most productive marine ecosystems are predicted to benefit from the increase in CO2 levels, but long-term effects of elevated CO2 on seagrass communities are less understood. Population reconstruction techniques was used to investigate the population dynamics of Cymodocea nodosa meadows, exposed to long term elevated CO2 at volcanic seeps off Greece and Italy. Effect of elevated CO2 was noticed on the growth, morphometry, density, biomass and age structure at CO2 seeps. Above to below ground biomass ratio of C. nodosa were higher at CO2 seeps than at reference sites. The plastochrome interval were similar at all CO2 seeps. The shoot age and shoot longevity of plants were lower at seeps than reference sites. The present recruitment (sampled year) of the seagrass were higher than long-term average recruitment of the communities near the seeps. Carbon to nitrogen ratios (%DW) of C. nodosa were higher in leaves at seeps. Annual leaf production was higher near the seeps. This study suggests increased production of C. nodosa under elevated CO2 levels, but other co-factors such as nutrients, trace metal toxicity must also be taken into consideration while predicting effects of future CO2 concentrations. Volcanic CO2 seeps are now being used as natural analogues for ocean acidification studies although these areas can be affected by trace element input and may alter ecosystem responses to gradient in carbonate chemistry. Here Fe and a range of trace elements (Cd, Co, Cu, Hg, Mn, Pb, Ni and Zn) were analysed from sediments and from the roots, rhizomes and leaves of seagrass at six CO2 seeps and reference sites off Greece and Italy. There were higher metal levels in sediment and seagrasses at all CO2 seeps than reference sites. Sediment Quality Guideline Quotient, a commonly used pollution index, indicated that some of the metals (Cd, Cu, Hg, Ni) were in high enough concentrations to have adverse biological effects, such as Cu at Ischia site and Hg at Vulcano. Higher accumulation of elements from sediments in roots and leaves at CO2 seeps were found from Bio Sediment Accumulation Factor index. There were higher levels of Cu, Fe, Mn and Zn in leaves and rhizomes for P. oceanica and higher levels of Cd, Co, Cu, Fe and Zn in C. nodosa compartments at CO2 seeps. Fe and Mn were found with positive correlation within sediment-roots and sediment-rhizomes, whereas Cd, Co and Pb were found with positive correlation in compartments of C. nodosa. In P. oceanica positive correlation were only observed for Cd within sediment-roots and plant compartments. Low pH and ocean acidification increased the concentration of elements at CO2 seeps than reference sites. Thus, caution is needed, when using volcanic seep systems as analogue for the effects of rising CO2, as metals can reach levels that are toxic to seagrass, masking any potential benefits of increased levels of carbon dioxide for seagrass productivity. Net community production (NCP) and community respiration (CR) were measured under air exposed and CO2 enriched conditions for intertidal Z. noltei meadows and unvegetated sediment communities during emersion in summer and winter seasons. Community production and respiration were measured in-situ using benthic chambers. CO2 flux under air and CO2 enriched conditions were measured over a series of short term incubations (30min) using an infra-red gas analyser. Incident photosynthetic active radiation (PAR) was recorded during the incubations covering the daily and seasonal variation. Linear regression model was used to test the effects of irradiance on net community production. NCP of Z. noltei community were higher under CO2 enriched conditions than air exposed conditions in both summer and winter seasons. There was no effect of CO2 on the CR rate of Z. noltei community in summer season. NCP of sediment community were higher in summer season and winter season under CO2 enriched conditions. Sediment CR rates were higher in winter than summer season. The light compensation point of Z. noltei and sediment community were lower in both seasons under CO2 enriched conditions. Seasonal budget of community production was higher in Z. noltei than sediment communities. A clear effect of PAR was noticed on the net community production of both communities. Higher PAR intensities resulted in higher NCP under CO2 enriched conditions for both communities. CO2 enrichment will have a positive effect on the intertidal communities during emersion.

Continue reading ‘Global change effects on seagrass ecosystem’

Reproductive trade-offs in a temperate reef fish under high pCO2 levels

Highlights

•Reproductive activity in two-spotted goby is stimulated under high  pCO2  levels.
•Females under high  pCO2  levels produce more eggs.
•Larvae of parental pairs under high  pCO2  levels hatch smaller.
•Different energy allocation strategies are used by females under .
•High  pCO2  levels.

Abstract

Fishes are currently facing novel types of anthropogenic stressors that have never experienced in their evolutionary history, such as ocean acidification. Under these stressful conditions, energetically costly processes, such as reproduction, may be sacrificed for increased chances of survival. This trade-off does not only affect the organism itself but may result in reduced offspring fitness. In the present study, the effects of exposure to high  pCO2 levels were tested on the reproductive performance of a temperate species, the two-spotted goby, Gobiusculus flavescens. Breeding pairs were kept under control (∼600 μatm, pH∼ 8.05) and high  pCO2  levels (∼2300 μatm, pH∼ 7.60) conditions for a 4-month period. Additionally, oxidative stress and energy metabolism-related biomarkers were measured. Results suggest that reproductive activity is stimulated under high  pCO2  levels. Parental pairs in the simulated ocean acidification conditions exhibited increased reproductive output, with 50% more clutches and 44% more eggs per clutch than pairs under control conditions. However, there was an apparent trade-off between offspring number and size, as larvae of parental pairs under high  pCO2  levels hatched significantly smaller, suggesting differences in parental provisioning, which could be related to the fact that these females produce more eggs. Moreover, results support the hypothesis of different energy allocation strategies used by females under high  pCO2  conditions. These changes might, ultimately, affect individual fitness and population replenishment.

Continue reading ‘Reproductive trade-offs in a temperate reef fish under high pCO2 levels’

Effects of ocean acidification and hydrodynamic conditions on carbon metabolism and dissolved organic carbon (DOC) fluxes in seagrass populations

Global change has been acknowledged as one of the main threats to the biosphere and its provision of ecosystem services, especially in marine ecosystems. Seagrasses play a critical ecological role in coastal ecosystems, but their responses to ocean acidification (OA) and climate change are not well understood. There have been previous studies focused on the effects of OA, but the outcome of interactions with co-factors predicted to alter during climate change still needs to be addressed. For example, the impact of higher CO2 and different hydrodynamic regimes on seagrass performance remains unknown. We studied the effects of OA under different current velocities on productivity of the seagrass Zostera noltei, using changes in dissolved oxygen as a proxy for the seagrass carbon metabolism, and release of dissolved organic carbon (DOC) in a four-week experiment using an open-water outdoor mesocosm. Under current pH conditions, increasing current velocity had a positive effect on productivity, but this depended on shoot density. However, this positive effect of current velocity disappeared under OA conditions. OA conditions led to a significant increase in gross production rate and respiration, suggesting that Z. noltei is carbon-limited under the current inorganic carbon concentration of seawater. In addition, an increase in non-structural carbohydrates was found, which may lead to better growing conditions and higher resilience in seagrasses subjected to environmental stress. Regarding DOC flux, a direct and positive relationship was found between current velocity and DOC release, both under current pH and OA conditions. We conclude that OA and high current velocity may lead to favourable growth scenarios for Z. noltei populations, increasing their productivity, non-structural carbohydrate concentrations and DOC release. Our results add new dimensions to predictions on how seagrass ecosystems will respond to climate change, with important implications for the resilience and conservation of these threatened ecosystems.

Continue reading ‘Effects of ocean acidification and hydrodynamic conditions on carbon metabolism and dissolved organic carbon (DOC) fluxes in seagrass populations’

Short-term spatial and temporal carbonate chemistry variability in two contrasting seagrass meadows: implications for pH buffering capacities

It has been hypothesized that highly productive coastal ecosystems, such as seagrass meadows, could lead to the establishment of ocean acidification (OA) refugia, or areas of elevated pH and aragonite saturation state (Ωa) compared to source seawater. However, seagrass ecosystems experience extreme variability in carbonate chemistry across short temporal and small spatial scales, which could impact the pH buffering capacity of these potential refugia. Herein, short-term (hourly to diel) and small-scale (across 0.01–0.14 km2) spatiotemporal carbonate chemistry variability was assessed within two seagrass meadows in order to determine their short-term potential to elevate seawater pH relative to source seawater. Two locations at similar latitudes were chosen in order to compare systems dominated by coarse calcium carbonate (Bailey’s Bay, Bermuda) and muddy silicate (Mission Bay, CA, USA) sediments. In both systems, spatial variability of pH across the seagrass meadow at any given time was often greater than diel variability (e.g., the average range over 24 h) at any one site, with greater spatial variability occurring at low tide in Mission Bay. Mission Bay (spatial ΔpH = 0.08 ± 0.08; diel ΔpH = 0.12 ± 0.01; mean ± SD) had a greater average range in both temporal and spatial seawater chemistry than Bailey’s Bay (spatial ΔpH = 0.02 ± 0.01; diel ΔpH = 0.03 ± 0.00; mean ± SD). These differences were most likely due to a combination of slower currents, a larger tidal range, and more favorable weather conditions for photosynthesis (e.g., sunny with no rain) in Mission Bay. In both systems, there was a substantial amount of time (usually at night) when seawater pH within the seagrass beds was lower relative to the source seawater. Future studies aimed at assessing the potential of seagrass ecosystems to act as OA refugia for marine organisms need to account for the small-scale, high-frequency carbonate chemistry variability in both space and time, as this variability will impact where and when OA will be buffered or intensified.

Continue reading ‘Short-term spatial and temporal carbonate chemistry variability in two contrasting seagrass meadows: implications for pH buffering capacities’

Nutrient loading fosters seagrass productivity under ocean acidification

The effects of climate change are likely to be dependent on local settings. Nonetheless, the compounded effects of global and regional stressors remain poorly understood. Here, we used CO2 vents to assess how the effects of ocean acidification on the seagrass, Posidonia oceanica, and the associated epiphytic community can be modified by enhanced nutrient loading. P. oceanica at ambient and low pH sites was exposed to three nutrient levels for 16 months. The response of P. oceanica to experimental conditions was assessed by combining analyses of gene expression, plant growth, photosynthetic pigments and epiphyte loading. At low pH, nutrient addition fostered plant growth and the synthesis of photosynthetic pigments. Overexpression of nitrogen transporter genes following nutrient additions at low pH suggests enhanced nutrient uptake by the plant. In addition, enhanced nutrient levels reduced the expression of selected antioxidant genes in plants exposed to low pH and increased epiphyte cover at both ambient and low pH. Our results show that the effects of ocean acidification on P. oceanica depend upon local nutrient concentration. More generally, our findings suggest that taking into account local environmental settings will be crucial to advance our understanding of the effects of global stressors on marine systems.

Continue reading ‘Nutrient loading fosters seagrass productivity under ocean acidification’


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Ocean acidification in the IPCC AR5 WG II

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