Posts Tagged 'phanerogams'

Resistance of seagrass habitats to ocean acidification via altered interactions in a tri-trophic chain

Despite the wide knowledge about prevalent effects of ocean acidification on single species, the consequences on species interactions that may promote or prevent habitat shifts are still poorly understood. Using natural CO2 vents, we investigated changes in a key tri-trophic chain embedded within all its natural complexity in seagrass systems. We found that seagrass habitats remain stable at vents despite the changes in their tri-trophic components. Under high pCO2, the feeding of a key herbivore (sea urchin) on a less palatable seagrass and its associated epiphytes decreased, whereas the feeding on higher-palatable green algae increased. We also observed a doubled density of a predatory wrasse under acidified conditions. Bottom-up CO2 effects interact with top-down control by predators to maintain the abundance of sea urchin populations under ambient and acidified conditions. The weakened urchin herbivory on a seagrass that was subjected to an intense fish herbivory at vents compensates the overall herbivory pressure on the habitat-forming seagrass. Overall plasticity of the studied system components may contribute to prevent habitat loss and to stabilize the system under acidified conditions. Thus, preserving the network of species interactions in seagrass ecosystems may help to minimize the impacts of ocean acidification in near-future oceans.

Continue reading ‘Resistance of seagrass habitats to ocean acidification via altered interactions in a tri-trophic chain’

Does ocean acidification benefit seagrasses in a mesohaline environment? a mesocosm experiment in the northern Gulf of Mexico

Ocean acidification is thought to benefit seagrasses because of increased carbon dioxide (CO2) availability for photosynthesis. However, in order to truly assess ecological responses, effects of ocean acidification need to be investigated in a variety of coastal environments. We tested the hypothesis that ocean acidification would benefit seagrasses in the northern Gulf of Mexico, where the seagrasses Halodule wrightii and Ruppia maritima coexist in a fluctuating environment. To evaluate if benefits of ocean acidification could alter seagrass bed composition, cores of H. wrightii and R. maritima were placed alone or in combination into aquaria and maintained in an outdoor mesocosm. Half of the aquaria were exposed to either ambient (mean pH of 8.1 ± 0.04 SD on total scale) or high CO2 (mean pH 7.7 ± 0.05 SD on total scale) conditions. After 54 days of experimental exposure, the δ13C values were significantly lower in seagrass tissue in the high CO2 condition. This integration of a different carbon source (either: preferential use of CO2, gas from cylinder, or both) indicates that plants were not solely relying on stored energy reserves for growth. Yet, after 41 to 54 days, seagrass morphology, biomass, photo-physiology, metabolism, and carbon and nitrogen content in the high CO2 condition did not differ from those at ambient. There was also no indication of differences in traits between the homospecific or heterospecific beds. Findings support two plausible conclusions: (1) these seagrasses rely heavily on bicarbonate use and growth will not be stimulated by near future acidification conditions or (2) the mesohaline environment limited the beneficial impacts of increased CO2 availability.

Continue reading ‘Does ocean acidification benefit seagrasses in a mesohaline environment? a mesocosm experiment in the northern Gulf of Mexico’

Global environmental changes negatively impact temperate seagrass ecosystems

The oceans are increasingly affected by multiple aspects of global change, with substantial impacts on ecosystem functioning and food-web dynamics. While the effects of single factors have been extensively studied, it has become increasingly evident that there is a need to unravel the complexities related to a multiple stressor environment. In a mesocosm experimental study, we exposed a simplified, multi-trophic seagrass ecosystem (composed of seagrass, two shrimp species, and two intermediate predatory fish species) to three global change factors consisting of simulated storm events (Storms), heat shocks (Heat), and ocean acidification (OA), and the combination of all three factors (All). The most striking result indicated that when all factors were combined, there was a negative influence at all trophic levels, while the treatments with individual factors revealed species-specific response patterns. It appeared, however, that single factors may drive the multi-stressor response. All single factors (i.e., Storms, Heat, and OA) had either negative, neutral, or positive effects on fish and shrimp, whereas no effect was recorded for any single stressor on seagrass plants. The findings demonstrate that when several global change factors appear simultaneously, they can have deleterious impacts on seagrass ecosystems, and that the nature of factors and food-web composition may determine the sensitivity level of the system. In a global change scenario, this may have serious and applicable implications for the future of temperate seagrass ecosystems.

Continue reading ‘Global environmental changes negatively impact temperate seagrass ecosystems’

Carbon budgets in coastal estuaries of the northwestern Gulf of Mexico under hydrologic control

Globally, estuaries are considered as important CO2 sources to the atmosphere. However, previous studies on estuarine carbon fluxes have mostly focused on temperate and high latitude regions, leaving a significant knowledge gap in subtropical and tropical estuaries. In addition, the drivers that cause large spatiotemporal variability in estuarine inorganic and organic carbon fluxes remain insufficiently explored. In this dissertation, carbon budgets in four northwestern Gulf of Mexico (nwGOM) estuaries along a climatic gradient, Lavaca-Colorado Estuary (LCE), Guadalupe Estuary (GE), Mission-Aransas Estuary (MAE), and Nueces Estuary (NE), were evaluated. All these estuaries, with annual CO2 emission ranging 2.7—35.9 mol·C·m-2·y-1, are moderate to strong CO2 sources. However, there was large spatiotemporal variability that corresponded to changes in hydrologic conditions. The two northern estuaries (LCE and GE), due to larger riverine discharges, had an order of magnitude higher CO2 emissions than the
southern estuaries (MAE and NE). In addition, episodic flooding made the entire regional CO2 fluxes differ significantly under dry (-0.7—20.9 mmol·C·m-2·d-1) and wet (11.6—170.0 mmol·C·m-2·d-1) conditions. A mass balance model for carbon budget predicted lateral transport of total organic matter (TOC) and dissolved inorganic carbon (DIC) from tidal wetlands, which accounted for ~95% and 70% of total TOC and DIC inputs to the open estuarine water, respectively. However, the loss of coastal saltmarsh-mangrove habitats due to sea level rise could result in ~1% per year decline in estuarine CO2 fluxes at the expense of decreasing lateral carbon transport. Finally, this dissertation suggested that the average estuarine CO2 flux from nwGOM was about 8 times higher than previously estimated North America estuarine CO2 flux. Additionally, flooding condition was estimated to elevate CO2 emission and lateral fluxes by 10 times in this region.

Continue reading ‘Carbon budgets in coastal estuaries of the northwestern Gulf of Mexico under hydrologic control’

Anthropogenic impacts on mangrove and saltmarsh communities in eastern Australia

The global phenomenon of mangrove encroachment into saltmarshes has been observed across five continents. It has been proposed that this encroachment is driven in part by rising atmospheric CO2 concentration and reduced salinity in saltmarshes resulting from rising sea levels enhancing the establishment success of mangrove seedlings. However, this is yet to be empirically tested at the community-level. In this study, we examined the effect of CO2 and salinity on seedling growth of two mangrove species, Aegiceras corniculatum and Avicennia
marina, grown individually and in a model saltmarsh community in a glasshouse experiment. We found that the shoot (210%) and root (91%) biomass of the saltmarsh species was significantly greater under elevated CO2. As a result, both mangrove species experienced a stronger competitive effect from the saltmarsh species under elevated CO2. Nevertheless, A. marina seedlings produced on average 48% more biomass under elevated CO2 when grown in competition with the saltmarsh species which they used to grow taller suggesting they were light
limited. In contrast, A. corniculatum growth did not significantly differ between CO2 treatments. However, it had on average 36% greater growth under seawater salinity compared to hypersaline conditions. Avicenna marina seedlings were not affected by salinity. From these results, we suggest that although CO2 and salinity are not universal drivers determining saltmarsh-mangrove boundaries, it is likely that rising atmospheric CO2 concentration and reduced salinity associated with sea level rise will enhance the establishment success of mangrove seedlings in saltmarshes, which may facilitate mangrove encroachment in the future.

Continue reading ‘Anthropogenic impacts on mangrove and saltmarsh communities in eastern Australia’

Water motion and vegetation control the pH dynamics in seagrass-dominated bays

Global oceanic pH is lowering, which is causing great concern for the natural functioning of marine ecosystems. Current pH predictions are based on open ocean models; however, coastal zones are dynamic systems with seawater pH fluctuating temporally and spatially. To understand how coastal ecosystems will respond in the future, we first need to quantify the extent that local processes influence the pH of coastal zones. With this study, we show that over a single diurnal cycle, the total pH can fluctuate up to 0.2 units in a shallow seagrass-dominated bay, driven by the photosynthesis and respiration of the vegetation. However, these biologically controlled pH fluctuations vary significantly over small distances. Monitoring conducted at neighboring sites with contrasting hydrodynamic regimes highlights how water motion controls the extent that the local pH is altered by the metabolism of vegetation. The interactive effects of hydrodynamics and vegetation were further investigated with an in situ experiment, where the hydrodynamics were constrained and thus the local water residence time was increased, displaying the counteractive effect of hydrodynamics on the pH change caused by vegetation. With this research, we provide detailed in situ evidence of the spatial variation of pH within marine ecosystems, highlighting the need to include hydrodynamic conditions when assessing the pH-effects of vegetation, and identifying potential high-pH refuges in a future low pH ocean.

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Ocean acidification alters meiobenthic assemblage composition and organic matter degradation rates in seagrass sediments

Seagrass meadows are an important organic matter (OM) reservoir but, are currently being lost due to global and regional stressors. Yet, there is limited research investigating the cumulative impacts of anthropogenic stressors on the structure and functioning of seagrass benthic assemblages, key drivers of OM mineralization and burial. Here, using a 16‐month field experiment, we assessed how meiobenthic assemblages and extracellular enzymatic activities (as a proxy of OM degradation) in Posidonia oceanica sediments responded to ocean acidification (OA) and nutrient loadings, at CO2 vents. P. oceanica meadows were exposed to three nutrient levels (control, moderate, and high) at both ambient and low pH sites. OA altered meiobenthic assemblage structure, resulting in increased abundance of annelids and crustaceans, along with a decline in foraminifera. In addition, low pH enhanced OM degradation rates in seagrass sediments by enhancing extracellular enzymatic activities, potentially decreasing the sediment carbon storage capacity of seagrasses. Nutrient enrichment had no effect on the response variables analyzed, suggesting that, under nutrient concentration unlikely to cause N or P imitation, a moderate increase of dissolved nutrients in the water column had limited influence on meiobenthic assemblages. These findings show that OA can significantly alter meiobenthic assemblage structure and enhance OM degradation rates in seagrass sediments. As meiofauna are ubiquitous key actors in the functioning of benthic ecosystems, we postulated that OA, altering the structure of meiobenthic assemblages and OM degradation, could affect organic carbon sequestration over large spatial scales.

Continue reading ‘Ocean acidification alters meiobenthic assemblage composition and organic matter degradation rates in seagrass sediments’


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Ocean acidification in the IPCC AR5 WG II

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