Posts Tagged 'flow'

Short periods of decreased water flow may modulate long-term ocean acidification in reef-building corals

Published 14 March 2024 Science Closed
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Ocean acidification (OA) poses a major threat to reef-building corals. Although water flow variability is common in coral reefs and modulates coral physiology, the interactive effects of flow and OA on corals remain poorly understood. Therefore, we performed a three-month OA experiment investigating the effect of changes in flow on coral physiology. We exposed the reef-building corals Acropora cytherea, Pocillopora verrucosa, and Porites cylindrica to control (pH 8.0) and OA (pH 7.8) conditions at moderate flow (6 cm s-1) and monitored OA effects on growth. Throughout the experiment, we intermittently exposed all corals to low flow (2 cm s-1) for 1.5 h and measured their photosynthesis:photosynthesis (P:R) ratio under low and moderate flow. On average, corals under OA calcified 18 % less and grew 23 % less in surface area than those at ambient pH. We observed species-specific interactive effects of OA and flow on coral physiology. P:R ratios decreased after 12 weeks of OA in A. cytherea (22 %) and P. cylindrica (28 %) under moderate flow, but were unaffected by OA under low flow. P:R ratios were stable in P. verrucosa. These results suggest that short periods of decreased water flow may modulate OA effects on some coral species, indicating that flow variability is a factor to consider when assessing long-term effects of climate change.

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The impact of extreme weather events exceeds those due to global-change drivers on coastal phytoplankton assemblages

Published 1 March 2024 Science Closed
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Highlights

  • Extreme wind and rainfall events have become frequent phenomena in coastal ecosystems.
  • We simulated these events under global change for five phytoplankton assemblages.
  • Extreme events were responsible for the bulk of variability on photosynthesis efficiency.
  • The impact of extreme events is low in assemblages with high diversity and evenness
  • Extreme events should be considered in global change studies.

Abstract

Extreme wind and rainfall events have become more frequent phenomena, impacting coastal ecosystems by inducing increased mixing regimes in the upper mixed layers (UML) and reduced transparency (i.e. browning), hence affecting phytoplankton photosynthesis. In this study, five plankton assemblages from the South Atlantic Ocean, from a gradient of environmental variability and anthropogenic exposure, were subjected to simulated extreme weather events under a global change scenario (GCS) of increased temperature and nutrients and decreased pH, and compared to ambient conditions (Control). Using multiple linear regression (MLR) analysis we determined that evenness and the ratio of diatoms/ (flagellates + dinoflagellates) significantly explained the variations (81–91 %) of the photosynthesis efficiency (i.e. Pchla/ETRchla ratio) for each site under static conditions. Mixing speed and the optical depth (i.e. attenuation coefficient * depth, kdz), as single drivers, explained 40–76 % of the variability in the Pchla/ETRchla ratio, while GCS drivers <9 %. Overall, assemblages with high diversity and evenness were less vulnerable to extreme weather events under a GCS. Extreme weather events should be considered in global change studies and conservation/management plans as even at local/regional scales, they can exceed the predicted impacts of mean global climate change on coastal primary productivity.

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Optimizing marine macrophyte capacity to locally ameliorate ocean acidification under variable light and flow regimes: insights from an experimental approach

Published 19 October 2023 Science Closed
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The urgent need to remediate ocean acidification has brought attention to the ability of marine macrophytes (seagrasses and seaweeds) to take up carbon dioxide (CO2) and locally raise seawater pH via primary production. This physiological process may represent a powerful ocean acidification mitigation tool in coastal areas. However, highly variable nearshore environmental conditions pose uncertainty in the extent of the amelioration effect. We developed experiments in aquaria to address two interconnected goals. First, we explored the individual capacities of four species of marine macrophytes (Ulva lactuca, Zostera marina, Fucus vesiculosus and Saccharina latissima) to ameliorate seawater acidity in experimentally elevated pCO2. Second, we used the most responsive species (i.e., S. latissima) to assess the effects of high and low water residence time on the amelioration of seawater acidity in ambient and simulated future scenarios of climate change across a gradient of irradiance. We measured changes in dissolved oxygen, pH, and total alkalinity, and derived resultant changes to dissolved inorganic carbon (DIC) and calcium carbonate saturation state (Ω). While all species increased productivity under elevated CO2, S. latissima was able to remove DIC and alter pH and Ω more substantially as CO2 increased. Additionally, the amelioration of seawater acidity by S. latissima was optimized under high irradiance and high residence time. However, the influence of water residence time was insignificant under future scenarios. Finally, we applied predictive models as a function of macrophyte biomass, irradiance, and residence time conditions in ambient and future climatic scenarios to allow projections at the ecosystem level. This research contributes to understanding the biological and physical drivers of the coastal CO2 system.

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Global environmental changes negatively impact temperate seagrass ecosystems

Published 14 February 2020 Science Closed
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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.

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Abiotic and biotic interactions in the diffusive boundary layer of kelp blades create a potential refuge from ocean acidification

Published 23 March 2018 Science Closed
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  1. Seaweeds are able to modify the chemical environment at their surface, in a micro‐zone called the diffusive boundary layer (DBL), via their metabolic processes controlled by light intensity. Depending on the thickness of the DBL, sessile invertebrates such as calcifying bryozoans or tube‐forming polychaetes living on the surface of the blades can be affected by the chemical variations occurring in this microlayer. Especially in the context of ocean acidification (OA), these microhabitats might be considered as a refuge from lower pH, because during the day photosynthesis temporarily raises the pH to values higher than in the mainstream seawater.
  2. We assessed the thickness and the characteristics of the DBL at two pH levels (today’s average surface ocean pH 8.1 and a reduced pH predicted for the end of the century, pH 7.7) and seawater flows (slow, 0.5 and fast, >8 cm/s) on Ecklonia radiata (kelp) blades. Oxygen and pH profiles from the blade surface to the mainstream seawater were measured with O2 and pH microsensors for both bare blades and blades colonized by the bryozoan Membranipora membranacea.
  3. The DBL was thicker in slow flow compared with fast flow and the presence of bryozoans increased the DBL thickness and shaped the DBL gradient in dark conditions. Net production was increased in the low pH condition, increasing the amount of oxygen in the DBL in both bare and epiphytized blades. This increase drove the daily pH fluctuations at the blade surface, shifting them towards higher values compared with today’s pH. The presence of bryozoans led to lower oxygen concentrations in the DBL and more complex pH fluctuations at the blade surface, particularly at pH 7.7.
  4. Overall, this study, based on microprofiles, shows that, in slow flow, DBL microenvironments at the surface of the kelps may constitute a refuge from OA with pH values higher than those of the mainstream seawater. For calcifying organisms, it could also represent training ground for harsh conditions, with broad daily pH and oxygen fluctuations. These chemical microenvironments, biologically shaped by the macrophytes, are of great interest for the resilience of coastal ecosystems in the context of global change.

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

Published 12 February 2018 Science Closed
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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.

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Differential growth responses to water flow and reduced pH in tropical marine macroalgae

Published 4 April 2017 Science Closed
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The physical environment plays a key role in facilitating the transfer of nutrients and dissolved gases to marine organisms and can alter the rate of delivery of dissolved inorganic carbon. For non-calcifying macroalgae, water motion can influence the physiological and ecological responses to various environmental changes such as ocean acidification (OA). We tested the effects of lowered pH under three different flow speeds on three dominant non-calcifying macroalgal species differing in their carbon-use and are commonly found in the back reefs of Moorea, French Polynesia. Relative growth rates (RGR) of two phaeophytes, Dictyota bartayresiana and Lobophora variegata (HCO3− users), and a rhodophyte, Amansia rhodantha (CO2 user) were measured to examine how the combined effects of OA and flow can affect algal growth. Growth rates were affected independently by pCO2 and flow treatments but there was no significant interactive effect. Additionally, growth rates among species varied within the different flow regimes. Of the three species, L. variegata had the overall greatest increase in RGR across all three flow speeds while A. rhodantha exhibited the greatest negative impact under elevated pCO2 at 0.1 cm·s− 1. These differential responses among algal species demonstrate the importance of flow when examining responses to a changing environment, and if the responses of macroalgae differ based on their carbon-use strategies, it may provide advantages to some macroalgal species in a future, more acidic ocean.

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Intertidal oysters reach their physiological limit in a future high-CO2 world

Published 7 March 2017 Science Closed
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Sessile marine molluscs living in the intertidal zone experience periods of internal acidosis when exposed to air (emersion) during low tide. Relative to other marine organisms, molluscs have been identified as vulnerable to future ocean acidification; however, paradoxically it has also been shown that molluscs exposed to high CO2 environments are more resilient compared with those molluscs naive to CO2 exposure. Two competing hypotheses were tested using a novel experimental design incorporating tidal simulations to predict the future intertidal limit of oysters in a high-CO2 world; either high-shore oysters will be more tolerant of elevated PCO2 because of their regular acidosis, or elevated PCO2 will cause high-shore oysters to reach their limit. Sydney rock oysters, Saccostrea glomerata, were collected from the high-intertidal and subtidal areas of the shore and exposed in an orthogonal design to either an intertidal or a subtidal treatment at ambient or elevated PCO2, and physiological variables were measured. The combined treatment of tidal emersion and elevated PCO2 interacted synergistically to reduce the haemolymph pH (pHe) of oysters, and increase the PCO2 in the haemolymph (Pe,CO2) and standard metabolic rate. Oysters in the intertidal treatment also had lower condition and growth. Oysters showed a high degree of plasticity, and little evidence was found that intertidal oysters were more resilient than subtidal oysters. It is concluded that in a high-CO2 world the upper vertical limit of oyster distribution on the shore may be reduced. These results suggest that previous studies on intertidal organisms that lacked tidal simulations may have underestimated the effects of elevated PCO2.

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Kelp and carbon: pathways and barriers to acquisition and transport

Published 7 February 2017 Science Closed
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Large brown algae in the class Phaeophyceae (Heterokontophyta) form the structural and energetic foundation of temperate and subtropical nearshore marine forests of high productivity and ecological diversity. This dissertation examines the carbon uptake and transport physiology of large brown algae with a particular focus on the plastic or adaptive responses of these physiological traits to their abiotic environment. Chapter 1 takes an anatomical and modeling approach to investigate the structure and function of photosynthate transport networks (analogous to phloem) in diverse members of the Laminariales. To evaluate the existence of scaling and optimization of the kelp vascular system, a model of optimized transport anatomy was developed and tested with a  diverse suite of kelp species in the Laminariales. Results revealed a surprising lack of universal scaling in the kelps and the presence of optimized transport anatomy in the giant kelp (Macrocystis pyrifera) only. Chapter 2 focuses on the dynamics of carbon uptake in M. pyrifera, which can acquire both carbon dioxide and bicarbonate as carbon substrates for photosynthesis. To evaluate whether the proportion of carbon dioxide and bicarbonate utilized by M. pyrifera is constant or a variable function of their fluctuating environment, oxygen evolution experiments were carried out  n entire blades from several targeted populations in the Monterey Bay. Results indicated that M. pyrifera possesses a plastic carbon uptake physiology in which proportionally more bicarbonate is used in high irradiance and high flow conditions, but that local populations have not yet developed fixed genetic differences. Chapter 3 investigates the mechanism and patterns of carbon stable isotope discrimination in M. pyrifera. Results of a dual field and laboratory incubation approach indicate that 13C discrimination patterns are determined by a complex interaction of light intensity, dissolved inorganic carbon limitation, and fractionation occurring during transport of polysaccharides. Overall, this dissertation informs patterns and mechanisms of carbon uptake and transport in kelps, and highlights the many ways in which kelps may impact and structure their ecosystems.

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Combined effects of ocean acidification with water flow and temperature on tropical non-calcareous macroalgae

Published 2 September 2016 Science Closed
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The vulnerability of coral reefs has substantially increased in the past few decades due to accelerating human-driven global change. The effects of ocean acidification (OA) and global warming individually and interactively have resulted in varying degrees of responses in benthic reef organisms. For non-calcareous macroalgae, the physiological and ecological responses to physical environmental changes can alter their relative abundances, which are often used as an indicator of the overall coral reef status. To better understand how fleshy macroalgae will respond to various physical parameters, three separate experiments were conducted from June 2014 to July 2015 in Moorea, French Polynesia. An important physical driver in transferring nutrients and dissolved gases to benthic reef organisms is water motion. In 2014, I tested the hypothesis that increased water motion and elevated pCO2 would benefit Amansia rhodantha (a CO2 user) more than Dictyota bartayresiana and Lobophora variegata (HCO3- users). The highest and lowest growth rates were at the intermediate and highest flow speed, respectively, for all three species. A. rhodantha exhibited the greatest reduction in biomass at reduced flow under ambient pCO2, indicating high sensitivity to mass transfer and carbon limitation. In 2015, the interactive effects of temperature and OA were tested in a two-part study on the metabolic (i.e. photosynthesis and respiration) and growth responses of D. bartayresiana and A. rhodantha. The first study in January 2015 showed that net photosynthesis in both species was affected by high pCO2 but not temperature, and the combination of temperature and OA affected respiration rates. In the second study in July 2015, metabolic rates were affected by temperature but not pCO2. Net photosynthesis and respiration of A. rhodantha were highest under OA conditions at 27.5 ºC, but were reduced at 30 ºC. There was no effect on metabolic rates of D. bartayresiana across all temperature treatments. The relative growth rates for D. bartayresiana were higher than A. rhodantha in the first study, while both species exhibited varying responses to treatments in the second study. Lastly, from May to June 2015, massive Porites spp. was paired with D. bartayresiana in competitive interactions at low and high flow speeds under ambient and elevated pCO2 levels. I tested the hypothesis that increased water flow would increase algal growth rates, enhancing the competitive ability of the alga against the coral. For corals, I predicted that OA and reduced water flow would negatively affect the corals, thus increasing susceptibility to algal overgrowth. Net calcification and the photosynthetic efficiency of corals were used as a proxy for fitness and health status, respectively, however neither was affected by water flow or OA. On the contrary, growth rates of D. bartayresiana were significantly reduced under low flow. The negative effects of reduced water motion on macroalgae may potentially compromise the ability of the alga to compete. The variation in water motion can affect resource acquisition and when combined with OA, can have significant implications on species interactions. These results indicate the importance of water motion in influencing macroalgal growth and provide insights to the varying responses in fleshy macroalgae to global change. Furthermore, their physiological responses may be attributed to their different carbon uptake strategies, as A. rhodantha was more sensitive to reduced flow and temperature than D. bartayresiana.

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Predicting carbon isotope discrimination in Eelgrass (Zostera marina L.) from the environmental parameters—light, flow, and [DIC]

Published 4 August 2015 Science Closed
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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.

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