Posts Tagged 'nutrients'

Impact of ocean acidification and ocean warming on the oxidation of dissolved Fe(II) in coastal and open Southern Ocean water

The Southern Ocean is the largest region where major nutrients such as nitrate, silicate and phosphate are present in excess, yet the crucial micronutrient element iron (Fe) is scarce. It is well established that the Southern Ocean is key in exporting carbon to greater depths through biomass production by phytoplankton, but Fe is metabolically required for photosynthesis. Changes in uptake of carbon and heat to the ocean will impact ocean acidification and ocean warming. These anthropogenically linked processes are projected to lead to a drop in ocean pH by 0.2 units and an increase in the ocean’s temperature by 2°C by the end of the century and are already known to have tremendous ecological impacts on the ocean’s flora and fauna. However, little is known about how changes in ocean temperature and pH could alter the nutrient composition in future oceans.

Regarding nutrients, this work focuses on the dissolved (d) element Fe. It is essential for photosynthesis, but also a limiting element in the Southern Ocean due to limiting sources leading to low availability. Iron exists in two redox states in seawater. While the species dFe(III) is stable in seawater and occurs in relatively higher concentrations, its redox partner dFe(II) is tied to several physico-chemical processes impacting its oxidation time and overall presence. The importance of dFe(II) also lies with its accessibility for phytoplankton in its reduced oxidative state. The overall aim of this study was to investigate changes in concentration, speciation, and availability of the ‘more’ bioavailable, rapidly oxidizing Fe species dFe(II) under a changing Southern Ocean scenario.

Chapter 2 addressed the redox behaviour of dFe(II) and dFe(III), where several questions were explored for further experimental planning. The main question was how the coastal and open ocean systems differ in their dFe(II) concentrations and how ocean acidification and ocean warming impact Fe redox chemistry in both systems. I therefore performed controlled acidification and temperature alteration experiments in coastal and open ocean water taken from the Tasmanian coast and the Southern Ocean. This large dataset enabled us to project for future ocean dFe(II) concentrations and oxidation rates. I observed that a reduction in ocean pH by 0.2 units doubles the dFe(II) oxidation time in the open ocean and tripled in coastal water through model-based experiments. In contrast to these high impacts from pH, an increase in temperature by 1°C accelerated the oxidation by ~ 1.1 times (13% in coastal water and 8% in open ocean water). Therefore, realistic changes in temperature are likely to have small impacts on the oxidation of dFe(II) in both water systems compared to the proposed changes in pH.

For phytoplankton, these results pose contradicting outcomes, and studies display mixed results once parameters such as ocean warming, and acidification are combined. An increase in temperature might lead to less or no growth once a certain temperature threshold is crossed. Similarly, a decrease in pH is also thought to impact phytoplankton physiology. It also depends on the severity of acidification and the phytoplankton species itself. Ocean warming could reduce phytoplankton growth, despite increased Fe availability due to higher solubility in warmer water. Regarding ocean acidification, on the other hand, dFe(II) could become available for an extended time, therefore enabling further uptake of dFe(II) by phytoplankton for that time. When comparing mixed effects of ocean acidification and warming, a reduction in pH might have a greater impact on the dFe(II) oxidation than just temperature. Temperature changes, however, might be a greater concern in the near future before ocean acidification becomes relevant.

Due to this projection of temperature being a more imminent concern, I targeted the limiting element Fe in its less investigated form dFe(II). I observed how temperature alone impacts growth of two Southern Ocean phytoplankton species. I therefore ran an dFe(II)-enrichment incubation experiment in Chapter 3 with differing temperatures (3°C, 5°C, and 7°C) in coastal and open ocean water from the Southern Ocean using the well-studied haptophyte Phaeocystis antarctica and the diatom Fragilariopsis cylindrus. These enrichment experiments with altered temperatures overall confirmed that phytoplankton growth was elevated once 5 nM dFe(II) were added. In other words, freely available dFe(II) was present, almost regardless of the temperature increase from 3°C to 7°C. This could implicate that an increase in temperature has beneficial effects on growth in the case of higher concentrations of freely available dFe(II). However, these values of future dFe(II) concentrations and oxidation rates under acidified and warmer scenarios are only laboratory-based projections, to better understand the dFe(II) presence and demand by phytoplankton species in a future Southern Ocean.

In Chapter 4, a one-month field study onboard the RV Investigator was conducted east of the Australian continent along the East Australian Current (EAC) into nutrient-rich but Fe poor water in the Southern Ocean. I observed the overall distribution of dFe(II) and hydrogen peroxide in this understudied region. The findings suggest that dFe(II) concentrations are very low in the observed area of the open Southern Ocean (< 0.1 nM) compared to coastal waters (> 0.5 nM), likely driven by differences in terrestrial Fe inputs. Hydrogen peroxide was generally higher in the southern stations within the upper 200 m (~60 nM) while the dFe(II) : dFe ratios are 10 % higher than reported for previous Southern Ocean studies. High biological activity in the upper water extending to the frontal mixing zone where the two major currents meet (EAC and STF), may further have led to the observed low dFe concentrations and high H22O22 concentrations. Occasional higher dFe(II) peaks found in this area in surface water may be the result of several external sources such as rain or vertical transport from seamounts but also due to biological or physico-chemical impacts such as photochemical reduction or uptake by phytoplankton.

Overall, the work in this study advances our understanding of the coupled effects of the climate change parameters ocean acidification and ocean warming on the dFe(II) oxidation, with implications for its availability to phytoplankton and overall sources in the region east and south-east of Tasmania in coastal and open ocean water. The experimental approaches taken suggest a higher impact of ocean acidification compared to ocean warming and a potential benefit for phytoplankton species preferring dFe(II).

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Thalassiosira weissflogii grown in various Zn levels shows different ecophysiological responses to seawater acidification

Highlights

  • Zn deficient encouraged cellular silicon and sinking rate under normal pCO2.
  • Higher pCO2 decreased cellular silicon and sinking rate of Zn-deficient T. weissflogii.
  • Higher pCO2 increased cellular silicon and sinking rate in Zn-replete T. weissflogii.
  • Silica and carbon cycle could be impacted by acidification and Zn levels.

Abstract

The presence of zinc (Zn), a vital element for algal physiological functions, coupled with the silicification of diatoms implies that it plays an integral role in the carbon and silicon cycles of the sea. In this study, we examined the effects of different pCO2 and Zn levels on growth rate, elemental compositions and silicification by Thalassiosira weissflogii. The results showed that under normal pCO2 (400 μatm), cultures of T. weissflogii were depressed for growth rate and silica incorporation rate, but encouraged for cellular silicon content, Si/C, Si/N, and sinking rate when Zn deficient (0.3 pmol L−1). However, cellular silicon and sinking rate of Zn-deficient and Zn-replete (25 pmol L−1T. weissflogii were decreased and increased at higher pCO2 (800 μatm), respectively. Thus, acidification may affect diatoms significantly differently depending on the Zn levels of the ocean and then alter the biochemical cycling of carbon and silica.

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Cascading effects augment the direct impact of CO2 on phytoplankton growth in a biogeochemical model

Atmospheric and oceanic CO2 concentrations are rising at an unprecedented rate. Laboratory studies indicate a positive effect of rising CO2 on phytoplankton growth until an optimum is reached, after which the negative impact of accompanying acidification dominates. Here, we implemented carbonate system sensitivities of phytoplankton growth into our global biogeochemical model FESOM-REcoM and accounted explicitly for coccolithophores as the group most sensitive to CO2. In idealized simulations in which solely the atmospheric CO2 mixing ratio was modified, changes in competitive fitness and biomass are not only caused by the direct effects of CO2, but also by indirect effects via nutrient and light limitation as well as grazing. These cascading effects can both amplify or dampen phytoplankton responses to changing ocean pCO2 levels. For example, coccolithophore growth is negatively affected both directly by future pCO2 and indirectly by changes in light limitation, but these effects are compensated by a weakened nutrient limitation resulting from the decrease in small-phytoplankton biomass. In the Southern Ocean, future pCO2 decreases small-phytoplankton biomass and hereby the preferred prey of zooplankton, which reduces the grazing pressure on diatoms and allows them to proliferate more strongly. In simulations that encompass CO2-driven warming and acidification, our model reveals that recent observed changes in North Atlantic coccolithophore biomass are driven primarily by warming and not by CO2. Our results highlight that CO2 can change the effects of other environmental drivers on phytoplankton growth, and that cascading effects may play an important role in projections of future net primary production.

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Phosphate limitation intensifies negative effects of ocean acidification on globally important nitrogen fixing cyanobacterium

Growth of the prominent nitrogen-fixing cyanobacterium Trichodesmium is often limited by phosphorus availability in the ocean. How nitrogen fixation by phosphorus-limited Trichodesmium may respond to ocean acidification remains poorly understood. Here, we use phosphate-limited chemostat experiments to show that acidification enhanced phosphorus demands and decreased phosphorus-specific nitrogen fixation rates in Trichodesmium. The increased phosphorus requirements were attributed primarily to elevated cellular polyphosphate contents, likely for maintaining cytosolic pH homeostasis in response to acidification. Alongside the accumulation of polyphosphate, decreased NADP(H):NAD(H) ratios and impaired chlorophyll synthesis and energy production were observed under acidified conditions. Consequently, the negative effects of acidification were amplified compared to those demonstrated previously under phosphorus sufficiency. Estimating the potential implications of this finding, using outputs from the Community Earth System Model, predicts that acidification and dissolved inorganic and organic phosphorus stress could synergistically cause an appreciable decrease in global Trichodesmium nitrogen fixation by 2100.

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High sclerobiont calcification in marginal reefs of the eastern tropical Pacific

Graphical abstract.

A sclerobiont is any organism capable of fouling hard substrates. Sclerobionts have recently received attention due to their notable calcium carbonate contributions to reef structures and potential to offset drops in carbonate budgets in degraded reefs. However, due to their encrusting nature, it is difficult to quantify net calcium carbonate production at the level of individual taxonomic groups, and knowledge regarding the main environmental factors that regulate their spatial distributions is limited. In addition, the material types used to create experimental substrates, their orientations, and their overall deployment times can influence settlement and the composition of the resulting communities. Thus, comparative evaluations of these variables are necessary to improve future research efforts. In this study, we used calcification accretion units (CAUs) to quantify the calcium carbonate contributions of sclerobionts at the taxonomic group level and evaluated the effects of two frequently used materials [i.e., polyvinyl chloride (PVC) and terracotta (TCT) tiles] on the recruitment and calcification of the sclerobiont community in the tropical Mexican Pacific and the Midriff Island Region of the Gulf of California over 6 and 15 months [n = 40; 5 CAUs x site (2) x deployment time (2) x material type (2)]. The net sclerobiont calcification rate (mean ± SD) reached maximum values at six months and was higher in the Mexican Pacific (2.15 ± 0.99 kg m−2 y−1) than in the Gulf of California (1.70 ± 0.67 kg m−2 y−1). Moreover, the calcification rate was slightly higher on the PVC-CAUs compared to that of the TCT-CAUs, although these differences were not consistent at the group level. In addition, cryptic microhabitats showed low calcification rates when compared to those of exposed microhabitatsCrustosecoralline algae and barnacles dominated the exposed experimental surfaces, while bryozoans, mollusks, and serpulid polychaetes dominated cryptic surfaces. Regardless of the site, deployment time, or material type, barnacles made the greatest contributions to calcimass production (between 41 and 88%). Our results demonstrate that the orientation of the experimental substrate, and the material to a lesser extent, influence the sclerobiont community and the associated calcification rate. Upwelling-induced surface nutrient levels, low pH levels, and the aragonite saturation state (ΩAr) limit the early cementation of reef-building organisms in the tropical Mexican Pacific and promote high bioerosion rates in corals of the Gulf of California. Our findings demonstrate that sclerobionts significantly contribute to calcium carbonate production even under conditions of high environmental variability.

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Design of a low-cost pH-Stat to study effects of ocean acidification on growth and nutrient consumption of diatoms

Highlights

  • A low-cost pH-stat was designed to evaluate the effect of pH variations on the growth rate and nutrient consumption in multiple microalgae cultures.
  • The current pH of the ocean resulted in the highest growth rate for P. tricornutum.
  • Nitrate was the limiting nutrient in the three pH levels evaluated.
  • Phosphate and iron were related to the acclimatization response of the microalgae.
  • Efficient pH control allowed for the observation of some of the effects of climate change on diatoms related to nutrient consumption.

Abstract

Increasing CO2 emissions has modified oceanic pH levels. These pH changes affect phytoplankton growth and composition. Diatom cells constitute almost 50% of phytoplankton, and they have significant importance in the ocean food chains and biotechnology industries. Therefore, knowledge of their response to pH changes could be useful for conservation and aquaculture of these species. There are different pH-Stat systems to supply CO2 gas to the culture medium, however, it is common to use one unit or pH probe for each culture. In this study, we designed a low-cost pH-stat to regulate the pH level in fifteen simultaneous cultures. It was evaluated with Phaeodactylum tricornutum at three pH setpoints:7.5 and 7.8 as acid treatments and 8.1 as control; each experiment lasted seven days, and growth rates, latency phases and nutrient consumption rates were determined. The accuracy and precision of the pH regulated was in an acceptable level compared with other systems. The growth rate and consumption of nitrate were higher at pH 8.1, moreover differences were observed in the duration of the latency phase, suggesting a longer acclimation process at lower pH. Changes in phosphate and iron consumption indicated a higher availability in acid treatments, however they did not enhance the growth. These denoted unfavorable effects of ocean acidification on diatoms growth.

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Quantification of the dominant drivers of acidification in the coastal Mid-Atlantic Bight

Abstract

In shallow coastal shelves like the Mid-Atlantic Bight (MAB), ocean acidification due to increased atmospheric carbon dioxide (CO2) is compounded by highly variable coastal processes including riverine freshwater inputs, nutrient loading, biogeochemical influence, coastal currents and water mass mixing, and seasonal transitions in physical parameters. Past deconstructions of carbonate system drivers in the MAB have focused on nearshore zones or single season data, and thus lack the spatial and temporal resolution required to assess impacts to important species occupying the shelf. Deconstructing highly resolved data collected during four seasonal Slocum glider deployments in the MAB, this study uses a Taylor Series decomposition to quantify the influence of temperature, salinity, biogeochemical activity, and water mass mixing on pH and aragonite saturation state from sea surface to bottom. Results show that water mass mixing and biogeochemical activity were the most significant drivers of the carbonate system in the MAB. Nearshore water was more acidic year-round due to riverine freshwater input, but photosynthesis reduced acidity at certain depths and times. Water mass mixing increased acidity in bottom water on the shelf, particularly in summer. Gulf Stream intrusions at the shelf break during fall acted to mitigate acidification on the shelf in habitats occupied by carbonate-bearing organisms. The relationships quantified here can be used to improve biogeochemical forecast models and determine habitat suitability for commercially important fin and shellfish species residing in the MAB.

Key Points

  • Water mass mixing and biogeochemical activity are the major drivers of seasonal carbonate system dynamics in the MAB
  • Water mass mixing has opposing effects on carbonate chemistry in the nearshore and at the continental shelf break

Plain Language Summary

The coastal ocean is experiencing changes in chemistry due to human activities, including carbon dioxide emissions, nutrient runoff, and seasonal changes in temperature, salinity, and coastal currents. These drivers have been studied close to shore and/or only during single seasons, leaving a gap in our understanding of seasonal changes across the entire economically important shelf region. Here, we use high-resolution data collected by a deep-sea robot that measures chemistry from ocean surface to the sea floor. We determined the importance of four key influences (temperature, salinity, water mass mixing, and biological activity) on changes in coastal chemistry over the course of a year. We found that the most important driver of shelf chemistry was mixing of freshwater at the coast and warm, salty water at the edge of the shelf. Biological activity was a secondary influence, which caused smaller scale changes in chemistry. These results can help to predict how coastal chemistry might change in the future, so that we can prepare for the effects on economically important animals and industries.

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Responses of elemental content and macromolecule of the coccolithophore Emiliania huxleyi to reduced phosphorus availability and ocean acidification depend on light intensity

Global climate change leads to simultaneous changes in multiple environmental drivers in the marine realm. Although physiological characterization of coccolithophores have been studied under climate change, there is limited knowledge on the biochemical responses of this biogeochemically important phytoplankton group to changing multiple environmental drivers. Here we investigate the interactive effects of reduced phosphorus availability (4 to 0.4 μmol L–1), elevated pCO2 concentrations (426 to 946 μatm) and increasing light intensity (40 to 300 μmol photons m–2 s–1) on elemental content and macromolecules of the cosmopolitan coccolithophore Emiliania huxleyi. Reduced phosphorus availability reduces particulate organic nitrogen and protein contents under low light intensity, but not under high light intensity. Reduced phosphorus availability and ocean acidification act synergistically to increase particulate organic carbon (POC) and carbohydrate contents under high light intensity but not under low light intensity. Reduced phosphorus availability, ocean acidification and increasing light intensity act synergistically to increase the allocation of POC to carbohydrates. Under future ocean acidification and increasing light intensity, enhanced carbon fixation could increase carbon storage in the phosphorus-limited regions of the oceans where E. huxleyi dominates the phytoplankton assemblages. In each light intensity, elemental carbon to phosphorus (C : P) and nitrogen to phosphorus (N : P) ratios decrease with increasing growth rate. These results suggest that coccolithophores could reallocate chemical elements and energy to synthesize macromolecules efficiently, which allows them to regulate its elemental content and growth rate to acclimate to changing environmental conditions.

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Denitrification and N2O emission in estuarine sediments in response to ocean acidification: from process to mechanism

Graphical abstract

Global estuarine ecosystems are experiencing severe nitrogen pollution and ocean acidification (OA) simultaneously. Sedimentary denitrification is an important way of reactive nitrogen removal but at the same time leads to the emission of large amounts of nitrous oxide (N2O), a potent greenhouse gas. It is known that OA in estuarine regions could impact denitrification and N2O production; however, the underlying mechanism is still underexplored. Here, sediment incubation and pure culture experiments were conducted to explore the OA impacts on microbial denitrification and the associated N2O emissions in estuarine sediments. Under neutral (in situ) conditions, fungal N2O emission dominated in the sediment, while the bacterial and fungal sources had a similar role under acidification. This indicated that acidification decreased the sedimentary fungal denitrification and likely inhibited the activity of fungal denitrifiers. To explore molecular mechanisms, a denitrifying fungal strain of Penicillium janthinellum was isolated from the sediments. By using deuterium-labeled single-cell Raman spectroscopy and isobaric tags for relative and absolute quantitation proteomics, we found that acidification inhibited electron transfers in P. janthinellum and downregulated expressions of the proteins related to energy production and conservation. Two collaborative pathways of energy generation in the P. janthinellum were further revealed, that is, aerobic oxidative phosphorylation and TCA cycle and anoxic pyruvate fermentation. This indicated a distinct energy supply strategy from bacterial denitrification. Our study provides insights into fungi-mediated nitrogen cycle in acidifying aquatic ecosystems.

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Dynamics of carbonate characteristics of the Kara Sea waters in the late autumn season of 2021

The field data characterizing the dynamics of the carbonate system, the level of surface water corrosivity with respect to aragonite, and CO2 fluxes in the ocean–atmosphere system in the Kara Sea in the late autumn season are investigated. The study of carbonate characteristics of waters was carried out on board the R/V Akademik Mstislav Keldysh in October 2021. At that time, the waters of the main part of the studied area of the Kara Sea absorbed carbon dioxide from the atmosphere intensely; in the invasion zone, the average CO2 flux was 30.2 ± 35.5 mmol m–2 day–1. It is shown that, among the Arctic seas in the season under study, the open water area of the Kara Sea was one of the most significant sinks for atmospheric CO2. The change in the flux direction occurred in a narrow coastal zone; supersaturation of waters with CO2 was recorded in the estuarine regions, where the contribution of inland waters exceeded 50%. It is found that, with a small contribution of river waters (<10%), the change in the surface water temperature determined more than 90% of the spatial variability of рСО2. In the late autumn season, the surface waters of the Kara Sea were mainly supersaturated with calcium carbonate; an extremely low aragonite saturation level was found only in the estuarine regions, the areas of maximum influence of river waters.

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Different responses of phytoplankton and zooplankton communities to current changing coastal environments

Marine plankton are faced with novel challenges associated with environmental changes such as ocean acidification, warming, and eutrophication. However, data on the effects of simultaneous environmental changes on complex natural communities in coastal ecosystems are relatively limited. Here we made a systematic analysis of biological and environmental parameters in the Bohai Sea over the past three years to suggest that plankton communities responded differently to current changing coastal environments, with the increase of phytoplankton and the decrease of zooplankton. These different changes of phyto- and zooplankton potentially resulted from the fact that both the effect of acidification as a result of pH decline and the effect of warming as a consequence of increasing temperature favored phytoplankton over zooplankton at present. Furthermore, water eutrophication and salinity as well as heavy metals Hg, Zn, and As had more or less diverse consequences for the dynamics of phytoplankton and zooplankton. Differently, with ongoing climate change, we also revealed that both phytoplankton and zooplankton would decrease in the future under the influence of interactions between acidification and warming.

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Interactive effects of CO2, temperature, irradiance, and nutrient limitation on the growth and physiology of the marine cyanobacterium Synechococcus (Cyanophyceae)

The marine cyanobacterium Synechococcus elongatus was grown in a continuous culture system to study the interactive effects of temperature, irradiance, nutrient limitation, and the partial pressure of CO2 (pCO2) on its growth and physiological characteristics. Cells were grown on a 14:10 h light:dark cycle at all combinations of low and high irradiance (50 and 300 μmol photons ⋅ m−2 ⋅ s−1, respectively), low and high pCO2 (400 and 1000 ppmv, respectively), nutrient limitation (nitrate-limited and nutrient-replete conditions), and temperatures of 20–45°C in 5°C increments. The maximum growth rate was ~4.5 · d−1 at 30–35°C. Under nutrient-replete conditions, growth rates at most temperatures and irradiances were about 8% slower at a pCO2 of 1000 ppmv versus 400 ppmv. The single exception was 45°C and high irradiance. Under those conditions, growth rates were ~45% higher at 1000 ppmv. Cellular carbon:nitrogen ratios were independent of temperature at a fixed relative growth rate but higher at high irradiance than at low irradiance. Initial slopes of photosynthesis–irradiance curves were higher at all temperatures under nutrient-replete versus nitrate-limited conditions; they were similar at all temperatures under high and low irradiance, except at 20°C, when they were suppressed at high irradiance. A model of phytoplankton growth in which cellular carbon was allocated to structure, storage, or the light or dark reactions of photosynthesis accounted for the general patterns of cell composition and growth rate. Allocation of carbon to the light reactions of photosynthesis was consistently higher at low versus high light and under nutrient-replete versus nitrate-limited conditions.

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Impact of intermittent convection in the northwestern Mediterranean Sea on oxygen content, nutrients and the carbonate system

Abstract

Using Argo profiling floats, cruises and mooring data, we reconstructed the dissolved oxygen (O2) dynamics in the Gulf of Lion and the Ligurian Sea, with a focus on the intermediate waters. By applying the CANYON-MED neural network-based method on the large network of O2-equipped Argo floats we derived nutrients and carbonate system variables in the Gulf of Lion and the Ligurian Sea at different depths in the water column and derived trends over the 2012-2020 period. In these waters, the O2 minimum is strongly affected by the intermittent convection process, and the two areas show dissimilar responses to the mixing events. In the absence of deep convection events, the O2-depleted layer tends to spread vertically and intensify even more so in the Ligurian than in the Gulf of Lion. In both areas, over the 2012-2020 period, nutrients increase overall in deep layers, with a concomitant impact on nutrient molar ratios tending towards an increase in P-limitation. Acidification estimates derived in different layers of the water column show an overall increase in dissolved inorganic carbon and a concurrent pH decrease. These trends were strongly affected by convection events slowing down the overall acidification trend.

Key Points

  • In the absence of deep convection events, the O2-depleted layer spreads vertically and intensifies more in the Ligurian than Gulf of Lion.
  • Nutrients increase in deep and to a lesser extent in intermediate waters with a decoupling between nitrate and phosphate trends.
  • Dissolved inorganic carbon increases in intermediate and deep waters with a concurrent pH decrease over the period of study, 2012-2020.
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Ocean acidification but not nutrient enrichment reduces grazing and alters diet preference in Littorina littorea

Ocean acidification and eutrophication have direct, positive effects on the growth of many marine macroalgae, potentially resulting in macroalgal blooms and shifts in ecosystem structure and function. Enhanced growth of macroalgae, however, may be controlled by the presence of grazers. While grazing under ocean acidification and eutrophication conditions has variable responses, there is evidence of these factors indirectly increasing consumption. We tested whether a common marine herbivorous snail, Littorina littorea, would increase consumption rates of macroalgae (Ulva and Fucus) under ocean acidification (increased pCO2) and/or eutrophication conditions, via feeding trials on live and reconstituted algal thalli. We found that increased pCO2 resulted in reduced grazing rates on live thalli, with snails feeding almost exclusively on Ulva. However, eutrophication did not impact consumption rates of live tissues. In addition, similarity in consumption of reconstituted Ulva and Fucus tissues across all treatments indicated that physical characteristics of algal tissues, rather than tissue chemistry, may drive dietary shifts in a changing climate. In this system, decreased consumption, coupled with increased growth of macroalgae, may ultimately enhance algal growth and spread.

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Pelagic calcifiers face increased mortality and habitat loss with warming and ocean acidification

Global change is impacting the oceans in an unprecedented way, and multiple lines of evidence suggest that species distributions are changing in space and time. There is increasing evidence that multiple environmental stressors act together to constrain species habitat more than expected from warming alone. Here, we conducted a comprehensive study of how temperature and aragonite saturation state act together to limit Limacina helicina, globally distributed pteropods that are ecologically important pelagic calcifiers and an indicator species for ocean change. We co-validated three different approaches to evaluate the impact of ocean warming and acidification (OWA) on the survival and distribution of this species in the California Current Ecosystem. First, we used colocated physical, chemical, and biological data from three large-scale west coast cruises and regional time series; second, we conducted multifactorial experimental incubations to evaluate how OWA impacts pteropod survival; and third, we validated the relationships we found against global distributions of pteropods and carbonate chemistry. OWA experimental work revealed mortality increases under OWA, while regional habitat suitability indices and global distributions of L. helicina suggest that a multi-stressor framework is essential for understanding pteropod distributions. In California Current Ecosystem habitats, where pteropods are living close to their thermal maximum already, additional warming and acidification through unabated fossil fuel emissions (RCP 8.5) are expected to dramatically reduce habitat suitability.

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Phycosphere pH of unicellular nano- and micro- phytoplankton cells and consequences for iron speciation

Surface ocean pH is declining due to anthropogenic atmospheric CO2 uptake with a global decline of ~0.3 possible by 2100. Extracellular pH influences a range of biological processes, including nutrient uptake, calcification and silicification. However, there are poor constraints on how pH levels in the extracellular microenvironment surrounding phytoplankton cells (the phycosphere) differ from bulk seawater. This adds uncertainty to biological impacts of environmental change. Furthermore, previous modelling work suggests that phycosphere pH of small cells is close to bulk seawater, and this has not been experimentally verified. Here we observe under 140 μmol photons·m−2·s−1 the phycosphere pH of Chlamydomonas concordia (5 µm diameter), Emiliania huxleyi (5 µm), Coscinodiscus radiatus (50 µm) and C. wailesii (100 µm) are 0.11 ± 0.07, 0.20 ± 0.09, 0.41 ± 0.04 and 0.15 ± 0.20 (mean ± SD) higher than bulk seawater (pH 8.00), respectively. Thickness of the pH boundary layer of C. wailesii increases from 18 ± 4 to 122 ± 17 µm when bulk seawater pH decreases from 8.00 to 7.78. Phycosphere pH is regulated by photosynthesis and extracellular enzymatic transformation of bicarbonate, as well as being influenced by light intensity and seawater pH and buffering capacity. The pH change alters Fe speciation in the phycosphere, and hence Fe availability to phytoplankton is likely better predicted by the phycosphere, rather than bulk seawater. Overall, the precise quantification of chemical conditions in the phycosphere is crucial for assessing the sensitivity of marine phytoplankton to ongoing ocean acidification and Fe limitation in surface oceans.

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Transitioning global change experiments on Southern Ocean phytoplankton from lab to field settings: insights and challenges

The influence of global change on Southern Ocean productivity will have major ramifications for future management of polar life. A prior laboratory study investigated the response of a batch-cultured subantarctic diatom to projected change simulating conditions for 2100 (increased temperature/CO2/irradiance/iron; decreased macronutrients), showed a twofold higher chlorophyll-derived growth rate driven mainly by temperature and iron. We translated this design to the field to understand the phytoplankton community response, within a subantarctic foodweb, to 2100 conditions. A 7-d shipboard study utilizing 250-liter mesocosms was conducted in March 2016. The outcome mirrors lab-culture experiments, yielding twofold higher chlorophyll in the 2100 treatment relative to the control. This trend was also evident for intrinsic metrics including nutrient depletion. Unlike the lab-culture study, photosynthetic competence revealed a transient effect in the 2100 mesocosm, peaking on day 3 then declining. Metaproteomics revealed significant differences in protein profiles between treatments by day 7. The control proteome was enriched for photosynthetic processes (c.f. 2100) and exhibited iron-limitation signatures; the 2100 proteome exposed a shift in cellular energy production. Our findings of enhanced phytoplankton growth are comparable to model simulations, but underlying mechanisms (temperature, iron, and/or light) differ between experiments and models. Batch-culture approaches hinder cross-comparison of mesocosm findings to model simulations (the latter are akin to “continuous-culture chemostats”). However, chemostat techniques are problematic to use with mesocosms, as mesozooplankton will evade seawater flow-through, thereby accumulating. Thus, laboratory, field, and modeling approaches reveal challenges to be addressed to better understand how global change will alter Southern Ocean productivity.

Continue reading ‘Transitioning global change experiments on Southern Ocean phytoplankton from lab to field settings: insights and challenges’

Impact of atmospheric dry deposition of nutrients on phytoplankton pigment composition and primary production in the coastal Bay of Bengal

Atmospheric deposition of pollutants decreases pH and increases the nutrient concentration in the surface water. To examine its impact on coastal phytoplankton composition and primary production, monthly atmospheric aerosol samples were mixed with coastal waters in the microcosm experiments. These experiments suggested that the biomass of Bacillariophyceae, Dinophyceae and Chlorophyceae were increased and primary production of the coastal waters increased by 3 to 19% due to the addition of aeolian nutrients. The increase in primary production displayed significant relation with a concentration of sulphate and nitrate in the atmospheric aerosols suggesting that both decreases in pH and fertilization enhanced primary production. The impact of acidification on primary production was found to be 22%, whereas 78% was contributed by the nutrient increase. The atmospheric pollution is increasing rapidly over the northern Indian Ocean since past two decades due to rapid industrialization. Hence, it is suggested that the impact of atmospheric pollution on the coastal ecosystem must be included in the numerical models to predict possible changes in the coastal ecosystem due to climate change.

Continue reading ‘Impact of atmospheric dry deposition of nutrients on phytoplankton pigment composition and primary production in the coastal Bay of Bengal’

Systematic review and meta-analysis of ocean acidification effects in Halimeda: implications for algal carbonate production

Highlights

  • Calcification responses to OA vary widely among Halimeda species (neutral, negative).
  • For some species, these responses also seem to be region-dependent.
  • Experimental evidence suggests future declines in Halimeda-derived CaCO3 production.
  • Occurrence and magnitude of declines will be determined by community composition.

Abstract

Ocean acidification (OA) has been identified as one of the major climate-change related threats, mainly due to its significant impacts on marine calcifiers. Among those are the calcareous green algae of the genus Halimeda that are known to be major carbonate producers in shallow tropical and subtropical seas. Hence, any negative OA impacts on these organisms may translate into significant declines in regional and global carbonate production. In this study, we compiled the available information regarding Halimeda spp. responses to OA (experimental, in situ), with special focus on the calcification responses, one of the most studied response parameters in this group. Furthermore, among the compiled studies (n = 31), we selected those reporting quantitative data of OA effects on algal net calcification in an attempt to identify potential general patterns of species- and/or regional-specific OA responses and hence, impacts on carbonate production. While obtaining general patterns was largely hampered by the often scarce number of studies on individual species and/or regions, the currently available information indicates species-specific susceptibility to OA, seemingly unrelated to evolutionary lineages (and associated differences in morphology), that is often accompanied by differences in a species’ response across different regions. Thus, for projections of future declines in Halimeda-associated carbonate production, we used available regional reports of species-specific carbonate production in conjunction with experimental OA responses for the respective species and regions. Based on the available information, declines can be expected worldwide, though some regions harbouring more sensitive species might be more impacted than others.

Continue reading ‘Systematic review and meta-analysis of ocean acidification effects in Halimeda: implications for algal carbonate production’

The Mediterranean Rhodes gyre: modelled impacts of climate change, acidification and fishing

The Mediterranean Rhodes gyre is a cyclonic gyre with high primary production due to local upwelling of nutrients, and occasional deep overturning up to 1km depth. This nutrient-rich state is in sharp contrast to other parts of the Eastern Mediterranean which are oligotrophic. Here we study the upwelling system central to the Rhodes gyre and the impact of different stressors like meteorological changes, acidification and fishing pressure up to the year 2100. A water column model spanning the physical, chemical and biological system up to top predators (GOTM-ERSEM-BFM-EwE) was used to simulate the pelagic environment under single and combined stressors. Results show that due to increasing winter temperatures deep overturning events are becoming more rare in the future, until they stop occurring around 2060 under the business-as-usual climate scenario (RCP8.5). Stratification becomes stronger as temperature effects outweigh salinity effects in the surface mixed layer. Together with the lack of deep overturning this limits the nutrient supply to the euphotic zone, significantly reducing primary production. Phytoplankton species shift towards smaller species as nutrients become more scarce, mimicking the situation found currently on the edge of the gyre. Climatic changes and fishing pressure affected higher trophic levels in an additive way for some species (sardines, dolphins), while in a synergistic way for others (anchovy, mackerel). Acidification impacts were negligible. Fish stocks reduced significantly under the 2 climate scenarios considered: ~30% under RCP4.5 and ~40% under RCP8.5, with limited beneficial impact of MSY-level fishing, indicating a need for mitigating measures beyond fleet control.

Continue reading ‘The Mediterranean Rhodes gyre: modelled impacts of climate change, acidification and fishing’

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