Posts Tagged 'North Atlantic'

Dissolved inorganic carbon export from rivers of Great Britain: spatial distribution and potential catchment-scale controls

Highlights

  • A survey of DIC was carried out across 41 rivers in Great Britain.
  • Results were examined in relation to land cover and natural gradients across Great Britain.
  • Estimated average yield of DIC from the survey catchments to the sea was 8.13 t ha−1 yr−1.
  • Free CO2 concentrations were strongly linked to catchment macro-nutrient status.
  • Free CO2 yield at was estimated to be 0.56 t C km2 yr−1.

Abstract

Dissolved inorganic carbon (DIC) fluxes from the land to ocean have been quantified for many rivers globally. However, CO2 fluxes to the atmosphere from inland waters are quantitatively significant components of the global carbon cycle that are currently poorly constrained. Understanding, the relative contributions of natural and human-impacted processes on the DIC cycle within catchments may provide a basis for developing improved management strategies to mitigate free CO2 concentrations in rivers and subsequent evasion to the atmosphere. Here, a large, internally consistent dataset collected from 41 catchments across Great Britain (GB), accounting for ∼36% of land area (∼83,997 km2) and representative of national land cover, was used to investigate catchment controls on riverine dissolved inorganic carbon (DIC), bicarbonate (HCO3) and free CO2 concentrations, fluxes to the coastal sea and annual yields per unit area of catchment. Estimated DIC flux to sea for the survey catchments was 647 kt DIC yr−1 which represented 69% of the total dissolved carbon flux from these catchments. Generally, those catchments with large proportions of carbonate and sedimentary sandstone were found to deliver greater DIC and HCO3 to the ocean. The calculated mean free CO2 yield for survey catchments (i.e. potential CO2 emission to the atmosphere) was 0.56 t C km−2 yr−1. Regression models demonstrated that whilst river DIC (R2 = 0.77) and HCO3 (R2 = 0.77) concentrations are largely explained by the geology of the landmass, along with a negative correlation to annual precipitation, free CO2 concentrations were strongly linked to catchment macronutrient status. Overall, DIC dominates dissolved C inputs to coastal waters, meaning that estuarine carbon dynamics are sensitive to underlying geology and therefore are likely to be reasonably constant. In contrast, potential losses of carbon to the atmosphere via dissolved CO2, which likely constitute a significant fraction of net terrestrial ecosystem production and hence the national carbon budget, may be amenable to greater direct management via altering patterns of land use.

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Sub-annual and inter-annual variations of total alkalinity in the northeastern Gulf of Mexico

We analyzed sub-annual and inter-annual variations of total alkalinity (TA) distributions in the northeastern Gulf of Mexico using TA data collected in seven cruises from 2012 to 2014, riverine TA records, and surface current and salinity data from Hybrid Coordinate Ocean Model reanalysis. Significant sub-annual and inter-annual TA variations were observed in the upper 150 m of the water column, mainly controlled by riverine TA inputs and ocean currents. Generally, the influence from riverine TA was strongest in the summer, when the influence of freshwater plume extended far from shore and riverine TA was high. Deep-water in the western part of our study area was mainly influenced by the Mississippi-Atchafalaya River plume, and the strength of riverine TA inputs mostly depended on distance from shore. Riverine inputs decreased from the Mississippi-Atchafalaya river-influenced area toward the West Florida Shelf. The riverine TA source for the coastal region near the West Florida Shelf was a composite of multiple local inputs that were highly modulated by both along-shore and offshore currents, and therefore exhibited larger sub-annual and inter-annual variations.

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Advancing real-time pH sensing capabilities to monitor coastal acidification as measured in a productive and dynamic estuary (Ría de Arousa, NW Spain)

Ocean acidification has critical impacts on marine ecosystems, but presents knowledge gaps on the ecological impacts requiring large-scale monitoring of physicochemical conditions to predict biological responses to ocean pH projections. The threat is especially significant in coastal regions like upwelling areas which are more sensitive and appear to respond more rapidly to anthropogenic perturbations. These ecosystems, such as the northwest coast of the Iberian Peninsula are characterized by complex physical and biogeochemical interactions, supporting enormous biological productivity and productive fisheries. The distribution of pH in upwelling systems has high variability on short temporal and spatial scales preventing a complete picture of acidification, which exhibit long-term pH rates markedly different from the measured in open waters. This motivation to significantly expand the coverage of pH monitoring in coastal areas has driven us to develop an autonomous pH monitoring instrument (from now on SURCOM) based on the Honeywell Durafet® pH electrode. A relevant feature is that SURCOM transmits near real-time pH and temperature measurements every 10.5 min through SIGFOX®, a low-power, low-bandwidth network for data transmission. This very careful design allows us to achieve a very low power consumption for the complete system resulting in 3 years of full autonomy with no other need than external cleaning and calibration. In this paper we describe the setup and the data set obtained by a SURCOM instrument over 240 days in a highly productive and dynamic coastal ecosystem, the Ría de Arousa embayment, providing valuable information on the performance of these low-cost and highly stable sensors, with potential for improving the pH variability description in nearshore systems and for reinforcing the monitoring-modeling of coastal acidification.

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Neural networks and the seawater CO2 system. From the global ocean to the Ría de Vigo

This doctoral dissertation is structured in six chapters and two appendices. From this point the reader is warned about the independent numbering in each chapter, both for sections and subsections as well as for figures and tables, that is, the numbering restarts at the beginning of each chapter. Chapter I is divided into two parts. On the one hand, the topic of the doctoral dissertation is introduced in a general way to put in context the different research studies that are part of it. This introduction presents the key concepts on climate change and ocean acidification necessary to approach the reading of the following chapters. On the other hand, the main and secondary objectives that are addressed in the next chapters are detailed. Chapter II develops the construction of a global and seasonal climatology of total alkalinity. The chapter details for the first time in the thesis the use of neural networks. This methodology is used throughout the manuscript, highlighting the peculiarities associated with each study in each of the chapters where it is applied. This chapter has been published in Earth System Science Data: https://doi.org/10.5194/essd-11-1109-2019 Chapter III describes the development of a total dissolved inorganic carbon climatology. In general terms, a methodology similar to that of chapter II is used, although with certain relevant nuances such as the inclusion of a new database. In this chapter, a pCO2 climatology is also generated in a secondary way to evaluate the consistency between the two climatologies previously generated in this thesis. This chapter has been published in Earth System Science Data: https://doi.org/10.5194/essd-12-1725-2020.


Chapter IV completely changes the scale of the previous two chapters and focuses on the study of sweater CO2 chemistry system on a regional scale. Specifically, neural networks are used to generate time series of total alkalinity and pH at various locations in the Ría de Vigo. From the time series, the magnitude of seasonal variability and interannual trends for these variables are analyzed. This chapter has been published in Biogeosciences Discussions: https://doi.org/10.5194/bg-2021-33, 2021 Chapter V contains an analysis of the variability of the hydrogen ion concentration and the aragonite saturation state in the Ría de Vigo. This analysis is carried out from the time series of these variables that are constructed thanks to the study developed in chapter IV. Chapters II to V are structured in the same way as a typical scientific article, thus containing an introduction, methodology, results, discussion and conclusions about each study. Finally, chapter VI summarizes the main conclusions derived from the complete work shown through this doctoral thesis. It is worth noting the inclusion of two appendices in the final part of the thesis. Appendix I details the meaning of each of the acronyms, abbreviations and symbols used throughout the manuscript. Appendix II contains a summary of the doctoral dissertation in Spanish

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Alkalinity generation from carbonate weathering in a silicate-dominated headwater catchment at Iskorasfjellet, northern Norway

The weathering rate of carbonate minerals is several orders of magnitude higher than for silicate minerals. Therefore, small amounts of carbonate minerals have the potential to control the dissolved weathering loads in silicate-dominated catchments. Both weathering processes produce alkalinity under the consumption of CO2. Given that only alkalinity generation from silicate weathering is thought to be a long-term sink for CO2, a misattributed weathering source could lead to incorrect conclusions about long- and short-term CO2 fixation. In this study, we aimed to identify the weathering sources responsible for alkalinity generation and CO2 fixation across watershed scales in a degrading permafrost landscape in northern Norway, 68.7–70.5° N, and on a temporal scale, in a subarctic headwater catchment on the mountainside of Iskorasfjellet, characterized by sporadic permafrost and underlain mainly by silicates as the alkalinity-bearing lithology. By analysing total alkalinity (AT) and dissolved inorganic carbon (DIC) concentrations, as well as the stable isotope signature of the latter (δ13C-DIC) in conjunction with dissolved cation and anion loads, we found that AT was almost entirely derived from weathering of the sparse carbonate minerals. We propose that in the headwater catchment, the riparian zone is a hotspot area of AT generation and release due to its enhanced hydrological connectivity, and that the weathering load contribution from the uphill catchment is limited by insufficient contact time of weathering agent and weatherable material. By using stable water isotopes, it was possible to explain temporal variations in AT concentrations following a precipitation event due to surface runoff. In addition to carbonic acid, sulphuric acid, probably originating from pyrite oxidation, is shown to be a potential corrosive reactant. An increased proportion of sulphuric acid as a potential weathering agent may have resulted in a decrease in AT. Therefore, carbonate weathering in the studied area should be considered not only as a short-term CO2 sink, but also as a potential CO2 source. Finally, we found that AT increased with decreasing permafrost probability, and attributed this relation to an increased water storage capacity associated with increasing contact of weathering agent and rock surfaces, and enhanced microbial activity. As both soil respiration and permafrost thaw are expected to increase with climate change, increasing the availability of weathering agent in the form of CO2 and water storage capacity, respectively, we suggest that future weathering rates and alkalinity generation will increase concomitantly in the study area.

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Aragonite saturation states in estuaries along a climate gradient in the northwestern Gulf of Mexico

In the northwestern Gulf of Mexico (nwGOM), the coastal climate shifts abruptly from the humid northeast to the semiarid southwest within a narrow latitudinal range. The climate effect plays an important role in controlling freshwater discharge into the shallow estuaries in this region. In addition to diminishing freshwater runoff down the coast, evaporation also increases substantially. Hence, these estuaries show increasing salinity along the coastline due to the large difference in freshwater inflow balance (river runoff and precipitation minus evaporation and diversion). However, this spatial gradient can be disrupted by intense storm events as a copious amount of precipitation leads to river flooding, which can cause temporary freshening of these systems in extreme cases, in addition to freshwater-induced ephemeral stratification. We examined estuarine water aragonite saturation state (Ωarag) data collected between 2014 and 2018, covering a period of contrasting hydrological conditions, from the initial drought to multiple flooding events, including a brief period that was influenced by a category 4 hurricane. Based on freshwater availability, these estuaries exhibited a diminishing Ωarag fluctuation from the most freshwater enriched Guadalupe Estuary to the most freshwater-starved Nueces Estuary. While Ωarag values were usually much higher than the threshold level (Ωarag = 1), brief freshwater discharge events and subsequent low oxygen levels in the lower water column led to episodic corrosive conditions. Based on previously obtained Ωarag temporal trends and Ωarag values obtained in this study, we estimated the time of emergence (ToE) for Ωarag. Not only did estuaries show decreasing ToE with diminishing freshwater availability but the sub-embayments of individual estuaries that had a less freshwater influence also had shorter ToE. This spatial pattern suggests that planning coastal restoration efforts, especially for shellfish organisms, should emphasize areas with longer ToE.

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Ocean acidification modifies behaviour of shelf seabed macrofauna: a laboratory study on two ecosystem engineers, Abra alba and Lanice conchilega

The feeding activity and burrow ventilation by benthic invertebrates importantly affect the biodiversity and functioning of seafloor sediments. Here we investigated how ocean acidification can modify these behavioural activities in two common and abundant macrofaunal ecosystem engineering species in temperate continental shelf communities: the white furrow shell Abra alba and the sand mason Lanice conchilega. Using time-lapse imagery and sediment porewater hydraulic signatures we show that both species adapt their behaviour in response to predicted future pH conditions (−0.3 units). During a three-week laboratory experiment, A. alba reduced the duration per feeding event when suspension and deposit feeding (by 86 and 53%, respectively), and almost completely ceased suspension feeding under reduced seawater pH in comparison to ambient seawater pH (pH ∼ 8.2). This behavioural change reduces the intake of low pH water during feeding and respiration. L. conchilega increased its piston-pumping frequency by 30 and 52%, respectively, after one and two weeks of exposure to future pH conditions (−0.3 units) relative to ambient conditions. This change in irrigation activity suggests higher metabolic demands under low seawater pH, and also extended low water column pH conditions deeper into the seafloor. Because the distribution of other populations depends on the physicochemical setting by our focal species, we argue that the demonstrated behavioural plasticity will likely have cascading effects on seafloor diversity and functioning, highlighting the complexity of how ocean acidification, and climate change in general, will affect seafloor ecology.

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Gadolinium ecotoxicity is enhanced in a warmer and acidified changing ocean as shown by the surf clam Spisula solida through a multibiomarker approach

Highlights

  • Spisula solida accumulated Gd after just one day.
  • Climate change did not impact Gd accumulation and elimination.
  • Gd was not proficiently eliminated in 7 days.
  • Lipid peroxidation was greater in clams exposed to warming and Gd.
  • Gd showed enhanced ecotoxicity in climate change conditions.

Abstract

Humans have exhaustively combusted fossil fuels, and released pollutants into the environment, at continuously faster rates resulting in global average temperature increase and seawater pH decrease. Climate change is forecasted to exacerbate the effects of pollutants such as the emergent rare earth elements. Therefore, the objective of this study was to assess the combined effects of rising temperature (Δ = + 4 °C) and decreasing pH (Δ = − 0.4 pH units) on the bioaccumulation and elimination of gadolinium (Gd) in the bioindicator bivalve species Spisula solida (Surf clam). We exposed surf clams to 10 µg L−1 of GdCl3 for seven days, under warming, acidification, and their combination, followed by a depuration phase lasting for another 7 days and investigated the Gd bioaccumulation and oxidative stress-related responses after 1, 3 and 7 days of exposure and the elimination phase. Gadolinium accumulated after just one day with values reaching the highest after 7 days. Gadolinium was not eliminated after 7 days, and elimination is further hampered under climate change scenarios. Warming and acidification, and their interaction did not significantly impact Gd concentration. However, there was a significant interaction on clam’s biochemical response. The augmented total antioxidant capacity and lipid peroxidation values show that the significant impacts of Gd on the oxidative stress response are enhanced under warming while the increased superoxide dismutase and catalase values demonstrate the combined impact of Gd, warming & acidification. Ultimately, lipid damage was greater in clams exposed to warming & Gd, which emphasizes the enhanced toxic effects of Gd in a changing ocean.

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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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Towards modelling cold-water coral reef-scale crumbling: including morphological variability in mechanical surrogate models

The structural complexity of cold-water corals is threatened by ocean acidification. Increased porosity and weakening of structurally critical parts of the reef framework may lead to rapid physical collapse on an ecosystem scale, reducing their potential for biodiversity support. We can use computational models to describe the mechanisms leading to reef-crumbling. How-ever, the implementation of such models into an efficient predictive tool that allows us to determine risk and timescales of reef collapse is missing. Here, we identified possible surrogate models to represent the branching architecture of the cold-water coral species Lophelia pertusa. For length scales greater than 13 cm, a continuum finite element mechanical approach can be used to analyse mechanical competence whereas at smaller length scales, mechanical surrogate models need to explicitly account for the statistical differences in the structure. We showed large morphological variations between L. pertusa colonies and branches, as well as dead and live skeletal structures, which need to be considered for the development of rapid monitoring tools for predicting risk of cold-water coral reefs crumbling. This will allow us to investigate timescales of changes, including the impact of exposure times to acidified waters on reef-crumbling.

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Benthic foraminifera and pore water carbonate chemistry on a tidal flat and salt marsh at Ria Formosa, Algarve, Portugal

Graphical abstract

Highlights

  • Foraminifera and halophytes showed a relationship with pore water properties.
  • Soil salinity and evaporation are the governing environmental factors.
  • Agglutinated foraminifera were rather related to pore water pCO2 than to submergence time or elevation.
  • Calcareous foraminifera specialised to tolerate carbonate-corrosive conditions prevailed at lowest saturation levels.

Abstract

Benthic foraminifera showed a vertical zonation in tidally influenced salt marshes, which has been used for sea level reconstructions. Growing evidence suggested that freshwater influx, salinity, or the pH of interstitial waters has also an impact on the foraminiferal distribution. A tidal flat and salt marsh transect was investigated in the north-western Ria Formosa coastal lagoon, Algarve, Portugal, to constrain the relationship of benthic foraminifera, halophytes, and pore water properties. The dominance of saltworts from the subfamily Salicornioideae and landward increasing soil salinities depicted evaporation as governing environmental factor. The carbonate chemistry from lagoonal and pore waters identified anoxic tidal flat sediments of as main source of total alkalinity. The alkalinity was lower in the salt marsh, where the pCO2 was extremely high. Salt marsh pore waters showed a high variability of carbonate system parameters, which mirrored small-scale spatial heterogeneities in the soil. The distribution of textulariid salt marsh foraminifera was confined to the vegetated zones, where their abundance increased with elevation. Calcareous species were frequent on the tidal flat and in the highest salt marsh. Many of them were specialised to high salinities or to extreme and variable environmental conditions. Two levels of faunal change in the salt marsh coincide with vegetation zonal boundaries, mean tide or mean high water levels. The two other faunal changes were related to changes in calcite saturation state or organic carbon concentrations. The proportion of textulariids showed a negative correlation with submergence time or elevation, and a significant correlation with pore water pCO2. The faunal distribution, pore water calcite saturation, and Ammonia dissolution patterns indicated that calcareous species specialised to tolerate carbonate-corrosive conditions prevailed even at lowest saturation levels.

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Inorganic carbon transport and dynamics in the Florida Straits

Abstract

Ocean heat and carbon are transported through the Florida Straits, contributing to the Atlantic Meridional Overturning Circulation, and playing an important role in climate. Insufficient observations of carbonate chemistry within the Florida Straits have limited our understanding of ocean acidification within this region. To examine carbonate chemistry and carbon transport dynamics within this region, we developed an algorithm to estimate dissolved inorganic carbon (DIC) using more routinely measured input parameters (temperature, salinity, and dissolved oxygen [DO]) and the corresponding sampling date, depth, and longitude. The developed DIC algorithm output demonstrates good agreement with limited existing in situ observations. By applying this algorithm, we developed a seasonally resolved time series of DIC spanning from 2002 to 2018 for the Florida Straits at 27°N. This time series suggests that short-term variations in surface water DO and DIC were strongly influenced by the Florida Current transport. The long-term increase in DIC was mainly caused by anthropogenic carbon accumulation and DO decrease. The highest increasing rate in DIC was found in North Atlantic Central Water where DO decrease was fastest while the decreasing rate in pH was highest in Antarctic Intermediate Water (AAIW) because of the lower buffer capacity of this water mass. The long-term pH decrease, especially in AAIW, can impact the health of deep corals in the Florida Straits. Quantifying carbon transport between the coast of Florida and the Bahamas is important to understanding the carbonate chemistry dynamics and the long-term acidification of this important region.

Key Points

  • Short-term variations in surface water oxygen and dissolved inorganic carbon concentrations are strongly influenced by the Florida Current transport
  • Anthropogenic carbon accumulation and dissolved oxygen decrease are two main factors that lead to the long-term increase in dissolved inorganic carbon of the Florida Straits
  • The highest long-term rate of increase in dissolved inorganic carbon of North Atlantic Central Water is likely due to the highest rate of increase in respiration
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Impact of local rivers on coastal acidification

Coastal ecosystems are highly dynamic areas for carbon cycling and are likely to be negatively impacted by increasing ocean acidification. This research focused on dissolved inorganic carbon (DIC) and total alkalinity (TA) in the Mississippi Sound to understand the influence of local rivers on coastal acidification. This area receives large fluxes of freshwater from local rivers, in addition to episodic inputs from the Mississippi River through a human-built diversion, the Bonnet Carré Spillway. Sites in the Sound were sampled monthly from August 2018 to November 2019 and weekly from June to August 2019 in response to an extended spillway opening. Prior to the 2019 spillway opening, the contribution of the local, lower alkalinity rivers to the Sound may have left the study area more susceptible to coastal acidification during winter months, with aragonite saturation states (Ωar) < 2. After the spillway opened, despite a large increase in TA throughout the Sound, aragonite saturation states remained low, likely due to hypoxia and increased CO2 concentrations in subsurface waters. Increased Mississippi River input could represent a new normal in the Sound’s hydrography during spring and summer months. The spillway has been utilized more frequently over the last two decades due to increasing precipitation in the Mississippi River watershed, which is primarily associated with climate change. Future increases in freshwater discharge and the associated declines in salinity, dissolved oxygen, and Ωar in the Sound will likely be detrimental to oyster stocks and the resilience of similar ecosystems to coastal acidification.

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Environmental change impacts on shell formation in the muricid Nucella lapillus

Environmental change is a significant threat to marine ecosystems worldwide. Ocean acidification, global warming and long-term emissions of anthropogenic effluents are all negatively impacting aquatic life. Marine calcifying organisms, in particular, are expected to be severely affected by decreasing seawater pH, resulting in shell dissolution and retardations during the formation and repair of shells. Understanding the underlying biological and environmental factors driving species vulnerabilities to habitat alterations is thus crucial to our ability to faithfully predict impacts on marine ecosystems under an array of environmental change scenarios. So far, existing knowledge about organism responses mainly stems from short to medium term laboratory experiments of single species or over- simplified communities. Although these studies have provided important insights, results may not translate to organism responses in a complex natural system requiring a more holistic experimental approach. In this thesis, I investigated shell formation mechanisms and shape and elemental composition responses in the shell of the important intertidal predatory muricid Nucella lapillus both in situ and across heterogeneous environmental gradients. The aim was to identify potential coping mechanisms of N. lapillus to environmental change and provide a more coherent picture of shell formation responses along large ecological gradients in the spatial and temporal domain. To investigate shell formation mechanisms, I tested for the possibility of shell recycling as a function to reduce calcification costs during times of exceptional demand using a multi-treatment shell labelling experiment. Reports on calcification costs vary largely in the literature. Still, recent discoveries showed that costs might increase as a function of decreasing calcification substrate abundance, suggesting that shell formation becomes increasingly more costly under future environmental change scenarios. However, despite the anticipated costs, no evidence was found that would indicate the use of functional dissolution as a means to recycle shell material for a more cost-efficient shell formation in N. lapillus. To investigate shell formation responses, I combined morphometric and shell thickness analyses with novel statistical methods to identify natural shape and thickness response of N. lapillus to large scale variability in temperature, salinity, wind speed and the carbonate system across a wide geographic range (from Portugal to Iceland) and through time (over 130 years). I found that along geographical gradients, the state of the carbonate system and, more specifically, the substrate inhibitor ratio ([HCO3−][H+]−1) (SIR) was the main predictor for shape variations in N. lapillus. Populations in regions with a lower SIR tend to form narrower shells with a higher spire to body whorl ratio. In contrast, populations in regions with a higher SIR form wider shells with a much lower spire to body whorl ratio. The results suggest a widespread phenotypic response of N. lapillus to continuing ocean acidification could be expected, affecting its phenotypic response patterns to predator or wave exposure regimes with profound implications for North Atlantic rocky shore communities. On the contrary, investigations of shell shape and thickness changes over the last 130 years from adjacent sampling regions on the Southern North Sea coast revealed that contrary to global predictions, N. lapillus built continuously thicker shells while maintaining a consistent shell shape throughout the last century. Systematic modelling efforts suggested that the observed shell thickening resulted from higher annual temperatures, longer yearly calcification windows, nearshore eutrophication, and enhanced prey abundance, which mitigated the impact of other climate change factors. An investigation into the trace elemental composition of common pollutant metals in the same archival N. lapillus specimens revealed that shell Cu/Ca and Zn/Ca concentration ratios remained remarkably constant throughout the last 130 years despite substantial shifts in the environmental concentration. However, Pb/Ca concentration ratios showed a definite trend closely aligned with leaded petrol emissions in Europe over the same period. Discussing physiological and environmental drivers for the observed shell bound heavy metal patterns, I argue that, unlike for Pb, constraints on environmental dissolved Cu species abundance and biologically mediated control on internal Zn levels were likely responsible for a decoupling of shell-bound to total ambient Cu and Zn concentrations. The results highlight the complexity of internal and external pathways that govern the uptake of heavy metals into the molluscan shell and suggest that the shell of N. lapillus could be a suitable archive for a targeted investigation of Pb pollution in the intertidal zone.

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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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Ocean acidification in the Gulf of Mexico: drivers, impacts, and unknowns

Highlights

  • We synthesize the current peer-reviewed literature on Gulf of Mexico (GOM) acidification across the ocean-estuarine continuum and identify critical knowledge, research, and monitoring gaps that limit our current understanding of environmental, ecological, and socioeconomic impacts from acidification.
  • The GOM remains a relatively understudied region with respect to ocean acidification, particularly with respect to regionally important organism and ecosystem responses.
  • Within the GOM, ocean acidification is spatially variable and numerous physical and biogeochemical processes contribute collectively to carbonate chemistry dynamics.

Abstract

Ocean acidification (OA) has resulted in global-scale changes in ocean chemistry, which can disturb marine organisms and ecosystems. Despite its extensively populated coastline, many marine-dependent communities, and valuable economies, the Gulf of Mexico (GOM) remains a relatively understudied region with respect to acidification. In general, the warm waters of the GOM are better buffered from acidification compared to higher latitude seas, yet long-term acidification has been documented in several GOM regions. OA within the GOM is recognized as spatially variable, particularly within the coastal zone where numerous physical and biogeochemical processes contribute to carbonate chemistry dynamics. The historical progression of OA within the entire GOM is difficult to assess because only a few dedicated long-term monitoring sites have recently been established, and full-water column observations are limited. However, environmental drivers on smaller scales that affect GOM acidification were found to include freshwater, nutrient, and carbonate discharge from large rivers; ocean warming, circulation and residence times; and episodic extreme weather events. GOM marine ecosystems provide essential services, including coastline protection and carbon dioxide removal, and habitats for many marine species that are economically and ecologically important. However, organismal and ecosystem responses to OA are not well constrained for the GOM due to a lack of studies examining the specific effects of OA on regionally relevant species under contemporary and projected conditions. Tackling the vast number of remaining scientific unknowns in this region can be coordinated through regional capacity networks, such as the Gulf of Mexico Coastal Acidification Network (GCAN), working to achieve a system-wide understanding of Gulf OA and its impacts. Here we synthesize the current peer-reviewed literature on GOM acidification across the ocean-estuarine continuum and identify critical knowledge, research, and monitoring gaps that limit our current understanding of environmental, ecological, and socioeconomic impacts from acidification.

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Deep-sea sponges in an Anthropocene ocean

Sponges (Porifera) are among the oldest multicellular animals on planet earth and are abundant throughout all oceans. From shallow, warm waters to the dark, cold deep sea. Sponges move large quantities of seawater through their body while efficiently removing dissolved and particulate nutrients. Their large filter capacity makes them important links in marine food webs as they are able to access nutrient sources that are unavailable to the majority of marine fauna and channel this energy to higher trophic levels. In the North Atlantic Ocean (NAO) sponges form dense aggregations, so called sponge rounds, that provide ecosystem services such as habitat provision, nutrient cycling and provision of novel, bioactive compounds. It remains unclear whether these deep-sea sponge grounds can continue to provide these services in a changing ocean that is increasingly industrialized. The physicochemical properties of the North Atlantic Ocean will be altered by human induced climate change. The aim of this thesis is to address this knowledge gaps by quantifying the basic eco-physiological processes such as oxygen consumption, clearance rate and uptake/release of inorganic nutrients of two habitat forming deep-sea sponges under the cumulative impacts of warmer, acidified seawater and the exposure to different types of re-suspended particles. The research underlying this thesis was part of the EU-funded research project Deep-sea Sponge Grounds Ecosystems of the North Atlantic: an integrated approach towards their preservation and sustainable exploitation (SponGES). Two model deep sea sponge species were used, Geodia barretti and Vazella pourtalesi. G. barretti collected from 300 m water depth in the Barents Sea, were exposed to four treatments resembling future ocean conditions (no treatment, 4 °C increase in seawater temperature, decrease of seawater pH by 0.3, and a combination of the high temperature, low pH). Over the course of 39 weeks, oxygen consumption, dissolved inorganic nutrient fluxes, and bacterioplankton clearance rates were measured as indicators of metabolic activity. All indicators within each sponge individual and per treatment were highly variable over time, and no effect of manipulated seawater treatments on these parameters could be demonstrated. Oxygen consumption rates in all groups closely followed a seasonal pattern, potentially caused by (a)biotic cues in the natural seawater flowing through the experimental aquaria. While similar metabolic rates across all treatments suggest that G. barretti physiologically coped with simulated future ocean conditions, tissue necrosis that developed in experimental animals might indicate that the response of the complex, high microbial G. barretti sponge (i.e., sponge host and microbial symbionts) to future ocean conditions may not be reflected in basic physiological processes. In addition to large scale changes of ocean conditions, also bottom trawling activities interact with the dense sponge aggregations. Bottom trawling has been identified as the most severe direct industrial threat to abundant sponge grounds by removing sponge biomass and indirect by re-suspending bottom sediments. Plumes of re-suspended sediment potentially smother and clog the aquiferous system of filter-feeding sponges with unknown implications for their health. The physiological responses of repeated exposure to natural sediment were studied in the glass sponge Vazella pourtalesii, which forms dense sponge grounds in Emerald Basin off Nova Scotia, Canada. Ex situ chamber-based measurements of bacterial clearance and oxygen consumption (respiration) rates indicated that the animals were able to cope with elevated concentrations of suspended sediment, as they expressed normal clearance and respiration rates over 7 days of sediment exposure. However, clearance rates significantly declined after 14 days of sediment exposure and the animals visibly accumulated sediment in their tissue. Therefore, long-term exposure to elevated concentrations of suspended sediment should be avoided in order to minimize adverse effects on the abundant Vazella sponge grounds. While sponges seem to cope with environmental changes and limited exposure to suspended particles as occurs in their natural environment, the response to cumulative stressors indicated impaired health. Exposure to a field relevant concentration of suspended sediment (50 mg L-1) and future ocean conditions (pH decrease of 0.2 units, temperature increase of 3 °C) on the physiological performance of Geodia barretti resulted in a cessation of pumping. Oxygen consumption rates remained unchanged under low pH and high temperature treatments and indicate mechanisms of pumping-independent mass transfer of oxygen. A small, but statistically significant shift in the microbiome associated with G. barretti was observed and possibly related to coping with cumulative stressors in this deep-sea sponge species. The synergistic nature of the treatment-specific effects has the potential to adversely affect the physiological fitness of this dominating sponge species in the North Atlantic Ocean. In addition to deep-sea fisheries the nascent industry of subsea mining is prospecting abundant mineral resources present in the deep sea. The extraction of subsea minerals, such as seafloor massive sulphide (SMS) deposits, will expose adjacent marine ecosystems to suspended particle plumes charged with elevated concentrations of heavy metals and other potentially toxic compounds. Up to date there is no information about the impact of mining activities on deep-sea benthic ecosystems such as abundant deep-sea sponge grounds in the North Atlantic Ocean. To simulate the effects of mining plumes on benthic life in the deep-sea, Geodia barretti and an associated brittle star genus were exposed to a field-relevant concentration of 30 mg L-1 suspended particles of crushed SMS deposits. Three weeks of exposure to suspended particles of crushed SMS resulted in a tenfold higher rate of tissue necrosis in sponges. All brittle stars in the experiment already perished within ten days of exposure. SMS particles were evidently accumulated in the sponge’s mesohyl and concentrations of iron and copper were 10 times elevated in SMS exposed individuals. Oxygen consumption and clearance rates were significantly retarded after the exposure to SMS particles, hampering the physiological performance of G. barretti. These adverse effects of crushed SMS deposits on G. barretti and its associated brittle star species potentially cascade in disruptions of benthic-pelagic coupling processes in the deep sea. More elaborate studies are advisable to identify threshold levels, management concepts and mitigation measures to minimize the impact of deep-sea mining plumes on benthic life. Sponges were shown to express high coping capacities towards fluctuations of environmental parameters within their habitat. However, additional stressors or persistence of sub-optimal conditions over extended time scales can challenge sponge’s ability to endure cumulative effects. Given the ecosystem services sponge grounds in the North Atlantic Ocean provide, industrial operations should ascertain refuges for deep-sea sponges faced with global ocean changes.

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A numerical reassessment of the Gulf of Mexico carbon system in connection with the Mississippi River and global ocean

Coupled physical–biogeochemical models can fill the spatial and temporal gap in ocean carbon observations. Challenges of applying a coupled physical–biogeochemical model in the regional ocean include the reasonable prescription of carbon model boundary conditions, lack of in situ observations, and the oversimplification of certain biogeochemical processes. In this study, we applied a coupled physical–biogeochemical model (Regional Ocean Modelling System, ROMS) to the Gulf of Mexico (GoM) and achieved an unprecedented 20-year high-resolution (5 km, 1/22°) hindcast covering the period of 2000 to 2019. The biogeochemical model incorporated the dynamics of dissolved organic carbon (DOC) pools and the formation and dissolution of carbonate minerals. The biogeochemical boundaries were interpolated from NCAR’s CESM2-WACCM-FV2 solution after evaluating the performance of 17 GCMs in the GoM waters. Model outputs included carbon system variables of wide interest, such as pCO2, pH, aragonite saturation state (ΩArag), calcite saturation state (ΩCalc), CO2 air–sea flux, and carbon burial rate. The model’s robustness is evaluated via extensive model–data comparison against buoys, remote-sensing-based machine learning (ML) products, and ship-based measurements. A reassessment of air–sea CO2 flux with previous modeling and observational studies gives us confidence that our model provides a robust and updated CO2 flux estimation, and NGoM is a stronger carbon sink than previously reported. Model results reveal that the GoM water has been experiencing a ∼ 0.0016 yr−1 decrease in surface pH over the past 2 decades, accompanied by a ∼ 1.66 µatm yr−1 increase in sea surface pCO2. The air–sea CO2 exchange estimation confirms in accordance with several previous models and ocean surface pCO2 observations that the river-dominated northern GoM (NGoM) is a substantial carbon sink, and the open GoM is a carbon source during summer and a carbon sink for the rest of the year. Sensitivity experiments are conducted to evaluate the impacts of river inputs and the global ocean via model boundaries. The NGoM carbon system is directly modified by the enormous carbon inputs (∼ 15.5 Tg C yr−1 DIC and ∼ 2.3 Tg C yr−1 DOC) from the Mississippi–Atchafalaya River System (MARS). Additionally, nutrient-stimulated biological activities create a ∼ 105 times higher particulate organic matter burial rate in NGoM sediment than in the case without river-delivered nutrients. The carbon system condition of the open ocean is driven by inputs from the Caribbean Sea via the Yucatan Channel and is affected more by thermal effects than biological factors.

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Camouflage and exploratory avoidance of newborn cuttlefish under warming and acidification

Ocean warming and acidification have been shown to elicit deleterious effects on cephalopod mollusks, especially during early ontogeny, albeit effects on behavior remain largely unexplored. This study aimed to evaluate, for the first time, the effect of end-of-the-century projected levels of ocean warming (W; + 3 °C) and acidification (A; 980 µatm pCO2) on Sepia officinalis hatchlings’ exploratory behavior and ability to camouflage in different substrate complexities (sand and black and white gravel). Cuttlefish were recorded in open field tests, from which mobility and exploratory avoidance behavior data were obtained. Latency to camouflage was registered remotely, and pixel intensity of body planes and background gravel were extracted from photographs. Hatching success was lowered under A and W combined (AW; 72.7%) compared to control conditions (C; 98.8%). Motion-related behaviors were not affected by the treatments. AW delayed camouflage response in the gravel substrate compared to W alone. Moreover, cuttlefish exhibited a higher contrast and consequently a stronger disruptive pattern under W, with no changes in background matching. These findings suggest that, although climate change may elicit relevant physiological challenges to cuttlefish, camouflage and mobility of these mollusks are not undermined under the ocean of tomorrow. 

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The influences of diurnal variability and ocean acidification on the bioerosion rates of two reef-dwelling Caribbean sponges

Ocean acidification (OA) is expected to modify the structure and function of coral reef ecosystems by reducing calcification, increasing bioerosion, and altering the physiology of many marine organisms. Much of our understanding of these relationships is based upon experiments with static OA treatments, though evidence suggests that the magnitude of diurnal fluctuations in carbonate chemistry may modulate the calcification response to OA. These light-mediated swings in seawater pH are projected to become more extreme with OA, yet their impact on bioerosion remains unknown. We evaluated the influence of diurnal carbonate chemistry variability on the bioerosion rates of two Caribbean sponges: the zooxanthellate Cliona varians and azooxanthellate Cliothosa delitrix. Replicate fragments from multiple colonies of each species were exposed to four precisely-controlled pH treatments: contemporary static (8.05 ± 0.00; mean pH ± diurnal pH oscillation), contemporary variable (8.05 ± 0.10), future OA static (7.80 ± 0.00), and future OA variable (7.80 ± 0.10). Significantly enhanced bioerosion rates, determined using buoyant weight measurements, were observed under more variable conditions in both the contemporary and future OA scenarios for C. varians, whereas the same effect was only apparent under contemporary pH conditions for C. delitrix. These results indicate that variable carbonate chemistry has a stimulating influence on sponge bioerosion, and we hypothesize that bioerosion rates evolve non-linearly as a function of pCO2 resulting in different magnitudes and directions of rate enhancement/reduction between day and night, even with an equal fluctuation around the mean. This response appeared to be intensified by photosymbionts, evident by the consistently higher percent increase in bioerosion rates for photosynthetic C. varians across all treatments. These findings further suggest that more variable natural ecosystems may presently experience elevated sponge bioerosion rates and that the heightened impact of OA enhanced bioerosion on reef habitat could occur sooner than prior predictions.

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