Increasing atmospheric CO2 is causing unprecedented changes in seawater chemistry, yet the uncertainty of the ecological response to these projected changes, termed ‘ocean acidification’, remains considerable at present. To predict the effects of these changes, we need to improve our understanding of the responses of marine primary producers since these drive biogeochemical cycles and determine the structure and function of benthic habitats. The majority of experiments on the effects of ocean acidification on photoautrophs to date have mainly focused on oceanic microalgae, leaving benthic assemblages largely overlooked. Carbon dioxide vents are providing a means for examining and predicting the impacts of ocean acidification on marine ecosystems. In this thesis a temperate CO2 volcanic vent gradient was used to investigate the responses of benthic microalgal assemblages (periphyton, epilithic, epipelic, epipsammic and endolithic) and macroalgae (a calcified phaeophyte, crustose coralline algae and turf algae) to increasing pCO2. The photosynthetic standing crop of microphytobenthic assemblages increased significantly with elevations in CO2 indicating that the productivity of shallow water habitats may be promoted over the course of this century. Some benthic diatoms appear to benefit in naturally CO2 enriched environments whilst benthic cyanobacteria in this study appear to be relatively insensitive to the levels of increase predicted for this century. Dramatic shifts in epilithic macroalgae assemblages were observed along the CO2 gradient and a calcified phaeophyte was revealed as an unexpected ecological winner under ocean acidification scenarios. These observations suggest that benthic algal assemblages have the potential to dramatically alter as CO2 levels continue to rise; this would have profound consequences for the structure and function of benthic ecosystems.

Planktonic organisms form the base of the marine food web and may be impacted by environmental change in many ways. The interactive effects of multiple, simultaneous climate-driven changes on these organisms are not well understood. This dissertation examined the impacts of ocean acidification in combination with other environmental stressors on marine plankton and determined spatial patterns of one of these potential interactive drivers. Chapter 2 investigated the synergistic effects of ocean acidification and hypoxia on the harmful dinoflagellate Amphidinium carterae. Findings indicated that empirical studies may be crucial to accurately predict organismal responses to multi-stressors. Results also suggested that photorespiration may serve a previously unrecognized role in dinoflagellate metabolism. Chapter 3 examined the combined effects of ocean acidification and lithogenic trace metals on the growth of another harmful dinoflagellate, Cochlodinium polykrikoides. Results indicated that high suspended sediment loads may deliver toxic concentrations of trace elements to marine phytoplankton in acidified coastal ecosystems. Chapter 4 examined the interactive effects of ocean acidification and bacteria on the severity and extent of dissolution in the shells of larval gastropods and the adult pteropod Limacina helicina. Research findings indicated that microbial communities on the shell surfaces of some planktonic molluscs may mediate certain types of shell dissolution in acidified, upwelled waters. Chapter 5 explored the use of thorium isotope fluxes as a proxy for dust and lithogenic iron in the Indian Ocean. Results suggested that the gradient of dust fluxes in the region could impose thresholds for biological productivity. Together, these interdisciplinary studies demonstrate coupled biological and chemical changes in marine ecosystems as a result of increased anthropogenic environmental change.

Since the industrial revolution, CO sub 2 has increased in the atmosphere and about 40% of the increase has been taken up by the ocean. An artifact of increasing CO sub 2 in the ocean is ocean acidification; it changes the calcium carbonate saturation state, which in turn alters the calcification rate of shelled organisms. The purpose of this dissertation is to estimate the changes in the carbonate system in the oceans, and whether these changes are due to natural (biological activity, chemical transformation or mixing of water masses) or anthropogenic (human activities) perturbations. The first hypothesis states that the presence of boric acid (B(OH) sub 3) in seawater changes the thermodynamic constants of CO sub 2, pK sup * sup 1 and pKsup * sup 2. Due to experimental limitations, the solubility of B(OH) sub 3 was determined in electrolyte solutions (LiCl, NaCl, KCl, RbCl and CsCl) instead of real or artificial seawater. The results can be used to estimate the B(OH) sub 3 activity coefficients gamma sub B and solubility [B] in natural mixed electrolyte solutions. The second hypothesis states that filtering seawater sampled in the open ocean is necessary for the determination of total alkalinity (TA). Measurement of 180 samples of surface, oxygen minimum, and deep waters in the Pacific and Indian oceans revealed that the at- sea measured TA of filtered and unfiltered samples were not statistically different. Finally, a synthesis and analysis of the carbonate parameters in the Atlantic and Indian oceans is undertaken. Results from repeat hydrographic cruises in these oceans were used for this task. Parameters TA and total CO sub 2 (TCO sub 2) are predicted using hydrographic properties and a multi-linear regression method to obtain a more homogenous dataset. The results of the predicted TA prove to be successful, which is not the case for TCO sub 2 at the surface of the ocean. Finally, it is found that the increase in anthropogenic CO sub 2 signal remineralization and mixing of water masses increase the acidity of the ocean at the surface and in deep waters, respectively. This causes the aragonite saturation horizon to shoal. Recommendations for further studies are provided in the "Summary and conclusion" chapter.

Human activities have contributed to the increase in atmospheric greenhouse gases such as carbon dioxide (CO2). This anthropogenic gas emission has led to a rise in the average Earth temperature. Moreover, the ocean constitutes the major sink for anthropogenic CO2 and its dissolution in surface waters has already resulted in an increase of seawater acidity since the beginning of the industrial revolution. This is commonly called ocean acidification. The increase in water temperature could induce modifications of the physical and chemical characteristics of the ocean. Also, the structure and the functioning of marine ecosystems may be altered as a result of ocean acidification. Phytoplankton productivity is one of the primary controls in regulating our climate, for instance via impact on atmospheric CO2 levels. Coccolithophores, of which Emiliania huxleyi is the most abundant species, are considered to be the most important pelagic calcifying organisms on Earth. Coccolithophores are characterized by calcium carbonate platelets (coccoliths) covering the exterior of the cells. They form massive blooms in temperate and sub-polar oceans and in particular along continental margin and in shelf seas. The intrinsic coupling of organic matter production and calcification in coccolithophores underlines their biogeochemical importance in the marine carbon cycle. Both processes are susceptible to change with ocean acidification and warming. Coccolithophores are further known to produce transparent exopolymer particles (TEP) that promote particle aggregation and related processes such as marine snow formation and sinking. Thus, the impact of ocean warming and acidification on coccolithophores needs to be studied and this can be carried out through a transdisciplinary approach. The first part of this thesis consisted of laboratory experiments on E. huxleyi under controlled conditions. The aim was to estimate the effect of increasing water temperature and acidity on E. huxleyi and especially on the calcification. Cultures were conducted at different partial pressures of CO2 (pCO2); the values considered were 180, 380 and 750 ppm corresponding to past, present and future (year 2100) atmospheric pCO2. These experiments were conducted at 13°C and 18°C. The cellular calcite concentration decreases with increasing pCO2. In addition, it decreases by 34 % at 380 ppm and by 7 % at 750 ppm with an increase in temperature of 5°C. Changes in calcite production at future pCO2 values are reflected in deteriorated coccolith morphology, while temperature does not affect coccolith morphology. Our findings suggest that the sole future increase of pCO2 may have a larger negative impact on calcification than its interacting effect with temperature or the increase in temperature alone. The evolution of culture experiments allows a better comprehension of the development of a bloom in natural environments. Indeed, in order to predict the future evolution of calcifying organisms, it is required to better understand the present-day biogeochemistry and ecology of pelagic calcifying communities under field conditions. The second part of this dissertation was dedicated to results obtained during field investigations in the northern Bay of Biscay, where frequent and recurrent coccolithophorid blooms were observed. Cruises, assisted by remote sensing, were carried out along the continental margin in 2006 (29 May – 10 June), 2007 (7 May – 24 May) and 2008 (5 May – 23 May). Relevant biogeochemical parameters were measured in the water column (temperature, salinity, dissolved oxygen, Chlorophyll-a and nutrient concentrations) in order to determine the status of the bloom at the time of the different campaigns. Calcification has been shown to be extremely important in the study area. In addition, TEP production was significant at some stations, suggesting that the northern Bay of Biscay could constitute an area of important carbon export. Mortality factors for coccolithophores were studied and the first results of lysis rates measured in this region were presented. Results obtained during culture experiments and comparison with data reported in the literature help to better understand and to predict the future of coccolithophores in a context of climate change. Data obtained during either culture experiments or field investigations allowed a better understanding of the TEP dynamics. Finally, the high lysis rates obtained demonstrate the importance of this process in bloom decline. Nevertheless, it is clear that we only begin to understand the effects of global change on marine biogeochemistry, carbon cycling and potential feedbacks on increasing atmospheric CO2. Thus, further research with a combination of laboratory experiments, field measurements and modelling are encouraged.

Ocean acidification is occurring in response to rapidly increasing concentrations of atmospheric CO2. Shallow-water hydrothermal vent systems have been proposed as natural laboratories for studying the effects of elevated pCO2 on benthic communities. Hydrothermal vents occur at depths of approximately 10m in Tutum Bay, Ambitle Island, Papua New Guinea; these vents are surrounded by a typical-appearing fringing coral-reef community. Groups of live specimens of seven species of reef-dwelling, larger benthic foraminifers, along with segments of calcareous green algae broken from live thalli, were collected from a reef location, placed in small mesh bags, and deployed for five days at six different sites along a gradient of temperature (29.6oC-59.3oC) and pH (5.9-8.1) with distance from a large hydrothermal vent in Tutum Bay. Foraminiferal taxa used in the experiment included Amphisorus hemprichii, a species with Mg-calcite porcelaneous shells, three species of Amphistegina that produce hyaline calcite shells, and three species with hyaline Mg-calcite shells (Heterostegina depressa and two Calcarina spp.). Several specimens of four of the seven foraminiferal species examined survived exposure to elevated temperatures of 59.3oC and low pH of 6.2 for five days, while at least one specimen of each of the seven species survived exposure to 39.9oC and pH 5.9. Examination of shells at 600-1000x magnification using scanning electron microscopy revealed fine-scale dissolution in specimens up to 30m from the vent. Results of this experiment, as well as previously reported observations from the study site, indicate that the calcifying reef-dwelling organisms examined can survive pH extremes that result in dissolution of their shells following death.

abstract / Biological Sciences / Doctoral / Doctor of Philosophy

The successful completion of the early developmental stages in organisms with complex life cycles is crucial to the persistence of a species both at the local and global scale. Thus changes in the abiotic environment experienced during larval and early benthic development can have profound effects on the development and ultimately dynamics of populations of marine invertebrates. The effects of elevated temperature and pCO2 in line with future predictions of anthropogenic climate change, ocean warming and ocean acidification (OA), on the survivorship and growth during early development of marine invertebrates is beginning to be understood, yet the underlying physiological ontogeny driving such changes, and the more subtle effects on physiological performance of climate change drivers, has yet to be distinguished. Therefore the aim of the present study is to investigate the effects of elevated temperature and pCO2 on the developmental eco-physiology of an economically and ecologically important species, the European lobster, Homarus gammarus, to characterise the underlying physiological responses of early development behind responses of survival and growth. The main findings relate to how changing optimal temperature conditions during larval development results in changes in metabolic performance and therefore aerobic scope, ultimately driving survival and growth. Larval stages which exhibit narrower aerobic scope were also sensitive to elevated pCO2 evident as reduced survival, changes to energetic demands and organic content, and reduced calcification. Furthermore, this is the first attempt to characterise the physiological response of early benthic juveniles to climate change drivers. Early benthic juveniles are quite different in underlying physiology to later juveniles and adults, cumulating in this stage being energy limited. Such limitations are expressed as a reduction in aerobic scope in relation to elevated temperature and pCO2, and associated sensitiveness to elevated pCO2 resulting in increased moult related mortalities and the breakdown of haemolymph buffering capacity under combinations of elevated temperature and pCO2. Throughout early development, elevated temperature and pCO2, through underlying physiological responses, may have dramatic effects on the geographic range and successful development of H. gammarus.

Decreases in ocean pH through ocean acidification has shown to have direct negative impacts on the early life stages of the Pacific purple sea urchin, Strongylocentrotus purpuratus. Research has suggested that multiple stressors could exacerbate, cancel, or even alleviate the impacts of ocean acidification on echinoderms. This study assessed the combined effects of changes in pCO2 concentrations (390, 800, 1500 ppm), salinities (28, 31, 34 ppt) and temperatures (12, 15, 18°C) on fertilization and larval development in S. purpuratus. Increased pCO2 was the predominant stressor, with additive and antagonistic effects from temperature changes, and no effect from salinity changes. Stressor combinations significantly decreased the rate of normal larval development by 28 – 68%, whereas fertilization and larval survival were unaffected. The strong impact on normal larval development likely indicates that later development stages could be detrimentally affected and could influence the population dynamics of S. purpuratus.

Rising atmospheric carbon dioxide (CO2) concentrations are increasing the rate at which anthropogenic CO2 is accumulating in the ocean, and thereby acidifying ocean water. However, accumulation of anthropogenic CO2 is not the only process affecting coastal oceans. Anthropogenic inputs of nutrients to coastal waters can result in massive algal blooms, a process known as eutrophication. Microbial consumption of this organic matter depletes bottom waters of oxygen and increases acidity through the release of CO2. This study assesses the synergistic effect of ocean acidification and eutrophication in the coastal ocean using data from six cruises to the northern Gulf of Mexico. In addition, this study investigates the effect of the 2011 Mississippi River flood on coastal pH and aragonite saturation states. Data from a model simulation using data collected from the northern Gulf of Mexico indicates that eutrophication is contributing to acidification of subsurface waters and plays a larger role than acidification from atmospheric CO2 uptake. Furthermore, results from the model simulation show that the decrease in pH since the industrial era is 0.04 units greater than expected from ocean acidification and eutrophication combined. The additional decrease was attributed to the reduced buffering capacity of the region and may be related to the uptake of atmospheric CO2 into O2-depleted and CO2-enriched waters, the addition of atmospheric CO2 into O2-rich and CO2-poor waters, the input of CO2 via respiration into waters in equilibrium with high atmospheric CO2, or a combination of all three processes.

As one of the most successful invasive species in the marine environment around the globe, the green crab Carcinus maenas possesses efficient regulatory mechanisms to quickly acclimate to environmental changes. The most important organs in this process are the nine pairs of gills that not only allow for osmoregulation, but have been shown to be involved in ammonia excretion and respiratory gas exchange. To date, however, little is known about the gills’ contribution to acid-base regulation that might become increasingly important in a “future ocean scenario” whereby surface ocean pH is predicted to drop by up to 0.5 units by the year 2100. The present thesis aims to characterize the green crab gills’ role in acid-base regulation and how it is linked to ammonia excretion. After exposure to hypercapnia (0.4 kPa pCO2 for 7 days), osmoregulating green crabs were capable of fully compensating for the resulting extracellular respiratory acidosis, while osmoconforming green crabs only partially buffered the accompanying drop in hemolymph pH after acclimation to 1% CO2 for 48 hours. Perfusion experiments on isolated green crab gills showed that different gills contributed to the excretion of H+ in an individual pattern and indicated that NH4+ is an important component of branchial acid excretion. Experiments on gill mRNA expression and pharmaceutical effects on isolated gills identified distinct epithelial transporters to play significant roles in branchial acid base regulation: Rhesus-like protein, basolateral bicarbonate transporter(s), cytoplasmic V-(H+)-ATPase, Na+/H+-exchanger, basolateral Na+/K+-ATPase, cytoplasmic and membrane bound carbonic anhydrase, and basolateral K+ channels. Regarding the latter, the present work provides the first sequence-based evidence for a potassium/sodium hyperpolarization-activated cyclic nucleotide-gated channel (CmHCN) capable of promoting NH4+ transport in the green crabs’ gill epithelium, and further demonstrates its direct involvement in branchial acid-base regulation. This highly conserved protein is a potentially important novel key-player in acid-base regulation in all animals. Interestingly, the observed principles linking acid-base to ammonia regulation in the decapod crustacean gill epithelium resemble many observations previously made in vertebrates. The data of the present thesis therefore provides valuable information for general acid-base regulation, while contributing substantially to our understanding of acid-base regulation in invertebrates. / February 2016