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We combine consistently dated benthic carbon isotopic records distributed over the entire Atlantic Ocean with numerical simulations performed by a glacial configuration of the Norwegian Earth System Model with active ocean biogeochemistry, in order to interpret the observed Cibicides δ13C changes at the stadial-interstadial transition corresponding to the end of Heinrich Stadial 4 (HS4) in terms of ocean circulation and remineralization changes. We show that the marked increase in Cibicides δ13C observed at the end of HS4 between ~2000 and 4200 m in the Atlantic can be explained by changes in nutrient concentrations as simulated by the model in response to the halting of freshwater input in the high latitude glacial North Atlantic. Our model results show that this Cibicides δ13C signal is associated with changes in the ratio of southern-sourced (SSW) versus northern-sourced (NSW) water masses at the core sites, whereby SSW is replaced by NSW as a consequence of the resumption of deep water formation in the northern North Atlantic and Nordic Seas after the freshwater input is halted. Our results further suggest that the contribution of ocean circulation changes to this signal increases from ~40 % at 2000 m to ~80 % at 4000 m. Below ~4200 m, the model shows little ocean circulation change but an increase in remineralization across the transition marking the end of HS4. The simulated lower remineralization during stadials than interstadials is particularly pronounced in deep subantarctic sites, in agreement with the decrease in the export production of carbon to the deep Southern Ocean during stadials found in previous studies.

Knowledge gaps pertaining to the remediation of freshwater lakes impacted by oil spills have persisted despite recent record highs for oil production and transportation across vulnerable regions in North America. The multiyear Freshwater Oil Spill Remediation Study (FOReSt), conducted at the IISD-Experimental Lakes Area in Canada, is focusing on the efficacy of minimally invasive methods for remediating oil spills in freshwater boreal lakes. In this thesis, the impacts and remediation of diluted bitumen (dilbit) and conventional heavy crude oil (CHV) spills were investigated (year 1), as were a variety of different remediation methods for spills of dilbit on different shoreline substrates (year 2). Two common small-bodied fish, fathead minnows (Promephales promelas) and finescale dace (Chrosomus neogaeus), were used to assess exposure to petrogenic polycyclic aromatic compounds (PACs) in model enclosed shoreline ecosystems impacted by spills and remediated using minimally invasive techniques. Short-term exposure to PACs, the most toxicologically relevant compounds in oil, was assessed in fish using biliary metabolite concentrations. In year one, finescale dace and fathead minnows residing in oil treated enclosures each had biliary pyrene metabolite concentrations that were positively correlated with pyrene concentrations in the water of the enclosures. Three months after the initial spills, fish in the enclosure receiving dilbit were significantly more exposed to PACs than fish in reference enclosures that did not receive oil. In year two, both finescale dace and fathead minnows residing in oil-treated exposures, regardless of shoreline substrate, showed increased exposure to PACs compared to fish in reference enclosures and the pristine lake environment two and a half months after the spills. No significant differences in exposure were observed among the remediation treatments. Biliary PAC metabolite concentrations were positively predicted by parent PAC concentrations in periphyton. PACs in periphyton two and a half months after oil introduction were positively correlated with PACs in the enclosures one week after spills, suggesting fish also had increased exposure to periphyton-bound alkyl-PACs. This thesis validates the use of small-bodied fish in assessing PAC exposure following freshwater oil spills and demonstrates the difficulties in estimating exposure using environmental concentrations in natural systems.

The environment during early life history strongly impacts phenotypic development in all organisms, which further influences developmental trajectory and ecological fitness later in life. Depending on the developmental stage and magnitude of change in the environment, phenotypes may become irreversible and thus have a long-lasting effect later in life. This thesis was designed to better understand how changes in the environment may influence plasticity and variation of metabolic phenotypes of Lake Sturgeon (Acipenser fulvescens) within the first year of life. Broadly speaking, the thesis tested two hypotheses that 1) all measured phenotypes would be plastic; and 2) durations of environmental effects on phenotypic development would be correlated with distinct developmental windows. Studies were developed to examine 1) short-term effects of temperature or diet on metabolic phenotypes such as metabolic rate, energy density, fatty acid profiles, and growth (Chapters 2 and 3) and 2) longer-term effects of temperature or diet during early life on these metabolic phenotypes (Chapters 4, 5 and 6). The first experimental chapter (Chapter 2) examined ontogenetic development of metabolic rate and demonstrated that dietary shifts between Artemia to bloodworm resulted in cessation of growth with elevated routine metabolic rate. Chapter 3 examined how fatty acid profiles and plasma cortisol concentration were influenced by environmental temperature and showed that decreasing temperature led to increases in mono- and polyunsaturated fatty acids in both phospholipids and triglycerides, and food deprivation resulted in lack of difference between baseline and peak cortisol concentrations. Chapter 4 examined how temperature during early life influenced plasticity of growth and showed that temperature post-dietary transition resulted in a transient effect on growth and energy metabolism without long-term effects post-winter. Chapter 5 examined how temperature during early life could influence growth and fatty acid metabolism when fish were exposed to colder temperatures later in life and demonstrated that elevated temperatures resulted in a longer-term effect on growth but lack of transcriptional responses of desaturating fatty acids when exposed to a cold temperature (3.5°C) later in life. The final experimental chapter, Chapter 6 examined longer-term effects of diet at the onset of exogenous feeding on metabolism and growth and demonstrated that an enriched diet resulted in prolonged effects on growth, digestive enzyme activity and survival prior to a simulated overwintering. This doctoral thesis research revealed that all measured metabolic phenotypes were plastic, but subtle changes in temperature and diet during early life history resulted in transient or prolonged effects on growth and metabolism in age-0 lake sturgeon. Results will aid our understanding of cohort and population dynamics as well as contribute to the development of conservation strategies for lake sturgeon, a species at risk or endangered across its natural range.

Palynological and sedimentological analyses were performed on the sediment core HH16-1205-GC retrieved from the central Isfjorden, West Spitsbergen in order to retrace the climate of the last 7000 years. The record revealed an overall cooling trend with an important climate shift between 4.4 and 3.8 cal. ka BP, and millennial-scale oscillations. Over the last 7000 years, sea-surface reconstruction from dinocyst assemblages indicates a decrease in summer sea-surface temperature, from 2.5 to 1.5 °C, and primary productivity, from 750 to 650 gC m-² a-1. Predominant sediment supply from the inner part of the fjord, ice rafting, dense sea-ice cover, strongly stratified water masses, and high primary productivity is observed in the sedimentological and palynological data between 6.8 and 5.8 cal. ka BP. The interval from 4.4 to 3.8 cal. ka BP is marked by a layer of coarser material followed by a significant decrease in the grain size mode, as well as changes in geochemical properties. large-amplitude fluctuations is observed in our geochemical data after 2.0 cal. ka BP, while an increase of the dinocysts Impagidinium pallidum and Spiniferites elongatus from 2.0 to 1.2 cal. ka BP suggests enhanced Atlantic Water inflow. The sea surface conditions reconstructions and XRF ratios also reveal large-amplitude millennial fluctuations. Wavelet analysis and cross-wavelet analysis performed on multiple variables confirm a strong cyclic signal with a periodicity of 1200 to 1500 years.

Increasing upwelling intensity and shoaling of the oxygen minimum zone (OMZ) is projected for Eastern Boundary Upwelling Systems (EBUSs) under ocean warming which may have severe consequences for mesopelagic food webs, trophic transfer, and fish production also in the Humboldt Current Upwelling System (HUS). To improve our mechanistic understanding, from February 23, 2017 until April 14, 2017 we performed a 50 days mesocosm experiment in the northern HUS (off Callao Bay, Peru) and monitored the zooplankton development prior to and following a simulated upwelling event through the addition of deeper water of two different OMZ-influenced subsurface waters to four of in total eight mesocosms. To elucidate plankton dynamics and trophic relationships, we followed the temporal development of the mesozooplankton community in relation to that of phytoplankton, analyzed the fatty acid composition and gut fluorescence of dominant copepods, and determined the stable isotope (SI) and elemental composition (C:N) of dominant zooplankton taxa. Zooplankton samples were collected from the mesocosms over the entire experiment duration using an Apstein net (17 cm diameter, 100 µm mesh) to determine abundance and taxonomic composition of the zooplankton community, and to analyze fatty acid composition, gut fluorescence and elemental composition of dominant zooplankton. Furthermore, abundance and biomass of zooplankton groups was estimated from scanned ZooScan images.

Buoyant Arctic cod (Boreogadus saida) eggs are found at the surface or at the ice-water interface in winter. While winter temperatures in saline waters fall below 0 degrees C, the temperature in areas affected by under-ice river plumes is slightly higher. Under-ice river plumes may therefore provide thermal refuges favoring the survival of the vulnerable early life stages of Arctic cod. Thermal refuges would allow early hatchers to survive, benefit from a long growing period, and add to the number of individuals recruiting to the adult population: These expectations define the freshwater winter refuge hypothesis. More than 42 rivers drain into Hudson Bay making it particularly well suited to test this hypothesis. Whereas the bulk of Arctic cod observed in Hudson Bay hatch between mid-April and June, some larvae hatch as early as January. We used two independent but complementary methods to test the hypothesis: (1) Lagrangian model simulations that traced back the planktonic trajectories of the sampled larvae and (2) measurements of the concentration of strontium-88 in the otolith cores. Throughout the Hudson Bay system, Lagrangian simulations revealed that early hatchers were more likely to hatch in lower surface salinities and that larvae reaching larger prewinter lengths were likely to have hatched near or within estuaries. Analysis of otolith microchemistry showed that larvae with low strontium-88 concentration in the otolith core, indicating a low salinity hatch location, had hatched earlier and thus had a longer growth period before freeze-up. These results show the potential for Arctic cod persistence in the Arctic where freshwater input is projected to increase and the ice regime is predicted to become more seasonal, provided that the surface temperatures remain below embryonic and larval lethal limits.

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n°90

As part of the Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP), several climate modeling centers performed a coordinated pre-study experiment with interactive stratospheric aerosol models simulating the volcanic aerosol cloud from an eruption resembling the 1815 Mt. Tambora eruption (VolMIP-Tambora ISA ensemble). The pre-study provided the ancillary ability to assess intermodel diversity in the radiative forcing for a large stratospheric-injecting equatorial eruption when the volcanic aerosol cloud is simulated interactively. An initial analysis of the VolMIP-Tambora ISA ensemble showed large disparities between models in the stratospheric global mean aerosol optical depth (AOD). In this study, we now show that stratospheric global mean AOD differences among the participating models are primarily due to differences in aerosol size, which we track here by effective radius. We identify specific physical and chemical processes that are missing in some models and/or parameterized differently between models, which are together causing the differences in effective radius. In particular, our analysis indicates that interactively tracking hydroxyl radical (OH) chemistry following a large volcanic injection of sulfur dioxide (SO2) is an important factor in allowing for the timescale for sulfate formation to be properly simulated. In addition, depending on the timescale of sulfate formation, there can be a large difference in effective radius and subsequently AOD that results from whether the SO2 is injected in a single model grid cell near the location of the volcanic eruption, or whether it is injected as a longitudinally averaged band around the Earth.

Mercury is a potent toxicant whose concentration in the environment has increased markedly due to human activity. However, uncertainties exist on sources and sinks of mercury, including those associated with mineral dust, with the Sahara Desert being the largest aeolian dust source worldwide. To characterize mercury associated with Saharan dust export to the Atlantic Ocean, aeolian dust (>2.2 µm in size) and soil samples were collected from the Canary Islands and Cabo Verde, ~400 and 800 km off the west coast of Africa. The sources of the dust samples were determined by back-trajectory analysis, and mercury content was characterized via direct mercury analysis. Mercury concentrations differed significantly (p 70%) over the Atlantic Ocean, had a mean mercury concentration of 29.8±31.3 ng/g (w/w), or 1.40±1.34 pg/m3 (w/v); this result is in the lower end of the data reported for remote coastal locations on a w/v basis, but lower than any on a w/w basis. Saharan-origin dust, defined as dust with a back trajectory predominantly (>70%) over the Sahara Desert, had a higher mean mercury concentration of 72.8±7.43 ng/g, or 4.64±2.15 pg/m3. Concentrations of mercury in dust samples were found to increase as back trajectories passed over a larger extent of the western African land surface relative to the Atlantic Ocean water surface. Finally, particulate-bound mercury (PBM) concentrations measured at Cabo Verde were used to estimate the flux of particulate mercury to major dust-impacted regions of the Atlantic Ocean, an area comprising 21.3 million km2. The PBM flux to this region is estimated at 6.5±2.2 t/y if all dust originates from the Sahara, or 3.6±1.6 t/y if the dust is of mixed Saharan and Atlantic origins. These numbers represent only ~1% to 3% of the total mercury deposition to the region estimated by current global mercury models. Surface soil mercury concentrations measured in the Canary Islands further support minimal mercury inputs from Saharan dust. Our study thus suggests the current models may have considerably overestimated Saharan dust contribution to mercury deposition.