Using modern and paleoceanographic isotopic systems to reconstruct Late Holocene temporal oceanographic variability in the rapidly warming Gulf of Maine

The Gulf of Maine, a semi-enclosed sea on the east coast of North America, has been warming faster than most of the rest of the world\u27s oceans over the last three decades. Since the installment of the Boothbay Harbor (Maine) sea surface temperature (SST) record in 1905, surface waters have warmed by more than 2°C, with more rapid warming seen recently. However, it is unclear when this pronounced warming in the Gulf of Maine began or whether the warming in recent decades is the result of natural variability, anthropogenic causes, or a combination of both. Such considerations are not only of grave concern for Gulf of Maine ecosystems and the many economically important fisheries in the region but also for other regions of the world\u27s oceans that are predicted to warm in the future with anthropogenic climate change. This dissertation uses stable and radiogenic isotopes, in both the modern waters of the Gulf of Maine and preserved Arctica islandica shells collected in the western Gulf of Maine, in order to reconstruct hydrographic variability in the region from several centuries before the instrumental record began up until present day. Isotopic investigations (d18Owater, d15NNO3-, d18ONO3-) of modern water samples collected at varying depths throughout the Gulf of Maine enable a better understanding of factors, including ocean circulation and nitrogen cycling processes, which dictate isotopic systems in today\u27s waters. Such investigations lend both insight into modern processes, such as the presence of nitrification in the upper water column, as well as aid in the interpretation of isotopic variability preserved in the geologic record. Furthermore, d15N in periostracum (the brown, protein rich layer on the outside of shells) of A. islandica shells is investigated as a new proxy for d15N and therefore water mass source variability. Using compound specific nitrogen isotopes of amino acid, we show that this proxy can be used in place of carbonate d15N to track changes in d15N of the clam diet and therefore d15N of nitrogen substrates in the water. Such findings enable a much higher resolution record of d15N in the Gulf of Maine through time than would be possible using d15N measured in carbonate. These analyses also suggest that the clams have not changed trophic level significantly over the last 300 years. Finally, this dissertation presents a 300-year reconstruction of hydrographic variability in the Gulf of Maine from crossdated A. islandica shells. Oxygen isotopes (d18O) and nitrogen isotopes (d15N) were measured in these shells and combined with radiocarbon (D14C) data from an earlier study. These latter two isotopic systems have been shown by others to vary with the major water masses that compose Gulf of Maine waters, with Warm Slope Water (WSW; originating from south of the Gulf of Maine) having lower d15N and higher D14C than Labrador Slope Water (LSW; originating in the Labrador Sea). Thus, these water mass tracers serve to distinguish among the different water masses entering the Gulf of Maine through time. Additionally, d18O varies with temperature, assuming a constant d18O of the water, and therefore is likely to also vary between warmer WSW and colder LSW. The d18O data, which have statistically significant correlations with the nearby Boothbay SST record that extends back to 1905, suggest that the Gulf of Maine has been warming since the 1870s, following a cooling of the region coming out of the Little Ice Age. SST output from a Community Earth System Model-Last Millennial Ensemble (CESM-LME) multi-ensemble mean for the region shows similar trends and suggests external forcing mechanisms for these changes. Likewise, the d15N and d14C reconstructions record comparable changes to the d18O record after the 1840s and therefore suggest, assuming isotopic signatures of water mass endmembers are constant, that the warming seen in the Gulf of Maine since the 1870s is in part a result of changes in the proportion of water masses entering the Gulf of Maine, with increasing northward flowing WSW entering the region over the last century. Such findings, coupled with statistically significant correlations between the d18O record and instrumental records of Florida Current strength and the Atlantic Meridional Overturning Circulation (AMOC) instrumental array at 26°N, suggest that recent changes in Gulf of Maine hydrographic variability can largely be attributed to changes in AMOC strength, which has been shown by several other paleoceanographic reconstructions to be weakening since the late 1800s due to increased freshwater flux into the northern North Atlantic. The records presented here suggest that this weakening of the AMOC followed a strengthening out of the Little Ice Age in the mid-to-late 1800s

Reconstructing Late Holocene Hydrographic Variability of the Gulf of Maine

I present an annually resolved reconstruction of seawater temperatures in the western North Atlantic from 1695-1915. This paleoclimate record was constructed using oxygen isotopes measured in precisely dated Arctica islandica shells collected off of Seguin Island in the western Gulf of Maine. The temperature reconstruction was derived from this oxygen isotope time series using a modern d18Ow-salinity mixing line developed for coastal waters in the Gulf of Maine from water samples collected over the last decade. The d18Ow and salinity composition of these water samples indicate that coastal surface waters consist of a mixture of Scotian Shelf Water and Maine River Water. The properties of these coastal waters are significantly influenced by seasonal changes in local river discharge. The Gulf of Maine oxygen isotope record suggests centennial-scale oscillations in seawater temperatures and therefore in the strength and position of the major ocean current systems that influence Gulf of Maine water properties. This record indicates that recent warming seen in the Gulf of Maine is not yet outside the natural seawater temperature variability of the region and therefore cannot be unequivocally linked to anthropogenic climate change. The positive and negative correlations between the Gulf of Maine oxygen isotope record and seawater temperature records from the subpolar gyre region of the North Atlantic and the western North Atlantic, respectively, are similar in pattern to the modeled and observed influence of the Atlantic meridional overturning circulation (AMOC) on seawater temperatures in these regions. This similarity suggests a possible association between AMOC variability and seawater temperatures in the Gulf of Maine. The association indicates that seawater temperature reconstructions from oxygen isotopes measured in A. islandica shells collected in the Gulf of Maine could provide an annually resolved, precisely dated reconstruction of AMOC variability. The oxygen isotope record I present in this thesis suggests centennial-scale oscillations in AMOC variability, with increased strength of the AMOC after the Little Ice Age

Spatial and temporal variability in the δ18Ow and salinity compositions of Gulf of Maine coastal surface waters

Hydrographic variability and dynamics in the Gulf of Maine are examined through the investigation of δ18Ow and salinity properties of coastal surface waters. Data from Gulf of Maine waters sampled over a decade, from 2003 to 2015, including a suite of samples that were collected monthly from April 2014 to March 2015, are presented. These water samples fall on a mixing line between Maine River Water (MRW) and Scotian Shelf Water (SSW). However, slope waters likely also contribute to these surface waters. The seasonal variability in water samples collected during 2014 and 2015 indicates the strong influence of river runoff on coastal Gulf of Maine surface water properties. The coastal surface Gulf of Maine mixing line presented in this paper is a needed baseline for reconstructing hydrographic variability in bicarbonates using oxygen isotopes

Rapid 20th century warming reverses 900-year cooling in the Gulf of Maine

The Gulf of Maine, located in the western North Atlantic, has undergone recent, rapid ocean warming but the lack of long-term, instrumental records hampers the ability to put these significant hydrographic changes into context. Here we present multiple 300-year long geochemical records (oxygen, nitrogen, and previously published radiocarbon isotopes) measured in absolutely-dated Arctica islandica shells from the western Gulf of Maine. These records, in combination with climate model simulations, suggest that the Gulf of Maine underwent a long-term cooling over most of the last 1000 years, driven primarily by volcanic forcing and North Atlantic ocean dynamics. This cooling trend was reversed by warming beginning in the late 1800s, likely due to increased atmospheric greenhouse gas concentrations and changes in western North Atlantic circulation. The climate model simulations suggest that the warming over the last century was more rapid than almost any other 100-year period in the last 1000 years in the region

Unexpected isotopic variability in biogenic aragonite: A user issue or proxy problem?

The present study seeks to investigate sources of isotopic variability in the commonly used paleoclimate archive, the marine bivalve Arctica islandica, with an emphasis on the potential of human-induced variability arising from sampling techniques. Stable carbon (d13Ccarbonate) and oxygen (d18Ocarbonate) isotopes were analyzed for split (intra-sample) and replicate (intra- and inter-shell) samples taken from a group of laboratory-reared individuals, a natural population from northern Norway, and a natural population from the Gulf of Maine, USA. Compared to analytical uncertainty of 0.17 ‰ and 0.30 ‰ for d13C and d18O, respectively, among the natural populations, the mean difference between shell splits and shell replicates ranged from 0.12 ‰ and 0.33 ‰ for d13C and d18O, respectively. Our data suggest that heterogeneity of the carbonate material (i.e., large range of isotopic composition within one sample due to seasonal environmental variability) may contribute to “unexpected” variability more than human-induced error from sampling imprecision when collecting whole annual increments. Furthermore, d13C from juvenile shells were highly variable (2s standard deviation = 0.65 ‰), approximately four times more variable than analytical precision. High precision among d18O measurements of the laboratory-reared shells confirm the presumption that shells reliably and consistently precipitate in isotopic equilibrium with ambient seawater. Monte Carlo simulations of measurements from this population allowed characterization of improvements in uncertainty at increasing levels of replication. Substantial reduction in uncertainty occurs when increasing from two to three shells, however replication using a total of four shells further decreased uncertainty to within the 99% confidence level. Published studies sometimes compensate for uncertainties by replicating records over multiple individuals or multiple transects within the one individual. Oftentimes, however, isotope records are constructed from single individuals or transects and therefore fail to provide thorough estimates of proxy error. Our findings suggest that replication of carbon and oxygen isotope measurements of contemporaneously produced aragonite is necessary in order to reduce proxy-derived noise. Furthermore, population-specific estimates of uncertainty related to natural variability among individuals should be investigated in order to provide more realistic representations of proxy noise when reporting isotope time series

Seasonal and interannual variation in high‑latitude estuarine fsh community structure along a glacial to non‑glacial watershed gradient in Southeast Alaska

Along the Gulf of Alaska, rapid glacier retreat has driven changes in transport of freshwater, sediments, and nutrients to estuary habitats. Over the coming decades, deglaciation will lead to a temporary increase, followed by a long-term decline of glacial influence on estuaries. Therefore, quantifying the current variability in estuarine fish community structure in regions predicted to be most affected by glacier loss is necessary to anticipate future impacts. We analyzed fish community data collected monthly (April through September) over 7 years (2013–2019) from glacially influenced estuaries along the southeastern Gulf of Alaska. River delta sites within estuaries were sampled along a natural gradient of glacial to non-glacial watersheds to characterize variation in fish communities exposed to varying degrees of glacial influence. Differences in seasonal patterns of taxa richness and abundance between the most and least glacially influenced sites suggest that hydrological drivers influence the structure of delta fish communities. The most glacially influenced sites had lower richness but higher abundance overall compared to those with least glacial influence; however, differences among sites were small compared to differences across months. Two dominant species—Pacific staghorn sculpin and starry flounder—contributed most to spatial and temporal variation in community composition; however, given only small interannual differences in richness and abundance over the period of the study, we conclude that year-to-year variation at these sites is relatively low at present. Our study provides an important benchmark against which to compare shifts in fish communities as watersheds and downstream estuaries continue to transform in the coming decades.This research was funded by the Alaska Established Program to Stimulate Competitive Research (EPSCoR) National Science Foundation award no. OIA-1208927 and award no. OIA-1757348 and by the State of Alaska. In addition, this publication is the result of research sponsored by Alaska Sea Grant with funds from the National Oceanic and Atmospheric Administration Ofce of Sea Grant, Department of Commerce, under grant no. NA14OAR4170079 (projects RR/14-01 & R/32-07 to AHB) and G00009215 (project 14CR-07 to CAB), and from the University of Alaska with funds appropriated by the state. Student support was also provided to DHD through a Ladd Macaulay Graduate Fellowship in Salmon Fisheries Research funded through an endowment and donations provided to the University of Alaska by Douglas Island Pink and Chum, Inc. (DIPAC), and to NCL by the North Pacifc Research Board through a Graduate Student Research Award. We are grateful to Franz Mueter for assistance with the analysis and to the many students and volunteers who participated in feldwork. Thanks to two anonymous reviewers whose comments improved the paper. This research was approved by the University of Alaska Institutional Animal Care and Use Committee (protocols 465729, 880562, 479533, 1238650).Ye

Seasonal and interannual variation in high‑latitude estuarine fsh community structure along a glacial to non‑glacial watershed gradient in Southeast Alaska

Along the Gulf of Alaska, rapid glacier retreat has driven changes in transport of freshwater, sediments, and nutrients to estuary habitats. Over the coming decades, deglaciation will lead to a temporary increase, followed by a long-term decline of glacial influence on estuaries. Therefore, quantifying the current variability in estuarine fish community structure in regions predicted to be most affected by glacier loss is necessary to anticipate future impacts. We analyzed fish community data collected monthly (April through September) over 7 years (2013–2019) from glacially influenced estuaries along the southeastern Gulf of Alaska. River delta sites within estuaries were sampled along a natural gradient of glacial to non-glacial watersheds to characterize variation in fish communities exposed to varying degrees of glacial influence. Differences in seasonal patterns of taxa richness and abundance between the most and least glacially influenced sites suggest that hydrological drivers influence the structure of delta fish communities. The most glacially influenced sites had lower richness but higher abundance overall compared to those with least glacial influence; however, differences among sites were small compared to differences across months. Two dominant species—Pacific staghorn sculpin and starry flounder—contributed most to spatial and temporal variation in community composition; however, given only small interannual differences in richness and abundance over the period of the study, we conclude that year-to-year variation at these sites is relatively low at present. Our study provides an important benchmark against which to compare shifts in fish communities as watersheds and downstream estuaries continue to transform in the coming decades.This research was funded by the Alaska Established Program to Stimulate Competitive Research (EPSCoR) National Science Foundation award no. OIA-1208927 and award no. OIA-1757348 and by the State of Alaska. In addition, this publication is the result of research sponsored by Alaska Sea Grant with funds from the National Oceanic and Atmospheric Administration Ofce of Sea Grant, Department of Commerce, under grant no. NA14OAR4170079 (projects RR/14-01 & R/32-07 to AHB) and G00009215 (project 14CR-07 to CAB), and from the University of Alaska with funds appropriated by the state. Student support was also provided to DHD through a Ladd Macaulay Graduate Fellowship in Salmon Fisheries Research funded through an endowment and donations provided to the University of Alaska by Douglas Island Pink and Chum, Inc. (DIPAC), and to NCL by the North Pacifc Research Board through a Graduate Student Research Award. We are grateful to Franz Mueter for assistance with the analysis and to the many students and volunteers who participated in feldwork. Thanks to two anonymous reviewers whose comments improved the paper. This research was approved by the University of Alaska Institutional Animal Care and Use Committee (protocols 465729, 880562, 479533, 1238650).Ye

Early gestational ethanol exposure in mice : Effects on brain structure, energy metabolism and adiposity in adult offspring

We examined whether an early-life event ethanol exposure in the initial stages of pregnancy affected offspring brain structure, energy metabolism, and body composition in later life. Consumption of 10% (v/v) ethanol by inbred C57BL/6J female mice from 0.5 to 8.5 days post coitum was used to model alcohol exposure during the first 3-4 weeks of gestation in humans, when pregnancy is not typically recognized. At adolescence (postnatal day [P] 28) and adulthood (P64), the brains of male offspring were scanned ex vivo using ultra-high field (16.4 T) magnetic resonance imaging and diffusion tensor imaging. Energy metabolism and body composition were measured in adulthood by indirect calorimetry and dual energy X-ray absorptiometry (DXA), respectively. Ethanol exposure had no substantial impact on white matter organization in the anterior commissure, corpus callosum, hippocampal commissure, internal capsule, optic tract, or thalamus. Whole brain volume and the volumes of the neocortex, cerebellum, and caudate putamen were also unaffected. Subtle, but non-significant, effects were observed on the hippocampus and the hypothalamus in adult ethanol-exposed male offspring. Ethanol exposure was additionally associated with a trend toward decreased oxygen consumption, carbon dioxide production, and reduced daily energy expenditure, as well as significantly increased adiposity, albeit with normal body weight and food intake, in adult male offspring. In summary, ethanol exposure restricted to early gestation had subtle long-term effects on the structure of specific brain regions in male offspring. The sensitivity of the hippocampus to ethanol-induced damage is reminiscent of that reported by other studies despite differences in the level, timing, and duration of exposure and likely contributes to the cognitive impairment that characteristically results from prenatal ethanol exposure. The hypothalamus plays an important role in regulating metabolism and energy homeostasis. Our finding of altered daily energy expenditure and adiposity in adult ethanol-exposed males is consistent with the idea that central nervous system abnormalities also underpin some of the metabolic phenotypes associated with ethanol exposure in pregnancy. (C) 2018 Elsevier Inc. All rights reserved.Peer reviewe

Pacific climate influences on ocean conditions and extreme shell growth events in the Northwestern Atlantic (Gulf of Maine)

The Gulf of Maine is undergoing rapid environmental and ecological changes, yet our spatial and temporal understanding of the climatic and hydrographic variability in this region, including extreme events, is limited and biased to recent decades. In this study, we utilize a highly replicated, multi-century master shell growth chronology derived from the annual increments formed in the shells of the long-lived bivalve Arctica islandica collected in 38 m from the central coastal region in the Gulf of Maine. Our results indicate that shell growth is highly synchronous and inversely related to local seawater temperatures. Using composite analyses of extreme shell growth events from CE 1900 to 2013, we extend our understanding of the factors driving oceanic variability and shell growth in the Northwestern Atlantic back to CE 1761. We suggest that extreme shell growth events are primarily controlled by Gulf of Maine sea surface temperature (SST) and stratification conditions, which in turn appear to be largely influenced by SST patterns in the Pacific Ocean through their influence on mid-latitude atmospheric circulation patterns and the location of the eddy-driven jet. The large-scale jet dynamics during these extreme years manifest as precipitation and moisture transport anomalies and regional SST conditions in the Gulf of Maine that either enhance or inhibit shell growth. Pacific climate variability is thus an important, yet understudied, influence on Gulf of Maine ocean conditions

Evolution of Fruit Traits in Ficus Subgenus Sycomorus (Moraceae): To What Extent Do Frugivores Determine Seed Dispersal Mode?

Fig trees are a ubiquitous component of tropical rain forests and exhibit an enormous diversity of ecologies. Focusing on Ficus subgenus Sycomorus, a phenotypically diverse and ecologically important Old World lineage, we examined the evolution of fruit traits using a molecular phylogeny constructed using 5 kilobases of DNA sequence data from 63 species (50% of global diversity). In particular, we ask whether patterns of trait correlations are consistent with dispersal agents as the primary selective force shaping morphological diversity or if other ecological factors may provide a better explanation? Fig colour, size and placement (axial, cauliflorous, or geocarpic) were all highly evolutionarily liable, and the same fruit traits have evolved in different biogeographic regions with substantially different dispersal agents. After controlling for phylogenetic autocorrelation, we found that fig colour and size were significantly associated with fig placement and plant-life history traits (maximum plant height and leaf area, respectively). However, contrary to prevailing assumptions, fig placement correlated poorly with known dispersal agents and appears more likely determined by other factors, such as flowering phenology, nutrient economy, and habitat preference. Thus, plant life-history, both directly and through its influence on fig placement, appears to have played a prominent role in determining fruit traits in these figs