Climate and oceanography of the Galapagos in the 21st century : expected changes and research needs

With the likelihood that carbon dioxide and other greenhouse-gas levels in the atmosphere will continue to increase for the next decades, and that the planet as a whole will likely warm as a result, we expect the oceanography and climate of the Galapagos to change. Based on an analysis of observational studies and climate models, the main changes are likely to include higher sea-surface temperatures, continued El Niño and La Niña events, some of which will be intense, a rise in sea level of several cm, increased precipitation, lower surface ocean pH, and a reduction in upwelling. These changes will likely alter the marine and terrestrial ecosystems of the Galapagos in ways that are difficult to predict. Major uncertainties exist concerning the relationship between the expected regional changes in ocean temperatures, precipitation, upwelling and seawater pH that most climate models consider, and the local changes in the Galapagos Islands

Tracing plant source water dynamics during drought by continuous transpiration measurements : An in-situ stable isotope approach

Publisher Copyright: © 2022 The Authors. Plant, Cell & Environment published by John Wiley & Sons Ltd.The isotopic composition of xylem water (δX) is of considerable interest for plant source water studies. In-situ monitored isotopic composition of transpired water (δT) could provide a nondestructive proxy for δX-values. Using flow-through leaf chambers, we monitored 2-hourly δT-dynamics in two tropical plant species, one canopy-forming tree and one understory herbaceous species. In an enclosed rainforest (Biosphere 2), we observed δT-dynamics in response to an experimental severe drought, followed by a 2H deep-water pulse applied belowground before starting regular rain. We also sampled branches to obtain δX-values from cryogenic vacuum extraction (CVE). Daily flux-weighted δ18OT-values were a good proxy for δ18OX-values under well-watered and drought conditions that matched the rainforest's water source. Transpiration-derived δ18OX-values were mostly lower than CVE-derived values. Transpiration-derived δ2HX-values were relatively high compared to source water and consistently higher than CVE-derived values during drought. Tracing the 2H deep-water pulse in real-time showed distinct water uptake and transport responses: a fast and strong contribution of deep water to canopy tree transpiration contrasting with a slow and limited contribution to understory species transpiration. Thus, the in-situ transpiration method is a promising tool to capture rapid dynamics in plant water uptake and use by both woody and nonwoody species.Peer reviewe

Chiral monoterpenes reveal forest emission mechanisms and drought responses

Monoterpenes (C10H16) are emitted in large quantities by vegetation to the atmosphere (>100 TgC year−1), where they readily react with hydroxyl radicals and ozone to form new particles and, hence, clouds, affecting the Earth’s radiative budget and, thereby, climate change1,2,3. Although most monoterpenes exist in two chiral mirror-image forms termed enantiomers, these (+) and (−) forms are rarely distinguished in measurement or modelling studies4,5,6. Therefore, the individual formation pathways of monoterpene enantiomers in plants and their ecological functions are poorly understood. Here we present enantiomerically separated atmospheric monoterpene and isoprene data from an enclosed tropical rainforest ecosystem in the absence of ultraviolet light and atmospheric oxidation chemistry, during a four-month controlled drought and rewetting experiment7. Surprisingly, the emitted enantiomers showed distinct diel emission peaks, which responded differently to progressive drying. Isotopic labelling established that vegetation emitted mainly de novo-synthesized (−)-α-pinene, whereas (+)-α-pinene was emitted from storage pools. As drought progressed, the source of (−)-α-pinene emissions shifted to storage pools, favouring cloud formation. Pre-drought mixing ratios of both α-pinene enantiomers correlated better with other monoterpenes than with each other, indicating different enzymatic controls. These results show that enantiomeric distribution is key to understanding the underlying processes driving monoterpene emissions from forest ecosystems and predicting atmospheric feedbacks in response to climate change

Leaf-level metabolic changes in response to drought affect daytime CO2 emission and isoprenoid synthesis pathways

In the near future, climate change will cause enhanced frequency and/or severity of droughts in terrestrial ecosystems, including tropical forests. Drought responses by tropical trees may affect their carbon use, including production of volatile organic compounds (VOCs), with implications for carbon cycling and atmospheric chemistry that are challenging to predict. It remains unclear how metabolic adjustments by mature tropical trees in response to drought will affect their carbon fluxes associated with daytime CO2 production and VOC emission. To address this gap, we used position-specific 13C-pyruvate labeling to investigate leaf CO2 and VOC fluxes from four tropical species before and during a controlled drought in the enclosed rainforest of Biosphere 2 (B2). Overall, plants that were more drought-sensitive had greater reductions in daytime CO2 production. Although daytime CO2 production was always dominated by non-mitochondrial processes, the relative contribution of CO2 from the tricarboxylic acid cycle tended to increase under drought. A notable exception was the legume tree Clitoria fairchildiana R.A. Howard, which had less anabolic CO2 production than the other species even under pre-drought conditions, perhaps due to more efficient refixation of CO2 and anaplerotic use for amino acid synthesis. The C. fairchildiana was also the only species to allocate detectable amounts of 13C label to VOCs and was a major source of VOCs in B2. In C. fairchildiana leaves, our data indicate that intermediates from the mevalonic acid (MVA) pathway are used to produce the volatile monoterpene trans-β-ocimene, but not isoprene. This apparent crosstalk between the MVA and methylerythritol phosphate pathways for monoterpene synthesis declined with drought. Finally, although trans-β-ocimene emissions increased under drought, it was increasingly sourced from stored intermediates and not de novo synthesis. Unique metabolic responses of legumes may play a disproportionate role in the overall changes in daytime CO2 and VOC fluxes in tropical forests experiencing drought

Interplay of community dynamics, temperature, and productivity on the hydrogen isotope signatures of lipid biomarkers

The hydrogen isotopic composition (δ2H) of lipid biomarkers has diverse applications in the fields of paleoclimatology, biogeochemistry, and microbial community dynamics. Large changes in hydrogen isotope fractionation have been observed among microbes with differing core metabolisms, while environmental factors including temperature and nutrient availability can affect isotope fractionation by photoautotrophs. Much effort has gone into studying these effects under laboratory conditions with single species cultures. Moving beyond controlled environments and quantifying the natural extent of these changes in freshwater lacustrine settings and identifying their causes is essential for robust application of δ2H values of common short-chain fatty acids as a proxy of net community metabolism and of phytoplankton-specific biomarkers as a paleohydrologic proxy. This work targets the effect of community dynamics, temperature, and productivity on 2H∕1H fractionation in lipid biomarkers through a comparative time series in two central Swiss lakes: eutrophic Lake Greifen and oligotrophic Lake Lucerne. Particulate organic matter was collected from surface waters at six time points throughout the spring and summer of 2015, and δ2H values of short-chain fatty acids, as well as chlorophyll-derived phytol and the diatom biomarker brassicasterol, were measured. We paired these measurements with in situ incubations conducted with NaH13CO3, which were used to calculate the production rates of individual lipids in lake surface water. As algal productivity increased from April to June, net discrimination against 2H in Lake Greifen increased by as much as 148 ‰ for individual fatty acids. During the same time period in Lake Lucerne, net discrimination against 2H increased by as much as 58 ‰ for individual fatty acids. A large portion of this signal is likely due to a greater proportion of heterotrophically derived fatty acids in the winter and early spring, which are displaced by more 2H-depleted fatty acids as phytoplankton productivity increases. Smaller increases in 2H discrimination for phytol and brassicasterol suggest that a portion of the signal is due to changes in net photoautotrophic 2H fractionation, which may be caused by increasing temperatures, a shift from maintenance to high growth, or changes in the community assemblage. The fractionation factors for brassicasterol were significantly different between the two lakes, suggesting that its hydrogen isotope composition may be more sensitive to nutrient regime than is the case for fatty acids or phytol.ISSN:1726-4170ISSN:1726-417

Fit and fatty freshwater fish: contrasting polyunsaturated fatty acid phenotypes between hybridizing stickleback lineages

Long-chain polyunsaturated fatty acids are biologically important lipids that are unevenly distributed between and throughout environments. This heterogeneity can affect the evolution of metabolic processes, as populations adapt to the resource landscape that they encounter. Here, we compare fatty acid phenotypes of stickleback over two time scales of evolutionary divergence: between two lineages with different metabolic capacities for fatty acid synthesis (i.e. different copy number of the fatty acid desaturase gene; FADS2) that independently colonized European freshwaters during the Pleistocene and Holocene; and between two ecotypes within each lineage that have diverged more recently (~150 years) in different habitats (i.e. lake and stream). We measured fatty acid profiles of wild-caught and lab-reared fish for each lineage and ecotype combination after rearing lab fish on a diet deficient in omega-3 long-chain polyunsaturated fatty acids. Since these lineages hybridize in nature, we also measured profiles of lab-reared hybrids and backcrosses raised on the same deficient diet. Wild fish showed strong compositional differences in fatty acids between habitats, lineages and sexes. Common garden fish had generally lower polyunsaturated fatty acid levels than wild fish, and females had lower omega-6:omega-3 than males. Fish from the lineage with fewer FADS2 copies also had lower levels of docosahexaenoic acid. Overall, we document divergence in fatty acid phenotypes between stickleback lineages with different histories of freshwater colonization, and between ecotypes in the early stages of adaptive population divergence

Lipid compound classes display diverging hydrogen isotope responses in lakes along a nutrient gradient

Compound specific hydrogen isotope ratios (H-2/H-1) of lipid biomarkers preserved in sediments are used as paleohydrologic proxies. However, several variables, including contributions from different source organisms and their growth rates, can influence H-2/H-1 fractionation between lipids and source water. Significant uncertainties remain about how these factors combine to produce the net H-2/H-1 signal exported to sediments.To assess the influence of phosphorus availability on H-2/H-1 ratios of lipids accumulating in lake sediments, we analyzed surface sediments and sediment traps from ten central Swiss lakes representing a wide range of trophic states. In agreement with results from laboratory cultures, H-2/H-1 fractionation for the diatom biomarker brassicasterol (24-methyl cholest-5,22-dien-3 beta-ol) increased in more productive lakes (0.6 +/- 0.1 parts per thousand per mu g/L total P in sediment traps and surface sediments). In contrast, H-2/H-1 fractionation for phytol, the isoprenoid side-chain moiety of chlorophyll, decreased with increasing total P (-0.4 +/- 0.1 parts per thousand per mu g/L total P in sediment traps), suggesting that different biochemical mechanisms are responsible for changes in H-2/H-1 fractionation for each type of isoprenoidal lipid. Opposing changes in H-2-fractionation for sterols and phytol cause their H-2/H-1 ratios to converge as total P increases. This response may be a new tracer for phytoplankton growth conditions and is not influenced by the source water isotope value.Interpreting the H-2/H-1 ratios of short to long chain (C-14-C-30) n-alkanoic acids and n-alkanols is complicated by likely contributions from heterotrophs and/or vascular plants. These values generally did not correlate with lake water isotopes, nor did their fractionation factors correlate with total P. For most lipids there was no significant difference between sediment trap and surface sediment H-2/H-1 ratios. However, n-C-14-n-C-18 fatty acids were H-2-enriched in the surface sediments, most likely due to degradation in the water column. Our results indicate that interpretations of short-chain fatty acid H-2/H-1 ratios as a water isotope signal likely require supporting information about ecological conditions and community structure, but that paired H isotope measurements of phytoplankton-derived sterols and phytol may be developed as a proxy for phytoplankton growth. (C) 2018 Elsevier Ltd. All rights reserved

Rapid atmospheric transport and large-scale deposition of recently synthesized plant waxes

Sedimentary plant wax H-2/H-1 ratios are important tools for understanding hydroclimate and environmental changes, but large spatial and temporal uncertainties exist about transport mechanisms from ecosystem to sediments. To assess atmospheric pathways, we collected aerosol samples for two years at four locations within a similar to 60 km radius in northern Switzerland. We measured n-alkane distributions and H-2/H-1 ratios in these samples, and from local plants, leaf litter, and soil, as well as surface sediment from six nearby lakes. Increased concentrations and H-2 depletion of long odd chain n-alkanes in early summer aerosols indicate that most wax aerosol production occurred shortly after leaf unfolding, when plants synthesize waxes in large quantities. During autumn and winter, aerosols were characterized by degraded n-alkanes lacking chain length preferences diagnostic of recent biosynthesis, and H-2/H-1 values that were in some cases more than 100 parts per thousand higher than growing season values. Despite these seasonal shifts, modeled deposition-weighted average H-2/H-1 values of long odd chain n-alkanes primarily reflected summer values. This was corroborated by n-alkane H-2/H-1 values in lake sediments, which were similar to deposition-weighted aerosol values at five of six sites. Atmospheric deposition rates for plant n-alkanes on land were similar to 20% of accumulation rates in lakes, suggesting a role for direct deposition to lakes or coastal oceans near similar production sources, and likely a larger role for deposition on land and transport in river systems. This mechanism allows mobilization and transport of large quantities of recently produced waxes as fine-grained material to low energy sedimentation sites over short timescales, even in areas with limited topography. Widespread atmospheric transfer well before leaf senescence also highlights the importance of the isotopic composition of early season source water used to synthesize waxes for the geologic record. (C) 2017 Elsevier Ltd. All rights reserved

Land-use evolution in the catchment of Lake Murten, Switzerland

Anthropogenic soil erosion is a problem of global concern and recently has become the focus of extensive research. In spite of this, our knowledge about the history of land-use and its long-term impact on soil erosion and the local environment remains limited. This study seeks to address this issue by investigating sediments of Lake Murten, Switzerland, using a multi-proxy approach to reconstruct the history of land-use and its impacts in the catchment. We analyzed pollen and charcoal to reconstruct past land-use and vegetation dynamics, and used the distributions of terrestrial leaf wax biomarkers, their δ13C isotopic composition and their soil retention time (compound-specific 14C) to evaluate long-term effects on past soil carbon dynamics. Arboreal pollen abundances, charcoal influx and cultural indicators match the archaeological evidence and reveal an eventful past around the lake. The first signs of human presence were detected around 5000 BCE, when Neolithic pile dwellers occupied the lake’s shores. However, human land-use had no significant effect on the pollen and the sedimentary organic matter (OM) composition during Neolithic times and the Bronze Age. This changed during the Late Iron Age and the Early Roman Period (ca. 70 BCE). Coincident with the rise of Aventicum, a Roman city, large-scale deforestation and agriculture began in the region. Severe soil degradation and outwash of soil organic carbon (SOC) at this time is documented by enhanced input of soil-derived and pre-aged leaf waxes, and resulted in cultural eutrophication ca. 2000 years ago. Soil erosion decreased after the fall of the Roman Empire and a short period of renaturation followed. Although the export of SOC returned to pre-Roman values after ca. 200 years, the forest never recovered to its past extent. The last two detected periods of land-use change correlate with the onset of Medieval agriculture (ca. 1000 CE) and the Industrial Period (ca. 1800 CE). Today, the mean transit time of leaf waxes is almost five times longer compared to the Roman Period, suggesting that substantial soil erosion has occurred and that an even longer time period would be necessary for the soil carbon dynamics to recover to their natural state

Hydrogen isotopes from lipid biomarkers record eutrophication induced changes in algal community assemblages

Phytoplankton are an important biogeochemical force, collectively impacting nutrient cycling as well as atmospheric and aquatic chemistry. However, it remains challenging to reconstruct changes in algal productivity and community assembly throughout the geologic past. Here, we lay the foundation for a sensitive proxy of past algal ecology based on compound-specific hydrogen isotope compositions (δ2H values) of common algal lipids. While such measurements have previously been used as indicators of water hydrogen isotope ratios, our results from laboratory cultures and experimental ponds demonstrate that changes in the δ2H values of ubiquitous lipids such as palmitic acid associated with taxonomic changes are an order of magnitude greater than those caused by hydrologic change. These results indicate that δ2H values of algal lipids, and the relative offset of these values among different compounds, can be used to reconstruct past changes in algal community assemblages, including those driven by changes in nutrient supply. We apply this approach to a ~180 year sedimentary record from Lake Greifen, a lake in the central Swiss Plateau that underwent well-documented eutrophication and partial recovery in the second half of the 20th century. As total phosphorus concentrations in the lake increased from > 100 mg/L to ~ 500 mg/L in the 1950s-1970s, palmitic acid d2H values increased by 40 ‰ and phytol δ2H values by 20 ‰; δ2H values of both compounds subsequently declined with total P following maximum values in the early 1980s. During this entire time interval, mean annual precipitation δ2H values fluctuated within a ~10 ‰ range and are not correlated with the changes in lipid δ2H values. Additionally, the decline in lipid δ2H values is correlated with declining relative abundance of green algae as the eutrophication pressure on Lake Greifen receded in the past decades. This correlation matches the prediction from our culturing and mesocosm results, where green algae produced exceptionally 2H-enriched fatty acids, and indicates that lipid δ2H can be applied to reconstruct nutrient induced shifts in algal populations