Myvatn Research Station


Myvatn Research Station

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Webert K.C.,University of Wisconsin - Madison | Herren C.M.,University of Wisconsin - Madison | Einarsson A.,Myvatn Research Station | Einarsson A.,University of Iceland | And 4 more authors.
Ecosphere | Year: 2017

The importance of environmental disturbances as drivers of ecological communities depends not only on the magnitude of the disturbance, but also on the disturbance-specific sensitivity of the community. Organisms that alter the physical structure of their surroundings can affect the sensitivity of their habitat to environmental disturbance, and may alter the potential for disturbance to shape ecological communities. Such organisms therefore act as ecosystem engineers by indirectly modifying the resources available to other species. The benthos of shallow, eutrophic Lake Mývatn, Iceland, is frequently disturbed by wind events that lead to sediment resuspension. The impact of wind, however, depends on the abundance of midges (Chironomidae) whose larval tubes bind sediment and reduce wind-driven resuspension. Here, we investigate the long-term effect of fluctuations in midge abundance on the benthic cladoceran community using two lake sediment cores representing 30 and 140 years of deposition. In both cores, midge remains show a significant positive correlation with abundance of a large benthic surface-dwelling cladoceran, Eurycercus lamellatus, relative to the abundance of a small within-sediment-dwelling cladoceran, Alona rectangula. To experimentally investigate whether this shift could have been caused by midges acting as ecosystem engineers, we subjected cladoceran communities to sediment resuspension events within mesocosms. We found a significant decrease in abundance of the large epibenthic E. lamellatus relative to the abundance of small infaunal Alona spp. when subjected to disturbance. These findings show that physical alteration of benthic sediment and hence the sensitivity of the sediment to disturbance may explain the community shift in cladocerans observed with fluctuating midge abundance in Lake Mývatn. © 2017 Webert et al.

Ascough P.L.,Scottish Enterprise | Cook G.T.,Scottish Enterprise | Church M.J.,Durham University | Dunbar E.,Scottish Enterprise | And 7 more authors.
Radiocarbon | Year: 2010

Lake Mývatn is an interior highland lake in northern Iceland that forms a unique ecosystem of international scientific importance and is surrounded by a landscape rich in archaeological and paleoenvironmental sites. A significant freshwater reservoir effect (FRE) has been identified in carbon from the lake at some Viking (about AD 870-1000) archaeological sites in the wider region (Mývatnssveit). Previous accelerator mass spectrometry (AMS) measurements indicated this FRE was about 1500-1900 14C yr. Here, we present the results of a study using stable isotope and 14C measurements to quantify the Mývatn FRE for both the Viking and modern periods. This work has identified a temporally variable FRE that is greatly in excess of previous assessments. New, paired samples of contemporaneous bone from terrestrial herbivores and omnivores (including humans) from Viking sites demonstrate at least some omnivore diets incorporated sufficient freshwater resources to result in a herbivore-omnivore age offset of up to 400 14C yr. Modern samples of benthic detritus, aquatic plants, zooplankton, invertebrates, and freshwater fish indicate an FRE in excess of 5000 14C yr in some species. Likely geothermal mechanisms for this large FRE are discussed, along with implications for both chronological reconstruction and integrated investigation of stable and radioactive isotopes. © 2010 by the Arizona Board of Regents on behalf of the University of Arizona.

Ascough P.L.,Scottish Enterprise | Cook G.T.,Scottish Enterprise | Hastie H.,Scottish Enterprise | Dunbar E.,Scottish Enterprise | And 4 more authors.
Holocene | Year: 2011

A freshwater radiocarbon ( 14C) reservoir effect (FRE) is a 14C age offset between the atmospheric and freshwater carbon reservoirs. FREs can be on the order of 10 000 14C yr in extreme examples and are a crucial consideration for 14C dating of palaeoenvironmental and archaeological samples. Correction for a FRE may be possible, provided the FRE and the proportion of FRE-affected carbon within a sample can be accurately quantified. However, although such correction is desirable for affected samples, it is essential that such correction is accurate in order to produce useful chronological information. Accuracy of FRE correction can be limited by spatial variation in FRE within a freshwater system, but despite this there is currently a paucity of information to identify and quantify such variability within affected systems. Here we present results of a study that investigates the effects of spatial FRE variation upon dating accuracy within the freshwater system of Lake Mývatn, northern Iceland. A substantial FRE (>10 000 14C yr) has previously been identified in archaeological and modern samples from the region, which shows the potential for considerable spatial variability. The study also assesses the use of δ 13C and δ 15N in age correction of affected samples. The results show that benthic detritus and organisms at primary trophic levels from locations within the lake are affected by a FRE of at least 3500 14C yr, with clear spatial variation resulting in 14C age differences of up to 7670 14C yr between samples. There is a broad correlation between stable isotope values and FRE within the data set. However, large associated uncertainties currently preclude highly accurate and precise stable isotope-based quantification of the proportion of FRE-affected carbon within archaeological and palaeoenvironmental samples from Mývatn and the surrounding region. © The Author(s) 2011.

Pogge von Strandmann P.A.E.,Birkbeck, University of London | Burton K.W.,Durham University | Opfergelt S.,Catholic University of Leuven | Eiriksdottir E.S.,University of Iceland | And 4 more authors.
Chemical Geology | Year: 2016

Lithium isotopes are rapidly becoming one of the most useful tracers of silicate weathering processes, but little is known on their behaviour in groundwaters and hydrothermal springs, and how these sources might influence the weathering signal in surface waters. This study presents lithium isotope compositions (δ7Li) for cold groundwaters (3-7°C) and hydrothermal springs that were at geothermal temperatures (200-300°C) but have cooled during transport (17-44°C). Both represent an important source of water and nutrients for Lake Myvatn, Iceland. We also present a time-series from the Laxa River, which is the single outflow from the lake. The δ7Li values in the input springs to Lake Myvatn are highly variable (5-27‰), and correlate inversely with temperature and total dissolved solids. These co-variations imply that even in such waters, the processes controlling δ7Li variations during weathering still operate: that is, the ratio of primary rock dissolution to secondary mineral formation, where the latter preferentially incorporates 6Li with a temperature-dependent fractionation factor. In high-temperature geothermal waters (>300°C) secondary mineral formation is inhibited, and has a low fractionation factor, leading to little δ7Li fractionation. Even in waters that have cooled considerably over several months from their geothermal temperatures, fractionation is still low, and δ7Li values are similar to those reported from waters measured at >350°C. In contrast, cooler groundwaters promote relatively high proportions of clay formation, which scavenge dissolved solids (including 6Li). The time series on the Laxa River, the single outflow from Lake Myvatn, shows little δ7Li variation with time over the 12month sampling period (17-21‰), demonstrating that in contrast to tracers such as Si isotopes, Li isotopes are unaffected by the significant seasonal phytoplankton blooms that occur in the lake. Thus, these results clearly illustrate that Li isotopes are ideally suited to constrain silicate weathering processes, because fractionation by secondary mineral formation operates even when groundwater and hydrothermal inputs are significant, and because Li isotopes are demonstrably unaffected by phytoplankton or plant growth. © 2016 Elsevier B.V.

Sayle K.L.,Scottish Enterprise | Cook G.T.,Scottish Enterprise | Ascough P.L.,Scottish Enterprise | Hastie H.R.,Scottish Enterprise | And 6 more authors.
Geochimica et Cosmochimica Acta | Year: 2013

Carbon and nitrogen stable isotope ratios (δ13C and δ15N) have been used widely in archaeology to investigate palaeodiet. Sulphur stable isotope ratios (δ34S) have shown great promise in this regard but the potential of this technique within archaeological science has yet to be fully explored. Here we report δ34S, δ13C and δ15N values for 129 samples of animal bone collagen from Skútusta{eth}ir, an early Viking age (landnám) settlement in north-east Iceland. This dataset represents the most comprehensive study to date of its kind on archaeological material and the results show a clear offset in δ34S values between animals deriving their dietary resources from terrestrial (mean=+5.6±2.8‰), freshwater (mean=-2.7±1.4‰) or marine (mean=+15.9±1.5‰) reservoirs (with the three food groups being significantly different at 2σ). This offset allows reconstruction of the dietary history of domesticated herbivores and demonstrates differences in husbandry practices and animal movement/trade, which would be otherwise impossible using only δ13C and δ15N values. For example, several terrestrial herbivores displayed enriched bone collagen δ34S values compared to the geology of the Lake Mývatn region, indicating they may have been affected by sea-spray whilst being pastured closer to the coast, before being traded inland. Additionally, the combination of heavy δ15N values coupled with light δ34S values within pig bone collagen suggests that these omnivores were consuming freshwater fish as a significant portion of their diet. Arctic foxes were also found to be consuming large quantities of freshwater resources and radiocarbon dating of both the pigs and foxes confirmed previous studies showing that a large freshwater radiocarbon (14C) reservoir effect exists within the lake. Overall, these stable isotope and 14C data have important implications for obtaining a fuller reconstruction of the diets of the early Viking settlers in Iceland, and may allow a clearer identification of the marine and/or freshwater 14C reservoir effects that are known to exist in human bone collagen. © 2013 Elsevier Ltd.

Millet A.,Holar University College | Kristjansson B.K.,Holar University College | Einarsson A.,Myvatn Research Station | Einarsson A.,University of Iceland | Rasanen K.,ETH Zurich
Ecology and Evolution | Year: 2013

Eco-evolutionary responses of natural populations to spatial environmental variation strongly depend on the relative strength of environmental differences/natural selection and dispersal/gene flow. In absence of geographic barriers, as often is the case in lake ecosystems, gene flow is expected to constrain adaptive divergence between environments - favoring phenotypic plasticity or high trait variability. However, if divergent natural selection is sufficiently strong, adaptive divergence can occur in face of gene flow. The extent of divergence is most often studied between two contrasting environments, whereas potential for multimodal divergence is little explored. We investigated phenotypic (body size, defensive structures, and feeding morphology) and genetic (microsatellites) structure in threespine stickleback (Gasterosteus aculeatus) across five habitat types and two basins (North and South) within the geologically young and highly heterogeneous Lake Mỳvatn, North East Iceland. We found that (1) North basin stickleback were, on average, larger and had relatively longer spines than South basin stickleback, whereas (2) feeding morphology (gill raker number and gill raker gap width) differed among three of five habitat types, and (3) there was only subtle genetic differentiation across the lake. Overall, our results indicate predator and prey mediated phenotypic divergence across multiple habitats in the lake, in face of gene flow. © 2013 The Authors.

Hauptfleisch U.,Myvatn Research Station | Hauptfleisch U.,University of Iceland | Einarsson A.,Myvatn Research Station | Einarsson A.,University of Iceland | And 3 more authors.
Freshwater Biology | Year: 2012

1. Monitoring of the ecosystem of Lake Mývatn, Iceland, since 1975 has revealed extreme fluctuations in important food web components, such as chironomids and cladocerans, with amplitudes of several orders of magnitude and a period of 5-8years. This study uses sediment cores from the lake to examine if the food web fluctuations appear in the microfossil record of the sediment. 2. Dating was achieved by means of a combination of 137Cs and volcanic tephra and was fine-tuned by wiggle-matching of chironomid microfossil and monitoring data. 3.Cladocera exuviae and chironomid egg capsules in the uppermost 34cm of sediment were compared with the monitoring record that consisted of 30years of window trap catches of flying chironomids and a 16-year record of chydorid Cladocera caught in activity traps. 4. The observed chironomid and cladoceran population fluctuations were reflected in the sediment record of chironomid eggs and of the exuviae of three of seven cladocerans: Alonella nana, Alona rectangula and Eurycercus lamellatus, which also had the most extreme fluctuations in the monitoring data (3-4 orders of magnitude). Chydorus sphaericus, and to some extent Alona quadrangularis and Acroperus harpae, showed regular fluctuations in the core that the monitoring did not reveal. Density of subfossil chironomid eggs correlated positively with that of larval head capsules but not with other microfossils. 5.This study shows a reasonably good correspondence between the fossil records of chironomids and cladocerans on the one hand and biomonitoring data on the other. The results pave the way for an extension of the food web history to much earlier time intervals of the ecosystem, allowing the study of long-term variation in the food web dynamics, including the impact of climatic variation and other external forcing. The results also indicate the usefulness of chironomid egg capsules in palaeolimnological studies. © 2010 Blackwell Publishing Ltd.

Hauptfleisch U.,Myvatn Research Station | Hauptfleisch U.,University of Iceland | Einarsson A.,Myvatn Research Station | Einarsson A.,University of Iceland
Radiocarbon | Year: 2012

Lake Mývatn and several smaller lakes in northern Iceland were formed by the large Holocene lava flow of the Younger Laxá Lava (YLL). The age of the YLL was estimated by tephrochronology at 1800-2300 BP (Thorarinsson 1951). Conventional radiocarbon dating of charred vegetation beneath the YLL (Thorarinsson 1964) yielded uncalibrated ages of 1940 ± 270 14C yr BP (Yale), 2110 ± 140 14C yr BP (Copenhagen), and 1990 ± 50 14C yr BP (Winnipeg). Ongoing paleolimnological research at Lake Mývatn requires a more precise age estimation of the YLL. Charcoal samples from beneath the YLL were collected at Presthvammur in 2007 and analyzed by accelerator mass spectrometry (AMS) 14C measurements. The reliability of the conventional 14C ages of the samples Yale, Copenhagen, and Winnipeg was re-evaluated, applying criteria from Pettit et al. (2003) and Graf (2009). The result of AMS 14C measurement (2170 ± 38 cal yr BP) underpins the local tephrochronology and provides a reliable age of the YLL and Lake Mývatn. © 2012 by the Arizona Board of Regents on behalf of the University of Arizona.

Opfergelt S.,University of Oxford | Opfergelt S.,Catholic University of Louvain | Eiriksdottir E.S.,University of Iceland | Burton K.W.,University of Oxford | And 5 more authors.
Earth and Planetary Science Letters | Year: 2011

Diatom productivity in the oceans plays a crucial role in the carbon cycle, but is strongly dependent upon the continental silicon supply. However, the relative influence of weathering and biological processes on continental Si fluxes remains poorly constrained. This study aims to quantify the impact of terrestrial diatom productivity on Si fluxes to the ocean. Lake Myvatn in North Iceland is one of the most productive lakes in the Northern Hemisphere, with nutrient-rich waters almost uniquely sourced by groundwater. The primary production is mainly controlled by diatom growth but also by cyanobacteria, and the lake output is via a single river, thereby providing a relatively simple natural laboratory to quantify the impact of diatom growth on the chemistry and Si budget of lake waters. Silicon stable isotopes (δ30Si) provide a tracer of this biocycling, and have been measured in groundwater inputs to the lake, and in time-series monitoring of waters at the lake outlet. The δ30Si values at the outlet range from +0.70±0.08 to +1.42±0.06%, which is significantly heavier than the groundwater input (average cold and hot springs: +0.50±0.17%, 2SD) and consistent with the preferential uptake of light Si isotopes by diatoms. The δ30Si value at the outlet increases by up to 0.9% in spring and autumn relative to the Si isotope composition of the inflow. These seasonal diatom blooms can be modeled by an open system of Si uptake and affect Si fluxes at the outlet of the lake by up to 79%, or 53% integrated over the year. In the summer a shift to lighter δ30Si values is correlated with a higher pH, which results in dissolution of diatoms releasing light Si isotopes. From mass balance, this seasonal diatom dissolution affects Si fluxes by up to 33%, but is limited to 3.7% integrated over the year. These results clearly illustrate that biological activity can have a significant impact on both isotope composition and elemental abundance of continental derived Si. They also demonstrate the pH dependency of diatom dissolution and/or preservation, which is likely to affect not only the continental Si fluxes to the ocean but also the Si recycling in the oceans themselves. © 2011 Elsevier B.V.

Einarsson A.,Myvatn Research Station | Einarsson A.,University of Iceland | Hauptfleisch U.,Myvatn Research Station | Hauptfleisch U.,University of Iceland | And 2 more authors.
Ecology | Year: 2016

Ecologists have long been fascinated by cyclic population fluctuations, because they suggest strong interactions between exploiter and victim species. Nonetheless, even for populations showing high-amplitude fluctuations, it is often hard to identify which species are the key drivers of the dynamics, because data are generally only available for a single species. Here, we use a paleoecological approach to investigate fluctuations in the midge population in Lake Mývatn, Iceland, which ranges over several orders of magnitude in irregular, multigeneration cycles. Previous circumstantial evidence points to consumer-resource interactions between midges and their primary food, diatoms, as the cause of these high-amplitude fluctuations. Using a pair of sediment cores from the lake, we reconstructed 26 years of dynamics of midges using egg remains and of algal groups using diagnostic pigments. We analyzed these data using statistical methods that account for both the autocorrelated nature of paleoecological data and measurement error caused by the mixing of sediment layers. The analyses revealed a signature of consumer-resource interactions in the fluctuations of midges and diatoms: diatom abundance (as inferred from biomarker pigment diatoxanthin) increased when midge abundance was low, and midge abundance (inferred from egg capsules) decreased when diatom abundance was low. Similar patterns were not found for pigments characterizing the other dominant primary producer group in the lake (cyanobacteria), subdominant algae (cryptophytes), or ubiquitous but chemically unstable biomarkers of total algal abundance (chlorophyll a); however, a significant but weaker pattern was found for the chemically stable indicator of total algal populations (β-carotene) to which diatoms are the dominant contributor. These analyses provide the first paleoecological evaluation of specific trophic interactions underlying high amplitude population fluctuations in lakes. © 2016 by the Ecological Society of America.

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