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Population cycles can be caused by consumer-resource interactions. Confirming the role of consumer-resource interactions, however, can be challenging due to an absence of data for the resource candidate. For example, interactions between midge larvae and benthic algae likely govern the high-amplitude population fluctuations of Tanytarsus gracilentus in Lake Mývatn, Iceland, but there are no records of benthic resources concurrent with adult midge population counts. Here, we investigate consumer population dynamics using the carbon stable isotope signatures of archived T. gracilentus specimens collected from 1977 to 2015, under the assumption that midge δ

The transfer of materials such as nutrients from one ecosystem to another can establish gradients that affect the distribution and performance of species. We examined how a natural resource gradient influences the density and flowering of butterwort (Pinguicula vulgaris), a carnivorous plant. Mývatn, a lake in Iceland, produces large midge populations, generating a gradient in midge abundance on shore. The midge gradient was associated with butterwort flowering probability, which decreased by roughly 50% moving away from the shore. In a midge addition experiment, plants receiving midges had a four-fold increase in their subsequent likelihood of flowering, implicating carnivory in the flowering gradient. Nonetheless, the density of butterworts increased with distance from shore, in negative association with soil nutrient content and the abundance of grass species. This suggests that the positive effect of the midge gradient on butterwort flowering is overwhelmed by the negative effect of competition with grasses.

The chironomids of Lake Mývatn show extreme population fluctuations that affect most aspects of the lake ecosystem. During periods of high chironomid densities, chironomid larvae comprise over 90% of aquatic secondary production. Here, we show that chironomid larvae substantially stimulate benthic gross primary production (GPP) and net primary production (NPP), despite consuming benthic algae. Benthic GPP in experimental mesocosms with 140,000 larvae/m2 was 71% higher than in mesocosms with no larvae. Similarly, chlorophyll a concentrations in mesocosms increased significantly over the range of larval densities. Furthermore, larvae showed increased growth rates at higher densities, possibly due to greater benthic algal availability in these treatments. We investigated the hypothesis that larvae promote benthic algal growth by alleviating nutrient limitation, and found that (1) larvae have the potential to cycle the entire yearly external loadings of nitrogen and phosphorus during the growing season, and (2) chlorophyll a concentrations were significantly greater in close proximity to larvae (on larval tubes). The positive feedback between chironomid larvae and benthic algae generated a net mutualism between the primary consumer and primary producer trophic levels in the benthic ecosystem. Thus, our results give an example in which unexpected positive feedbacks can lead to both high primary and high secondary production.

The Mývatn area in northeast Iceland has been occupied by farming communities since the arrival of Viking Age settlers in the late ninth century. Despite its inland location and relatively high elevation, this lake basin was affected by continuous human occupation through periods of harsh climate, volcanic eruptions, epidemics, and world system impacts. Mývatn’s residents have practised farming, fishing, egg-collecting, and hunting activities for over a millennium. They managed the landscape and its resources with the use of traditional knowledge, which included the story of the troll woman, Kraka, who lived in a cave in the mountain Blafjall (“Blue Mountain”). The story of Kraka and the river Kraka that bears her name provides a striking metaphor for the landscape history including water resources and environmental changes the agricultural community sustained over time.

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 12 month 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.

Soon after the settlement of Iceland in the late 9th and early 10th century, an extensive system of turf walls was erected and was functional for about 200 years. After its collapse in the 12th century, few if any walls were built until the late 18th century. The remnants of this Viking Age fence system have been mapped in northeast Iceland, revealing enclosures with a characteristic geometry, broadly reflecting property boundaries. The fences are the product of a pastoral society, which then as now was based on dispersed farmsteads subsisting on animal husbandry. There was a strong seasonal component, most notably a need for harvesting and storing winter fodder. The short-lived fence system offers a rare snapshot of the division of a newly colonized landscape into functional units. This paper presents a framework for evaluating those units based on current thinking in behavioral ecology. The settlement of Iceland by the Viking Age colonists may be described by principles of habitat-selection the...

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., National Science Foundation Graduate Research Fellowship. Grant Number: DGE-1256259 LTREB. Grant Number: DEB-1052160

During the Viking Age, Norse peoples established settlements across the North Atlantic, colonizing the pristine and near-pristine landscapes of the Faroe Islands, Iceland, Greenland, and the short-lived Vinland settlement in Newfoundland. Current North Atlantic archaeological research themes include efforts to understand human adaptation and impact in these environments. For example, early Icelandic settlements persisted despite substantial environmental impacts and climatic change, while the Greenlandic settlements were abandoned ca. AD 1450 in the face of similar environmental degradation. The Norse settlers utilized both imported domestic livestock and natural fauna, including wild birds and aquatic resources. The stable isotope ratios of carbon and nitrogen (expressed as δ13C and δ15N) in archaeofaunal bones provide a powerful tool for the reconstruction of Norse economy and diet. Here we assess the δ13C and δ15N values of faunal and floral samples from sites in North Iceland within the conte...

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 Skutustaðir, an early Viking age (landnam) 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.

Soil profiles and archaeological sites in Northeast Iceland contain a sequence of basaltic tephra layers coinciding in time with the first human settlement of the area during the Viking Age, known as the Landnam tephra sequence. The chronology of these layers is useful when reconstructing the history of human settlement in Iceland and its environmental impact. The only properly dated tephra layer from this tephra sequence is the Landnam tephra layer (LTL), formed in the A.D. 870s. Sedimentation rates calculated from the interval between tephra layers of known age (A.D. 871 ± 2, 1158, and 1300) in high-resolution sediment cores from Lake Mývatn were used to establish the approximate age of six tephra layers in the medieval period. One of the main objectives of the study was to improve dating of a younger tephra layer, formed in the mid-10th century according to previous studies. This tephra layer, originating from the Veiðivotn volcanic system, has proven to be a very important marker bed for arch...

n recent decades, sustainability research and historical ecology research have made the incorporation of local and traditional ecological knowledge (LTK) a priority for the purpose of understanding recent environmental change and achieving long-term perspectives on local resource interactions. This chapter brings together the evidence from archaeological, ecological, historical, and ethnographic sources to document the 1,100 year management of wild birds around Lake Mývatn in northern Iceland. In doing so, it sheds light on specific long-term resource management strategies applied by the community: precise, limited egg collection, and regulated bird hunting. These methods seem to have been effective in sustaining large, local bird populations and ensuring continual access to an important economic and dietary resource.

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 palcolimnological 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.

Summary 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 5–8 years. 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 34 cm of sediment were compared with the monitoring record that consisted of 30 years 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.

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.

Barrow's Goldeneye females with broods in the Lake Mývatn district, Iceland, concentrated in a small area in the lake's outlet where the density of blackfly Simulium vittatum larvae, the principal food of the young, was highest. A downstream migration of broods took place as predicted from the growth pattern and emergence of blackflies in the river.

This paper examines 2 potential sources of the radiocarbon offset between human and terrestrial mammal (horse) bones recovered from Norse (∼AD 870–1000) pagan graves in Mývatnssveit, north Iceland. These are the marine and freshwater 14C reservoir effects that may be incorporated into human bones from dietary sources. The size of the marine 14C reservoir effect (MRE) during the Norse period was investigated by measurement of multiple paired samples (terrestrial mammal and marine mollusk shell) at 2 archaeological sites in Mývatnssveit and 1 site on the north Icelandic coast. These produced 3 new δR values for the north coast of Iceland, indicating a δR of 106 ± 10 14C yr at AD 868–985, and of 144 ± 28 14C yr at AD 1280–1400. These values are statistically comparable and give an overall weighted mean δR of 111 ± 10 14C yr.The freshwater reservoir effect was similarly quantified using freshwater fish bones from a site in Mývatnssveit. These show an offset of between 1285 and 1830 14C yr, where the fish are depleted in 14C relative to the terrestrial mammals. This is attributed to the input of geothermally derived CO2 into the groundwater and subsequently into Lake Mývatn. We conclude the following: i) some of the Norse inhabitants of Mývatnssveit incorporated non-terrestrial resources into their diet that may be identified from the stable isotope composition of their bone collagen; ii) the MRE off the north Icelandic coast during the Norse period fits a spatial gradient of wider North Atlantic MRE values with increasing values to the northwest; and iii) it is important to consider the effect that geothermal activity could have on the 14C activity of samples influenced by groundwater at Icelandic archaeological sites.

The first stratigraphically continuous pollen profile spanning the Norse and Medieval periods from the archaeologically-rich My´vatnssveit region of northern Iceland is presented. Detailed analyses were made of the tephra, sediment characteristics, pollen and chironomids of a 3 kyr sediment sequence from Helluvaðstjo¨ rn, a small, shallow lake. The pollen data show a steady decline in the percentage abundance of tree birch (Betula pubescens) pollen between the Norse settlement (landna´m, c. AD 870) and c. AD 1300, a pattern that contrasts with the abrupt fall in birch pollen percentages immediately following the Norse colonization at almost all previously studied sites in Iceland. Some lines of evidence suggest that the gradual birch decline could be a result of reworking of soil pollen, but independent evidence suggests that this may not necessarily be the case. The pollen record indicates that birch woodland was replaced by acidophilic taxa (notably Empetrum nigrum and Sphagnum), again contrasting with the more usual pattern of Poaceae expansion seen in post-landna´m pollen diagrams from mires close to farm sites. Chironomid and Pediastrum accumulation data show that the limnic environment became more productive immediately after landna´m, probably because of anthropogenic disturbance. An increase in sedimentation rate after landna´m appears initially to have been caused by increased lake productivity, while reworked inorganic soil materials became a significant contributor to the sediments after c. AD 1200. The data suggest that the impact of settlement on terrestrial vegetation may have been more variable than previously thought, while freshwater ecosystems experienced significant and rapid change.

The 'Landscapes of Settlement Project' has carried out archaeological and paleoenvironmental research in the Lake Mývatn region of N. Iceland since 1996. Animal bone collections dating from the late 9th century to the early 13th century AD have been recovered from five sites in different ecological zones around the lake, and three of these sites provide multiple phases datable through radiocarbon, artefacts, and volcanic tephra. Modern systematic biological and geological investigations in the Mývatn district date to the 19th century and a detailed picture of the recent ecology can be combined with both archaeological and historical evidence for long term resource exploitation by humans in this inland region. Analysis of bird bones and bird eggshell suggests that the locally managed sustainable harvest of migratory waterfowl eggs carried out over the last 150 years extends back to the 9th century. These inland archaeofauna also include significant numbers of marine fish and sea birds, marine mollu...

We monitored numbers and production of diving ducks on Lake Myvatn and its outlet river, the Laxa, over a period of 25 years, 1975–1999. Aquatic insects, a major food source, were monitored for 23 years, 1977–1999, using window traps. In this paper, we focus on the relationship between the demography of five species of diving ducks and the abundance of aquatic insect species, mainly Chironomidae and Simuliidae. Production of ducks was estimated when most young were 2–4 weeks old, either as young per female (Aythya marila and Aythya fuligula) or as total young found (Melanitta nigra, Bucephala islandica, Histrionicus histrionicus). Variation between years in production of young ducks was strongly correlated with variation in the abundance of aquatic insects. Procladius islandicus and Psectrocladius barbimanus, both of which are univoltine with peak emergence in late July (when many young ducks hatch), as well as the bivoltine Orthocladius consobrinus, showed particularly high correlations with duckling numbers. In one year, 1992, catastrophic weather depressed production. Food appeared to determine production, primarily through its availability to the young. Changes in spring density of breeding ducks of all five species were negatively associated with density in the year before. Resources, estimated as food availability or reproductive success in the year before, were positively correlated with change in breeding numbers in A. marila, A. fuligula and B. islandica. Production three years earlier had a weak but significant effect on change in numbers of M. nigra.

Numbers of moulting diving ducks at Myvatn, north Iceland, were monitored over a period of 25 years; aquatic insects (Chironomidae and Simuliidae), a major food resource, were monitored with window traps for 23 years. The response of the duck populations to changes in the food situation during the moulting period varied between species. Histrionicus histrionicus and Melanitta nigra invariably go to the sea to moult. Three species (Aythya fuligula, Clangula hyemalis and Mergus serrator) have shown long term variation in moult migration habits. Aythya marila and Bucephala islandica stay for moulting, their numbers supplemented by birds from elsewhere. A. marila is a generalist feeder and therefore at low risk of starving, the strategy of Bucephala islandica is to move within the water system where the food situation tends to alternate between the lake and the river. Numbers of Bucephala islandica males moulting on Lake Myvatn were strongly and positively correlated with chironomid numbers and those moulting on the river Laxa with Simulium vittatum, the main food resource there. We did not find convincing evidence that numbers of moulting A. marila and C. hyemalis responded to variation in the food supply during the study period. This applied also to A. fuligula in the South Basin of Myvatn, but numbers in the North Basin were positively associated with chironomid numbers. M. serrator moulting on Myvatn showed negative correlations with the chironomids, perhaps reflecting a negative association between chironomids and its main food, Gasterosteus aculeatus. Apart from safety considerations for a flightless bird, the choice of a moulting site is apparently influenced by the local food conditions on one hand and by the opportunities and risks involved in migrating to distant moulting sites with an unknown food situation on the other hand.

Studies on habitat selection by waterbirds usually describe large-scale distribution of populations or species but the dispersion within lakes is an understudied aspect. Detailed mapping of horned grebes (Podiceps auritus (L.)) on the North Basin of Lake Myvatn, Iceland, was carried out in late summer 2000 in order to compare their dispersion with water depth and submerged vegetation. Depth and vegetation were measured on sampling sites that were evenly distributed over the North Basin, and the information obtained compared to the distribution of the grebes. Four species of submerged macrophytes dominated in the North Basin; Myriophyllum spicatum, M. alterniflorum, Potamogeton filiformis and P. perfoliatus, mainly at depths < 2.5 m. The grebes seemed to prefer to dive in shallow areas vegetated with stands of Myriophyllum spp. Densities of three-spined sticklebacks Gasterosteus aculeatus were compared in stands of M. spicatum and P. filiformis in order to estimate food availability. No difference was found, so the sticklebacks could not explain the preference of the grebes for areas vegetated with Myriophyllum spp. Nevertheless, this study indicates that shallow areas, vegetated with Myriophyllum spp., are important feeding areas for the horned grebe.

Benthic Cladocera were studied with a modified type of an activity trap in Lake Myvatn in 1990–1992. After feasibility experiments, the operation time and the distance of the traps from the benthic substrate were adjusted in order to minimize the effects of diurnal variation and the trapping of planktonic organisms. The trap catches of Eurycercus lamellatus were positively correlated with their abundance at the bottom as estimated by grab sampling (r = 0.910, P < 0.001). The usefulness of the activity trap was demonstrated by: (1) a lake-wide survey of the benthic Cladocera; (2) a study of the seasonal variation in the size distribution, abundance and sex ratio of E. lamellatus; and (3) a study of the seasonal succession of Chydorus sphaericus and Alona affinis. The variation of benthic Cladocera among 21 trap sites distributed on a 1–2-km scale across the lake exceeded the within-site variation. The sampling sites could be divided into five main groups based on cluster analysis. Eurycercus lamellatus was the most common species in the mat of filamentous green algae (Cladophorales). Alonella nana dominated the area of spring water inflow in the north basin and Macrothrix hirsuticornis the area of spring water inflow in the southeastern part of the lake. In other parts of the lake either Chydorus sphaericus or Alona quadrangularis tended to dominate. The size distribution and sex ratio of E. lamellatus was followed at two sites through one summer. In early summer most individuals were females less than 1.45 mm long. Around mid-summer they had grown to 0.69–3.1 mm. By the end of August the size distribution had become bimodal, with a large number of small males and a smaller number of much larger females. The seasonal succession in the abundance of E. lamellatus, A. affinis and C. sphaericus was followed at four sites over two seasons. With some exceptions the abundance of a species followed a similar seasonal trajectory on the different stations in any one year. There was, however, a marked difference between the two years (1991 and 1992), probably related to different temperatures.

In July 1999 and May 2001 a group of limnologists and other scientists involved in the research of Lake Myvatn gathered in the village of Skutustadir by the lake for two symposia to discuss and sum up the results based on the research and monitoring that has been conducted there over the past quarter of a century. The ensuing papers now appear in this issue of Aquatic Ecology under the heading 'The Ecology of Lake Myvatn and the River Laxa– Temporal and Spatial Variation'.

Aquatic dipterans dominate the macrozoobenthos of Lake Myvatn and the River Laxa and are important in the food web. As benthic sampling is time-consuming and expensive, a window trap was developed in order to facilitate the monitoring of the zoobenthos resource. Four window traps were operated at Myvatn and Laxa during 1977–1996. The results yielded population indices for about 20 species of chironomids and one simuliid. Numbers of the most abundant midge species showed close correlations between 2 trap sites on the lake shore, 5 km apart; Cricotopus tibialis did not show such synchrony. At the River Laxa, the dominant blackfly (Simulium vittatum) showed correlation between 2 trap sites, at the outlet and 3 km downstream; while the chironomids, apart from Micropsectra atrofasciata, did not. The dominant midge of Lake Myvatn, Tanytarsus gracilentus showed 3 cyclic oscillations during the 20-year period. The fluctuations were tracked by Micropsectra lindrothi, Procladius islandicus, Orthocladius consobrinus, Psectrocladius barbimanus and Chironomus islandicus. A group of non-cyclic species included Cricotopus sylvestris and Orthocladius oblidens.

Benthic Cladocera were monitored at five sites in Lake Myvatn, Iceland, over a decade (1990–1999), as part of a programme documenting the population fluctuations of animals at different trophic levels in the lake. The species composition remained relatively stable over the first seven years, but in 1997 the population of Eurycercus lamellatus was greatly reduced at all sites. The following year saw a mass occurrence of Alona rectangula and Alonella nana that were previously abundant only locally and rare at most sites. Alona affinis, A. quadrangularis, Acroperus harpae and Chydorus sphaericus were not affected. In 1999 the Cladocera assemblages returned to the pre-1997 situation. The shift was from large-bodied epibenthic and planktonic species (Eurycercus, Daphnia) to small infaunal (Alona rectangula) and ubiquitous (Alonella nana) species. Medium sized (Alona, Acroperus) and some small cladocerans (Chydorus) were not affected. The course of events was reminiscent of a trophic cascade caused by a change in size-selective predation pressure. If so, the impact of a predatory fish population (three-spined stickleback, Gasterosteus aculeatus) depended on whether cyclic chironomid populations were in a high or a low phase. The change in the Cladocera coincided with profound changes in the sediment characteristics associated with low chironomid abundance. We suggest that the relative competitive ability of the Cladocera species is reversed when the sediment has become homogeneous and nutrient-poor after overexploitation by the dominant, tube building and detritivorous chironomid Tanytarsus gracilentus.

In situ paired light and dark-stirred benthic flux chambers were used to estimate dissolved oxygen flux across the sediment–water interface in Lake Mývatn, Iceland. Three sampling stations were selected, each station reflecting a specific sedimentary environment, benthic communities, and water depth. During this study the phytoplankton density was low. Spatial and seasonal variations of bottom DO concentration and DO flux have been observed during this study. The oxygen consumption rate at all study sites had a mean of −89 (± 44) mmol m−2 d−1 while the oxygen production rate due to benthic algae had a mean of 131 (± 103) mmol m−2 d−1. There was a strong correlation (r = 0.91) between oxygen consumption rate and temperature. This was presumably because of the temperature influence on rates of microbial and macrobenthic processes. The mean benthic primary production rate at all study sites was 1216 (± 957) mg C m−2 d−1 between June 2000 and February 2001. Annual gross benthic primary production was estimated from the gross mean daily benthic DO production (P) and Redfield's C:O2 ratio of 106:138 to be 420 g C m−2 y−1 at station HO, 250 g C m−2 y−1 at B2 and 340 g C m−2 y−1 at station 95. Thus, the mean gross benthic primary production was estimated as 1151 mg C m−2 d−1 at station HO, 685 mg C m−2 d−1 at station B2, and 932 mg C m−2 d−1 at station 95.

Ecological features of Lake Myvatn and the outflowing River Laxa show a wide range of spatial and temporal variations. The physical division of the lake into three main basins and the variation in chemical composition and temperature of the artesian springs feeding this shallow productive lake have large spatial effects. Variation in groundwater characteristics depends on percolation time and proximity to geothermal sources. Variation in precipitation is evened out by the porous volcanic soil and bedrock and the spring-water discharge is therefore very stable. A pulse of volcanic activity in 1975–1984 (the Krafla Fires) heated the groundwater entering the North Basin of the lake and changed its chemistry. Although much reduced, these effects have not disappeared yet, but overall the impact of the volcanic activity on the biota seemed minimal. Recycling of nutrients through internal loading is important and occurs on various time scales. In winter, when the lake is ice-covered, the topmost 5-cm layer of sediment pore water has a hundredfold concentration of nutrients relative to the overlying lake water. The nutrients are released during the ice-free period by sediment resuspension, diffusion, bioturbation and recycling. In spring, resuspension events sometimes lead to spikes in dissolved phosphorus and nitrogen, but there is little evidence of any major desorption of nutrients from suspended particles during such events later in the summer. In contrast to the stable groundwater, the biota show more or less regular fluctuations with no straightforward correlation with external signals. The most prominent fluctuations, those of the chironomid Tanytarsus gracilentus seem to be driven by interactions between the species and its sediment resources. Fluctuations in other invertebrates could be a consequence of the Tanytarsus cycles due to the large impact this species has on the benthic environment of this detritus-driven ecosystem. Temporal variation in epibenthic chironomids and Cladocera translates into variable production of vertebrate predators (Arctic charr, Salvelinus alpinus, and ducks), body condition and mortality of fish and sometimes into return rates of migrating adult ducks. The waterfowl show large temporal variation on a centennial scale, e.g., the invasion of the tufted duck (Aythya fuligula) which arrived by the end of the 19th century and has by now outnumbered other species. Fluctuations of Cyanobacteria (Anabaena) and the fish Gasterosteus aculeatus (three-spined stickleback) harmonize with the cycles in the benthic community. Palaeolimnological studies indicate that primary production in the South Basin became increasingly benthic as the lake depth was reduced by sedimentation (around 2 mm year−1). Other trends include a decrease in Tanytarsus and Daphnia and an exponential increase in green algae (Cladophorales, Pediastrum) and associated organisms.

Spatial patterns in the abundance of species are determined by local abiotic and biotic conditions, and by the movement of individuals among localities. For species distributed among discrete habitat ‘‘islands’’, such as zooplankton distributed among lakes, local conditions within lakes often dominate low movement rates among lakes to determine the composition of communities. Here, we ask whether the same abiotic and biotic environmental conditions can generate spatial patterns in the distribution of zooplankton within a lake where there are high horizontal movement rates. We conducted three spatial surveys of zooplankton communities in Lake My´vatn, Iceland, a moderately sized (37 km2) shallow lake with a high outflow rate. The pelagic zooplankton community showed strong spatial structure (spatial autocorrelation), with species composition varying with spatial variation in chlorophyll-a, the abundance of Anabaena (cyanobacteria), lake depth, light extinction coefficient, and temperature. These factors are known from other studies to be strong drivers of among-lake variation in freshwater zooplankton communities. However, in contrast with among-lake studies, fish (stickleback) abundance had no measureable effect on the abundance or species composition of the zooplankton community, although high local stickleback abundance was associated with low zooplankton:phytoplankton biomass ratios. Finally, a parallel study of the underlying benthic crustacean community showed much finer spatial variation (spatial autocorrelation to a range 0.6 km vs. 9 km for pelagic zooplankton), suggesting that the stationary character of the benthos allows finer grained spatial patterns. Given the high flow rate of water in My´vatn (.200 m/d), the generation of spatial patterns suggests very strong effects of variation in abiotic and biotic environmental conditions on the population dynamics of zooplankton in the lake.

This report addresses changes in population size in five species of diving ducks breeding and wintering in the Baltic Sea. Declines in breeding Greater Scaup, Eider and Velvet Scoters on the Baltic coast are verified. Long-tailed Ducks and Eiders have declined in winter. Breeding Eiders have declined in Norway. Monitoring programs in all Nordic countries are partly inadequate for detecting changes in numbers of birds both in winter and during breeding, but large-scale trends appear reliable. The reasons for the declines observed are largely unknown. For breeding Eiders in the northern part of the Baltic Sea a change in predation regimes on the breeding islands may be important. If the negative trends are to be stopped, a deeper understanding of the drivers involved and better knowledge about the ecological status of the Baltic Sea food webs utilized by the diving ducks are needed.

Summary 1. Tanytarsus gracilentus population dynamics in Lake Myvatn show a tendency to cycle, with three oscillations occurring between 1977 and 1999 having periods of roughly 7 years. The population abundance fluctuated over four orders of magnitude. 2. A partial autocorrelation function (PACF) accounting for measurement error revealed a strong positive lag-1 autocorrelation and a moderate negative lag-2 partial autocorrelation. This suggests that the dynamics can be explained by a simple secondorder autoregressive process. 3. We tested the alternative hypotheses that the cyclic dynamics of T. gracilentus were driven by consumer‐resource interactions in which T. gracilentus is the consumer, or predator‐prey interactions in which T. gracilentus is the prey. We analysed autoregressive models including both consumer‐resource interactions and predator‐prey interactions. 4. Wing length of T. gracilentus was used as a surrogate for resource abundance and/or quality, because body size is known to fluctuate with resource abundance and quality in dipterans. Furthermore, the wing lengths of Micropsectra lindrothi , a species ecologically similar to T. gracilentus , fluctuated synchronously with T. gracilentus wing lengths, thereby indicating that the shared resources of these two species were indeed cycling. Wing lengths of other chironomid species were not synchronized. 5. The predators of T. gracilentus included midges in the genera Procladius and Macropelopia , and the fish Gasterosteus aculeatus (three-spined stickleback). 6. The autoregressive models supported the hypothesis that T. gracilentus dynamics were driven by consumer‐resource interactions, and rejected the hypothesis that the dynamics were driven by predator‐prey interactions. 7. The models also revealed the consequences of consumer‐resource interactions for the magnitude of fluctuations in T. gracilentus abundance. Consumer‐resource interactions amplified the exogenous variability affecting T. gracilentus per capita population growth rates (e.g. temperature, rainfall, etc.), leading to variability in abundance more than two orders of magnitude greater than the exogenous variability.

A growing collection of mtDNA genetic information from waterfowl species across North America suggests that larger-bodied cavity-nesting species exhibit greater levels of population differentiation than smaller-bodied congeners. Although little is known about nest-cavity availability for these species, one hypothesis to explain differences in population structure is reduced dispersal tendency of larger-bodied cavity-nesting species due to limited abundance of large cavities. To investigate this hypothesis, we examined population structure of three cavity-nesting waterfowl species distributed across much of North America: Barrow's Goldeneye (Bucephala islandica), Common Goldeneye (B. clangula), and Bufflehead (B. albeola). We compared patterns of population structure using both variation in mtDNA control-region sequences and band-recovery data for the same species and geographic regions. Results were highly congruent between data types, showing structured population patterns for Barrow's and Commo...

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 Mývatn, 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., Ecology and Evolution, 3 (10), ISSN:2045-7758

第3回極域科学シンポジウム/第34回極域生物シンポジウム 11月27日（火） 国立極地研究所 ３階ラウンジ

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 δ13C and δ15N 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.

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 (d30Si) 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 d30Si 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 d30Si 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 d30Si 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.

We examined annual variation in production, recruitment and density of the three most abundant vertebrate species of the River Laxa at Lake Mývatn, Iceland: Barrow’s goldeneye, Bucephala islandica, harlequin duck, Histrionicus histrionicus, and brown trout, Salmo trutta, in relation to food resources and other environmental variables. The study is largely based on correlations from long-term monitoring series in the period 1975–2002. Production of young in the harlequin duck was significantly correlated with food resources (the blackfly, Simulium vittatum) of the river, as was the recruitment of brown trout to the angling stock. In Barrow’s goldeneye, which uses both the lake and the river, dispersion of adults in spring and young in August was influenced by the availability of aquatic insects in each habitat. The dispersion of Barrow’s goldeneye tracks the availability of aquatic insects in each of these two main habitats. Introduced Amercian mink, Mustela vison, may have affected spring numbers and dispersion of harlequin ducks, but the evidence was not conclusive. Numbers of both duck species and the trout (as CPUE) were relatively stable, although a sharp drop in numbers followed by slow recovery was observed in Barrow’s goldeneye, and an increase was observed in harlequin ducks in the first year of study.

Iceland's Lake Myvatn ecosystem is remarkable in that it is so dominated by a single species. Myvatn translates as 'midge lake', and it is the midge Tanytarsus gracilentus that dominates, comprising two-thirds of the lake's secondary productivity. Midge numbers undergo extreme fluctuations of almost six orders of magnitude with an irregular period of 4 to 7 years. A new analysis of 25 years of population monitoring shows that this phenomenon can be explained by alternative dynamical states with the amplitude of the fluctuations set by small subsidies of food entering the habitat. Small decreases in food supply due to human disturbances could explain recent increases in midge fluctuations. In conservation terms, midge populations are inherently unpredictable and are much more vulnerable to small disturbances in the lake than was expected. The Lake Myvatn midges illustrate the fundamental complexities of natural ecosystems and the difficulties in managing them. The cover shows mating swarms of male midges waiting for females to join them. An example of complex population dynamics (the midge Tanytarsus gracilentus in Lake Myvatn, Iceland) is described and modelled. Complex dynamics are often shown by simple ecological models1,2 and have been clearly demonstrated in laboratory3,4 and natural systems5,6,7,8,9. Yet many classes of theoretically possible dynamics are still poorly documented in nature. Here we study long-term time-series data of a midge, Tanytarsus gracilentus (Diptera: Chironomidae), in Lake Myvatn, Iceland. The midge undergoes density fluctuations of almost six orders of magnitude. Rather than regular cycles, however, these fluctuations have irregular periods of 4–7 years, indicating complex dynamics. We fit three consumer–resource models capable of qualitatively distinct dynamics to the data. Of these, the best-fitting model shows alternative dynamical states in the absence of environmental variability; depending on the initial midge densities, the model shows either fluctuations around a fixed point or high-amplitude cycles. This explains the observed complex population dynamics: high-amplitude but irregular fluctuations occur because stochastic variability causes the dynamics to switch between domains of attraction to the alternative states. In the model, the amplitude of fluctuations depends strongly on minute resource subsidies into the midge habitat. These resource subsidies may be sensitive to human-caused changes in the hydrology of the lake, with human impacts such as dredging leading to higher-amplitude fluctuations. Tanytarsus gracilentus is a key component of the Myvatn ecosystem, representing two-thirds of the secondary productivity of the lake10 and providing vital food resources to fish and to breeding bird populations11,12. Therefore the high-amplitude, irregular fluctuations in midge densities generated by alternative dynamical states dominate much of the ecology of the lake.

We describe the main results of a monitoring study at Lake Myvatn, northern Iceland, begun in 1975. The aims were to find factors that limit production of young and cause changes in density of breeding ducks of several species. We estimated numbers of ducks in spring, before nesting, numbers of ducklings produced, and numbers moulting. Chironomid and simuliid dipterans were monitored with window traps. In all duck species studied, production of young was correlated with food abundance. Reproductive performance determined subsequent changes in spring population density of Eurasian Wigeon, Tufted Duck, Greater Scaup, Common Scoter and Harlequin Duck. The spring population of Barrow’s Goldeneye apparently did not respond to variation in reproductive success. Moulting numbers of male Tufted Duck were related to chironomid abundance, but not those of Scaup and Barrow’s Goldeneye. Moulting numbers were not associated with previous reproductive output.

Fossil plant pigments and Cladophora fragments were analyzed in a 6.53 m long sediment profile from Lake Mývatn, Iceland, covering most of its history of about 2300 years. A decrease in myxoxanthophyll (produced by Cyanobacteria) with time and an increase in the benthic Cladophora reflects a gradual shift from planktonic to benthic primary production as water depth is reduced (to 3.15 m at the core site) because of sediment accumulation.

1. A migratory breeding population of Eurasian wigeon Anas penelope L., was studied over the period 1975-95 on 54 km 2 of freshwater habitat at Lake Myvatn, northern Iceland. Spring numbers of males averaged 1076 (range 526-1887). The sex ratio was 0.97 females per male and the proportion of yearling males averaged 0.21. The mean brood was 3.93 and a mean of 2.83 young per female (range 0.09-5.14) was produced. Production of young was positively related to chironomid abundance and negatively to cold and wet weather. 2. Both sexes seem to be faithful to their winter range, but males are much less faithful to the summer range than are females. The apparent return rate of males is presumably an expression of the return rate of females, which had previously bred locally or been produced at Myvatn. 3. The apparent return rate of old males (2 years or older), mean 0.84 (range 0.58-l.22), was positively correlated with chironomid abundance in the previous summer, which varied greatly. It is suggested that abundant chironomids in late summer may provide a source of highly nutritious food which would increase the probability of wigeon returning to Myvatn or favour the survival of Myvatn wigeon on winter grounds shared with birds originating from other breeding areas. Chironomid numbers showed serial correlation, hence it may be advantageous for the birds to assess the food supply before departure and return accordingly. 4. The apparent return rate of yearling males, mean 0.22 (range 0.06-0.56), was negatively related to the number of young produced in the year before. 5. The density of breeding wigeon at Lake Myvatn was mainly determined by food conditions in the breeding area, as experienced by the adults 1 year previously, and events in the winter range appeared to have less influence. Flyway populations may be limited by a complex set of local and global fluctuations in resources in (i) breeding localities, determining density and production in each, coupled with (ii) staging and wintering localities in each of which local resources would determine density.

SUMMARY. Sediment from a precursor of the 2300-year old Lake Mývatn was traced by diatoms in pseudocrater scoria. This lake (Mývatn I) was deeper than the present Mývatn but similar in size. Tephrochronology confirms an equal age of the Ytrlfloi and Sydrifloi basins of the present Mývatn. Diatom succession in the sediment of the Sydrifloi basin was divided into four zones, of which the lowest corresponds to inherited and highly disturbed Mývatn I sediment. At 90 cm depth, i.e. around 1600 AD, in the Sydrifloi sediment a simultaneous increase occurred in Cladophora aegagropila (Chlorophyta), Eurycercus lamellatus (Cladocera) and three diatom species epiphytic on Cladophora. The density of Cladocera also followed this trend. The increase in Cladophora was perhaps caused by improved light conditions on the bottom, either because of a decrease in water depth due to sedimentation, or a decrease in Anabaena blooms, or both.