Southwest Iceland Research Articles

Biochemical characteristics and demography of the marine calanoid copepod Calanus finmarchicus during spring in Icelandic waters

Abstract The chemical composition of Calanus finmarchicus was analyzed at 21 stations from three regions where 12 transects were sampled from 0 to 50 m depth, including stage composition analysis at the Siglunes and Selvogsbanki transects in relation to the effects of different water masses, during a survey in Icelandic waters during 11–29 May 2018. The stage composition of C. finmarchicus at the northern transect of Siglunes was mostly made up of copepodites CI–CIII, around 76%, whereas older copepods CV–CVI dominated, around 64%, at the southern transect of Selvogsbanki. The dry weight (dw) and lipid content of C. finmarchicus were overall highest in the southwest (160 ± 60 μg dw ind−1 and 60 ± 20 μg lipids ind−1) and lowest in the east (120 ± 20 μg dw ind−1 and 30 ± 10 μg lipids ind−1). Total astaxanthin content ranged from 0.14 to 0.27 μg mg−1 dw. Chitin made up 2–4% of the dw. The diatom marker C20:5n3 was prolific in the copepods southwest of Iceland, while the dinoflagellate markers C18:4n3 and C22:6n3 dominated in the copepods east of Iceland. The results indicate that C. finmarchicus found in the south of Iceland developed faster than those in the north, with the variation in chemical composition and developmental time highly influenced by hydrographic regions.

On the Potential of 3D Transdimensional Surface Wave Tomography for Geothermal Prospecting of the Reykjanes Peninsula

Seismic travel time tomography using surface waves is an effective tool for three-dimensional crustal imaging. Historically, these surface waves are the result of active seismic sources or earthquakes. More recently, however, surface waves retrieved through the application of seismic interferometry have also been exploited. Conventionally, two-step inversion algorithms are employed to solve the tomographic inverse problem. That is, a first inversion results in frequency-dependent, two-dimensional maps of phase velocity, which then serve as input for a series of independent, one-dimensional frequency-to-depth inversions. As such, a set of localized depth-dependent velocity profiles are obtained at the surface points. Stitching these separate profiles together subsequently yields a three-dimensional velocity model. Relatively recently, a one-step three-dimensional non-linear tomographic algorithm has been proposed. The algorithm is rooted in a Bayesian framework using Markov chains with reversible jumps, and is referred to as transdimensional tomography. Specifically, the three-dimensional velocity field is parameterized by means of a polyhedral Voronoi tessellation. In this study, we investigate the potential of this algorithm for the purpose of recovering the three-dimensional surface-wave-velocity structure from ambient noise recorded on and around the Reykjanes Peninsula, southwest Iceland. To that end, we design a number of synthetic tests that take into account the station configuration of the Reykjanes seismic network. We find that the algorithm is able to recover the 3D velocity structure at various scales in areas where station density is high. In addition, we find that the standard deviation of the recovered velocities is low in those regions. At the same time, the velocity structure is less well recovered in parts of the peninsula sampled by fewer stations. This implies that the algorithm successfully adapts model resolution to the density of rays. It also adapts model resolution to the amount of noise in the travel times. Because the algorithm is computationally demanding, we modify the algorithm such that computational costs are reduced while sufficiently preserving non-linearity. We conclude that the algorithm can now be applied adequately to travel times extracted from station–station cross correlations by the Reykjanes seismic network.

Inflation-Deflation Episodes in the Hengill and Hrómundartindur Volcanic Complexes, SW Iceland

Non-eruptive uplift and subsidence episodes remain a challenge for monitoring and hazard assessments in active volcanic systems worldwide. Sources of such deformation may relate to processes such as magma inflow and outflow, motion and phase changes of hydrothermal fluids or magma volatiles, heat transfer from magmatic bodies and heat-mining from geothermal extraction. The Hengill area, in southwest Iceland, hosts two active volcanic systems, Hengill and Hrómundartindur, and two high-temperature geothermal power plants, Hellisheiði and Nesjavellir. Using a combination of geodetic data sets (GNSS and InSAR; Global Navigation Satellite Systems and Interferometry Synthetic Aperture Radar, respectively) and a non-linear inversion scheme to estimate the optimal analytical model parameters, we investigate the ground deformation between 2017–2018. Due to other ongoing deformation processes in the area, such as plate motion, subsidence in the two geothermal production fields, and deep-seated source of contraction since 2006, we estimate 2017–2018 difference velocities by subtracting background deformation, determined from data spanning 2015–2017 (InSAR) or 2009–2017 (GNSS). This method highlights changes in ground deformation observed in 2017–2018 compared to prior years: uplift signal of ∼10 km diameter located in the eastern part of the Hengill area, and geothermal production-related temporal changes in deformation near Húsmúli, in the western part of the Hengill area. We find an inflation source located between the Hengill and Hrómundartindur volcanic complexes, lasting for ∼5 months, with a maximum uplift of ∼12 mm. Our model inversions give a source at depth of ∼6–7 km, located approximately in the same crustal volume as an inferred contracting source in 2006–2017, within the local brittle-ductile transition zone. No significant changes were observed in local seismicity, borehole temperatures and pressures during the uplift episode. These transient inflation and deflation sources are located ∼3 km NW from a source of non-eruptive uplift in the area (1993–1999). We consider possible magmatic and hydrothermal processes as the causes for these inflation-deflation episodes and conclude that further geophysical and geological studies are needed to better understand such episodes.

Data Echoes: Sound, Evidence, and Acoustic Methods in Energy Landscapes

This paper explores an informal acoustic method developed by a group of industrial geologists working in geothermal energy landscapes in the southwest of Iceland. Through a series of ethnographic descriptions, this paper renders the work these geologists carry out in sonic terms, emphasizing how they use their bodies as sonic detectors in the production of geological evidence. Sound, the paper argues, is what allows geologists to make the intractable problem of volcanic cooling doable. It does this by differentiating two forms of evidence. Primary evidence, which ends up as data in geological reports, and secondary sonic evidence, which is what establishes that this primary evidence is, in fact, evidence. The paper introduces the concept data echoes as a way to think about how sound articulates between these evidential protocols. As echo, sound works as an outside, which, while remaining external to official protocols of knowledge production, nevertheless helps to constitute distinctions that are meaningful to the production of those categories. As data echoes through the various moments of data capture, analysis, and model building, sound’s temporal form helps to predict the time frame of volcanic cooling, as it affects both the immediate energy production scenarios and the long durée of volcanic time.

Winter–spring warming in the North Atlantic during the last 2000 years: evidence from southwest Iceland

Abstract. Temperature reconstructions from the Northern Hemisphere (NH) generally indicate cooling over the Holocene, which is often attributed to decreasing summer insolation. However, climate model simulations predict that rising atmospheric CO2 concentrations and the collapse of the Laurentide Ice Sheet caused mean annual warming during this epoch. This contrast could reflect a seasonal bias in temperature proxies, and particularly a lack of proxies that record cold (late fall–early spring) season temperatures, or inaccuracies in climate model predictions of NH temperature. We reconstructed winter–spring temperatures during the Common Era (i.e., the last 2000 years) using alkenones, lipids produced by Isochrysidales haptophyte algae that bloom during spring ice-out, preserved in sediments from Vestra Gíslholtsvatn (VGHV), southwest Iceland. Our record indicates that winter–spring temperatures warmed during the last 2000 years, in contrast to most NH averages. Sensitivity tests with a lake energy balance model suggest that warmer winter and spring air temperatures result in earlier ice-out dates and warmer spring lake water temperatures and therefore warming in our proxy record. Regional air temperatures are strongly influenced by sea surface temperatures during the winter and spring season. Sea surface temperatures (SSTs) respond to both changes in ocean circulation and gradual changes in insolation. We also found distinct seasonal differences in centennial-scale, cold-season temperature variations in VGHV compared to existing records of summer and annual temperatures from Iceland. Multi-decadal to centennial-scale changes in winter–spring temperatures were strongly modulated by internal climate variability and changes in regional ocean circulation, which can result in winter and spring warming in Iceland even after a major negative radiative perturbation.

Concentrations of sulphur and trace elements in subarctic soils and mosses in relation to geothermal power plants at Hengill, Iceland – ecological implications

Amidst the globally accelerated plans to increase geothermal energy utilization, knowledge of ecological responses to power plant emissions is limited. The aim of this study is to investigate ecosystem accumulation of elements emitted from power plants in the Hengill geothermal field in Southwest Iceland, in relation to patterns of plant growth and health. The moss Racomitrium lanuginosum was used as a bio-indicator, a dominating plant in our study areas. Concentrations of sulphur, arsenic, boron, antimony, and mercury in soil and shoots of R. lanuginosum, were determined, and growth and other moss characteristics (moss damage, physiology and mat depth) assessed at different distances and directions from two geothermal power plants, Hellisheidi (303 MWe, operated since 2006) and Nesjavellir (120 MWe, operated since 1990). Higher concentrations of these elements were detected around Hellisheidi than Nesjavellir. Sulphur, antimony, and mercury concentrations in moss decreased with increasing distance from the power plant around Nesjavellir, while arsenic concentrations increased with increasing distance away and boron concentrations were relatively low, below detection. Similar trends for sulphur and antimony followed in soils. Arsenic concentrations in soil on the other hand, increased with increasing distance from the Nesjavellir power plant while boron concentrations were, below detection limit except at 250 m downwind and at 1000 m and 4000 m upwind. At Hellisheidi, sulphur and boron concentrations, in moss decreased with increasing distance away from the power plant and showed an opposite trend in soil. Arsenic, antimony and mercury concentrations in moss showed a general increase in concentrations with increasing distance away from the power plant; while in soil, trace element concentrations did not change significantly with distance, and there were no clear patterns related to the direction of prevailing winds. On average, moss growth and other moss trait values were higher at Hellisheidi than Nesjavellir. At Nesjavellir, moss mat depth and biomass moss response variables increased with increasing distance away and were greater upwind than downwind; while shoot turnover (%) and chlorophyll concentration decreased with increasing distance away from the power plant and did not vary with direction. Moss shoot length did not vary with distance but showed a decrease downwind. At Hellisheidi, trends were non-linear with distance, however biomass increase was the only response variable significantly higher downwind than upwind: other variables did not vary with direction. The frequency of moss damage was quite low around both power plants. We thus conclude that emitted sulphur from the geothermal power plants is deposited in the surrounding ecosystem and has so far no clear indications of harmful effects. In addition, the emerging patterns of arsenic, boron, antimony and mercury concentrations in the ecosystem were not clear to indicate deposition from the geothermal power plants and there were no related harmful effects. Nonetheless, further long-term monitoring of potential environmental impacts is advised. We recommend experimental studies to establish in detail if and how different levels of sulphur deposition may affect ecosystems for appropriate mitigation.

Wild fowl egg consumption in postmedieval Iceland. SEM analysis of archaeological eggshells from the Bishop’s seat at Skálholt, southwest Iceland

An analysis of 20 samples of eggshells using SEM imaging successfully identified the samples to species level. The samples came from an archaeological site in Iceland dating to the 17th and 18th century, an elite ecclesiastical centre and Bishop’s household. The eggs are all from wildfowl, specifically mallards, Arctic skua, common guillemot and razorbill, and we know from historical sources that the harvesting of wild bird eggs for consumption was a common practice. The present study contributes to a better understanding of everyday life at the bishopric of Skálholt during the Early Modern period in Iceland.

Centennial relationships between ocean temperature and Atlantic puffin production reveal shifting decennial trends.

The current warming of the oceans has been shown to have detrimental effects for a number of species. An understanding of the underlying mechanisms may be hampered by the non-linearity and non-stationarity of the relationships between temperature and demography, and by the insufficient length of available time series. Most demographic time series are too short to study the effects of climate on wildlife in the classical sense of meteorological patterns over at least 30years. Here we present a harvest time series of Atlantic puffins (Fratercula arctica) that goes back as far as 1880. It originates in the world's largest puffin colony, in southwest Iceland, which has recently experienced a strong decline. By estimating an annual chick production index for 128years, we found prolonged periods of strong correlations between local sea surface temperature (SST) and chick production. The sign of decennial correlations switches three times during this period, where the phases of strong negative correlations between puffin productivity and SST correspond to the early 20th century Arctic warming period and to the most recent decades. Most of the variation (72%) in chick production is explained by a model in which productivity peaks at an SST of 7.1°C, clearly rejecting the assumption of a linear relationship. There is also evidence supporting non-stationarity: The SST at which puffins production peaked has increased by 0.24°C during the 20th century, although the increase in average SST during the same period has been more than three times faster. The best supported models indicate that the population's decline is at least partially caused by the increasing SST around Iceland.

Microbial intrusion and seasonal dynamics in the groundwater microbiome of a porous basaltic rock aquifer used as municipal water reservoir.

Groundwater is a key resource for safe drinking water supply. Yet unconfined aquifers can be vulnerable to microbial contamination during extreme weather events that lead to surface runoff. The current study characterises the groundwater microbiome of a porous basaltic rock aquifer in South-West Iceland used for drinking water extraction and analyses the microbial community dynamics during surface runoff. The groundwater microbial community sampled from 12 wells across the extraction area contained over 745 prokaryotic genera and was phylogenetically similar between wells and most seasons, representing a diverse but homogenous ecosystem. The largest seasonal variation in the microbial community composition was detected during a period of concurrent snow melt and high precipitation leading to surface runoff. This period was characterised by an increased abundance of soil-associated taxa in the groundwater microbiome and specifically of taxa assigned to Aeromonas and Bacillus. A field experiment simulating high surface runoff around a groundwater well confirmed the increased abundance of surface soil microorganisms in the well water, indicating vulnerability of groundwater towards surface microbial intrusion during extreme weather events. As such events are likely to increase due to climate change, novel water management tools such as microbial community analysis could help ensure drinking water safety.

Impacts of Norse settlement on terrestrial and aquatic ecosystems in Southwest Iceland

Norse colonization of North Atlantic islands in the 1st millennium of the Common Era led to drastic prehistoric environmental changes in these previously “pristine” landscapes. In Iceland, Norse settlement is associated with a rapid decline in birch trees and heightened soil erosion, yet the timing of Norse exploration in the North Atlantic coincided with large climate changes that also influenced Icelandic environments. To date, there are few records that disentangle climatic and human impacts on terrestrial ecosystems, and there has been very little work on the impacts of Norse arrival on Iceland’s aquatic ecosystems. Here we use a high-resolution lake-sediment record from Vestra Gislholtsvatn (VGHV), southwest Iceland to investigate these processes during the last 2,000 years. Norse arrival (c. 870 CE) in Iceland is followed by a rapid increase in sedimentation rate and a transition in leaf wax n-alkanes indicating a decrease in trees and expansion of grasses. This transition coincides with limnological changes, including increased primary productivity (i.e. C17 n-alkane and biogenic opal fluxes) and shifts in the haptophyte algal community. Many of these changes are still apparent today. Comparisons with a new winter-spring alkenone temperature reconstruction from VGHV and marine sea surface temperature records show little to no correlation between terrestrial and aquatic ecological changes and climate at this time. Similarly, volcanic eruptions recorded in VGHV are not associated with any long-term environmental changes. Rather, the VGHV record suggests that human settlement had a lasting impact on the catchment area of VGHV and changes within the lake ecosystem.

Decline of Leach’s Storm Petrels Hydrobates leucorhous at the largest colonies in the northeast Atlantic

Leach’s Storm Petrel Hydrobates leucorhous has undergone substantial population declines at North Atlantic colonies over recent decades, but censusing the species is challenging because it nests in burrows and is only active at colonies at night. Acoustic playback surveys allow birds present in nest sites to be detected when they respond to recordings of vocalisations. However, not all birds respond to playback on every occasion, response rate is likely to decline with increasing distance between the bird and the playback location, and the observer may not detect all responses. As a result, various analysis methods have been developed to measure and correct for these imperfect response and detection probabilities. We applied two classes of methods (calibration plot and hierarchical distance sampling) to acoustic survey data from the two largest colonies of breeding Leach’s Storm Petrels in the northeast Atlantic: the St Kilda archipelago off the coast of northwest Scotland, and the island of Elliðaey in the Vestmannaeyjar archipelago off the southwest of Iceland. Our results indicate an overall decline of 68% for the St Kilda archipelago between 2000 and 2019, with a current best estimate of ~8,900 (95% CI: 7,800–10,100) pairs. The population on Elliðaey appears to have declined by 40 –49% between 1991 and 2018, with a current best estimate of ~5,400 (95% CI: 4,300–6,700) pairs. We also discuss the relative efficiency and precision of the two survey methods.

Boron isotopic signatures of melt inclusions from North Iceland reveal recycled material in the Icelandic mantle source

Trace element and volatile heterogeneity in the Earth’s mantle is influenced by the recycling of oceanic lithosphere through subduction. Oceanic island basalts commonly have high concentrations of volatiles compared to mid-ocean ridge basalts, but the extent to which this enrichment is linked to recycled mantle domains remains unclear. Boron is an ideal tracer of recycled subducted material, since only a small percentage of a recycled component is required to modify the bulk δ11B of the source mantle. Boron isotopic compositions of primary melts thus have potential to trace the fate of recycled subducted material in the deep mantle, and to constrain the lengthscales of lithologic and compositional heterogeneities in diverse tectonic settings.We present new measurements of volatiles, light elements and boron isotopic ratios in basaltic glasses and melt inclusions that sample the mantle at two endmember spatial scales. Submarine glasses from the Reykjanes Ridge sample long-wavelength mantle heterogeneity on the broad scale of the Iceland plume. Crystal-hosted melt inclusions from the Askja and Bárðarbunga volcanic systems in North Iceland sample short-wavelength mantle heterogeneity close to the plume centre. The Reykjanes Ridge glasses record only very weak along-ridge enrichment in B content approaching Iceland, and there is no systematic variability in δ11B along the entire ridge segment. These observations constrain ambient Reykjanes Ridge mantle to have a δ11B of −6.1‰ (2SD = 1.5‰, 2SE = 0.3‰). The North Iceland melt inclusions have widely variable δ11B between −20.7 and +0.6‰. We screen melt inclusions against influence from crustal contamination, identifying high [B] and low δ18O as fingerprints of assimilation processes. Only the most primitive melt inclusions with MgO ⩾ 8 wt.% reliably record mantle-derived δ11B. In North Iceland, incompatible trace element (ITE)-depleted primitive melt inclusions from Holuhraun record a δ11B of −10.6‰, a signal that has also been seen in melt inclusions from southwest Iceland (Gurenko and Chaussidon, 1997). In contrast, primitive ITE-enriched melt inclusions from nearby Askja volcano record a δ11B of −5.7‰, overlapping with our new constraint on the δ11B of Reykjanes Ridge mantle. Coupled [B], δ11B and δ18O signatures of more evolved melt inclusions from North Iceland are consistent with primary melts assimilating <5–20% of hydrothermally altered basaltic hyaloclastite as they ascend through the upper crust.Our data reveal the presence of a depleted, low-δ11B and an enriched, higher-δ11B mantle component, both intrinsic to the Icelandic mantle source and distinct from Reykjanes Ridge mantle. Non-modal melting calculations suggest that the enriched and depleted mantle components both contain ∼0.085 μg/g B, slightly lower than the 0.10–0.11 μg/g calculated for Reykjanes Ridge mantle. These data are consistent with the Icelandic mantle containing B-depleted dehydrated recycled oceanic lithosphere, in keeping with the low B/Pr of Icelandic melt inclusions in comparison to Reykjanes Ridge glasses or MORB. Our new data provide strong support for the role of recycled subducted lithosphere in melt generation at ocean islands, and highlight the need for careful screening of melt inclusion compositions in order to study global volatile recycling in ocean island basalts.

Soil carbon status after vegetation restoration in South West Iceland

Reclamation of degraded land using revegetation is one way of sequestering carbon into the soil. In this study an assessment was done to estimate the status of soil carbon amounts after revegetation with trees and grass in South West Iceland (Hafnarmelar). Natural woodland and eroded plots were part of the assessed plots as controls making four treatments. Soil samples were analysed for bulk density, carbon content and soil texture. Total % carbon (C) was analysed using vario MAX CN analyser (measured % C) and Loss on Ignition (calculated % C). The results showed that natural woodland had higher (9.32%) C than the tree treatment (4.91%), and both had significantly higher C than the grass (1.12%) and the eroded (0.76%) treatments (p < 0.0001). The amount of C in the grass and the eroded treatments were not statistically different (p > 0.0566). Notably, the grass treatment had carbon below the minimum expected level of 1.5% in Icelandic Andosol under vegetation. The natural woodland and the tree treatment had fine soil texture than the grass and eroded plots. Results suggest that where land has been properly restored or kept in natural condition, soil properties improve significantly especially when trees are part of the restored vegetation. The natural woodland had possibly not lost the old carbon-rich soil, as was the case with the tree, grass and eroded plots hence more time for development of various soil properties. Moreover, more litter deposits in natural woodlands and partly in the tree treatments might have contributed to higher carbon than in the grass and eroded treatments. Furthermore clay content variations (natural woodlands and the tree treatments had finer soil texture) might also be responsible for C limitations in the grass and the eroded treatments. Therefore, more restoration efforts are encouraged. The results also showed that LOI is a good method for C estimation but not very accurate estimator of soil organic carbon unless equations are developed with respect to known carbon content of particular soil type.

Dispersal of juvenile cod (Gadus morhua L.) in Icelandic waters

Knowledge of dispersal and migration of juvenile fish is essential for understanding the underlying processes responsible for population structure and recruitment variation. We explored the dispersal patterns of juvenile cod in waters around Iceland from putative nursery grounds to provide insights into their population structure. About 6000 juvenile cod were tagged at five inshore locations in 1993, 1997 and 2002. Of these, 767 juveniles were recaptured after 27–3297 days at liberty. The recaptures were weighted using standardized fishing effort and revealed high fidelity of juvenile cod to their putative nursery grounds. Cod tagged west of Iceland dispersed southwards to the main spawning grounds, south and southwest of Iceland, at the time of maturation. In contrast, juveniles tagged northwest of Iceland demonstrated no affiliation with the main spawning grounds. However, the juveniles tagged north of Iceland dispersed over a large geographical area and co-occurred in the same areas as those from areas northwest and west of Iceland during their third year at liberty. Different dispersal patterns for cod tagged at different locations indicated different life history strategies of Icelandic cod with limited mixing of juveniles followed by variable migration patterns towards spawning, supporting earlier studies showing the complexity of the population dynamics of the Icelandic cod.

Contrasting responses of black fly species (Diptera: Simuliidae) to experimental whole‐stream warming

Abstract As global temperatures continue to rise, assessment of how species within ecological communities respond to shifts in temperature has become increasingly important. However, such assessments require detailed long‐term observations or ecosystem‐level manipulations that allow for interactions among species and the potential for species dispersal and exchange with the regional species pool. We examined the effects of experimental whole‐stream warming on a larval black fly assemblage in southwest Iceland. We used a paired‐catchment design, in which we studied the warmed stream and a nearby reference stream for 1 year prior to warming and 2 years during warming and estimated population abundance, biomass, secondary production, and growth rates for larvae of three black fly species. Experimental warming by 3.8°C had contrasting effects on the three black fly species in the assemblage. The abundance, biomass, growth, and production of Prosimulium ursinum decreased in the experimental stream during the warming manipulation. Despite increasing in the reference stream, the abundance, biomass, and production of another species, Simulium vernum, decreased in the experimental stream during warming. In contrast, warming had an overall positive effect on Simulium vittatum. While warming had little effect on the growth of overwintering cohorts of S. vittatum, warming led to an additional cohort during the summer months and increased its abundance, biomass, and production. Overall, family‐level production was enhanced by warming, despite variation in species‐level responses. Our study illustrates that the effects of climate warming are likely to differ even among closely related species. Moreover, our study highlights the need for further investigation into the uneven effects of warming on individual species and how those variable effects influence food web dynamics and ecosystem function.

Population dynamics of three brachyuran crab species (Decapoda) in Icelandic waters: impact of recent colonization of the Atlantic rock crab (Cancer irroratus)

AbstractThe Atlantic rock crab (Cancer irroratus) was first found in Icelandic waters in 2006. Since then, the species has dispersed rapidly and is currently found clockwise from the southwest coast of Iceland to the east, corresponding to &amp;gt;70% of the coastline. Here, we present a monitoring study on this non-indigenous crab species in Iceland from 2007 to 2019. The study shows that the rock crab is now the most abundant brachyuran crab species on soft substrate bottoms in Southwest Iceland, both as adults and planktonic larvae, indicating that it is outcompeting its rival native species, the European green crab (Carcinus maenas) and the spider crab (Hyas araneus). The average size of the rock crab was similar over time (2007–2019), although it fluctuated between years in a pattern similar to that for the green crab, while significant reduction in size was observed for male spider crabs. The rock crab population is still in a growth phase in Icelandic waters, as seen in increasing distributional range, and can be found in densities comparable to the highest reported for the species in its native range in North America.

Thermal niche diversity and trophic redundancy drive neutral effects of warming on energy flux through a stream food web.

Climate warming is predicted to alter routing and flows of energy through food webs because of the critical and varied effects of temperature on physiological rates, community structure, and trophic dynamics. Few studies, however, have experimentally assessed the net effect of warming on energy flux and food web dynamics in natural intact communities. Here, we test how warming affects energy flux and the trophic basis of production in a natural invertebrate food web by experimentally heating a stream reach in southwest Iceland by ~4°C for 2yr and comparing its response to an unheated reference stream. Previous results from this experiment showed that warming led to shifts in the structure of the invertebrate assemblage, with estimated increases in total metabolic demand but no change in annual secondary production. We hypothesized that elevated metabolic demand and invariant secondary production would combine to increase total consumption of organic matter in the food web, if diet composition did not change appreciably with warming. Dietary composition of primary consumers indeed varied little between streams and among years, with gut contents primarily consisting of diatoms (72.9%) and amorphous detritus (19.5%). Diatoms dominated the trophic basis of production of primary consumers in both study streams, contributing 79-86% to secondary production. Although warming increased the flux of filamentous algae within the food web, total resource consumption did not increase as predicted. The neutral net effect of warming on total energy flow through the food web was a result of taxon-level variation in responses to warming, a neutral effect on total invertebrate production, and strong trophic redundancy within the invertebrate assemblage. Thus, food webs characterized by a high degree of trophic redundancy may be more resistant to the effects of climate warming than those with more diverse and specialized consumers.

Distribution and abundance of the invasive tunicate Ciona intestinalis (Linnaeus, 1767) in Icelandic harbours

Ciona intestinalis is within the group of taxa that are spreading globally and is one of the most thriving invasive marine species, known to depress both species richness and abundance at a local scale in distinct geographic areas. It is a dominant biofouling agent, associated with the decreasing economic incomes from aquaculture operations around the world. Ciona intestinalis was first observed in 2007 at Straumsvík in Southwest Iceland. The present study was designed to provide general information on its current distribution across Icelandic harbours. The species was found only on the SW coast of Iceland, in dense aggregations reaching up to 876 ind/m2 in all harbours from Grindavík to Akranes. The current work provides information that can be used to track the dispersal of C. intestinalis populations along the Icelandic coast and for the development of effective management decisions.

Processing automatic seismic event detections: an iterative sorting algorithm improving earthquake hypocentres using interevent cross-correlation

SUMMARYWe present an iterative classification scheme using interevent cross-correlation to update an existing earthquake catalogue with similar events from a list of automatic seismic event detections. The algorithm automatically produces catalogue quality events, with improved hypocentres and reliable P- and S-arrival time information.Detected events are classified into four event categories with the purpose of using the top category, with the highest assessed event quality and highest true-to-false ratio, directly for local earthquake tomography without additional manual analysis. The remaining categories have varying proportions of lower quality events, quality being defined primarily by the number of observed phase onsets, and can be viewed as different priority groups for manual inspection to reduce the time spent by a seismic analyst.A list of 3348 event detections from the geothermally active volcanic region around Hengill, southwest Iceland, produced by our migration and stack detector (Wagner et al. 2017), was processed using a reference catalogue of 1108 manually picked events from the same area.P- and S-phase onset times were automatically determined for the detected events using correlation time lags with respect to manually picked phase arrivals from different multiple reference events at the same station. A significant improvement of the initial hypocentre estimates was achieved after relocating the detected events using the computed phase onset times. The differential time data set resulting from the correlation was successfully used for a double-difference relocation of the final updated catalogue.The routine can potentially be implemented in real-time seismic monitoring environments in combination with a variety of seismic event/phase detectors.

Sediment geochemistry and mineralogy from a glacial terrain river system in southwest Iceland

Abstract The weathering of basalt has implications for the global carbon cycle on Earth as well as for understanding sedimentary processes on other terrestrial bodies dominated by a basaltic crust (e.g., Mars), but yet studies in mafic terrains are limited in comparison to their felsic counterpart. Our work details the compositional transformations resulting from the sedimentation process of first-cycle sediments generated in a basalt dominated watershed of southwest Iceland. By sampling multiple sites along the Hvita S river transport pathway and analyzing the sedimentology, geochemistry, and mineralogy, this work provides a geologically integrated approach to understanding a fluvial source-to-sink system in mafic terrains. Environmental conditions such as climate, hydrology, and transport distance all influence the overall sediment composition, and in response, chemical weathering, physical abrasion, sorting, transport, and mixing are all sedimentary processes unraveled in the geochemical and mineralogical relationships of individual grain size bins. Chemical weathering initiates in the upper reaches of the watershed with the alteration of primary mafic phases to form secondary weathering products. As sediment continues to move downstream, the entire sediment suite becomes more altered, and the extent of chemical weathering is intensified, as evidenced by a higher abundance of clay minerals and glassy material. Fluvial sorting separates detritus by particle size and differentiates them by compositional gradients in geochemistry and mineralogy. As grain size decreases, mafic minerals become less abundant and the finer grain sizes are preferentially enriched in immature weathering minerals and mineraloids (e.g., smectite clays and X-ray amorphous phases). This grain size trend is also correlated with elemental fractionation, illustrated most notably by the clay size fraction (