Gísli Már Gíslason

Impacts of Warming on the Structure and Functioning of Aquatic Communities : Individual-to Ecosystem-Level Responses

Environmental warming is predicted to rise dramatically over the next century, yet few studies have investigated its effects in natural, multi-species systems. We present data collated over an 8-year period from a catchment of geothermally heated streams in Iceland, which acts as a natural experiment on the effects of warming across different organisational levels and spatiotemporal scales. Body sizes and population biomasses of individual species responded strongly to temperature, with some providing evidence to support temperature size rules. Macroinvertebrate and meiofaunal community composition also changed dramatically across the thermal gradient. Interactions within the warm streams in particular were characterised by food chains linking algae to snails to the apex predator, brown trout These chains were missing from the colder systems, where snails were replaced by much smaller herbivores and invertebrate omnivores were the top predators. Trout were also subsidised by terrestrial invertebrate prey, which could have an effect analogous to apparent competition within the aquatic prey assemblage. Top-down effects by snails on diatoms were stronger in the warmer streams, which could account for a shallowing of mass-abundance slopes across the community. This may indicate reduced energy transfer efficiency from resources to consumers in the warmer systems and/or a change in predator-prey mass ratios. All the ecosystem process rates investigated increased with temperature, but with differing thermal sensitivities, with important implications for overall ecosystem functioning (e.g. creating potential imbalances in elemental fluxes). Ecosystem respiration rose rapidly with temperature, leading to increased heterotrophy. There were also indications that food web stability may be lower in the warmer streams.

The ecology of Lake Myvatn and the River Laxá: Variation in space and time

Ecological features of Lake Myvatn and the outflowing River Laxá 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.

Body Size, Diet and Growth of Landlocked Brown Trout, Salmo trutta, in the Subarctic River Laxá, North-East Iceland

Studies on diet preferences of stream-dwelling salmonids have mostly been limited to a relatively small range in body size. This study examined the influence of salmonid body size on prey size and diet composition, and its consequences for growth, in landlocked stream-resident brown trout, Salmo trutta (2.5–61.3 cm) in the Laxá River, N-E Iceland. The most common prey of the 1622 trout sampled, were blackflies, Simulium vittatum, chironomid midges and the freshwater snail, Lymnaea peregra, which represented 56.3%, 21.8%, 10.8% of the stomach content volume, respectively. In general, the Laxá trout showed a consistent, but moderate, shift towards larger prey with increased body size. The relatively stable growth and the large body size attained were probably due to the high production of small benthic invertebrates and only secondarily to the ontogenetic shift towards larger prey.

Distribution and Habitat Preferences of Icelandic Trichoptera

Eleven species of Trichoptera are found in Iceland. Four species occur both in running and stagnant waters, six species prefer very stagnant waters, shallow ponds and swamps and one species occurs mainly in streams. In general each species occupies a much greater range of habitats in Iceland than they do elsewhere in Europe, where the number of species is 20 to 30 times greater. Distributional patterns within Iceland fall into 4 categories: (a) found all over Iceland (5 species). (b) found where summer temperatures are high (4 species), (c) found in the East and North (1 species) and (d) found all over Iceland, except in the East (1 species). Species in the first category are those that occupy the widest range of habitats. In the second category are species that are found only in shallow stagnant waters, which freeze solid during the winter, and all larval growth takes place in the summer. The single species in the third category is a recent immigrant which is profoundly affecting the distribution of the single species in the fourth category.

Glacier shrinkage driving global changes in downstream systems

Glaciers cover ∼10% of the Earth’s land surface, but they are shrinking rapidly across most parts of the world, leading to cascading impacts on downstream systems. Glaciers impart unique footprints on river flow at times when other water sources are low. Changes in river hydrology and morphology caused by climate-induced glacier loss are projected to be the greatest of any hydrological system, with major implications for riverine and near-shore marine environments. Here, we synthesize current evidence of how glacier shrinkage will alter hydrological regimes, sediment transport, and biogeochemical and contaminant fluxes from rivers to oceans. This will profoundly influence the natural environment, including many facets of biodiversity, and the ecosystem services that glacier-fed rivers provide to humans, particularly provision of water for agriculture, hydropower, and consumption. We conclude that human society must plan adaptation and mitigation measures for the full breadth of impacts in all affected regions caused by glacier shrinkage.

Temperature effects on fish production across a natural thermal gradient

Abstract Global warming is widely predicted to reduce the biomass production of top predators, or even result in species loss. Several exceptions to this expectation have been identified, however, and it is vital that we understand the underlying mechanisms if we are to improve our ability to predict future trends. Here, we used a natural warming experiment in Iceland and quantitative theoretical predictions to investigate the success of brown trout as top predators across a stream temperature gradient (4–25 °C). Brown trout are at the northern limit of their geographic distribution in this system, with ambient stream temperatures below their optimum for maximal growth, and above it in the warmest streams. A five‐month mark‐recapture study revealed that population abundance, biomass, growth rate, and production of trout all increased with stream temperature. We identified two mechanisms that contributed to these responses: (1) trout became more selective in their diet as stream temperature increased, feeding higher in the food web and increasing in trophic position; and (2) trophic transfer through the food web was more efficient in the warmer streams. We found little evidence to support a third potential mechanism: that external subsidies would play a more important role in the diet of trout with increasing stream temperature. Resource availability was also amplified through the trophic levels with warming, as predicted by metabolic theory in nutrient‐replete systems. These results highlight circumstances in which top predators can thrive in warmer environments and contribute to our knowledge of warming impacts on natural communities and ecosystem functioning.

Populations of ducks and trout of the River LaxÁ, Iceland, in relation to variation in food resources

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.

Stock size and movements of landlocked brown trout i (Salmo trutta L.) in the subarctic River Laxa, north-east Iceland

fisheries are managed as high quality rod fishing and r it is permitted to land only brown trout larger than 35 cm long; smaller trout are returned alive to the river. The maximum number of brown trout caught ~ per rod is limited to 12 per day. The angling season is between 1 June and 31 August, with a maximum of24 rods leased daily, each allocated to a certain site along the river. The aim of this study was to estimate the size of the stock of brown trout, the fishing pressure, the cause of the catch fluctuations that have been observed and the migration of trout within the river Loke Myvain and to Lake Mfvatn. The information will be used for further understanding of the ecology of the River Laxa and to improve the management of the trout population. 0

Functional diversity and community assembly of river invertebrates show globally consistent responses to decreasing glacier cover

Global change threatens invertebrate biodiversity and its central role in numerous ecosystem functions and services. Functional trait analyses have been advocated to uncover global mechanisms behind biodiversity responses to environmental change, but the application of this approach for invertebrates is underdeveloped relative to other organism groups. From an evaluation of 363 records comprising >1.23 million invertebrates collected from rivers across nine biogeographic regions on three continents, consistent responses of community trait composition and diversity to replicated gradients of reduced glacier cover are demonstrated. After accounting for a systematic regional effect of latitude, the processes shaping river invertebrate functional diversity are globally consistent. Analyses nested within individual regions identified an increase in functional diversity as glacier cover decreases. Community assembly models demonstrated that dispersal limitation was the dominant process underlying these patterns, although environmental filtering was also evident in highly glacierized basins. These findings indicate that predictable mechanisms govern river invertebrate community responses to decreasing glacier cover globally.Analysing >1 million river invertebrates from nine biogeographic regions, the authors show that functional trait diversity increases consistently as glacier cover decreases.

Spatial heterogeneity in lotic chironomids and simuliids in relation to catchment characteristics in Iceland

AI; an active volcanic island, with a great variery of bedrock formations and heterogeneous carchmem characteristics, Iceland offers a valuable opportuniry to examine the effect of rhe bedrock origin on the biota. Iceland straddles rhe Mid-Adantic Ridge, where rwo tectonic plares are moving aparr. The Ridge runs across rhe island in a south-west ro norrh-easr direcrion and is marked by a volcanically active belt (Fig. 1). On moving wesror easrwards from rhis neovolcanic zone, rhe bedrock gers older. The oldesr rock is more than 13 million years old, and is locared in rhe north-wesrern and easrern regions of the island (EINARSSON 1994). The biota of river ecosysrems is usually srrongly influenced by irs catchment characterisrics, e.g. irs geology, topography, soil and vegerarion cover (HYNES 1975). The origin and age of rhe bedrock are known to influence the fertiliry of freshwarer ecosys-