Scandinavian Lakes Research Articles

Declining calcium concentration drives shifts toward smaller and less nutritious zooplankton in northern lakes.

Zooplankton community composition of northern lakes is changing due to the interactive effects of climate change and recovery from acidification, yet limited data are available to assess these changes combined. Here, we built a database using archives of temperature, water chemistry and zooplankton data from 60 Scandinavian lakes that represent broad spatial and temporal gradients in key parameters: temperature, calcium (Ca), total phosphorus (TP), total organic carbon (TOC), and pH. Using machine learning techniques, we found that Ca was the most important determinant of the relative abundance of all zooplankton groups studied, while pH was second, and TOC third in importance. Further, we found that Ca is declining in almost all lakes, and we detected a critical Ca threshold in lake water of 1.3 mg L-1 , below which the relative abundance of zooplankton shifts toward dominance of Holopedium gibberum and small cladocerans at the expense of Daphnia and copepods. Our findings suggest that low Ca concentrations may shape zooplankton communities, and that current trajectories of Ca decline could promote widespread changes in pelagic food webs as zooplankton are important trophic links from phytoplankton to fish and different zooplankton species play different roles in this context.

New determination of Pediastrum orientale in polar lake sediments and its palaeoecological implications – Reindeer Lake, Bellsund, Spitsbergen

The main component of microfossils in the bottom sediments of Reindeer Lake from Spitsbergen, documenting the almost 8,000-year history of this water body, are algae representing colonies of a very rare species Pediastrum orientale (Skuja) Jankovská et Komárek 1995 with only a very small (lower than 1%) admixture of other green algae species. It has also been recorded in other lake sediments in Spitsbergen. The specimens belong to the morphotype “b” of the species, known previously from the Scandinavian lakes. Due to its unique nature and scarce information on ecological conditions, this study analysed the palaeoenvironmental factors favouring the occurrence of Pediastrum orientale together with an assessment of its bioindication value. The affinity of the species to oligotrophic lakes, where blooms of this alga are observed, was confirmed. The presence of numerous colonies of Pediastrum orientale in fossil lake sediments can provide a rationale for inferring palaeoecological conditions, including the trophic state of water bodies, thus expanding knowledge of their evolutionary directions and documenting key events in the geological history of the catchment. Further studies of lake gyttjas in Spitsbergen should pay more attention to the taxonomic composition of Pediastrum and other green algae, as well as to the determinants of sediment deposition in different lake catchment environments.

Acidification of freshwater lakes in Scandinavia: impacts and recovery of chironomid communities under accelerating environmental changes

We re-examined the effects of acidification on Scandinavian freshwater lakes using paleolimnological reconstructions focused on subfossil chironomids. Our study showed a widespread shift in chironomid community composition occurring at around 1960 and corresponding with the maximum fallout of anthropogenic SO42− deposition. Results also showed that taxonomic turn-over was higher in chironomid records from nitrogen-limited lakes, likely due to the cumulative effects of acidification and eutrophication driven by atmospheric nitrogen deposition. Despite strong evidence of chemical recovery from acidification, all lake records failed to show a return to pre-acidified community compositions and most chironomid communities instead continue to follow the trajectories triggered by acidification. We hypothesized that when SO42− deposition started to decrease after 1980, a period also marked by rapid environmental and climatic changes, the influence of pH and/or other acidification-related variables in structuring chironomid community became less important, thus, giving more relative importance to other drivers, such as brownification and climatic processes. Results, therefore, suggest the key role of acidification in shaping the response of chironomid communities to future environmental changes. Future paleolimnological studies will contribute to better manage aquatic ecosystems recovering from acidification worldwide by allowing managers to quantify the efficacy of different management actions taken to mitigate acidification as well as to redefine appropriate restoration targets.

Plastic pollution fosters more microbial growth in lakes than natural organic matter

Plastic debris widely pollutes freshwaters. Abiotic and biotic degradation of plastics releases carbon-based substrates that are available for heterotrophic growth, but little is known about how these novel organic compounds influence microbial metabolism. Here we found leachate from plastic shopping bags was chemically distinct and more bioavailable than natural organic matter from 29 Scandinavian lakes. Consequently, plastic leachate increased bacterial biomass acquisition by 2.29-times when added at an environmentally-relevant concentration to lake surface waters. These results were not solely attributable to the amount of dissolved organic carbon provided by the leachate. Bacterial growth was 1.72-times more efficient with plastic leachate because the added carbon was more accessible than natural organic matter. These effects varied with both the availability of alternate, especially labile, carbon sources and bacterial diversity. Together, our results suggest that plastic pollution may stimulate aquatic food webs and highlight where pollution mitigation strategies could be most effective.

Multiple thresholds and trajectories of microbial biodiversity predicted across browning gradients by neural networks and decision tree learning

Ecological association studies often assume monotonicity such as between biodiversity and environmental properties although there is growing evidence that nonmonotonic relations dominate in nature. Here, we apply machine-learning algorithms to reveal the nonmonotonic association between microbial diversity and an anthropogenic-induced large-scale change, the browning of freshwaters, along a longitudinal gradient covering 70 boreal lakes in Scandinavia. Measures of bacterial richness and evenness (alpha-diversity) showed nonmonotonic trends in relation to environmental gradients, peaking at intermediate levels of browning. Depending on the statistical methods, variables indicative for browning could explain 5% of the variance in bacterial community composition (beta-diversity) when applying standard methods assuming monotonic relations and up to 45% with machine-learning methods taking non-monotonicity into account. This non-monotonicity observed at the community level was explained by the complex interchangeable nature of individual taxa responses as shown by a high degree of nonmonotonic responses of individual bacterial sequence variants to browning. Furthermore, the nonmonotonic models provide the position of thresholds and predict alternative bacterial diversity trajectories in boreal freshwater as a result of ongoing climate and land-use changes, which in turn will affect entire ecosystem metabolism and likely greenhouse gas production.

The biology and feeding ecology of Arctic charr in the Kerguelen Islands.

Subsequent to their introduction in the 1950s, Arctic charr Salvelinus alpinus have been able to establish a self-sustaining population that has adapted to the unique conditions of the sub-Antarctic Kerguelen Islands. Here, 48 individuals (198-415 mm) were caught with gillnets and their basic biology and feeding ecology were examined using stable isotope analysis. The Lac des Fougères population split use of littoral and pelagic resources evenly, although larger fish relied more heavily on littoral production and appear to follow the size-dependent life history habitat template seen in many Scandinavian lakes where smaller sized individuals occupy the pelagic zone and larger individuals dominate the littoral habitat. In Kerguelen, Arctic charr mature at the same ages (5.6 years) as Arctic charr in both sub-Arctic and Arctic lakes. Although mortality was average in comparison to comparator sub-Arctic lakes, it was high in comparison to Arctic lakes. Maximal age (>7+) was at the lower end of the range typically seen in sub-Arctic lakes. Although they inhabit a resource-poor environment, Kerguelen Arctic charr showed no evidence of cannibalism. Thus, while Arctic charr can survive and reproduce in the relatively unproductive Kerguelen lake environments, survival and growth nevertheless appear to be traded off against survival and longevity. The uniqueness of the population location and the recency of its introduction suggest that further monitoring of the population has the potential to yield valuable insights into both the adaptability of the species and its likely responses to ongoing large-scale environmental change as represented by climate change.

Phosphorus Availability Promotes Bacterial DOC-Mineralization, but Not Cumulative CO2-Production.

The current trend of increasing input of terrestrially derived dissolved organic carbon (DOC) to boreal freshwater systems is causing increased levels of carbon dioxide (CO2) supersaturation and degassing. Phosphorus (P) is often the most limiting nutrient for bacterial growth and would thus be expected to increase overall mineralization rates and CO2 production. However, high carbon (C) to P ratios of terrestrially derived DOC could also cause elevated cell-specific respiration of the excess C in heterotrophic bacteria. Using data from a survey of 75 Scandinavian lakes along an ecosystem gradient of DOC, we estimated in situ CO2 production rates. These rates showed a unimodal response with DOC-specific CO2 production negatively related to DOC:total phosphorus (TP) ratio, and a turning point at 5 mg C L−1, indicating higher DOC turnover rates in productive than in unproductive lakes. To further assess the dependency of bacterial respiration (BR) on DOC and P, we monitored CO2 production in incubations of water with a gradient of DOC crossed with two levels of inorganic P. Finally, we crossed DOC and P with a temperature gradient to test the temperature dependency of respiration rates [as oxygen (O2) consumption]. While total CO2 production seemed to be unaffected by P additions, respiration rates, and growth yields, as estimated by ribosomal gene copy numbers, suggest increased bacterial growth and decreased cell-specific respiration under non-limited P conditions. Respiration rates showed a sigmoid response to increasing DOC availability reaching a plateau at about 20 mg C L−1 of initial DOC concentrations. In addition to these P and DOC level effects, respiration rates responded in a non-monotonic fashion to temperature with an increase in respiration rates by a factor of 2.6 (±0.2) from 15 to 25°C and a decrease above 30°C. The combined results from the survey and experiments highlight DOC as the major determinant of CO2 production in boreal lakes, with P and temperature as significant modulators of respiration kinetics.

The role of photomineralization for CO2 emissions in boreal lakes along a gradient of dissolved organic matter

AbstractMany boreal lakes are experiencing an increase in concentrations of terrestrially derived dissolved organic matter (DOM)—a process commonly labeled “browning.” Browning affects microbial and photochemical mineralization of DOM, and causes increased light attenuation and hence reduced photosynthesis. Consequently, browning regulates lake heterotrophy and net CO2‐efflux to the atmosphere. Climate and environmental change makes ecological forecasting and global carbon cycle modeling increasingly important. A proper understanding of the magnitude and relative contribution from CO2‐generating processes for lakes ranging in dissolve organic carbon (DOC) concentrations is therefore crucial for constraining models and forecasts. Here, we aim to study the relative contribution of photomineralization to total CO2 production in 70 Scandinavian lakes along an ecosystem gradient of DOC concentration. We combined spectral data from the lakes with regression estimates between optical parameters and wavelength specific photochemical reactivity to estimate rates of photochemical DOC mineralization. Further, we estimated total in‐lake CO2‐production and efflux from lake chemical and physical data. Photochemical mineralization corresponded on average to 9% ± 1% of the total CO2‐evasion, with the highest contribution in clear lakes. The calculated relative contribution of photochemical mineralization to total in‐lake CO2‐production was about 3% ± 0.2% in all lakes. Although lakes differed substantially in color, depth‐integrated photomineralization estimates were similar in all lakes, regardless of DOC concentrations. DOC concentrations were positively related to CO2‐efflux and total in‐lake CO2‐production but negatively related to primary production. We conclude that enhanced rates of photochemical mineralization will be a minor contributor to increased heterotrophy under increased browning.

Congruence, but no cascade-Pelagic biodiversity across three trophic levels in Nordic lakes.

Covariation in species richness and community structure across taxonomical groups (cross‐taxon congruence) has practical consequences for the identification of biodiversity surrogates and proxies, as well as theoretical ramifications for understanding the mechanisms maintaining and sustaining biodiversity. We found there to exist a high cross‐taxon congruence between phytoplankton, zooplankton, and fish in 73 large Scandinavian lakes across a 750 km longitudinal transect. The fraction of the total diversity variation explained by local environment alone was small for all trophic levels while a substantial fraction could be explained by spatial gradient variables. Almost half of the explained variation could not be resolved between local and spatial factors, possibly due to confounding issues between longitude and landscape productivity. There is strong consensus that the longitudinal gradient found in the regional fish community results from postglacial dispersal limitations, while there is much less evidence for the species richness and community structure gradients at lower trophic levels being directly affected by dispersal limitation over the same time scale. We found strong support for bidirectional interactions between fish and zooplankton species richness, while corresponding interactions between phytoplankton and zooplankton richness were much weaker. Both the weakening of the linkage at lower trophic levels and the bidirectional nature of the interaction indicates that the underlying mechanism must be qualitatively different from a trophic cascade.

Heteroxanthin as a pigment biomarker for Gonyostomum semen (Raphidophyceae).

The ability to identify drivers responsible for algal community shifts is an important aspect of environmental issues. The lack of long-term datasets, covering periods prior to these shifts, is often limiting our understanding of drivers responsible. The freshwater alga, Gonyostomum semen (Raphidophyceae), has significantly increased distribution and mass occurrences in Scandinavian lakes during the past few decades, often releasing a skin irritating slime that causes discomfort for swimmers. While the alga has been extensively studied, long-term data from individual lakes are often absent or greatly limited and drivers behind this species’ success are still not clear. However, if specific and persistent taxa biomarkers for G. semen could be detected in dated sediment cores, long-term data would be improved and more useful. To test for biomarkers, we examined the pigment composition of several G. semen strains in culture. Further, dated sediment core samples from Lake Lundebyvann, Norway, were used to test the pigments’ suitability as biomarkers in paleolimnological studies. Modifications to a common analysis allowed for the successful detection of the polar xanthophyll heteroxanthin and the non-polar chlorophyll a, as well as several other algal pigments by using high performance liquid chromatography-photometric diode arrays (HPLC-PDA). Heteroxanthin was confirmed by liquid chromatography-mass spectrometry (LC-MS) and detected by HPLC-PDA in all examined G. semen strains, along with chlorophyll a. Using HPLC-PDA, we also identified and confirmed the presence of the biomarker, xanthophyll heteroxanthin, in sediment core samples up to 60 years of age. The specificity of this xanthophyll was also tested by examining a wide range of algal strains from common Norwegian phytoplankton species. Heteroxanthin was not detected in any species commonly occurring in significant amounts in Norwegian lakes. We therefore conclude that heteroxanthin is a suitable pigment biomarker for G. semen and that this pigment can be successfully used for paleolimnological studies.

Holocene Hydroclimate Variability in Central Scandinavia Inferred from Flood Layers in Contourite Drift Deposits in Lake Storsjön

Despite the societal importance of extreme hydroclimate events, few palaeoenvironmental studies of Scandinavian lake sediments have investigated flood occurrences. Here we present a flood history based on lithological, geochemical and mineral magnetic records of a Holocene sediment sequence collected from contourite drift deposits in Lake Storsjön (63.12° N, 14.37° E). After the last deglaciation, the lake began to form around 9800 cal yr BP, but glacial activity persisted in the catchment for ~250 years. Element concentrations and mineral magnetic properties of the sediments indicate relatively stable sedimentation conditions during the Holocene. However, human impact in the form of expanding agriculture is evident from about 1100 cal yr BP, and intensified in the 20th century. Black layers containing iron sulphide appear irregularly throughout the sequence. The increased influx of organic matter during flood events led to decomposition and oxygen consumption, and eventually to anoxic conditions in the interstitial water preserving these layers. Elevated frequencies of black layer occurrence between 3600 and 1800 cal yr BP reflect vegetation changes in the catchment as well as large-scale climatic change. Soil erosion during snowmelt flood events increased with a tree line descent since the onset of the neoglacial period (~4000 cal yr BP). The peak in black layer occurrence coincides with a prominent solar minimum ~2600 cal yr BP, which may have accentuated the observed pattern due to the prevalence of a negative NAO index, a longer snow accumulation period and consequently stronger snowmelt floods.

Temperature-precipitation relationship of the Common Era in northern Europe

Due to the lack of knowledge on dynamics of the North Atlantic Oscillation (NAO) prior to the last millennium, synchronized records of air temperature and precipitation variability are needed to understand large-scale drivers of the hydroclimate. Here, we use completely synchronized paleolimnological proxy-based records of air temperature and effective precipitation from two Scandinavian lakes with ∼2000-year sediment profiles. We show that the relationship between air temperature and precipitation (T/P ratio) is synchronous in both study sites throughout the records suggesting warm and dry conditions at ∼300–1100 CE and cold and wet conditions at ∼1200–1900 CE. Owing to the significantly increased air temperatures, the most recent T/P ratio has again turned positive. During the first millennium of the Common Era, the T/P mimics patterns in Southern Oscillation index, whereas the second millennium shows response to the NAO index but is also concurrent with solar irradiance shifts. Since our T/P reconstruction is mostly linked with the NAO, we propose the T/P ratio as an indicator of the NAO. Our results from the coherent records provide first-time knowledge on the long-term temperature-precipitation relationship in Northern Europe that increase understanding of the comprehensive hydroclimate system in the region and the NAO dynamics also further back in time.

Fungal communities in Scandinavian lakes along a longitudinal gradient

Abstract This study investigates the diversity and distribution of fungal communities in 77 oligotrophic lakes in southern Norway and Sweden using 454-sequencing with fungal-specific primers targeting ITS2 region of the rRNA gene. A total of 232 operational taxonomic units (OTUs) belonging to four recognized phyla were detected. A large proportion (70.69%) of the detected OTUs were Dikarya (Ascomycota and Basidiomycota), while Chytridiomycota dominated quantitatively (63.37% reads). The most abundant aquatic fungi were taxonomically assigned to Chytridiomycota, whose members are known to be saprobes on a large variety of substrates and parasites of phytoplankton, zooplankton, fungi and invertebrates, suggesting that resident fungi strictly depend on surfaces and, therefore, are closely associated with other types of aquatic organisms. Our results indicate that surface waters of oligotrophic lakes harbour a diverse pool of fungi, both with tentative terrestrial and true aquatic origin. Longitude and environmental factors were important in structuring the fungal community composition.

Coupling dissolved organic carbon, CO2 and productivity in boreal lakes

Summary Lakes worldwide are in rapid change because of direct or indirect climate impacts. In boreal areas, the increased concentrations of dissolved organic matter (DOM) are profoundly affecting light climate and productivity in multiple ways. Photochemical and microbial mineralisation of DOM are major sources of CO2 in these lakes. It has been suggested that this CO2 could potentially promote primary production and offset negative impacts of increased light attenuation. A synoptic survey of 76 Scandinavian lakes along gradients of DOM and total phosphorus (TP) revealed a major negative impact of DOM on primary production and algal biomass primarily related to light attenuation, whilst a strong positive impact of TP. The negative impact of DOM on lake productivity is thus partly offset by DOM‐associated P. Concentrations of CO2 did not relate significantly to either production, biomass, or seston stoichiometry, and thus whilst elevated CO2 may promote primary production in more productive lakes, it does not in these CO2‐supersaturated boreal lakes. Zooplankton biomass correlated strongly with TP and total algal biomass, less so with primary production, and was negatively related to DOM, likely reflecting the autotroph responses to DOM.

Small Boreal Lake Ecosystem Evolution under the Influence of Natural and Anthropogenic Factors: Results of Multidisciplinary Long-Term Study

Small aquatic ecosystems of the boreal zone are known to be most sensitive indicators of on-going environmental change as well as local anthropogenic pressure, while being highly vulnerable to external impacts. Compared to rather detailed knowledge of the evolution of large and small lakes in Scandinavia and Canada, and large lakes in Eurasia, highly abundant small boreal lakes of northwest Russia have received very little attention, although they may become important centers of attraction of growing rural population in the near future. Here we present the results of a multidisciplinary, multi-annual study of a small boreal humic lake of NW Russia. A shallow (3 m) and a deep (16 m) site of this lake were regularly sampled for a range of chemical and biological parameters. Average multi-daily, summer-time values of the epilimnion (upper oxygenated) layer of the lake provided indications of possible trends in temperature, nutrients, and bacterio-plankton concentration that revealed the local pollution impact in the shallow zone and overall environmental trend in the deep sampling point of the lake. Organic phosphorus, nitrate, and lead were found to be most efficient tracers of local anthropogenic pollution, especially visible in the surface layer of the shallow site of the lake. Cycling of trace elements between the epilimnion and hypolimnion is tightly linked to dissolved organic matter speciation and size fractionation due to the dominance of organic and organo-ferric colloids. The capacity of lake self-purification depends on the ratio of primary productivity to mineralization of organic matter. This ratio remained >1 both during winter and summer periods, which suggests a high potential of lake recovery from the input of allochthonous dissolved organic matter and local anthropogenic pollution.

The trophic niche of Arctic charr in large southern Scandinavian lakes is determined by fish community and lake morphometry

The influence of interspecific interactions and habitat characteristics on the ecological niche of species’ populations is a controversial issue in ecology. We explored the niche use of Arctic charr (Salvelinus alpinus L.) in seven relatively large and deep lakes in southern Norway. We aimed to investigate how the presence of potentially competitive fish species, such as whitefish (Coregonus lavaretus L.), perch (Perca fluviatilis L.) and brown trout (Salmo trutta L.), and the lake morphometry affects the littoral reliance and niche breadth of Arctic charr. Stable isotope (δ13C and δ15N) analyses showed that Arctic charr relied less on littoral food resources than brown trout and perch but had largely overlapping niche with whitefish. Arctic charr showed narrower niche use (i.e. less individual variation in δ13C and δ15N) than the other fish species in all seven lakes, particularly in communities where both littoral and pelagic competitors were present. Our results demonstrate that Arctic charr rely much less on littoral resources (<50%) in large and deep lakes in southern Scandinavia than in small, shallower subarctic lakes, and that interspecific interactions with both littoral and pelagic resource competitors can lead to niche compression of this generalist salmonid fish.

Efterkrigsritualet – En komparativ analyse af de danske krigsbytteofringer

The post-war ritual – a comparative analysis of the Danish war booty offerings and the Roman triumphsThis article is inspired by a theory, which postulates that the Roman triumphs should be seen as an inspirational source relative to the interpretation of war booty offerings found in Danish bogs, and aims to examine the validity of this claim. The so-called triumph theory originates in the discussion of whether the military equipment recovered from Danish bogs stems from defensive or offensive battles. According to the triumph theory, the war booty was brought home from battles fought in foreign parts by a victorious Germanic people. It was then was placed in a triumph procession, following the Roman model, with display of the booty, and finally sacrificed in a lake according to traditional Germanic custom.After a general introduction to the Roman triumphs (fig. 1) and the Danish war booty offerings (fig. 2), some of the elements making up these two ceremonies are presented, specifically the actual material war booty, the human captives and the animals (figs. 4-5). There are distinct differences between the two ceremonies, for example in the war booty included in the Roman triumphs (fig. 3) and the Germanic war booty offerings, respectively. One discrepancy, which is difficult to ignore, is the treatment of, and further potential for reuse of, the captured weapons of the enemy displayed in the two ceremonies. There are examples where booty from the Romans’ triumphs was later reused. This could not be the case for the Germanic war booty offerings, whereby weapons underwent such a massive process of destruction and, furthermore, were then deposited in lakes, making reuse seem impossible.The locations where the respective ceremonies took place – the city of Rome and South Scandinavian lakes – are then presented.An alternative explanation for the war booty offerings is then examined. Instead of seeing them as a Germanic version of the Roman triumphs, could the offerings be interpreted as Roman victory trophies (fig. 6)? This idea is rejected due to the lack of evidence for the sites of the offerings also being that of the actual battle field.Finally, attention is turned to the classical writers, who wrote about the Germanic and Gallic custom of depositing of military equipment, in order to extract what they can say about the gods whom the two ceremonies were intended to benefit.In the conclusion, the arguments are summed up in support of the author’s view that the triumph theory does not provide a convincing explanation for the Danish war booty offerings.Pernille Mosholm Boye ThulstrupForhistorisk ArkæologiInstitut for Kultur og SamfundAarhus Universitet

Sterols of the Green‐Pigmented, Freshwater Raphidophyte, Gonyostomum semen, from Scandinavian Lakes

Sterols are a class of membrane-reinforcing, ringed lipids which have a long history of examination in algae as a means of deriving chemotaxonomic relationships and as potential lipidic biomarkers. The Raphidophyceae represent a class of harmful, bloom-forming, marine and freshwater algae. To date, there have been four published examinations of their sterol composition, focusing primarily on brown-pigmented, marine species within the genera, Chattonella, Fibrocapsa, and Heterosigma. Lacking in these examinations has been the species Gonyostomum semen Ehrenb., which is a green-pigmented, freshwater raphidophyte with a worldwide distribution. The goal of this study was to examine the sterol composition of this nuisance alga, determine the potential of using its sterol profile as a biomarker, and finally to determine if there is any intraspecific variability between isolates. We have examined 21 isolates of G. semen from a number of Scandinavian lakes, and all were found to produce two major sterols, 24-ethylcholesta-5,22E-dien-3β-ol and 24-ethylcholest-5-en-3β-ol, and 24-methylcholest-5-en-3β-ol as a minor sterol; the presence of 24-ethylcholesta-5,22E-dien-3β-ol differentiates G. semen from brown-pigmented, marine raphidophytes which generally lack it. The results of this study indicate that isolates of G. semen from geographically separate lakes across Finland and Scandinavia have the same sterol biosynthetic pathway, and that there is no evolutionary divergence between the isolates with regard to sterol composition. The sterols of G. semen are not considered to be useful biomarkers for this particular organism because they are commonly found in other algae and plants.

Migration confers survival benefits against avian predators for partially migratory freshwater fish

The importance of predation risk in shaping patterns of animal migration is not well studied, mostly owing to difficulties in accurately quantifying predation risk for migratory versus resident individuals. Here, we present data from an extensive field study, which shows that migration in a freshwater fish (roach, Rutilus rutilus) that commonly migrates from lakes to streams during winter confers a significant survival benefit with respect to bird (cormorant, Phalacrocorax carbo spp.) predation. We tagged over 2000 individual fish in two Scandinavian lakes over 4 years and monitored migratory behaviour using passive telemetry. Next, we calculated the predation vulnerability of fish with differing migration strategies, by recovering data from passive integrated transponder tags of fish eaten by cormorants at communal roosts close to the lakes. We show that fish can reduce their predation risk from cormorants by migrating into streams, and that probability of being preyed upon by cormorants is positively related to the time individuals spend in the lake during winter. Our data add to the growing body of evidence that highlights the importance of predation for migratory dynamics, and, to our knowledge, is one of the first studies to directly quantify a predator avoidance benefit to migrants in the field.

Forecasting the ecological impacts of the Hemimysis anomala invasion in North America: Lessons from other freshwater mysid introductions

The bloody red mysid shrimp Hemimysis anomala is the most recent Ponto-Caspian invader of the Great Lakes. At present, there is a paucity of data that can be used to forecast its ecological impacts in North America; however, published studies of other invasive freshwater mysids (i.e. Mysis diluviana introduced to North American lakes and Mysis relicta introduced to Scandinavian lakes) reveal potentially valuable predictive information. A frequent impact of Mysis introductions is a drastic decline in zooplankton (particularly cladoceran) abundance, which can release phytoplankton populations from grazer control and alter the abundance and growth of pelagic fishes. Other impacts include enhanced bioaccumulation of contaminants, new pathways of parasite transmission to fishes, and altered nutrient cycling. Like Mysis spp., H. anomala has the potential to affect all trophic levels in lakes. It has reduced the abundance and diversity of zooplankton in European reservoirs, where it achieved mean densities higher than those recorded for other introduced mysids elsewhere. It can colonize a broader range of habitats than Mysis throughout the year, including the nearshore areas of lakes and large rivers. Within river systems, the impacts of H. anomala will likely be most pronounced in backwater lakes, impounded areas and reservoirs. Understanding the environmental factors that limit the local abundance of H. anomala and mediate its species interactions is essential for developing predictive models of its impact.