Determinants of Eurasian otter (Lutra lutra) diet in a seasonally changing reservoir

Otter diet in reservoirs is known to experience seasonal changes. We selected a reservoir with a large population of exclusively wintering great cormorants and seasonal changes in stored water volume to test the relative influence of abiotic and biotic factors on otter foraging ecology. DNA metabarcoding of otter spraints revealed a dietary change from autumn to winter. Otters had a diet dominated by the exotic goldfish in autumn, but predated intensively on the native northern straight-mouth nase in winter. This change was likely caused by predation of cormorants on goldfish and to fish biology. Secondly, macroscopic analysis of spraints revealed that otters shifted from a diet dominated by fish (in terms of biomass) to a diet dominated by red swamp crayfish during spring–summer, when the latter became overabundant. As revealed by modelling, this second shift was most likely influenced by the sudden increase in stored water volume in spring, but also by the cumulative effect of cormorant predation on fish during autumn–winter. Macroscopic analyses of otter spraints collected in a second reservoir with no cormorants revealed a lack of seasonality. Hence, the combined influence of both biotic and abiotic factors explained otter diet seasonality in a lentic-water novel ecosystem.

     

Species-specific probe,based on 18S rDNA sequence,could be used for identification of the mucilage producer microalga <Emphasis Type="Italic">Gonyaulax fragilis</Emphasis> (Dinophyta)

Brook trout (Salvelinus fontinalis) in Appalachia experience prolonged periods of poor feeding conditions, particularly during summer and fall. To determine which prey organisms are important in sustaining brook trout populations, we monitored the feeding patterns of a population of brook trout over the course of 2 years with an emphasis on seasonal change. We employed a bioenergetics model to estimate whether or not each fish had obtained enough energy to meet daily metabolic demand. As a result, qualitative comparisons between fish feeding above maintenance ration (successfully feeding fish) and fish feeding below maintenance ration (unsuccessfully feeding fish) were possible. With the exception of winter, brook trout derived significantly more energy from terrestrial organisms than aquatic organisms. During each season, successfully feeding brook trout fed on greater proportions of specific prey types. Terrestrial Coleoptera and Lepidoptera consistently proved to be important prey during warmer seasons, while large organisms such as vertebrates and crayfish appeared to be important during winter. Our findings suggest that terrestrial organisms are more important than aquatic organisms in sustaining brook trout populations. Further, certain large and abundant terrestrial taxa are critical in providing energy to brook trout.     

Seasonal variations in the physiological status and energy content of somatic and reproductive tissues of chub

The seasonal fluctuations in condition, nutrition and somatic energy content, and gonad development cycle were investigated in chub Leuciscus pyrenaicus from the headwaters of the Guadalete River, a freshwater ecosystem characterized by strong seasonal fluctuations in discharge and water level, temperature and food supply. The relationship between somatic stage and gonad cycle was also investigated and discussed. Condition, nutrition and somatic energy cycles could be divided into two distinct periods: from April to January with summer decreases and autumn increments, which is common for juveniles and mature fish; and from January to March, when juveniles and mature fish displayed different temporal variations related to the reproductive cycle. Gonad development took place from the end of the winter into the summer, the testes developing before the ovaries. Spawning started in late spring (May) and continued into summer (June and July), with fish quiescent by autumn (September). The results suggest that, for L. pyrenaicus , both environmental factors (e.g. food supply, water temperature) and reproduction needs affect the condition, nutrition and somatic energy storage of fish, which have been used as indicators of the physiological status of the population.     

Seasonal changes in eelgrass functions: current velocity reduction,prevention of sediment resuspension,and control of sediment–water column nutrient flux in relation to eelgrass dynamics

We assessed seasonal changes in eelgrass (Zostera marina) functions, i.e., reduction of current velocity, buffering of sediment resuspension, and control of dissolved inorganic nitrogen flux between the sediments and the water column, using field observations and experiments in the Akkeshi-ko estuary, Hokkaido, Japan. We also analyzed the relationships between eelgrass traits and functions. The efficiency of the reduction in current velocity increased with the development of the eelgrass canopy. Sediment resuspension was inhibited from May to August, during which time the eelgrass canopy developed. Eelgrass controlled the NH₄ ⁺ concentration of sediment porewater through root nutrient uptake, affecting NH₄ ⁺ flux between the sediments and the water column. Fluctuations in eelgrass functions and coincident changes in dynamics resulted in seasonal changes in the eelgrass environment, which may in turn affect the dynamics of organisms inhabiting eelgrass beds, e.g., mysids and epiphytic algae. Moreover, the developed eelgrass canopy trapped a large amount of material during spring and summer, which was resuspended into the surrounding ecosystem in autumn when the canopy and its functions (i.e., reduction of current and sediment resuspension) diminished. These results suggest that seasonal changes in eelgrass functions also affect communities within marginal coastal ecosystems through the control of allochthonous resources. Handling editor: P. Viaroli     

Biogeographic Variability in the Value of Mussel Beds as Ecosystem Engineers on South African Rocky Shores

Ecological engineers have important effects on biodiversity because they often increase habitat complexity and moderate environmental conditions, implying that their influence on associated fauna will vary across gradients of environmental stress. To test this, we quantified the positive effects of mussel beds on associated benthic communities around the entire South African coastline (~3500 km). We hypothesised that molluscan assemblages would show stronger affinities to the presence of mussel beds with increasing levels of heat-stress. Biomimetic loggers used to characterise thermal properties within and outside mussel beds found that solitary mussels experienced significantly greater daily maximum temperatures than mussels within beds across all locations. However, the magnitude of such differences did not appear to vary with latitude or time of year but rather was strongly influenced by biogeographic region. Differences in the abundance, diversity and community structure of molluscs within and outside mussel beds showed similar biogeographic variability, with differences in total molluscan abundances being most pronounced along the cool temperate west coast during summer and least pronounced along the warm temperate south coast during winter. Greater affinity of molluscan assemblages for mussel beds within cooler biogeographic regions suggests that evolutionary history and/or other abiotic factors may be the primary cause for the stronger influence of mussel beds on the west coast. This highlights the complex, context-dependant nature of ecosystem engineering and the varying degrees to which associated organisms affiliate with these biogenic structures. Such findings have important implications for the use of ecosystem engineers as umbrella species in ecological conservation.     

Effects of an invasive crayfish on trophic relationships in north‐temperate lake food webs

1. The introduction of invasive species is one of the main threats to global biodiversity, ecosystem structure and ecosystem processes. In freshwaters, invasive crayfish alter macroinvertebrate community structure and destroy macrophyte beds. There is limited knowledge on how such invasive species‐driven changes affect consumers at higher trophic levels. 2. In this study, we explore how the invasive rusty crayfish Orconectes rusticus, a benthic omnivore, affects benthic macroinvertebrates, as well as the broader consequences for ecosystem‐level trophic flows in terms of fish benthivory and trophic position (TP). We expected crayfish to decrease abundance of benthic macroinvertebrates, making most fish species less reliant on benthic resources. We expected crayfish specialists (e.g. Lepomis sp. and Micropterus sp.) to increase their benthic dependence. 3. In 10 northern Wisconsin lakes, we measured rusty crayfish relative abundance (catch per unit effort, CPUE), macroinvertebrate abundance, and C and N stable isotope ratios of 11 littoral fish species. We used stable isotope data and mixing models to characterise the trophic pathways supporting each fish species, and related trophic structure to crayfish relative abundance, fish body size and abiotic predictors using hierarchical Bayesian models. 4. Benthic invertebrate abundance was negatively correlated with rusty crayfish relative abundance. Fish benthivory increased with crayfish CPUE for all 11 fish species; posterior probabilities of a positive effect were >95%. TP also increased slightly with crayfish CPUE for some species, particularly smallmouth bass, largemouth bass, rock bass and Johnny darter. Moreover, both fish body size and lake abiotic variables explained variation in TP, while their effects on benthivory were small. 5. Rusty crayfish abundance explained relatively little of the overall variation in fish benthivory and TP. Although rusty crayfish appear to have strong effects on abundances of benthic macroinvertebrates, energy flow pathways and trophic niches of lentic fishes were not strongly influenced by invasive rusty crayfish.     

Nonconsumptive predator effects modify crayfish‐induced bioturbation as mediated by limb loss: Field and mesocosm experiments

1. We addressed the implications of limb loss and regeneration for multispecies interactions and their impacts on ecosystem engineering in freshwater stream environments.
2. We included regenerative and nonregenerative crayfish as well as fish predators in a 2 × 2 factorial design to assess the effects on water turbidity of interactions between crayfish ecosystem engineers differing in regenerative status and their fish predators.
3. We demonstrated that crayfish limb loss and predation risks lead to more turbidity in field and mesocosm conditions. Moreover, ongoing regeneration of crayfish increased turbidity, while fish presence seemed to hinder crayfish turbidity‐inducing behaviors (such as tail‐flipping and burrowing) in the mesocosm experiment.
4. We confirmed that greater numbers of crayfish produce a greater amount of turbidity in situ in streams.
5. Although mechanical burrowing crayfish capacities may depend on crayfish burrowing classification (primary, secondary, or tertiary), our work emphasizes the implication for turbidity levels of crayfish autotomy in freshwater streams.

     

A study of the occurrence and distribution of bdellovibrios in estuarine sediment over an annual cycle

The recovery of bdellovibrios from estuarine sediments over an annual cycle was studied. Greater numbers of the predators were recovered in sediment than in the water column. Increases in the number of bdellovibrios recovered from sediment over various periods of time suggest that multiplication of the predators occurred. Sediment was observed to be an important ecosystem for the survival of bdellovibrios in the winter months. As has been observed in water, the number of bdellovibrios in sediment fluctuated, with seasonal and temperature changes declining to very low numbers during the winter months. In the colder months, low numbers of the predators appeared to winter-over in sediment, with greater numbers of the organisms being recovered from deeper sediment. As the water temperature warmed in the spring, increases in the number of bdellovibrios occurred first in sediment and subsequently in water. This increase of bdellovibrios in sediment may have resulted in the shedding of the organisms into the water column where their numbers subsequently increased. Population fluctuations of bdellovibrios were similar in both water and sediment. Although the temperature may account for much of the observed fluctuation in the number of bdellovibrios, other factors, including salinity and the number of host bacteria, may also play a major role. The number of bdellovibrios recovered from sediment correlated positively with the water temperature, and negatively with the water salinity and the number of bacterial colony-forming units from sediment. The results of this study revealed the significance of sediment to the seasonal cycle, survival, and growth of the bdellovibrios in an estuarine environment.     

Contrasting effects of an invasive crayfish (Procambarus clarkii) on two temperate stream communities

1. The effects of omnivorous exotic species on native communities are often difficult to predict because of the broad diets and behavioural flexibility of the omnivore, and the diverse abiotic and biotic characteristics of invaded systems. We investigated experimentally the effects of a gradient of density of the introduced, omnivorous red swamp crayfish Procambarus clarkii (Decapoda: Cambaridae) on two stream communities in southern California, U.S.A. 2. The Ventura River is a clear, flowing stream with a cobble substratum, with abundant algae but low densities of large invertebrates, small herbivores and snails. The Santa Ynez River at the time of the study consisted of a series of drying pools underlain by sand, with abundant charophytes, large predatory invertebrates and herbivores, including snails. 3. In the Ventura River, periphyton biomass and inorganic sediment decreased with increasing crayfish abundance, but in the Santa Ynez River, periphyton and sediment were unrelated to crayfish densities. 4. In the Ventura River, the biomass and density of all benthic invertebrates combined, chironomids, micropredators, the meiofauna (chydorid cladocerans, copepods and ostracods), and specific predatory and herbivorous taxa, as well as taxon richness, were negatively related to crayfish density. In the Santa Ynez River, the biomass and average body size of benthic invertebrates, predatory invertebrates, herbivores and chironomids, but not total invertebrate density or taxon richness, were negatively related to crayfish density. 5. Fewer large predatory invertebrates and snails (Physella gyrina) in both streams, and baetid mayflies in the Ventura River, were visible at night in channels where crayfish were abundant. Snails responded to crayfish by moving above the water line in the Santa Ynez River, but not in the Ventura River. 6. We suggest that the same omnivore had different effects on these neighbouring streams because of crayfish predation on large invertebrates in the Santa Ynez River and the scarcity of such prey in the Ventura River, leading to increased crayfish grazing on periphyton, and reductions in periphyton‐associated invertebrates, in the Ventura River.     

Intertidal community structure differs significantly between substrates dominated by native eelgrass (<Emphasis Type="Italic">Zostera marina</Emphasis> L.) and adjacent to the introduced oyster <Emphasis Type="Italic">Crassostrea gigas</Emphasis> (Thunberg) in British Columbia,Canada

Eelgrass beds represent important habitats for marine organisms, but are in decline in many coastal areas around the world. On Cortes Island, British Columbia, Canada, oysters coexist regionally with native eelgrass (Zostera marina L.), but eelgrass is typically absent directly seaward of oyster beds (the “below-oyster cobble zone”). We compared assemblage structure of nekton (fish and swimming macroinvertebrates) and epibenthos (macroinvertebrates and macroalgae) between eelgrass bed and below-oyster habitats. We sampled the intertidal zone on Cortes Island at low tide using two methods: quadrats to enumerate epibenthic macroinvertebrates and macroalgae, and beach seines to enumerate fish and swimming macroinvertebrates. Using multivariate analysis of similarity (ANOSIM), we found that the structure of nektonic and epibenthic assemblages associated with below-oyster cobble zones were significantly different from those in eelgrass-beds. Univariate measures showed that nektonic species richness and abundance were significantly higher in eelgrass beds than in below-oyster cobble habitat, whereas epibenthic species richness and abundance were significantly higher in below-oyster habitat. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: J. Trexler     

Influence of habitat dynamics on the distribution and abundance of the federally threatened Santa Ana Sucker, <Emphasis Type="Italic">Catostomus santaanae</Emphasis>, in the Santa Ana River

Habitat degradation affects native stream fish populations worldwide. We examined the impact of fluctuation in environmental variables on the population dynamics of the federally threatened Santa Ana sucker, Catostomus santaanae, in the Santa Ana River, California through: 1) annual quantitative surveys of C. santaanae abundance and habitat at three 100-m sites between 2001 and 2008 and 2) annual surveys of habitat composition within a 30-km stretch of the Santa Ana River between 2006 and 2008. We used Akaike’s Information Criterion (AIC) to evaluate competing models that used environmental variables to explain variation in C. santaanae abundance among sites and years. The most plausible model identified a positive relationship between C. santaanae abundance and both the amount of coarse substrate (i.e., gravel and cobble) and rate of discharge among site-years. Surveys at the 30-km scale indicated that the prevalence of coarse substrate declined in a downstream direction in each year and that the total amount of this habitat type varied annually. Specifically, cobble/gravel habitat was mostly confined to the upstream 4 km, 9.6 km, and 5.1 km of the survey area in 2006, 2007, and 2008, respectively. Fine sediment comprised the bulk of downstream habitat every year. This large-scale flux in the distribution of coarse sediment likely has a large effect on the population dynamics of C. santaanae in the Santa Ana River. Our results underscore the need to maintain and enhance suitable C. santaanae habitat to ensure its long-term persistence in the Santa Ana River.     

Iron and copper in Plagioscion squamosissimus (Piscis: Sciaenidae) from the Orinoco river, Venezuela

Bauxite explotation of the Orinoco River in recent years is an important source of heavy metals discharge in the ecosystem, changing the natural biochemical flow of these elements and their concentrations in water, sediment and organisms. Iron and copper concentrations were measured in the fish Plagioscion squamosissimus in the Orinoco river, by sampling the fish population for three months (September-November 1998) in the main channel of the middle Orinoco (07 degrees 38' 21.2" N; 66 degrees 19' 10.9" W) and in Castillero lagoon (7 degrees 39' 09" N; 66 degrees 09' 00" W) with 2 and 4 cm mesh sizes. The internal organs of 30 fishes per month and site were stove-dried at 80 degrees C, pulverized and dried in disecator for 30 min to use as indicators with the acid digestion method for predicting the effect of heavy metals. We found relatively high values of iron and copper concentrations in fishes of the lagoon, and high seasonal variations in the iron concentration.     

Disruption of seasonality in growth hormone-transgenic coho salmon (Oncorhynchus kisutch) and the role of cholecystokinin in seasonal feeding behavior

Seasonal variation in daily food intake is a well-documented phenomenon in many organisms including wild-type coho salmon where the appetite is noticeably reduced during periods of decreased day length and low water temperature. This reduction may in part be explained by altered production of cholecystokinin (CCK) and growth hormone (GH). CCK is a hormone produced in the brain and gut that mediates a feeling of satiety and thus has an inhibitory effect on food intake and foraging behaviour. Growth hormone (GH) enhances feeding behaviour and consequently growth, but its production is reduced during winter. The objectives of this study were: first, to compare the seasonal feeding behaviour of wild and GH-transgenic coho salmon; second, to determine the behavioural effect of blocking the action of CCK (by using devazepide) on the seasonal food intake; and third, to measure CCK expression in brain and gut tissues between the two genotypes across seasons. We found that, in contrast to wild salmon, food intake in transgenic salmon was not reduced during winter indicating that seasonal control of appetite regulation has been disrupted by constitutive production of GH in transgenic animals. Blocking of CCK increased food intake in both genotypes in all seasons. The increase was stronger in wild genotypes than transgenic fish; however blocking CCK in wild-type fish in winter did not elevate appetites to levels observed in the summer. The response to devazepide was generally faster in transgenic than in wild salmon with more rapid effects observed during summer than during winter, possibly due to a higher temperature in summer. Overall, a seasonal effect on CCK mRNA levels was observed in telencephalon with levels during winter being higher compared to the summer in wild fish, but with no seasonal effect in transgenic fish. No differences in seasonal CCK expression were found in hypothalamus. Higher levels of CCK were detected in the gut of both genotypes in winter compared to summer. Thus, CCK appears to mediate food intake among seasons in both wild-type and GH-transgenic salmon, and an altered CCK regulation may be responsible at least in part for the seasonal regulation of food intake.     

SEASONAL PERIODICITY OF CHRYSOPHYCEAE AND SYNUROPHYCEAE IN A SMALL NEW ENGLAND LAKE: IMPLICATIONS FOR PALEOLIMNOLOGICAL RESEARCH1

The seasonal periodicity of taxa of Chrysophyceae and Synurophyceae from a small New England lake is described for the period September 1983 through June 1988. We found 51 taxa, including 29 that accounted for over 10% of the total in at least one collection. The taxa were fitted into one of five seasonal patterns. Patterns I and II represented taxa restricted to warm (pattern I) or cold (pattern II) months, respectively. Pattern HI represented organisms that began growth in the summer, persisted through autumn and disappeared with the onset of an ice cover. Pattern IV was an extension of pattern III, in which the taxon remained in the plankton throughout the winter and disappeared soon after ice out. Species without a clear seasonal pattern were grouped as pattern V. The seasonal periodicity of the flora, as examined with ordination analyses, was found to remain remarkedly similar during the 58–month study. Except for episodes of low pH during spring snow melt and unseasonally warm or cold weather, sample scores followed a fairly consistent pattern along the first and second primary axes. Water temperature, specific conductance, and pH were important variables that controlled changes in the species composition during the course of a given year. The flora was used to develop an inference model for water temperature. According to the analyses, the remains of a surface sediment sample represented a flora that grew primarily during the late autumn period at 7.6 ° C. Ways in which seasonal data could be utilized to improve paleolimnological inference work are discussed.     

Are fish populations in temperate streams affected by crayfish? – A field survey and prospects

Synopsis Fish populations may be affected by predation and competition from various types of organisms, among which crayfish have been suggested as important actors. We here present results from stream surveys, suggesting that neither native noble, Astacus astacus, nor introduced signal crayfish, Pacifastacus leniusculus, necessarily affect fish population densities in temperate stream communities. Comparisons of fish densities within stream sites between years with absence and presence of crayfish showed no effect of either crayfish species. A further analysis of changes in fish densities between periods without and with crayfish in low, intermediate and high densities revealed that crayfish density did neither have an effect on fish densities. Our study is one of exceptionally few that consider the above aspects in long-term perspectives in natural systems, and we discuss that previously reported divergent results of crayfish effects on fish may be highly dependent on specific species and methods used, and that the effects of crayfish on fish populations deserve further attention to enable reliable predictions of community processes in streams.     

Regime shifts in shallow lakes: the importance of seasonal fish migration

Shallow eutrophic lakes commonly exist in two alternative stable states: a clear-water state and a turbid water state. A number of mechanisms, including both abiotic and biotic processes, buffer the respective states against changes, whereas other mechanisms likely drive transitions between states. Our earlier research shows that a large proportion of zooplanktivorous fish populations in shallow lakes undertake seasonal migrations where they leave the lake during winter and migrate back to the lake in spring. Based on our past research, we propose a number of scenarios of how feedback processes between the individual and ecosystem levels may affect stability of alternative stable states in shallow lakes when mediated by fish migration. Migration effects on shallow lakes result from processes at different scales, from the individual to the ecosystem. Our earlier research has shown that ecosystem properties, including piscivore abundance and zooplankton productivity, affect the individual state of zooplanktivorous fish, such as growth rate or condition. Individual state, in turn, affects the relative proportion and timing of migrating zooplanktivorous fish. This change, in turn, may stabilize states or cause runaway processes that eventually lead to state shifts. Consequently, such knowledge of processes coupled to seasonal migration of planktivorous fish should increase our understanding of shallow lake dynamics.     

Effects of stream predator richness on the prey community and ecosystem attributes

It is important to understand the role that different predators can have to be able to predict how changes in the predator assemblage may affect the prey community and ecosystem attributes. We tested the effects of different stream predators on macroinvertebrates and ecosystem attributes, in terms of benthic algal biomass and accumulation of detritus, in artificial stream channels. Predator richness was manipulated from zero to three predators, using two fish and one crayfish species, while density was kept equal (n = 6) in all treatments with predators. Predators differed in their foraging strategies (benthic vs. drift feeding fish and omnivorous crayfish) but had overlapping food preferences. We found effects of both predator species richness and identity, but the direction of effects differed depending on the response variable. While there was no effect on macroinvertebrate biomass, diversity of predatory macroinvertebrates decreased with increasing predator species richness, which suggests complementarity between predators for this functional feeding group. Moreover, the accumulation of detritus was affected by both predator species richness and predator identity. Increasing predator species richness decreased detritus accumulation and presence of the benthic fish resulted in the lowest amounts of detritus. Predator identity (the benthic fish), but not predator species richness had a positive effect on benthic algal biomass. Furthermore, the results indicate indirect negative effects between the two ecosystem attributes, with a negative correlation between the amount of detritus and algal biomass. Hence, interactions between different predators directly affected stream community structure, while predator identity had the strongest impact on ecosystem attributes.     

Trace elements in the Upper Fly River, Papua New Guinea

SUMMARY. Measurements were made of calcium, magnesium and eight trace elements (Cu, Zn, Fe, Mn, Ni, Pb, Cd, Co) in waters, sediment and biota at eight sites in the Upper Fly River, Papua New Guinea. Determinations from streams draining an area rich in copper ore showed relatively low values for soluble copper and this was attributed to the small amount of ore exposed to weathering, heavy rainfall and high pH of water associated with the limestone bedrock. More iron was transported in a soluble form down a headwater tributary (Alice River) than in the Fly River but higher iron concentrations were associated with particulate matter in the Fly than in the Alice. The geochemistry of the catchment and the distance from the headwaters were important influences on the amount of particulate iron and its distribution between soluble and particulate forms.
Levels of metals in the biota generally reflected the background concentrations of metals encountered at each site. Concentrations of Cu, Cd, and Zn were lower in starved invertebrates compared with unstarved individuals but no consistent changes were observed in Fe or Mn content.     

Habitat, predation, and coexistence between invasive and native crayfishes: prioritizing lakes for invasion prevention

Conditions fostering coexistence of native species with invasive species have received little attention in invasion biology, especially for closely related invasive and native species. We used long-term datasets on multiple replicate invasions to define conditions under which native virile crayfish (Orconectes virilis) can coexist with invasive rusty crayfish (O. rusticus). We examined multiple drivers of coexistence involving habitat use and predation at between-lake and within-lake scales to derive predictions that could guide prioritization efforts to prevent future introductions of rusty crayfish and mitigate impacts of existing invasions. Lakes in which native species persisted for many years had significantly less cobble and sand habitats, and significantly more vegetated habitats compared to lakes from which native crayfish have been displaced. In the presence of rusty crayfish, virile crayfish alter their habitat use to vegetated habitats relative to habitat use in the absence of rusty crayfish. Such vegetated habitats had greater plant standing crop, plant species richness, and sediment percent organic matter compared to vegetated sites occupied by rusty crayfish. Our results suggest that low abundance of cobble habitat and altered habitat use allows native crayfish to coexist with the rusty crayfish invader. At the within-lake scale, virile crayfish persist by escaping predation in the vegetated habitats, despite suboptimal abiotic conditions. By understanding these abiotic and biotic conditions that promote coexistence, managers could enhance native crayfish persistence by targeting high cobble lakes for efforts to prevent the introduction of invasive crayfish, and targeting vegetated habitats for protection in already invaded lakes.     

Climate-Induced Changes in Spring Snowmelt Impact Ecosystem Metabolism and Carbon Fluxes in an Alpine Stream Network

Although stream ecosystems are recognized as an important component of the global carbon cycle, the impacts of climate-induced hydrological extremes on carbon fluxes in stream networks remain unclear. Using continuous measurements of ecosystem metabolism, we report on the effects of changes in snowmelt hydrology during the anomalously warm winter 2013/2014 on gross primary production (GPP), ecosystem respiration (ER), and net ecosystem production (NEP) in an Alpine stream network. We estimated ecosystem metabolism across 12 study reaches of the 254 km² subalpine Ybbs River Network (YRN), Austria, for 18 months. During spring snowmelt, GPP peaked in 10 of our 12 study reaches, which appeared to be driven by PAR and catchment area. In contrast, the winter precipitation shift from snow to rain following the low-snow winter in 2013/2014 increased spring ER in upper elevation catchments, causing spring NEP to shift from autotrophy to heterotrophy. Our findings suggest that the YRN transitioned from a transient sink to a source of carbon dioxide (CO₂) in spring as snowmelt hydrology differed following the high-snow versus low-snow winter. This shift toward increased heterotrophy during spring snowmelt following a warm winter has potential consequences for annual ecosystem metabolism, as spring GPP contributed on average 33% to annual GPP fluxes compared to spring ER, which averaged 21% of annual ER fluxes. We propose that Alpine headwaters will emit more within-stream respiratory CO₂ to the atmosphere while providing less autochthonous organic energy to downstream ecosystems as the climate gets warmer.