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Spatial and temporal heterogeneity interact to make the foraging rates of individuals more variable than expected from models assuming that spatial and temporal dimensions of habitat conditions operate independently. For example, trophic resource pulses produce temporal patches of prey superabundance, yet little is known about how spatial heterogeneity in habitat conditions mediates the ability of consumers to exploit these high-quality foraging opportunities. We studied how spatial variation in water temperature regulates the potential for juvenile coho salmon to exploit a seasonal pulsed subsidy of eggs produced by anadromous sockeye salmon. Streams within the Wood River watershed, southwestern Alaska, USA, ranged in mean summer temperature from 3.6[degrees] to 14.5[degrees]C. Growth of juvenile coho prior to the arrival of the seasonal egg subsidy was positively related to water temperature among streams. An in situ experiment combined with field samples of diets revealed a size threshold for egg consumption; only individuals longer than ~70 mm could consume eggs due to gape limitation of smaller individuals. A bioenergetics simulation demonstrated that water temperature regulated whether age-0 coho salmon could grow large enough to exceed the size threshold for egg consumption. Coho salmon that consumed eggs had energy rations that were five times higher than fish that did not consume eggs, resulting in a positive feedback of water temperature on their integrated seasonal growth. Across this landscape, heterogeneity in water temperature mediates individual- and population-level responses to seasonally available resource pulses. Our study illustrates that ecological mechanisms, such as size-based foraging asymmetries, can magnify the effects of climate change compared to predictions based on physiology alone. Key words: bioenergetics; climate change, coho salmon; heterogeneity; Oncorhynchus spp.; opportunistic foraging; resource pulse; size structure; sockeye salmon; spatial subsidy; water temperature; Wood River, southwestern Alaska, USA.

Understanding how abundance regulates the effects of organisms on their ecosystems remains a critical goal of ecology, especially for understanding inter-ecosystem transfers of energy and nutrients. Here we examined how territoriality and nest-digging by anadromous salmon mediate trophic subsidies to stream fishes. Salmon eggs become available for consumption primarily by the digging of salmon that superimpose their nests on previous nests. An individual-based model of spawning salmon predicted that territoriality and habitat saturation produce a nonlinear effect of salmon density on numbers of available eggs to resident predators. Field studies in Alaskan streams found that higher densities of salmon produce disproportionately more eggs in stream drift and in diets of resident fishes (Arctic grayling and rainbow trout). Bioenergetics model simulations indicated that these subsidies produce substantially enhanced growth rates of trout. These results demonstrate that small changes in salmon abundance can drive large changes in subsidies to stream food webs. Thus, the ecological consequences of population declines of keystone species, such as salmon, will be exacerbated when behavior generates nonlinear impacts. Key words: density dependence; ecosystem-based management; eggs; fisheries; food web; foraging; functional extinction; grayling; marine-derived nutrients; salmon; superimposition; trout.

Organisms can control movements of nutrients and matter by physically modifying habitat. We examined how an ecosystem engineer, sockeye salmon (Oncorhynchus nerka), influences seasonal fluxes of sediments, nitrogen (N), and phosphorus (P) in streams of southwestern Alaska. The purpose of this study was to investigate whether salmon act as net importers or net exporters of matter and nutrients from streams and how these roles change as a function of salmon population density. We measured discharge and concentrations of suspended sediments and total N and P every 7-14 days for up to four summers in 10 streams spanning a gradient in salmon densities. We statistically allocated whole-season fluxes to salmon activities, such as excretion and bioturbation, and to export by hydrologic discharge. In addition, we used counts of spawning salmon to estimate nutrient and matter imports by salmon to streams. Large seasonal pulses of suspended sediments, P, and N were associated with salmon spawning activities, often increasing export an order of magnitude higher than during pre-salmon levels. Years and streams with more salmon had significantly higher levels of export of sediments and nutrients. In addition, years with higher precipitation had higher background export of P and N. Salmon exported an average of the equivalent of 189%, 60%, and 55% of total matter, P, and N that salmon imported in their bodies. The relative magnitude of export varied; salmon exported more than their bodies imported in 80%, 20%, and 16% across all streams and years for sediments, P, and N, respectively. A bioassay experiment indicated that the P exported by salmon is directly available for use by primary producers in the downstream lake. These results demonstrate that salmon not only move nutrients upstream on large spatial scales via their migration from the ocean and subsequent death, but also redistribute matter and nutrients on finer spatial scales through their spawning activities. Key words: Alaska, USA; bioturbation; disturbance; ecosystem engineer; excretion; marine-derived nutrients; mass balance; Oncorhynchus nerka; redd; sockeye salmon; subsidy.

For decades ecologists have recognized the potential importance of marine-derived nutrients (MDN) deposited in freshwater ecosystems by spawning anadromous salmon. Previous studies have shown that some MDN are retained in freshwater ecosystems. A popular hypothesis linking MDN to salmon population productivity posits that MDN provided by post-spawning mortality of salmon are critical for salmon population dynamics because they enhance prey populations in the freshwater ecosystems used as nursery habitats. We tested this hypothesis by reconstructing historical sockeye salmon populations for the last 300 years in Bristol Bay, Alaska. Stable nitrogen isotope chronologies in lake sediments and sockeye catch and escapement histories show that commercial fisheries intercepted about two-thirds of MDN bound for freshwater spawning grounds since about 1900. Reconstruction of lake algal production using fossil pigments shows that this loss of MDN has reduced lake algal productivity to about one-third of its level before commercial fishing. However, contrary to expectation, recent sockeye population sizes (sum of spawning escapement and fishery catch) in the last century were equivalent to those before the advent of commercial fishing. These data demonstrate that the MDN subsidy is important for the productivity of coastal lakes but that some sockeye salmon populations are limited by other features of ecosystems such as the amount of suitable spawning habitat. Key words: biogeochemistry; marine-derived nutrients; mixing model; [sup.15] N; paleolimnology; primary production; salmon; salmon enhancement; sediments; stable isotopes; sustainability.

Detecting and forecasting the effects of changing climate on natural and exploited populations represent a major challenge to ecologists and resource managers. These efforts are complicated by underlying density-dependent processes and the differential responses of predators and their prey to changing climate. We explored the effects of density-dependence and changing climate on growth of juvenile sockeye salmon and the densities of their zooplankton prey in the Wood River system of southwestern Alaska. We fit dynamic time-series models to data collected between 1962 and 2002 describing growth of juvenile sockeye, timing of spring ice breakup, and summer zooplankton densities. The timing of spring breakup has moved about seven days earlier now than it was in the early 1960s. Our analyses suggest that most of this shift has been a response to the warm phase of the Pacific Decadal Oscillation that persisted from the mid-1970s to the late 1990s. This progression toward earlier spring breakup dates was associated with warmer summer water temperatures and increased zooplankton (especially Daphnia) densities, which translated into increased sockeye growth during their first year of life. The number of spawning adults that produced each year class of sockeye had a strong negative effect on juvenile sockeye growth rates, so that the size of the density-dependent effect was, on average, twice as large as the effect of spring breakup date. These results highlight the complexity of ecological responses to changing climate and suggest that climate warming may enhance growing conditions for juvenile salmonids in large lakes of Alaska. Key words: Bristol Bay, southwest Alaska, USA; climate change; cryophenology; fish; global change; global warming; ice phenology; Pacific Decadal Oscillation; sockeye salmon; Wood River, southwest Alaska. USA; zooplankton.

The largest uncertainty in forecasting the effects of climate change on ecosystems is in understanding how it will affect the nature of interactions among species. Climate change may have unexpected consequences because different species show unique responses to changes in environmental temperatures. Here we show that increasingly warmer springs since 1962 have disrupted the trophic linkages between phytoplankton and zooplankton in a large temperate lake because of differing sensitivity to vernal warming. The timing of thermal stratification and the spring diatom bloom have advanced by more than 20 days during this time period. A long-term decline in Daphnia populations, the keystone herbivore, is associated with an expanding temporal mismatch with the spring diatom bloom and may have severe consequences for resource flow to upper trophic levels. Key words: Daphnia pulicaria; diatoms; energy flux; food web; Keratella cochlearis; match; mismatch; phenology; plankton; timing.

Diel vertical migration (DVM) is a widespread phenomenon in aquatic organisms, yet the adaptive significance of this behavior is still unclear. In particular, diel vertical migration by juvenile sockeye salmon (Oncorhynchus nerka) has received considerable attention. We studied how changes in the light environment affect juvenile sockeye DVM in Alaskan lakes through changes in foraging rates and predation risk. Using hydroacoustics to track temporal changes in fish distribution, we found clear patterns of DVM and a strong, significant correlation between the mean depth of the sockeye and the amount of light at the lake surface. However, we could not detect diel changes in the depth distribution of fish large enough to be sockeye predators. Given a lack of diel vertical migration in the zooplankton community, it appears that juvenile sockeye were not simply tracking their food supply. Calculations of the in situ light experienced by individual juvenile sockeye suggested that they migrate to maintain a constant light environment. This light environment allowed the sockeye to exploit an antipredation window whereby they could forage on zooplankton while reducing the odds of visual detection by their predators. Furthermore, this antipredation window was continuous in early summer, but was split into two discrete time periods during the crepuscular hours in late summer. These data support the hypothesis that changes in habitat use by juvenile sockeye salmon reflect a dynamic strategy to minimize the ratio of predation risk to foraging gain that changes dramatically over the course of diel cycles in pelagic ecosystems. Key words: antipredation window; diel vertical migration; feeding rate vs. predation risk; foraging gain; kokanee; light environment; Oncorhynchus nerka; predation risk; sockeye salmon.

Increased exposure to ultraviolet-B (UV-B) radiation has been proposed as a major environmental stressor leading to global amphibian declines. Prior experimental evidence from the U.S. Pacific Northwest (PNW) indicating the acute embryonic sensitivity of at least four amphibian species to UV-B has been central to the literature about amphibian decline. However, these results have not been expanded to address population-scale effects and natural landscape variation in UV-B transparency of water at amphibian breeding sites: both necessary links to assess the importance of UV-B for amphibian declines. We quantified the UV-B transparency of 136 potential amphibian breeding sites to establish the pattern of UV-B exposure across two montane regions in the PNW. Our data suggest that 85% of sites are naturally protected by dissolved organic matter in pond water, and that only a fraction of breeding sites are expected to experience UV-B intensities exceeding levels associated with elevated egg mortality. Thus, the spectral characteristics of natural waters likely mediate the physiological effects of UV-B on amphibian eggs in all but the clearest waters. These data imply that UV-B is unlikely to cause broad amphibian declines across the landscape of the American Northwest. Key words: Ambystoma gracile; Ambystoma macrodactylum; amphibian declines; Bufo boreas; Cascades frog; dissolved organic matter, DOM; long-toed salamander; morthwestern salamander; Pacific Northwest; Rana cascadae; UV-B radiation; western toad.

Fishes are important pools of nitrogen (N) and phosphorus (P), and they have important direct and indirect effects on cycling of these potentially limiting nutrients in lakes. We used a model of fish bioenergetics coupled to a mass balance model of fish N and P budgets to investigate the stoichiometric interactions among fishes, their food, and the nutrient recycling that results from this predator-prey interaction. We tested the hypothesis that the N:P ratios in fish excretion (supply ratios) are high but potentially variable because of fishes' high P requirement and the likelihood of P limitation of fish growth. A survey of 186 cases representing 18 fish species demonstrated that limitation of fish growth due to availability of P in food was exceedingly rare in natural ecosystems. Fish growth rates were energy limited in almost all systems. Fish grew at an average of 26% of the maximum potential rate determined from bioenergetics. As a result, the predicted N:P supply ratio from fish excretion was low and relatively constant (the median mass ratio for piscivores was 6.1, and 13.1 for all other fishes). Excretion rates for N and P by fishes were predictable from body size, food nutrient concentration, and environmental temperature. Nutrient recycling by fishes will tend to alleviate P limitation of phytoplankton growth, but the importance of this effect will be directly proportional to the magnitude of nutrient regeneration rates from fishes relative to other sources available to phytoplankton. Key words: bioenergetics; fishes; limnology; nitrogen; nutrient cycling; phosphorus; stoichiometa'; supply ratio., INTRODUCTION Integrating the approaches of population/community ecology and ecosystem ecology remains an important but difficult goal of current ecological research (Jones and Lawton 1995). Studies of ecosystems have traditionally evaluated [...]

A Planktivore Lake with small herbivorous zooplankton and a Piscivore Lake with large grazers were monitored for 2 yr and then enriched for 2 yr with inorganic phosphorus and nitrogen. An unenriched lake with an undisturbed fish community served as a reference ecosystem. Added nutrients increased biomass of both phytoplankton and zooplankton. In the Planktivore Lake, phytoplankton were stimulated more than zooplankton. This situation was reversed in the Piscivore Lake. Time series models predicting chlorophyll from P input rate and crustacean length fit well for total chlorophyll and the edible fraction (35 [µmeter]). A 1-mm change in mean crustacean length had about the same effect on chlorophyll as a decrease in P input rate of 1 mg[center dot][m.sup.-3][center dot][d.sup.-1]. There was no evidence of interaction between grazer and P input effects. Although effects of grazers and P input are equal and additive, the range of mean crustacean lengths among lakes is ≅1 mm, while the range of P input rates is substantially >1 mg[center dot][m.sup.-3][center dot][d.sup.-1]. Therefore, the potential for increasing eutrophication by P input exceeds the potential for controlling eutrophication by food web manipulation. Nevertheless, biomanipulation can reduce chlorophyll concentration at P input rates of 1.5 mg[center dot][m.sup.-3][center dot][d.sup.-1] or more; these rates characterize a large fraction of the world's stratified lakes. Key words: Bayesian analysis; biomanipulation; cascading trophic interactions; chlorophyll; ecosystem; grazing; lake; phosphorus; time series analysis., INTRODUCTION Ecologists have adopted both deterministic and stochastic views of trophic cascades. Most analyses have emphasized deterministic, reciprocal responses of alternating trophic levels (e.g., increased predation decreases herbivory and leads [...]

Responses of zooplankton, pelagic primary producers, planktonic bacteria, and [CO.sub.2] exchange with the atmosphere were measured in four lakes with contrasting food webs under a range of nutrient enrichments during a seven-year period. Prior to enrichment, food webs were manipulated to create contrasts between piscivore dominance and planktivore dominance. Nutrient enrichments of inorganic nitrogen and phosphorus exhibited ratios of N:P [is greater than] 17:1, by atoms, to maintain P limitation. An unmanipulated reference lake, Paul Lake, revealed baseline variability but showed no trends that could confound the interpretation of changes in the nearby manipulated lakes. Herbivorous zooplankton of West Long Lake (piscivorous fishes) were large-bodied Daphnia spp., in contrast to the small-bodied grazers that predominated in Peter Lake (planktivorous fishes). At comparable levels of nutrient enrichment, Peter Lake's areal chlorophyll and areal primary production rates exceeded those of West Long Lake by factors of approximately three and six, respectively. Grazers suppressed pelagic primary producers in West Long Lake, relative to Peter Lake, even when nutrient input rates were so high that soluble reactive phosphorus accumulated in the epilimnions of both lakes during summer. Peter Lake also had higher bacterial production (but not biomass) than West Long Lake. Hydrologic changes that accompanied manipulation of East Long Lake caused concentrations of colored dissolved organic carbon to increase, leading to considerable variability in fish and zooplankton populations. Both trophic cascades and water color appeared to inhibit the response of primary producers to nutrients in East Long Lake. Carbon dioxide was discharged to the atmosphere by Paul Lake in all years and by the other lakes prior to nutrient addition. During nutrient addition, only Peter Lake consistently absorbed [CO.sub.2] from the atmosphere, due to high rates of carbon fixation by primary producers. In contrast, [CO.sub.2] concentrations of West Long Lake shifted to near-atmospheric levels, and net fluxes were near zero, while East Long Lake continued to discharge [CO.sub.2] to the atmosphere. Key words: bacteria; carbon dioxide flux; chlorophyll; lake ecosystem; fish; food web; nutrient effects; Paul Lake, Peter Lake, East and West Long Lakes; phosphorus input; trophic cascades; zooplankton.

KATHRYN L. COTTINGHAM [1,3] DANIEL E. SCHINDLER [1,2] Abstract. Sensitivity, the magnitude of change following perturbation, and return rate, the rate of recovery, are two key components of ecological stability. [...]

Interest is growing in developing conservation strategies to restore and maintain coral reef ecosystems in the face of mounting anthropogenic stressors, particularly climate warming and associated mass bleaching events. One such approach is to propagate coral colonies ex situ and transplant them to degraded reef areas to augment habitat for reef-dependent fauna, prevent colonization from spatial competitors, and enhance coral reproductive output. In addition to such "demographic restoration" efforts, manipulating the thermal tolerance of outplanted colonies through assisted relocation, selective breeding, or genetic engineering is being considered for enhancing rates of evolutionary adaptation to warming. Although research into such "assisted evolution" strategies has been growing, their expected performance remains unclear. We evaluated the potential outcomes of demographic restoration and assisted evolution in climate change scenarios using an eco-evolutionary simulation model. We found that supplementing reefs with pre-existing genotypes (demographic restoration) offers little climate resilience benefits unless input levels are large and maintained for centuries. Supplementation with thermally resistant colonies was successful at improving coral cover at lower input levels, but only if maintained for at least a century. Overall, we found that, although demographic restoration and assisted evolution have the potential to improve long-term coral cover, both approaches had a limited impact in preventing severe declines under climate change scenarios. Conversely, with sufficient natural genetic variance and time, corals could readily adapt to warming temperatures, suggesting that restoration approaches focused on building genetic variance may outperform those based solely on introducing heat-tolerant genotypes., (© 2022 The Authors. Ecological Applications published by Wiley Periodicals LLC on behalf of The Ecological Society of America.)

Global environmental change is challenging species with novel conditions, such that demographic and evolutionary trajectories of populations are often shaped by the exchange of organisms and alleles across landscapes. Current ecological theory predicts that random networks with dispersal shortcuts connecting distant sites can promote persistence when there is no capacity for evolution. Here, we show with an eco-evolutionary model that dispersal shortcuts across environmental gradients instead hinder persistence for populations that can evolve because long-distance migrants bring extreme trait values that are often maladaptive, short-circuiting the adaptive response of populations to directional change. Our results demonstrate that incorporating evolution and environmental heterogeneity fundamentally alters theoretical predictions regarding persistence in ecological networks., (© 2021 The Authors. Ecology published by Wiley Periodicals LLC on behalf of Ecological Society of America.)

The deuterium ratio ( 2 H/ 1 H) in tissue is often used to estimate terrestrial subsidies to aquatic consumers because of strongly differentiated values between terrestrial and aquatic primary producers. However, quantitative deuterium-based analyses of terrestrial resource assimilation are highly dependent on several poorly defined assumptions. We explored the sensitivity of these estimates to assumptions regarding environmental water contributions to consumer deuterium content (ω) and algal photosynthetic hydrogen discrimination (ε H ). We also tested whether 13 C/ 12 C and 2 H/ 1 H-based estimates of terrestrial resource assimilation give similar outcomes. The average of the 12 experiments that have directly estimated proportional contributions of environmental water to consumer tissue 2 H/ 1 H was 0.27 ± 0.11 (mean ± SD), with similar values for invertebrates and fish. Conversely, of the 28 field studies that have used 2 H/ 1 H to characterize aquatic food webs, all but one assume a value that is less than our current best estimate, usually substantially less. A reanalysis of the raw data from four recent case studies indicates the calculated terrestrial contribution to aquatic consumers is extremely sensitive to this assumption. When the authors' original assumptions were used (i.e., ω = 0.16 ± 0.05), the estimated proportional contribution of terrestrial resources to aquatic consumers (θ T ) averaged 29 ± 17%, and when ω = 0.27 was used the average estimated assimilation of allochthonous resources was ≈0.00. A compilation of published photosynthetic hydrogen discrimination values for microalgae averaged ε H  = -150 ± 27‰ (SD, n = 99), and a sensitivity analysis showed the outcomes of these calculations were also strongly influenced by uncertainty in ε H . There was no statistical association between 13 C/ 12 C and 2 H/ 1 H-based estimates of terrestrial subsidies (r = -0.12, n = 274). This analysis indicates that the assumptions in deuterium-based estimates of terrestrial resource assimilation are highly influential but poorly constrained; therefore, the impact of these assumptions on calculated outputs must be carefully assessed and thoroughly reported. Due to the highly uncertain assumptions inherent in deuterium-based analyses, we urge much more caution when using this approach to estimate terrestrial subsidies to consumers in aquatic ecosystems., (© 2018 by the Ecological Society of America.)

Production patterns of highly mobile species, such as anadromous fish, often exhibit high spatial and temporal heterogeneity across landscapes. Such variability is often asynchronous in time among habitats, which stabilizes production at aggregate scales of complexity. Reconstructing production patterns explicitly in space and time across multiple scales, however, remains difficult but is important for prioritizing habitat conservation. This is especially true for fishes inhabiting river basins due to long-range dispersal, high mortality at early life stages, complex population structure and elusive life history variation. We develop a new approach for mapping production patterns of Pacific salmon across a large river basin by integrating otolith microchemistry and dendritic isoscape models. The geographically continuous Bayesian assignment framework presented here yielded high accuracies (>90%) and relatively high precisions (precisions <4%; i.e., assignment areas of <530 river km of the 13 100 km total river length) when used to determine the natal source of known-origin juvenile Chinook salmon captured throughout the study region. Integrating these methods enabled us to base estimates of provenance and habitat use of individuals on a per location basis using strontium isotopic data throughout the continuous spatial domain of a river network. Such a framework provides substantial advantages over the more common nominal approach to employing otolith microchemistry to reconstruct movement patterns of fish. In doing so, we reconstructed the spatial production patterns of adult Chinook salmon returning to a large watershed in Bristol Bay, Alaska and illustrate the power of such an approach to conservation efforts., (© 2016 by the Ecological Society of America.)

Predator-prey interactions shape ecosystem structure and function, potentially limiting the productivity of valuable species. Simultaneously, stochastic environmental forcing affects species productivity, often through unknown mechanisms. The interacting effects of trophic and environmental conditions complicate management of exploited ecosystems and have motivated calls for more holistic management via ecosystem-based approaches, yet the limitations to these approaches are not widely appreciated. The Chignik salmon fishery in Alaska is managed to achieve maximum sustainable yield for sockeye salmon, though research suggests that predation by less economically valuable, and thus not targeted, coho salmon during juvenile rearing limits the productivity of sockeye salmon. We examined the relationship between historical sockeye salmon recruitment and coho salmon abundance observed in the Chignik system and could not detect a clear effect of coho salmon abundance on sockeye salmon productivity, given existing data. Using simulation models, we examined the probability of detecting a known predation effect on sockeye salmon recruitment in the presence of observation error in coho salmon abundance and stochasticity in sockeye salmon recruitment. Increased recruitment stochasticity reduced the ability to detect predator effects in recruitment, an effect further strengthened when low frequency environmental variation was added to the system. Further, increased observation error biased estimates of predator effects towards zero. Thus, in systems with high observation error on predator abundances, estimates of predation effects will be substantially weaker than true effects. We examined the effects of stochasticity on the ability of an adaptive management program to learn about ecosystem structure and detect an effect of management actions intended to release a prey species from its predators. Simulation models revealed that even under scenarios of large predation effects on sockeye salmon, stochastic recruitment masked detection of an effect of increased coho salmon harvest for nearly a decade. These results highlight the challenges inherent in ecosystem-based management of predator-prey systems due to mismatched timescales of ecosystem dynamics and the willingness of stakeholders to risk losses in order to test uncertain hypotheses. It is critical for stakeholders considering EBFM (ecosystem-based fisheries management) and adaptive management strategies to be aware of the potential timelines of perceiving ecosystem change., (© 2016 by the Ecological Society of America.)

Time can be a limiting constraint for consumers, particularly when resource phenology mediates foraging opportunity. Though a large body of research has explored how resource phenology influences trophic interactions, this work has focused on the topics of trophic mismatch or predator swamping, which typically occur over short periods, at small spatial extents or coarse resolutions. In contrast many consumers integrate across landscape heterogeneity in resource phenology, moving to track ephemeral food sources that propagate across space as resource waves. Here we provide a conceptual framework to advance the study of phenological diversity and resource waves. We define resource waves, review evidence of their importance in recent case studies, and demonstrate their broader ecological significance with a simulation model. We found that consumers ranging from fig wasps (Chalcidoidea) to grizzly bears (Ursus arctos) exploit resource waves, integrating across phenological diversity to make resource aggregates available for much longer than their component parts. In model simulations, phenological diversity was often more important to consumer energy gain than resource abundance per se. Current ecosystem-based management assumes that species abundance mediates the strength of trophic interactions. Our results challenge this assumption and highlight new opportunities for conservation and management. Resource waves are an emergent property of consumer-resource interactions and are broadly significant in ecology and conservation.

Abstract. Physical attributes of rivers control the quantity and quality of energy sources available to consumers, but it remains untested whether geomorphic conditions of whole watersheds affect the assimilation of different resources by stream organisms. We compared the fatty acid (FA) compositions of two invertebrate taxa (caddisflies, mayflies) collected from 16 streams in southwest Alaska, USA, to assess how assimilation of terrestrial organic matter (OM) and algae varied across a landscape gradient in watershed features. We found relatively higher assimilation of algae in high-gradient streams compared with low-gradient streams, and the opposite pattern for assimilation of terrestrial OM and microbes. The strength of these patterns was more pronounced for caddisflies than mayflies. Invertebrates from low-gradient watersheds had FA markers unique to methane-oxidizing bacteria and sulfate-reducing microbes, indicating a contribution of anaerobic pathways to primary consumers. Diversity of FA composition was highest in watersheds of intermediate slopes that contain both significant terrestrial inputs as well as high algal biomass. By controlling the accumulation rate and processing of terrestrial OM, watershed features influence the energetic base of food webs in boreal streams.

Few studies have considered the management implications of mortality to target fish stocks caused by non-retention in commercial harvest gear (escape mortality). We demonstrate the magnitude of this previously unquantified source of mortality and its implications for the population dynamics of exploited stocks, biological metrics, stock productivity, and optimal management. Non-retention in commercial gillnet fisheries for Pacific salmon (Oncorhynchus spp.) is common and often leads to delayed mortality in spawning populations. This represents losses, not only to fishery harvest, but also in future recruitment to exploited stocks. We estimated incidence of non-retention in Alaskan gillnet fisheries for sockeye salmon (O. nerka) and found disentanglement injuries to be extensive and highly variable between years. Injuries related to non-retention were noted in all spawning populations, and incidence of injury ranged from 6% to 44% of escaped salmon across nine river systems over five years. We also demonstrate that non-retention rates strongly correlate with fishing effort. We applied maximum likelihood and Bayesian approaches to stock-recruitment analyses, discounting estimates of spawning salmon to account for fishery-related mortality in escaped fish. Discounting spawning stock estimates as a function of annual fishing effort improved model fits to historical stock-recruitment data in most modeled systems. This suggests the productivity of exploited stocks has been systematically underestimated. It also suggests that indices of fishing effort may be used to predict escape mortality and correct for losses. Our results illustrate how explicitly accounting for collateral effects of fishery extraction may improve estimates of productivity and better inform management metrics derived from estimates of stock-recruitment analyses.

Global change is leading to shifts in the seasonal timing of growth and maturation for primary producers. Remote sensing is increasingly used to measure the timing of primary production in both aquatic and terrestrial ecosystems, but there is often a poor correlation between these results and direct observations of life-history responses of individual species. One explanation may be that, in addition to phenological shifts, global change is also causing shifts in community composition among species with different seasonal timing of growth and maturation. We quantified how shifts in species phenology and in community composition translated into phenological change in a diverse phytoplankton community from 1962 to 2000. During this time, the aggregate community spring-summer phytoplankton peak has shifted 63 days earlier. The mean taxon shift was only 3 days earlier, and shifts in taxa phenology explained only 40% of the observed community phenological shift. The remaining community shift was attributed to dominant early-season taxa increasing in abundance while a dominant late-season taxon decreased in abundance. In diverse producer communities experiencing multiple stressors, changes in species composition must be considered to fully understand and predict shifts in the seasonal timing of primary production.

Vertical heterogeneity in the physical characteristics of lakes and oceans is ecologically salient and exploited by a wide range of taxa through diel vertical migration to enhance their growth and survival. Whether analogous behaviors exploit horizontal habitat heterogeneity in streams is largely unknown. We investigated fish movement behavior at daily timescales to explore how individuals integrated across spatial variation in food abundance and water temperature. Juvenile coho salmon made feeding forays into cold habitats with abundant food, and then moved long distances (350-1300 m) to warmer habitats that accelerated their metabolism and increased their assimilative capacity. This behavioral thermoregulation enabled fish to mitigate trade-offs between trophic and thermal resources by exploiting thermal heterogeneity. Fish that exploited thermal heterogeneity grew at substantially faster rates than did individuals that assumed other behaviors. Our results provide empirical support for the importance of thermal diversity in lotic systems, and emphasize the importance of considering interactions between animal behavior and habitat heterogeneity when managing and restoring ecosystems.

Habitat heterogeneity can generate intraspecific diversity through local adaptation of populations. While it is becoming increasingly clear that population diversity can increase stability in species abundance, less is known about how population diversity can benefit consumers that can integrate across population diversity in their prey. Here we demonstrate cascading effects of thermal heterogeneity on trout-salmon interactions in streams where rainbow trout rely heavily on the seasonal availability of anadromous salmon eggs. Water temperature in an Alaskan stream varied spatially from 5 degrees C to 17.5 degrees C, and spawning sockeye salmon showed population differentiation associated with this thermal heterogeneity. Individuals that spawned early in cool regions of the 5 km long stream were genetically differentiated from those spawning in warmer regions later in the season. Sockeye salmon spawning generates a pulsed resource subsidy that supports the majority of seasonal growth in stream-dwelling rainbow trout. The spatial and temporal structuring of sockeye salmon spawn timing in our focal stream extended the duration of the pulsed subsidy compared to a thermally homogeneous stream with a single population of salmon. Further, rainbow trout adopted movement strategies that exploited the multiple pulses of egg subsidies in the thermally heterogeneous stream. Fish that moved to track the resource pulse grew at rates about 2.5 times higher than those that remained stationary or trout in the reference stream with a single seasonal pulse of eggs. Our results demonstrate that habitat heterogeneity can have important effects on the population diversity of dominant species, and in turn, influence their value to species that prey upon them. Therefore, habitat homogenization may have farther-reaching ecological effects than previously considered.

In coastal areas of the North Pacific Ocean, annual returns of spawning salmon provide a substantial influx of nutrients and organic matter to streams and are generally believed to enhance the productivity of recipient ecosystems. Loss of this subsidy from areas with diminished salmon runs has been hypothesized to limit ecosystem productivity in juvenile salmon rearing habitats (lakes and streams), thereby reinforcing population declines. Using five to seven years of data from an Alaskan stream supporting moderate salmon densities, we show that salmon predictably increased stream water nutrient concentrations, which were on average 190% (nitrogen) and 390% (phosphorus) pre-salmon values, and that primary producers incorporated some of these nutrients into tissues. However, benthic algal biomass declined by an order of magnitude despite increased nutrients. We also measured changes in stream ecosystem metabolic properties, including gross primary productivity (GPP) and ecosystem respiration (ER), from three salmon streams by analyzing diel measurements of oxygen concentrations and stable isotopic ratios (delta O-O2) within a Bayesian statistical model of oxygen dynamics. Our results do not support a shift toward higher primary productivity with the return of salmon, as is expected from a nutrient fertilization mechanism. Rather, net ecosystem metabolism switched from approximately net autotrophic (GPP > or = ER) to a strongly net heterotrophic state (GPP << ER) in response to bioturbation of benthic habitats by salmon. Following the seasonal arrival of salmon, GPP declined to <12% of pre-salmon rates, while ER increased by over threefold. Metabolism by live salmon could not account for the observed increase in ER early in the salmon run, suggesting salmon nutrients and disturbance enhanced in situ heterotrophic respiration. Salmon also changed the physical properties of the stream, increasing air-water gas exchange by nearly 10-fold during peak spawning. We suggest that management efforts to restore salmon ecosystems should consider effects on ecosystem metabolic properties and how salmon disturbance affects the incorporation of marine-derived nutrients into food webs.