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Resource polymorphism has been suggested to be a platform for speciation. In some cases resource polymorphism depends on phenotypic plasticity but in other cases on genetic differences between morphotypes, which in turn has been suggested to be the ongoing development of a species pair. Here we study environmentally induced morphological differences in two age classes of Arctic char (Salvelinus alpinus) influencing char performance and diet in relation to resource availability. We found that structurally complex habitats with relatively lower zooplankton densities gave rise to individuals with a deeper body, and a downward positioned tip of the snout compared with individuals from structurally simple habitats with relatively higher zooplankton densities for both age classes. Environment also had an effect on foraging efficiency on zooplankton, with fish from structurally simple habitats had a higher foraging rate than fish from structurally complex habitats. Diet analyses showed that resource use in char mainly depends on the relative abundance of different resources. Therefore, to gain further understanding of resource polymorphism we suggest that future studies must include population dynamic feedbacks by the resources on the consumers.

Temperature impact all biota and ecosystems through its direct effect on the vital rates of primary producers and consumers. Still, how these changes in rates translates through ecosystem feed backs to the population level of top consumers are poorly understood. Here, we studied effects of temperature increase (+ 3 ̊C during ice free season) on fish population dynamics in a replicated large scale pond ecosystem experiment over 3 years. Increased temperature had no significant effect on whole ecosystem gross primary production while top down effects of warming on intermediate consumers changed from negative to positive due to negative effects of warming on fish population abundance. Total fish density and biomass and abundance of both mature and old fish decreased with warming, while proportion of young fish increased, with warming. The effects of warming on fish population demographics were likely due to that temperature increased cohort competition and fish energy requirements relative to resource production. Our results suggest that global warming may increase competition, favor young individuals and overall decrease fish population densities and biomass.

Climate change is predicted to increase water temperatures and export of terrestrial dissolved matter (TDOM) to aquatic ecosystems with concomitant effects on ecosystem productivity and production of consumers at the top of the food web. Here we study how increased level of TDOM and temperature alone or in combination affect benthic and pelagic primary production and biomass production of fish in replicated large scale outdoor pond experiment. We increased temperature (+3 ̊C) and TDOM concentrations (×2.5) using natural high TDOM water from a small creek and used naturally reproducing three-spine sticklebacks (Gasterosteus aculeatus) populations as model fish species. Gross primary production (GPP) was similar to ambient control conditions in all treatments although increased TDOM concentrations deceased benthic but stimulated pelagic GPP, presumably as a result of decreased light availability and increased nutrient availability, respectively. The production of young-of-the-year (YOY), older, and total fish population were negatively affected by warming, whereas increased TDOM caused decreased YOY and total fish production. Effect of temperature on fish production were likely a result of increased metabolic cost in relation to resource production and intake rates whereas the negative effect of TDOM on fish production was likely a result of decreasing benthic resource production. However, the combined effect on fish production of warming and TDOM was only intermediate compared to individual treatments. Our study suggest climate change will lead to decreased fish production and that the effect may be most pronounced in clear systems dominated by benthic GPP .

Water temperature and export of terrestrial dissolved organic carbon (DOC) to recipient aquatic ecosystems have strong impacts on ecosystem productivity. Increased DOC concentration causing brownification of water is argued to reduce fish production by decreasing light availability for autotrophic production. On the other hand, higher temperatures have been shown to correlate positively with fish recruitment. This increase has been related to increased growth rates and survival of young-of-the-year (YOY) fish with warming. However, whether or not increased temperature results in higher growth depends on resource availability, suggesting a relationship between individual gain and recruitment mediated via the interplay between resource production and temperature. In a replicated, large scale experimental pond ecosystem, we tested the effects of increased temperature (+3oC) and terrestrial DOC concentrations (+4 mg/l) on recruitment (size, density and biomass) of three-spined sticklebacks over one growth season. Gross primary production (GPP) was similar between treatments, whereas zooplankton and benthic invertebrate biomass were negatively affected by increased temperature and if any higher at increased DOC levels. Increased temperature had no effect on individual size but negative effects on body condition and recruitment of YOY sticklebacks, while increased DOC concentration had no effect on recruitment. No positive effect of temperature increase on GPP and decreased resource levels in combination with higher metabolic costs, are suggested to increase starvation mortality and to be the main mechanism behind observed negative effects of warming on recruitment. Based on our results, we suggest that climate change may, counter intuitively, have negative effects on fish recruitment due to decreased carrying capacity of nursery habitats as a consequence of increased energy requirements in juvenile fish in relation to the net effects on resource production and ecosystem

Flows of energy and matter across habitat boundaries can be major determinants of the functioning of recipient ecosystems. It is currently debated whether terrestrial dissolved organic matter (tDOM) is a resource subsidy or a resource subtraction in recipient lakes. In a long-term field experiment, pelagic phosphorus concentration and aquatic primary production increased with increasing tDOM input, suggesting that tDOM acted primarily as a direct nutrient subsidy. Piecewise structural equation modeling supports, however, an approximately equally important role for a second mechanism: colored tDOM acted also as a resource subtraction by shading benthic algae, preventing them from intercepting nutrients released across the sediment-water interface. Inhibition of benthic algae by colored tDOM thus indirectly promoted pelagic algae and whole-ecosystem primary production. We conclude that cross-ecosystem terrestrial DOM inputs can modify light and nutrient flows between aquatic habitats and alter the outcome of resource competition between benthic and pelagic producers. These results are particularly relevant for shallow northern lakes, which are facing increased tDOM runoff with climate change.

The effects of temperature on top consumer populations in subarctic lake communities were studied by contrasting two lake pairs in different climate regimes: one pair on the low alpine tundra and one pair in the subalpine birch forest. We measured zooplankton and macroinvertebrate biomasses over the season and estimated population density and size structure of the top consumer Arctic char (Salvelinus alpinus). Furthermore, we modelled char growth using literature data on temperature dependent search rate, handling time and metabolic demands. The forest lakes were warmer than the tundra lakes. Char in the forest lakes were larger and had a higher individual growth compared to char in the tundra lakes, while population density and biomasses of char were not different between the forest and the tundra lakes. There were no differences in macroinvertebrate and zooplankton resource levels available for char between lake pairs. Our modeling of char growth revealed that higher temperature increased growth of char at the observed resource densities, suggesting that the higher temperature in the forest lakes was primarily the cause of the higher growth of char in these lakes. We suggest that cannibalism in char may regulate char recruitment and thereby population density and biomass of char leading to effects of increasing temperature on consumer biomass and consumer individual growth different from what is expected in pure consumer-resource systems. Our results emphasize the importance of feedbacks within ecosystems when addressing effects of climate change and increasing temperature on lake communities.

Climate change results in changes in the geographical distribution of species. Species invasion success into a new area is dependent both on the dispersal ability of species as well as the strength and identity of biotic interactions between resident and invading species. Coexistence in intraguild predation (IGP) systems depends on the relative strength of predation and competition interactions which in turn are temperature dependent. We investigated the effects of introducing an intermediate consumer, ninespine stickleback (Pungitius pungitius), into allopatric populations of the omnivorous top predator Arctic char (Salvelinus alpinus). Introductions were performed in lakes with different climate regimes, two tundra lakes and two forest lakes that differed in average summer temperatures with 1.4 ± 0.5 ºC (average ± 1SD). We found that sticklebacks were able to invade and increase in density in both tundra and forest lakes. Sticklebacks had strong negative effects on resource densities which also was reflected in a decreased growth of small char. Increasing stickleback density had a positive effect on growth of large adults and on the maximum size of char. We conclude that stickleback presence is not limited by biological interactions in these systems but rather by dispersion ability. We suggest that the size dependency in the response of char to the invasion of sticklebacks is fundamental for the successful invasion of sticklebacks, and that size dependent interactions including cannibalism play important roles for coexistence in natural IGP-systems.

Planktivorous fish affect the species composition, morphology and size of their zooplankton prey species by size selective predation. Microfossils of cladocerans are preserved in the lake sediment and hence cladoceran fossil characteristics may be used to reconstruct historical variation of planktivory in lakes. In lakes dominated by fish that are not obligate planktivores (e.g. perch, Perca fluviatilis), the individual size determines to large extent the type of resources consumed. Therefore the size distribution and density of such fish populations determine overall planktivory impact in these lakes.In a Northern Swedish lake over a 9-year period, we compared the carapace length and mucro length of the cladoceran Bosmina spp. from the contemporary samples with the sediment archive. We estimated the temporal variation in overall planktivory pressure on Bosmina by calculating a population planktivore capacity (PPC) measure on zooplankton using the population density and size structure of the perch population together with size dependent attack rate functions on Bosmina.Our results show that temporal changes in body size characteristics of Bosmina in the contemporary and fossil remains were strongly correlated. Contemporary lake samples and fossil remains of Bosmina showed similar responses in body size characteristics to changes in estimated PPC. Initially, when PPC was low in the lake, both size of carapace and mucro were large. As PPC increased due to strong recruitment of perch during the middle of the study period both carapace and mucro size were small, to increase again towards the end of the study when PPC decreased again. Small perch, mainly young-of-the-year (YOY) perch contribution to PPC was high, more than 80% of to the total PPC in most years. This suggests that the contribution of the smallest size classes of fish to PPC can be high and needs to be taken into account when trying to relate changes in zooplankton community characteristics to variation in

Based on morphological measurements on Bosmina in a sediment core from a boreal lake we estimated 60 years back in time historical changes in Planktivore Community Capacity (PCC), a recent derived approach to estimate past planktivore pressure in lakes. We then validated the PCC approach with available information on changes in the lake fish community based on historical knowledge and more recent fish population studies in the lake. Based on available data on fish density, size structure and size dependent attack rate functions on zooplankton we calculated a measure of planktivore pressure in the lake, the Planktivore Population Capacity (PPC). The reconstructed PCC development over the time from Bosmina remains in the sediment core correlated well with PPC estimates (r2 = 0.89 p<0.05) as well as qualitatively with historical information on the development of fish community in the lake. The results from our study suggest that the derived PCC function based on morphological measurements on Bosmina remains in sediments can successfully be used to track historical changes in planktivore communities in lakes.