Productivity denotes the rate of biomass synthesis in ecosystems and is a fundamental characteristic that frames ecosystem function and management. Limitation of productivity by nutrient availability is an established paradigm for lake ecosystems. Here, we assess the relevance of this paradigm for a majority of the world's small, nutrient-poor lakes, with different concentrations of coloured organic matter. By comparing small unproductive lakes along a water colour gradient, we show that coloured terrestrial organic matter controls the key process for new biomass synthesis (the benthic primary production) through its effects on light attenuation. We also show that this translates into effects on production and biomass of higher trophic levels (benthic invertebrates and fish). These results are inconsistent with the idea that nutrient supply primarily controls lake productivity, and we propose that a large share of the world's unproductive lakes, within natural variations of organic carbon and nutrient input, are limited by light and not by nutrients. We anticipate that our result will have implications for understanding lake ecosystem function and responses to environmental change. Catchment export of coloured organic matter is sensitive to short-term natural variability and long-term, large-scale changes, driven by climate and different anthropogenic influences. Consequently, changes in terrestrial carbon cycling will have pronounced effects on most lake ecosystems by mediating changes in light climate and productivity of lakes.

We investigated productivity at the basal trophic level in 15 unproductive lakes in a gradient ranging from clear-water to brown-water (humic) lakes in northern Sweden. Primary production and bacterial production in benthic and pelagic habitats were measured to estimate the variation in energy mobilization from external energy sources (primary production plus bacterial production on allochthonous organic carbon) along the gradient. Clear-water lakes were dominated by autotrophic energy mobilization in the benthic habitat, whereas humic lakes were dominated by heterotrophic energy mobilization in the pelagic habitat. Whole-lake (benthic + pelagic) energy mobilization was negatively correlated to the light-extinction coefficient, which was determined by colored terrestrial organic matter in the lake water. Thus, variation in the concentration of terrestrial organic matter and its light-absorbing characteristics exerts strong control on the magnitude, as well as on the processes and pathways, of energy mobilization in unproductive lakes. We suggest that unproductive lakes in general are sensitive to input of terrestrial organic matter because of its effects on basal energy mobilization in both benthic and pelagic habitats.

Trophic cascades have been a central paradigm in explaining the structure of ecological communities but have been demonstrated mainly through comparative studies or experimental manipulations. In contrast, evidence for shifts in trophic cascades caused by intrinsically driven population dynamics is meager. By using empirical data of a cannibalistic fish population covering a 10-year period and a size-structured population model, we show the occurrence of a dynamic trophic cascade in a lake ecosystem, in which the community over time alternates between two different configurations. The intrinsically driven change in the size structure of the fish population from a dominance of stunted individuals to a dominance of gigantic cannibals among adult individuals is the driving force behind distinct abundance switches observed in zooplankton and phytoplankton. The presence of the phase with gigantic cannibals depends critically on the energy they extract from their victims, allowing strong reproduction for a number of years. C ommunity-wide trophic cascades, the propagation of indirect mutualism between nonadjacent trophic levels in food webs, have been suggested to occur more frequently in aquatic than in terrestrial systems (1-4). This suggestion is based on the arguments that terrestrial systems have a higher heterogeneity, a higher overall species diversity, and more chemical defenses among primary producers (higher plants vs. algae; ref. 1). Although the validity of all of these arguments has been questioned (2, 4), undoubtedly the empirical evidence for community-wide trophic cascades is, at present, substantially stronger for aquatic than for terrestrial systems. The empirical evidence largely stems from two sources: comparative studies of different systems in which the trophic structure, such as food chain length, differs (5-6) and experimental manipulations of top predators, either intentional or unintentional (species invasions; refs. 2, 3, and 7-9). In contrast, there is hardly any evidence for dynamic trophic cascades, in which major shifts in overall food-web structure are intrinsically driven by population dynamics. Only a few studies on recruitment variation have considered this aspect (10-12).Cannibalism has been shown to have a number of diverse effects on population dynamics and persistence (13-17). These effects, among others, include a potential for alternative stable states (18,19) and chaotic dynamics (20). Although many cannibalistic models ignore the energy that cannibals gain from cannibalism and, thus, are essentially ''infanticide'' models (13,15,20), some theoretical studies have shown that the energy extracted by the cannibal may have substantial impact on population persistence and individual life history (14, 16). Empirical evidence also suggests such an effect of energy extraction on population dynamics because of increased growth and thereby increased per-capita fecundity of cannibals (17).Here we present strong evidence for a whole-lake trophic cascade that is dynamic and intrinsically ...

We quantified the utilization of terrestrial organic matter (OM) in the food web of a humic lake by analyzing the metabolism and the consumers' stable isotopic (C, H, N) composition in benthic and pelagic habitats. Terrestrial OM inputs (3 g C m 22 d 21 ) to the lake greatly exceeded autochthonous OM production (3 mg C m 22 d 21 ) in the lake. Heterotrophic bacterial growth (19 mg C m 22 d 21 ) and community respiration (115 mg C m 22 d 21 ) were high relative to algal photosynthesis and were predominantly (. 85%) supported by terrestrial OM in both habitats. Consequently, terrestrial OM fueled most (85%) of the total production at the base of the lake's food web (i.e., the sum of primary and bacterial production). Despite the uncertainties of quantitatively estimating resource use based on stable isotopes, terrestrial OM clearly also supported around half the zooplankton (47%), macrozoobenthos (63%), and fish (57%) biomass. These results indicate that, although rates of terrestrial OM inputs were around three orders of magnitude greater than that of autochthonous OM production, the use of the two resources by higher trophic levels was roughly equal. The disproportionally low reliance on terrestrial OM at higher trophic levels, compared with its high rates of input and high support of basic biomass production in the lake, suggests that autochthonous resources could not be completely replaced by terrestrial resources and indicates an upper limit to terrestrial support of lake food webs.

Abstract. Recent size-structured cannibalistic models point to the importance of the energy gain by cannibals and also show that this gain may result in the emergence of giant individuals. We use a combination of a 10-year field study of a perch (Perca fluviatilis) population and quantitative within-season modeling of individual and population-level dynamics to investigate which mechanisms are most likely to drive the dynamics of the studied perch population. We focused on three main aspects to explain observed discrepancies between earlier model predictions and data: (1) introduction of more than one shared resource between cannibals and victims, (2) whether or not several victim age cohorts are necessary to allow giant growth, and (3) the intensity of inter-cohort competition between young-of-the-year (YOY) perch and 1-yr-old perch.At the start of the study period, the perch population was dominated by ''stunted'' perch individuals, and recruitment of perch to an age of 1-yr-old was negligible. Following a major death in adult perch, strong recruitments of perch to 1-yr-old were thereafter observed for a number of years. As 1-yr-olds these successful recruiters subsequently starved to death due to competition with the new YOY. The few surviving adult perch accelerated substantially in growth and became ''giants.'' At the end of the study period, the perch population moved back to the situation with stunted individuals. There was a high agreement between observed diets of cannibalistic perch and those predicted by the model for both the stunted and the giant phases. Analyses of growth rates showed that cannibalistic perch could become giants on a diet of YOY perch only, but that a supplement with the second shared resource (macroinvertebrates) was needed to reach the observed sizes. Modeling of growth and diet in the giant phase showed an exploitative competitive effect of YOY perch on 1-yr-old perch, but a restriction in habitat use of 1-yr-old perch had to be assumed to yield the observed growth rate and diet. The resource dynamics of zooplankton and macroinvertebrates were both accurately predicted by the model. Also, YOY perch mortality was accurately predicted and, furthermore, suggested that one of the trawling methods used may underestimate the number of YOY perch when they increase in size.We conclude that the presence of a second shared resource and the restricted habitat use and absence of cannibalistic consumption by 1-yr-old perch individuals are two important mechanisms to explain the discrepancy between model predictions and data. Our results also point to the fact that that the dynamics observed may be explained by complex dynamics not involving the presence of a giant and dwarf cycle.

This study quantified new biomass production of algae and bacteria in both benthic and pelagic habitats of clear-water lakes to contrast how carbon from the atmosphere and terrestrial sources regulates whole-lake metabolism. We studied four small unproductive lakes in subarctic northern Sweden during one summer season. The production of new biomass in both benthic and pelagic habitats was calculated as the sum of autotrophic production by algae and heterotrophic production by bacteria using allochthonous organic carbon (OC). Whole-lake production of new biomass was dominated by the benthic habitat (86% +/- 4% [mean +/- SD]) and by primary production (77% +/- 9%). Still, heterotrophic bacteria fueled by allochthonous OC constituted a significant portion of the new biomass production in both benthic (19% +/- 11%) and pelagic habitats (51% +/- 24%). In addition, overall net production (primary production minus respiration) was close to zero in the benthic habitats but highly negative (-163 +/- 81 mg C x m(-2) x d(-1)) in pelagic regions of all lakes. We conclude (1) that allochthonous OC supported a significant part of total production of new biomass in both pelagic and benthic habitats, (2) that benthic habitats dominated the whole-lake production of new biomass, and (3) that respiration and net CO2 production dominated the carbon flux of the pelagic habitats and biomass production dominated the benthic carbon flux. Taken together, these findings suggest that previous investigations have greatly underestimated the productivity of clear-water lakes when benthic autotrophic production and metabolism of allochthonous OC have not been measured.

In many cannibalistic populations, cannibals share resources with their victims, leading to a size‐dependent mixture of cannibalistic and competitive interactions. We analyze the impacts of such interactions on the population dynamics of Eurasian perch (Perca fluviatilis) by considering effects of intercohort competition, habitat heterogeneity, habitat selection, and energy gain made by cannibals. Over a six‐year period, we measured mortality and recruitment patterns, individual growth, body condition, resource levels, diets, and habitat use as functions of density for an allopatric perch population in a low‐productivity lake. During the course of the study, two major die‐offs took place, selectively affecting larger cannibalistic individuals, followed by several years of successful recruitment of young fish. Habitat use of perch ≥2 yr old was density dependent, and these fish used only the inshore region at low densities. The appearance of young fish followed the die‐offs of cannibalistic perch and their subsequent absence from the offshore area, both of which decreased cannibalism on pelagic recruits. Whereas die‐offs of larger perch could not be related to competition with young‐of‐the‐year (YOY) perch, evidence for a competitive impact of YOY perch on Age‐1 perch was present due to substantial food overlap. A strong depression in pelagic zooplankton was observed during summer in years with strong recruitment, which resulted in reduced consumption of zooplankton, slow growth, and reduced body condition in Age‐1 perch and suggested high mortality of Age‐1 perch in autumn. Age‐1 perch did not appear to profit substantially from cannibalism on YOY perch because of the short time period that they could efficiently prey on YOY perch. The few larger perch that survived the die‐offs gained substantial energy from cannibalism in years with strong recruitments, which increased both growth rates and per capita fecundity. Size‐dependent intercohort competition may have strong impacts on cannibal–victim interactions when victims share resources with cannibals. Furthermore, habitat heterogeneity, combined with habitat selection, may limit the extent to which cannibals have a stabilizing effect on population dynamics. Finally, the energy gained by cannibals may have important consequences on population dynamics as this energy is allocated into new recruits.

In size-structured populations, prey have the potential to restrict the recruitment of their predator by decreasing the growth rate of young predators through interspecific competition, a situation referred to as a ''competitive juvenile bottleneck.'' Two mechanisms have been advanced by which a decrease in growth may increase the mortality of young predators: (1) increased time during which juvenile predators are susceptible to gape-limited predation and (2) increased susceptibility to starvation. To study the effects of competition from a prey fish (roach, Rutilus rutilus) on the recruitment of a piscivorous fish (perch, Perca fluviatilis), roach were introduced in two of four small adjacent unproductive lakes inhabited by natural populations of perch. We thereafter studied the diet, growth, and survival of the new-born cohorts of perch during a 13-mo period.Growth and survival of larval perch were not affected by roach whereas during the latter part of the growing season, growth and condition of young-of-the-year (YOY) perch were negatively affected by roach. The growth retardation of YOY perch in roach-treatment lakes coincided with a shift in diet from free-swimming zooplankton to benthic cladocerans and chironomids, which in turn was related to a stronger decline in pelagic zooplankton resources in these lakes. The small size and poor condition of YOY perch in the roachtreatment lakes resulted in almost all these perch dying from starvation during the winter and subsequent spring. There was a tendency for there to be higher densities of YOY perch in the roach-treatment lakes, possibly due to a strong die-off of adult perch in these lakes that in turn may have reduced cannibalism in these lakes. The results from an enclosure experiment on intra-cohort competition among YOY perch, however, suggested that the discrepancy in growth of YOY perch between the lakes was mainly due to direct competition from roach for the zooplankton resource.Temporal variation in competition intensity and in duration of periods for growth and energy gain (e.g., summer) vs. periods of only energy loss (e.g., winter) are suggested to have major effects on juvenile predator growth and subsequent recruitment to the adult stage through competitive juvenile bottlenecks, and thus to affect overall community dynamics. The relative importance of size-dependent winter starvation mortality compared to mortality caused by gape-limited predators as a mortality agent on juveniles is hypothesized to increase with latitude. Whole-lake experiments lend themselves both to testing the importance in natural systems of mechanisms previously identified at small spatial and short temporal scales, and also to identifying important mechanisms that cannot be studied at smaller scales.