The Logic and Realism of the Hypothesis of Exploitation Ecosystems

Hypotheses on trophic dynamics in terrestrial ecosystems fall into two major categories: those in which plants are assumed to be invulnerable to their consumers and those in which the build-up of plant biomass is assumed to require top-down control of folivores. The hypothesis of exploitation ecosystems (EEH) belongs to the latter category and focuses particularly on the consequences of the high energetic costs of maintenance of endotherms. Carnivorous endotherms require relatively high prey densities in order to break even. Moreover, they are dependent on folivorous prey during the limiting season, at least at high latitudes. The endotherm branch of the grazing web is thus predicted to collapse from three-link trophic dynamics (carnivores --> folivores --> plants --> inorganic resources) to two-link dynamics (folivores --> plants --> inorganic resources) along gradients of decreasing primary productivity. Consequently, the vegetation of cold and unproductive areas is predicted to be under intense winter grazing pressure, which prevents the accumulation of aboveground plant biomass and excludes erect woody plants. In the most extreme habitats (e.g., polar deserts and their high alpine counterparts), even folivorous endotherms are predicted to be absent, and the scanty vegetation is predicted to be structured by preemptive competition. Within temperature-determined productivity gradients, EEH is corroborated by biomass patterns, by patterns in the structure and dynamics of carnivore, folivore, and plant communities, and by experimental results. The general idea of top-down trophic dynamics is supported for other autotroph-based systems, too, but the relevance and sufficiency of the energy constraint in explaining patterns in trophic dynamics appears to be variable. Moreover, critical empirical evidence for or against the capacity of folivorous insects to regulate plant biomass has not yet been obtained. Another open question is the ability of boreal and temperate browsers, evolved in productive environments with intense predation pressure and abundance of forage, to prevent the regeneration of the least palatable tree species. There are, thus, many open questions waiting to be answered and many exciting experiments waiting to be conducted.     

Strength of a Trophic Cascade Between an Apex Predator,Mammalian Herbivore and Grasses in a Desert Ecosystem Does Not Vary with Temporal Fluctuations in Primary Productivity

There has long been debate regarding the primacy of bottom-up and top-down effects as factors shaping ecosystems. The exploitation ecosystems hypothesis (EEH) predicts that predators indirectly benefit plants because their top-down effects limit herbivores’ consumption of plants, and that the strength of trophic cascade increases with increasing primary productivity. However, in arid environments, pulses of primary productivity produced by irregular rainfall events could decouple herbivore–plant and predator–prey dynamics if high conversion efficiency from seed biomass to consumers allows the rapid build-up of consumer populations. Here, we test predictions of the EEH in an arid environment. We measured activity/abundances of dingoes, red kangaroos and grasses, and diet of dingoes, in landscapes where dingoes were culled or not culled over 3 years. Dingo activity was correlated with rainfall, and their tracks were less frequent at culled sites. Kangaroo abundance was greater at sites where dingoes were culled and increased with rainfall in the previous 6 months. Grass cover was greater at sites where dingoes were not culled and increased with rainfall in the previous 3 months. During a period of average rainfall, dingoes primarily consumed rodents and increased their consumption of kangaroos during a period of drier conditions. Our results are consistent with the hypothesis that suppression of an apex predator triggers a trophic cascade, but are at odds with the EEH’s prediction that the magnitude of trophic cascades should increase with primary productivity. Our study demonstrates that temporal fluctuations in primary productivity can have effects on biomasses of plants and consumers which are in many ways analogous to those observed along spatial gradients of primary productivity.     

Exploitation ecosystems and trophic cascades in non‐equilibrium systems: pasture – red kangaroo – dingo interactions in arid Australia

The exploitation ecosystems hypothesis (EEH) proposes that 1) plant biomass reflects the primary productivity of an ecosystem modified by the regulating effect of herbivory, and 2) herbivore abundance reflects the productivity of plants modified by the regulating effect of predation. Primary productivity thus determines the number of trophic levels in an ecosystem and the extent to which bottom–up and top–down regulation influence the biomass ratios of adjacent and non‐adjacent trophic levels (i.e. trophic cascading). We constructed an interactive model of plant (pasture), herbivore (red kangaroo Macropus rufus) and predator (dingo Canis lupus dingo), a system in which trophic cascades have been suggested to occur, and used it to test the effects of increasing stochastic variation in primary productivity and dingo culling on predictions of the EEH. The model contained four feedback loops: the predator–herbivore and herbivore–plant feedback loops, and the predator and plant density‐dependent feedback loops. The equilibrium conditions along the primary productivity gradient reproduced the three zones of trophic dynamics predicted by the EEH, plus an additional zone at productivities above which the maximum density of a predator is achieved due to social regulation: that zone is characterized by increasing herbivore density and decreasing plant biomass. Culling dingoes produced trophic cascades that were strongly attenuated at primary productivities below which the maximum density of dingoes was attained. Results were robust to uncertainty in kangaroo off‐take by dingoes and to the efficacy of dingo culling, but prey switching by dingoes from red kangaroos to reptiles would weaken trophic cascades. We conclude that social regulation of carnivores has important implications for expression of the EEH and trophic cascades, and that attenuation of trophic cascades increases with increasing stochasticity in primary productivity. Our model also provides a framework for understanding the conditions in which dingo‐mediated trophic cascades might be expected to occur, and generates testable predictions about the effects of higher dingo densities (e.g. by stopping culling or reintroduction to former range) on kangaroo and pasture dynamics.     

Patterns in the abundance of kangaroo populations in arid Australia are consistent with the exploitation ecosystems hypothesis

The exploitation ecosystems hypothesis (EEH) makes predictions about trophic interactions along gradients of primary productivity. The EEH has been shown to apply to a wide range of terrestrial environments but its applicability to arid environments has received little attention. One reason for this is that arid environments may not satisfy the assumptions of the EEH because dearth of water may limit biological activity in both temporal and spatial contexts. The EEH predicts that herbivore biomass should increase linearly with primary productivity in the absence of predators; but when predators are present herbivore biomass will remain relatively constant due to top down regulation. We tested this prediction in an arid environment using rainfall as a proxy of primary productivity and an index of the abundance of the dominant herbivores (kangaroos Macropus spp.). We compared an index of kangaroo abundance at 18 areas situated along a gradient of mean annual rainfall in areas where a top predator (the dingo Canis lupus dingo) was rare and common. We also explored the relationship between the density of artificial water points (AWPs) and kangaroo abundance to investigate if the resource subsidy provided by AWPs allows kangaroos to persist in high numbers. Consistent with the EEH, kangaroo abundance showed a weak relationship with mean annual rainfall in the presence of dingoes but increased with increasing annual rainfall in the absence of dingoes. The density of AWPs was a poor predictor of kangaroo abundance. Our analysis of macro‐ecological patterns suggests that kangaroo populations are primarily top down regulated in the presence of dingoes, but are bottom up regulated in the absence of dingoes. Our findings provide evidence that top down regulation can prevail over bottom up regulation of herbivore populations in arid ecosystems and highlights the usefulness of the EEH as a predictor of macro‐ecological patterns of species abundance.     

Spatial patterns and dynamic responses of arctic food webs corroborate the exploitation ecosystems hypothesis (EEH)

According to the exploitation ecosystems hypothesis (EEH), productive terrestrial ecosystems are characterized by community-level trophic cascades, whereas unproductive ecosystems harbor food-limited grazers, which regulate community-level plant biomass. We tested this hypothesis along arctic-alpine productivity gradients at the Joatka field base, Finnmark, Norway. In unproductive habitats, mammalian predators were absent and plant biomass was constant, whereas herbivore biomass varied, reflecting the productivity of the habitat. In productive habitats, predatory mammals were persistently present and plant biomass varied in space, but herbivore biomass did not. Plant biomass of productive tundra scrublands declined by 40% when vegetation blocks were transferred to predation-free islands. Corresponding transfer to herbivore-free islands triggered an increase in plant biomass. Fertilization of an unproductive tundra heath resulted in a fourfold increase in rodent density and a corresponding increase in winter grazing activity, whereas the total aboveground plant biomass remained unchanged. These results corroborate the predictions of the EEH, implying that the endotherm community and the vegetation of the North European tundra behaves dynamically as if each trophic level consisted of a single population, in spite of local co-occurrence of >20 plant species representing different major taxonomic groups, growth forms, and defensive strategies.     

All wet or dried up? Real differences between aquatic and terrestrial food webs

Ecologists have greatly advanced our understanding of the processes that regulate trophic structure and dynamics in ecosystems. However, the causes of systematic variation among ecosystems remain controversial and poorly elucidated. Contrasts between aquatic and terrestrial ecosystems in particular have inspired much speculation, but only recent empirical quantification. Here, we review evidence for systematic differences in energy flow and biomass partitioning between producers and herbivores, detritus and decomposers, and higher trophic levels. The magnitudes of different trophic pathways vary considerably, with less herbivory, more decomposers and more detrital accumulation on land. Aquatic-terrestrial differences are consistent across the global range of primary productivity, indicating that structural contrasts between the two systems are preserved despite large variation in energy input. We argue that variable selective forces drive differences in plant allocation patterns in aquatic and terrestrial environments that propagate upward to shape food webs. The small size and lack of structural tissues in phytoplankton mean that aquatic primary producers achieve faster growth rates and are more nutritious to heterotrophs than their terrestrial counterparts. Plankton food webs are also strongly size-structured, while size and trophic position are less strongly correlated in most terrestrial (and many benthic) habitats. The available data indicate that contrasts between aquatic and terrestrial food webs are driven primarily by the growth rate, size and nutritional quality of autotrophs. Differences in food-web architecture (food chain length, the prevalence of omnivory, specialization or anti-predator defences) may arise as a consequence of systematic variation in the character of the producer community.     

Non‐consumptive effects of a top‐predator decrease the strength of the trophic cascade in a four‐level terrestrial food web

The fear of predators can strongly impact food web dynamics and ecosystem functioning through effects on herbivores morphology, physiology or behaviour. While non‐consumptive predator effects have been mostly studied in three‐level food chains, we lack evidence for the propagation of non‐consumptive indirect effects of apex predators in four level food‐webs, notably in terrestrial ecosystems. In experimental mesocosms, we manipulated a four‐level food chain including top‐predator cues (snakes), mesopredators (lizards), herbivores (crickets), and primary producers (plants). The strength of the trophic cascade induced by mesopredators through the consumption of herbivores decreased in the presence of top‐predator cues. Specifically, primary production was higher in mesocosms where mesopredators were present relative to mesocosms with herbivores only, and this difference was reduced in presence of top‐predator cues, probably through a trait‐mediated effect on lizard foraging. Our study demonstrates that non‐consumptive effects of predation risk can cascade down to affect both herbivores and plants in a four‐level terrestrial food chain and emphasises the need to quantify the importance of such indirect effects in natural communities.     

The distribution of deer biomass in North America supports the hypothesis of exploitation ecosystems

The hypothesis of exploitation ecosystems (EEH) predicts that, along a productivity gradient in terrestrial environments, predators will regulate herbivores at a relatively constant density whenever primary productivity exceeds 700 g m⁻² y⁻¹; under this threshold, or if predators are absent, forage production determines herbivore density. I tested EEH using the pattern of deer biomass distribution over North America, the dominant family of large herbivores. Deer biomass increased from the High Arctic to the north of the boreal forest and remained in the same range southward within the gray wolf range; for the same latitude, deer biomass increased by a factor of 5 in the absence of wolves. South of the wolf range, there existed a clear relationship between actual evapotranspiration, a proxy of primary productivity, and deer biomass. Highest deer densities occurred in the south-east of the continent where only white-tailed deer are present. The observed pattern lends support to EEH and suggests that the removal of large predators in southern North America may have imposed an unprecedented pressure on plants eaten by deer.     

Pathways between primary production and fisheries yields of large marine ecosystems

The shift in marine resource management from a compartmentalized approach of dealing with resources on a species basis to an approach based on management of spatially defined ecosystems requires an accurate accounting of energy flow. The flow of energy from primary production through the food web will ultimately limit upper trophic-level fishery yields. In this work, we examine the relationship between yield and several metrics including net primary production, chlorophyll concentration, particle-export ratio, and the ratio of secondary to primary production. We also evaluate the relationship between yield and two additional rate measures that describe the export of energy from the pelagic food web, particle export flux and mesozooplankton productivity. We found primary production is a poor predictor of global fishery yields for a sample of 52 large marine ecosystems. However, chlorophyll concentration, particle-export ratio, and the ratio of secondary to primary production were positively associated with yields. The latter two measures provide greater mechanistic insight into factors controlling fishery production than chlorophyll concentration alone. Particle export flux and mesozooplankton productivity were also significantly related to yield on a global basis. Collectively, our analyses suggest that factors related to the export of energy from pelagic food webs are critical to defining patterns of fishery yields. Such trophic patterns are associated with temperature and latitude and hence greater yields are associated with colder, high latitude ecosystems.     

Detritus, trophic dynamics and biodiversity

Traditional approaches to the study of food webs emphasize the transfer of local primary productivity in the form of living plant organic matter across trophic levels. However, dead organic matter, or detritus, a common feature of most ecosystems plays a frequently overlooked role as a dynamic heterogeneous resource and habitat for many species. We develop an integrative framework for understanding the impact of detritus that emphasizes the ontogeny and heterogeneity of detritus and the various ways that explicit inclusion of detrital dynamics alters generalizations about the structure and functioning of food webs. Through its influences on food web composition and dynamics, detritus often increases system stability and persistence, having substantial effects on trophic structure and biodiversity. Inclusion of detrital heterogeneity in models of food web dynamics is an essential new direction for ecological research.     

Invasive plants have different effects on trophic structure of green and brown food webs in terrestrial ecosystems: a meta‐analysis

Although invasive plants are a major source of terrestrial ecosystem degradation worldwide, it remains unclear which trophic levels above the base of the food web are most vulnerable to plant invasions. We performed a meta‐analysis of 38 independent studies from 32 papers to examine how invasive plants alter major groupings of primary and secondary consumers in three globally distributed ecosystems: wetlands, woodlands and grasslands. Within each ecosystem we examined if green (grazing) food webs are more sensitive to plant invasions compared to brown (detrital) food webs. Invasive plants have strong negative effects on primary consumers (detritivores, bacterivores, fungivores, and/or herbivores) in woodlands and wetlands, which become less abundant in both green and brown food webs in woodlands and green webs in wetlands. Plant invasions increased abundances of secondary consumers (predators and/or parasitoids) only in woodland brown food webs and green webs in wetlands. Effects of invasive plants on grazing and detrital food webs clearly differed between ecosystems. Overall, invasive plants had the most pronounced effects on the trophic structure of wetlands and woodlands, but caused no detectable changes to grassland trophic structure.     

Thermal acclimation modulates the impacts of temperature and enrichment on trophic interaction strengths and population dynamics

Global change affects individual phenotypes and biotic interactions, which can have cascading effects up to the ecosystem level. However, the role of environmentally induced phenotypic plasticity in species interactions is poorly understood, leaving a substantial gap in our knowledge of the impacts of global change on ecosystems. Using a cladoceran–dragonfly system, we experimentally investigated the effects of thermal acclimation, acute temperature change and enrichment on predator functional response and metabolic rate. Using our experimental data, we next parameterized a population dynamics model to determine the consequences of these effects on trophic interaction strength and food‐chain stability. We found that (1) predation and metabolic rates of the dragonfly larvae increase with acute warming, (2) warm‐acclimated larvae have a higher maximum predation rate than cold‐acclimated ones, and (3) long‐term interaction strength increases with enrichment but decreases with both acclimation and acute temperatures. Overall, our experimental results show that thermal acclimation can buffer negative impacts of environmental change on predators and increase food‐web stability and persistence. We conclude that the effect of acclimation and, more generally, phenotypic plasticity on trophic interactions should not be overlooked if we aim to understand the effects of climate change and enrichment on species interaction strength and food‐web stability.     

Effects of Multi-chain Omnivory on the Strength of Trophic Control in Lakes

Omnivory has been implicated in both diffusing and intensifying the effects of consumer control in food chains. Some have postulated that the strong, community level, top-down control apparent in lakes is not expressed in terrestrial systems because terrestrial food webs are reticulate, with high degrees of omnivory and diverse plant communities. In contrast, lake food webs are depicted as simple linear chains based on phytoplankton-derived energy. Here, we explore the dynamic implications of recent evidence showing that attached algal (periphyton) carbon contributes substantially to lake primary and secondary productivity, including fish production. Periphyton production represents a cryptic energy source in oligotrophic and mesotrophic lakes that is overlooked by previous theoretical treatment of trophic control in lakes. Literature data demonstrate that many fish are multi-chain omnivores, exploiting food chains based on both littoral and pelagic primary producers. Using consumer-resource models, we examine how multiple food chains affect fourth-level trophic control across nutrient gradients in lakes. The models predict that the stabilizing effects of linked food chains are strongest in lakes where both phytoplankton and periphyton contribute substantially to production of higher trophic levels. This stabilization enables a strong and persistent top down control on the pelagic food chain in mesotrophic lakes. The extension of classical trophic cascade theory to incorporate more complex food web structures driven by multi-chain predators provides a conceptual framework for analysis of reticulate food webs in ecosystems.     

Cascading predator control interacts with productivity to determine the trophic level of biomass accumulation in a benthic food web

Large-scale exploitation of higher trophic levels by humans, together with global-scale nutrient enrichment, highlights the need to explore interactions between predator loss and resource availability. The hypothesis of exploitation ecosystems suggests that top–down and bottom–up control alternate between trophic levels, resulting in a positive relationship between primary production and the abundance of every second trophic level. Specifically, in food webs with three effective trophic levels, primary producers and predators should increase with primary production, while in food webs with two trophic levels, only herbivores should increase. We provided short-term experimental support for these model predictions in a natural benthic community with three effective trophic levels, where the number of algal recruits, but not the biomass of gastropod grazers, increased with algal production. In contrast, when the food web was reduced to two trophic levels by removing larger predators, the number of algal recruits was unchanged while gastropod grazer biomass increased with algal production. Predator removal only affected the consumer-controlled early life-stages of algae, indicating that both the number of trophic levels and the life-stage development of the producer trophic level determine the propagation of trophic cascades in benthic systems. Our results support the hypothesis that predators interact with resource availability to determine food-web structure.     

Carrion cycling in food webs: comparisons among terrestrial and marine ecosystems

In light of current global changes to ecosystem function (e.g. climate change, trophic downgrading, and invasive species), there has been a recent surge of interest in exploring differences in nutrient cycling among ecosystem types. In particular, a growing awareness has emerged concerning the importance of scavenging in food web dynamics, although no studies have focused specifically on exploring differences in carrion consumption between aquatic and terrestrial ecosystems. In this forum we synthesize the scavenging literature to elucidate differences in scavenging dynamics between terrestrial and marine ecosystems, and identify areas where future research is needed to more clearly understand the role of carrion consumption in maintaining ecosystem function within each of these environments. Although scavenging plays a similar functional role in terrestrial and aquatic food webs, here we suggest that several fundamental differences exist in scavenging dynamics among these ecosystem types due to the unique selection pressures imposed by the physical properties of water and air. In particular, the movement of carcasses in marine ecosystems (e.g. wave action, upwelling, and sinking) diffuses biological activity associated with scavenging and decomposition across large, three‐dimensional spatial scales, creating a unique spatial disconnect between the processes of production, scavenging, and decomposition, which in contrast are tightly linked in terrestrial ecosystems. Moreover, the limited role of bacteria and temporal stability of environmental conditions on the sea floor appears to have facilitated the evolution of a much more diverse community of macrofauna that relies on carrion for a higher portion of its nutrient consumption than is present in terrestrial ecosystems. Our observations are further discussed as they pertain to the potential impacts of climate change and trophic downgrading (i.e. removal of apex consumers from ecosystems) on scavenging dynamics within marine and terrestrial ecosystems.     

Trophic status based on nutrient concentration scales and primary producers community of tropical coastal lagoons influenced by groundwater discharges

Changes in the rate of primary production as an indication of trophic status of aquatic ecosystems have been one of the major indicators of their health. Despite different approaches been devised for evaluating this process, few are useful for comparison and generalization of their results. The coastal zone ecosystems of Yucatan (SE Mexico) exhibit hydrological variability closely associated with the groundwater discharges, which are the only freshwater sources in the coastal ecosystems. In order to learn about the trophic status of karstic tropical coastal lagoons, three of them (Celestun, Chelem and Dzilam) located in Yucatan (SE, Mexico) were monitored for inorganic nutrients, Chl-a, phytoplankton and macrophyte productivity. The nitrate concentrations suggest that the trophic status was influenced by fresh water springs, being meso-eutrophic in Celestun, oligotrophic in Chelem, and mesotrophic in Dzilam. In the case of ammonium ions, the three lagoons were mesotrophic, indicating that processes such as remineralization play an important role in the trophic dynamic of these shallow ecosystems. According to phosphate concentrations, Celestun and Dzilam were mesotrophic, and Chelem was oligotrophic. External inputs of phosphate and bioturbation by waterfowl may be responsible of these differences. Primary productivity at Celestun was greater than at Chelem and Dzilam lagoons and the contribution by seagrasses was significant in all three. It is found that indices based on nutrient concentration and phytoplankton biomass are useful as an indication of trophic status in groundwater influenced coastal lagoons. Moreover, estimations on the total system productivity, and the relative contribution of each primary producer, is a holistic approach useful for understanding trophic dynamic in shallow tropical coastal ecosystems.     

Interplay between the paradox of enrichment and nutrient cycling in food webs

Nutrient cycling is fundamental to ecosystem functioning. Despite recent major advances in the understanding of complex food web dynamics, food web models have so far generally ignored nutrient cycling. However, nutrient cycling is expected to strongly impact food web stability and functioning. To make up for this gap, we built an allometric and size structured food web model including nutrient cycling. By releasing mineral nutrients, recycling increases the availability of limiting resources for primary producers and links each trophic level to the bottom of food webs. We found that nutrient cycling can provide a significant part of the total nutrient supply of the food web, leading to a strong enrichment effect that promotes species persistence in nutrient poor ecosystems but leads to a paradox of enrichment at high nutrient inputs. The presence of recycling loops linking each trophic level to the basal resources weakly affects species biomass temporal variability in the food web. Recycling loops tend to slightly dampen the destabilising effect of nutrient enrichment on consumer temporal variability while they have opposite effects for primary producers. By considering nutrient cycling, this new model improves our understanding of the response of food webs to nutrient availability and opens perspectives to better link studies on food web dynamics and ecosystem functioning.     

Temperature‐driven selection on metabolic traits increases the strength of an algal–grazer interaction in naturally warmed streams

Trophic interactions are important determinants of the structure and functioning of ecosystems. Because the metabolism and consumption rates of ectotherms increase sharply with temperature, there are major concerns that global warming will increase the strength of trophic interactions, destabilizing food webs, and altering ecosystem structure and function. We used geothermally warmed streams that span an 11°C temperature gradient to investigate the interplay between temperature‐driven selection on traits related to metabolism and resource acquisition, and the interaction strength between the keystone gastropod grazer, Radix balthica, and a common algal resource. Populations from a warm stream (~28°C) had higher maximal metabolic rates and optimal temperatures than their counterparts from a cold stream (~17°C). We found that metabolic rates of the population originating from the warmer stream were higher across all measurement temperatures. A reciprocal transplant experiment demonstrated that the interaction strengths between the grazer and its algal resource were highest for both populations when transplanted into the warm stream. In line with the thermal dependence of respiration, interaction strengths involving grazers from the warm stream were always higher than those with grazers from the cold stream. These results imply that increases in metabolism and resource consumption mediated by the direct, thermodynamic effects of higher temperatures on physiological rates are not mitigated by metabolic compensation in the long term, and suggest that warming could increase the strength of algal–grazer interactions with likely knock‐on effects for the biodiversity and productivity of aquatic ecosystems.     

On the relationship between productivity and food chain length at different ecological levels

The effects of energy on food web structure have been debated for at least 80 years. Nevertheless, the empirical evidence is meager, especially from terrestrial ecosystems. We analyzed long-term temporal variation in food chain length in a semiarid continental ecosystem, where productivity shows large interannual variations. Incidence of nonherbivorous prey in predator diet was used as a proxy of trophic position, allowing us to analyze the effect of productivity on food chain length within the assemblage of top predators (which comprises the most abundant and persistent top predators in the system) and to compare observed patterns at the species and assemblage levels. At the species level, the relationship between trophic position and productivity took different forms, varying in magnitude and shape. This pattern contrasts with the consistent increase in food chain length, with productivity observed at the assemblage level. Our results indicate that productivity can be a main determinant of food chain length, but not necessarily because of energy limitation. Further, the increase in food chain length with available energy probably represents an aggregate attribute, driven to a large extent by predators with higher consumption rates, rather than being the result of compensatory responses among predators.     

Nutrient dynamics and pelagic food web interactions in oligotrophic and eutrophic environments: an overview

The concept of limiting nutrients is a cornerstone of theories concerning the control of production, structure and dynamics of freshwater and marine plankton. The current dogma is that nitrogen is limiting in most marine environments while freshwater ecosystems are mostly phosphorus-limited, although evidence of phytoplankton limitation by either N or P has been found in both environments.However, the same considerations apply to the availability of phosphorus in freshwater as to nitrogen in oceans. In resource-limited environments the plankton dynamics depend mostly on the internal mechanisms which act to recycle the limiting nutrient many times over within the surface waters. As the overall productivity increases, this dependence on nutrient regeneration decreases.The relationship between the stock of limiting nutrient, rates of supply and plankton dynamics must therefore be seen in the light of the processes operating within the entire food chain over quite different time scales. There is strong evidence that process-rates are mostly size-dependent and that food web interactions at the microbial level (picophytoplankton, bacteria, microheterotrophs) strongly effect the production of carbon and the regeneration of nutrients in the pelagic zone.