Modelling nutrient cycles in relation to food web structure in a biomanipulated shallow lake

The modelPCLAKE describes the phosphorus and nitrogen cycles within a shallow lake ecosystem, including the sediment and a simplified biological food web. All components are modelled in a generalized way rather than a very detailed one. This model has been applied to Lake Zwemlust, a small biomanipulated lake in The Netherlands. Formerly, this highly eutrophic lake was dominated by cyanobacteria and devoid of macrophytes. Biomanipulation was carried out in 1987 by pumping-out of the water, removal of all fish, and refilling of the lake with seepage water. The lake was restocked with some rudd, pike, zooplankton and seedlings of macrophytes, and then monitored up to 1992. Macrophytes developed rather quickly and reached their maximum biomass during the six-years period in 1989. Despite the continuously high nutrient (N and P) loading, algal biomass remained low due to nitrogen limitation, caused by competition with the macrophytes. From 1990 onwards, the macrophytes declined again and a species shift occurred, following an increase of herbivorous birds on the lake and the development of herbivorous fishes.Model simulations grossly reproduced the observed developments in Lake Zwemlust before and after the biomanipulation measures. The existence of multiple steady states at the same trophic state and the possible shift between them could be simulated well. This study also demonstrates the interrelation between system structure and the distribution and cycling of nutrients. It is concluded, that within general boundary conditions set by the trophic state of the system, the food web structure determines the actual nutrient flows and the occurrence of nutrient limitations of the primary producers. It is shown that both aspects can be integrated in one mathematical model. The long-term stability of the macrophyte dominance in the lake is discussed.     

Cascading effects of predator diversity and omnivory in a marine food web

Over‐harvesting, habitat loss and exotic invasions have altered predator diversity and composition in a variety of communities which is predicted to affect other trophic levels and ecosystem functioning. We tested this hypothesis by manipulating predator identity and diversity in outdoor mesocosms that contained five species of macroalgae and a macroinvertebrate herbivore assemblage dominated by amphipods and isopods. We used five common predators including four carnivores (crabs, shrimp, blennies and killifish) and one omnivore (pinfish). Three carnivorous predators each induced a strong trophic cascade by reducing herbivore abundance and increasing algal biomass and diversity. Surprisingly, increasing predator diversity reversed these effects on macroalgae and altered algal composition, largely due to the inclusion and performance of omnivorous fish in diverse predator assemblages. Changes in predator diversity can cascade to lower trophic levels; the exact effects, however, will be difficult to predict due to the many complex interactions that occur in diverse food webs.     

The potential of artificial refugia for maintaining a community of large-bodied cladocera against fish predation in a shallow eutrophic lake

The Norfolk Broads are a series of shallow lakes which are highly eutrophic and typified by dense populations of phytoplankton and an absence of submerged aquatic plants. The zooplankton community is subject to intense predation pressure by young fish and is dominated by small-bodied organisms which have a low potential for reducing phytoplankton populations through grazing. Various designs and densities of artificial refugia for zooplankton against fish predation were established in Hoveton Great Broad in order to enhance populations of large-bodied Cladocera. Initially some of the refuges contained higher densities and larger individuals ofDaphnia andCeriodaphnia than the surrounding open water. However, towards the end of the first season and throughout the subsequent two years, population densities and size-structure were similar both within and outside the refuges, although there was still evidence of enhanced body-size ofDaphnia within the refuges compared with the open water. The provision of habitat structures designed as refugia from fish predation did not enhance large-bodied cladoceran populations enough to promote this restoration strategy as feasible for eutrophic and shallow lakes.     

Three years of experience in biomanipulating a small eutrophic lake: Lago di Candia (Northern Italy)

Results obtained from a step by step approach to the biomanipulation of a natural lacustrine environment (Lago di Candia, Northern Italy) are presented. Since the diversion of the municipal sewage of the small town of Candia, runoff and precipitation have been the sole contributors of nutrient to the lake. Fish population is mainly characterized by rudd (Scardinius erythrophthalmus) overstocking and by a low density of large-mouth-bass (Micropterus salmoides) and pike (Esox lucius). During 1986 about 12t of rudd (1–2 year old) were removed from the lake. Considering 1986 as ‘control year’, average Secchi disc transparency improved from 2.3 m in 1986 to 3.3 m in 1988; phytoplankton biovolume decreased from 114 to 58 mm³ l⁻¹ but zooplankton biovolume increased from 8 to 11.5 mm³ l⁻¹. The results achieved show that a grodual biomanipulation treatment can have a satisfactory outcome, and has the advantage of not producing catastrophic situations either in the biotic or in the abiotic compartments of the lake.     

Interactive and cumulative effects of multiple human stressors in marine systems

Humans impact natural systems in a multitude of ways, yet the cumulative effect of multiple stressors on ecological communities remains largely unknown. Here we synthesized 171 studies that manipulated two or more stressors in marine and coastal systems and found that cumulative effects in individual studies were additive (26%), synergistic (36%), and antagonistic (38%). The overall interaction effect across all studies was synergistic, but interaction type varied by response level (community: antagonistic, population: synergistic), trophic level (autotrophs: antagonistic, heterotrophs: synergistic), and specific stressor pair (seven pairs additive, three pairs each synergistic and antagonistic). Addition of a third stressor changed interaction effects significantly in two‐thirds of all cases and doubled the number of synergistic interactions. Given that most studies were performed in laboratories where stressor effects can be carefully isolated, these three‐stressor results suggest that synergies may be quite common in nature where more than two stressors almost always coexist. While significant gaps exist in multiple stressor research, our results suggest an immediate need to account for stressor interactions in ecological studies and conservation planning.     

Global change effects on a mechanistic decomposer food web model

Global change may affect the structure and functioning of decomposer food webs through qualitative changes in freshly fallen litter. We analyzed the predicted effects of a changing environment on a dynamic model of a donor‐controlled natural decomposer ecosystem near Wekerom, the Netherlands. This system consists of fungi, bacteria, fungivores, bacterivores and omnivores feeding on microbiota and litter as well. The model concentrates on carbon and nitrogen flows through the trophic niches that define this decomposer system, and is designed to predict litter masses and abundances of soil biota. For modeling purposes, the quality of freshly fallen leaf litter is defined in terms of nitrogenous and non‐nitrogenous components, of which refractory and labile forms are present. The environmental impacts of elevated CO₂, enhanced UV‐B and eutrophication, each with their own influence on leaf litter quality, are studied. The model predicts steady‐state dynamics exclusively, for all three scenarios. Environmental changes impact most demonstratively on the highest trophic niches, and affect microbiotic abundances and litter decomposition rates to a lesser extent. We conclude that the absence of trophic cascade effects may be attributed to weak trophic links, and that non‐equilibrium dynamics occurring in the system are generally because of encounter rates based on fractional substrate densities in the litter. We set out a number of experimentally testable hypotheses that may improve understanding of ecosystem dynamics.     

Carbon flow in the littoral food web of an oligotrophic lake

Benthic food web dynamics and carbon flow were examined in the littoral zone of Lake Coleridge, a large deep oligotrophic lake, using radioactive and stable isotope techniques in conjunction with analyses of stomach contents of the fauna. We specifically address two hypotheses: (1) that macrophytes only contribute to the carbon flow to higher trophic levels when they have decayed; and (2) that epiphytic algae is the major source of carbon for macroinvertebrates, and thus fish, with only minor contributions from phytoplankton or terrestrial sources. Epiphytic diatoms were a major component of the stomach contents of the gastropod snail Potamopyrgus antipodarum, and of chironomids. Animal remains were also common in the diet of some chironomids, while amorphous organic matter predominated in the stomachs of oligochaetes. A variety of epiphytic algal taxa was found in trichopteran larvae. Feeding rate of P. antipodarum measured with radioactive tracers increased by 10× on decayed macrophytes (Elodea) compared with live material, while feeding rates on characean algae increased by a factor of 3 when decayed material was presented. However, assimilation rates were less than 20% on decayed material compared with 48–52% on live material. Potential carbon sources were easily distinguished based on their ¹³C values, although isotopic ratios showed significant variation among sites. Epiphytic algae showed less variation among sites than macrophytes and were depleted by 4–5 compared with macrophytes. Detrital material, organic matter in the sediments and plankton were significantly depleted in ¹³C relative to macrophytes and slightly depleted relative to epiphytic algae. Most macroinvertebrate taxa showed a similar pattern among sites to macrophytes and epiphytic algae. P. antipodarum and chironomids were slightly enriched compared with epiphytic algae. Ratios for the common bully (Gobiomorphus cotidianus) were generally consistent with a diet dominated by chironomids, while there was some evidence for terrestrial inputs for koaro (Galaxias brevipinnis) and juvenile brown trout. Epiphytic algae appear to underpin much of the production in the littoral zone of this oligotrophic lake, with trichopteran and chironomid larvae mediating carbon flows from algae to fish. Macrophytes do not make a major contribution directly to carbon flow to higher trophic levels even when decayed. The lack of a direct link between macrophytes and higher trophic levels is due to the faunal composition, including a lack of large herbivores.     

Food web manipulation in Lake Zwemlust: Positive and negative effects during the first two years

The hypertrophic Lake Zwemlust, a small water body used as a swimming pool, was characterized by algal blooms in summer, reducing the Secchi disk transparency to less than 0.3 m. Since in The Netherlands a Secchi disk transparency of 1 m is obligatory for swimming waters, corrective measures were called for to improve the light climate of the lake. In March, 1987, as an experiment, the lake was drained by pumping out the water to facilitate fish elimination. Planktivorous and benthivorous fish species, which were predominant, were removed by seine- and electro-fishing. After the lake had refilled by seepage it was restocked by a new simple fish community comprising pike (Esox lucius) and rudd (Scardinius erythrophthalmus) only. Stacks of willow twigs (Salix) and macrophytes (roots ofNuphar lutea and seedlings ofChara globularis) were introduced into the lake as spawning grounds and refuges for the pike against cannibalism and as shelter for the zooplankton. The effects of this food web manipulation on the light climate, phytoplankton, zooplankton, fish, macrophytes, macrofauna and on the nutrient concentrations were monitored during 1987 and 1988. In summer 1987, despite of high nutrient concentrations, the phytoplankton density was low, due to control by zooplankton, causing a Secchi disk transparency of 2.5 m, the maximum depth. Chlorophyll-a concentrations were low (<5 g Chl.l^–1), blooms of cyanobacteria did not occur and a shift from rotifers to cladocerans took place. In 1988, however, also some negative effects were noticed. Macrophytes and filamentous green algae reached a much higher biomass (50–60% cover of the lake bottom) than in 1987; some species, growing through the entire water column, interfered with the lake's recreational use. Associated with the macro-vegetation and possibly with the absence of larger cyprinids, the diet of which also comprises snails, a large scale development of the snail population, among themLymnaea peregra var.ovata took place. This species is known to act as an intermediate host of the bird-parasitizing trematodeTrichobilharzia ocellata, the cercariae of which cause an itching sensation at the spot of penetration of the human skin, accompanied by rash (schistosome dermatitis or swimmers' itch); in July, 1988, about 40% of the bathers complained about this itching. A positive effect of the macrophytes and filamentous green algae was the high uptake of nitrogen, resulting in a low nitrogen concentration in the lake and growth limitation of the phytoplankton population by nitrogen in the summer of 1988. In 1988 the cladocerans were abundant in April only; and unlike in 1987, in the summer of 1988 there was a shift from cladocerans to rotifers. Therefore, only in early spring (April) zooplankton grazing controlled phytoplankton growth and in summer nitrogen limitation was the major controlling factor, keeping chlorophyll-a concentrations low.     

Experimental warming transforms multiple predator effects in a grassland food web

This experimental study tests new theory for multiple predator effects on communities by using warming to alter predator habitat use and hence direct and indirect interactions in a grassland food web containing two dominant spider predator species, a dominant grasshopper herbivore and grass and herb plants. Experimental warming further offers insight into how climate change might alter direct and indirect effects. Under ambient environmental conditions, spiders used habitat in spatially complementary locations. Consistent with predictions, the multiple predator effect on grasshoppers and on plants was the average of the individual predator effects. Warming strengthened the single predator effects. It also caused the spider species to overlap lower in the vegetation canopy. Consistent with predictions, the system was transformed into an intraguild predation system with the consequent extinction of one spider species. The results portend climate caused loss of predator diversity with important consequences for food web structure and function.     

Linearity in the aggregate effects of multiple predators in a food web

Theory in community ecology often assumes that predator species have similar indirect effects and thus can be treated mathematically as a single functional unit (e.g. guild or trophic level). This assumption is questionable biologically because predator species typically differ in their effects, creating the potential for nonlinearities when they coexist. We evaluated the nature of indirect effects caused by three species of hunting spider predators, singly and in multiple species combinations, on grass and herb plants in experimental old-field food webs. Despite the potential for nonlinearity, indirect effects in different multiple predator combinations consistently did not differ significantly from the respective means of the single species effects. Thus, for this experimental system, the whole was simply the average of the parts. Consequently, models which abstract predator species as single trophic levels would successfully predict indirect effects in this system regardless of the composition of the predator fauna.     

全球变化对食物网结构影响机制的研究进展

食物网主要依靠基于不同营养级间物种互作形成的上行与下行调控维持其结构。全球变化能够改变种间关系, 威胁生物多样性的维持, 然而目前对全球变化改变食物网结构的机制仍处于探索阶段。近年来通过大时空格局与多营养级食物网研究, 发现全球变化的作用机制主要可归结为3种: 物候错配、关键种丧失与生物入侵。该文聚焦于这3种机制, 综述各种机制造成的食物网结构变化并探讨相关的进化与生态驱动因素。三种干扰机制均通过改变原有种间关系, 影响食物网调控, 改变食物网结构。不同的是, 物候错配造成的种间关系变化是由于不同物种的物候对全球变化产生非同步响应所致; 关键种丧失则使营养级间取食/捕食关系发生变化甚至缺失; 而入侵物种通过竞争排除同营养级物种改变种间关系。最后, 该文提出食物网结构变化的实质是物种是否能够适应快速变化的生态环境, 并据此展望未来研究方向。随着全球变化影响日益加剧, 急需继续深入探索导致全球变化下食物网结构改变的机制, 为制定合理的生物多样性保护与生态修复规划提供重要理论支撑。     

Influence of food‐web structure on the biodegradability of lake sediment

1. Sediment plays a key role in internal nutrient cycling and eutrophication in lakes. However, studies focusing on the efficiency of the biomanipulation techniques for improving the control of primary producers have rarely examined the effects of changes in food‐web structure on the sediment biochemical composition and biodegradability. 2. In a 1‐year experiment conducted in large replicated mesocosms, we tested how the absence or presence of a zooplanktivorous fish (roach, Rutilus rutilus) affected the elemental composition and the potential biodegradability of recently deposited sediment in a eutrophic system. The potential biodegradability of these sediments was assessed in laboratory microcosms by measuring the production of CO₂ during 44‐day incubations. 3. The potential biodegradability of recently deposited sediment from the fish treatment was 60% higher than that from the fishless treatment. This higher biodegradability was corroborated by a higher annual loss of sediment in fish enclosures (36%) than in fishless ones (16%). Annual losses of carbon, nitrogen and organic phosphorous were higher for sediment from fish enclosures. 4. Carbon and nitrogen contents of sediment were higher for the fish treatment. In contrast, the sediment C/N ratio, one of the proxies used to estimate sediment biodegradability, did not differ between treatments. No relationship was observed between elemental composition of sediment and its potential biodegradability. This latter appeared to be more probably dependent on the biochemical composition of the sediment and especially on the content of labile compounds such as proteins, sugars and polyunsaturated fatty acids. The use of sterols as biomarkers revealed an important degradation by microorganisms of 1‐year‐old sediment from both fish and fishless treatments. 5. Our results revealed that fish biomanipulations might favour clear water states not only through a stronger top–down control on phytoplankton but also through a lower biodegradability of sediment reducing internal nutrient cycling.     

Planktonic functional diversity changes in synchrony with lake ecosystem state

Managing ecosystems to effectively preserve function and services requires reliable tools that can infer changes in the stability and dynamics of a system. Conceptually, functional diversity (FD) appears as a sensitive and viable monitoring metric stemming from suggestions that FD is a universally important measure of biodiversity and has a mechanistic influence on ecological processes. It is however unclear whether changes in FD consistently occur prior to state responses or vice versa, with no current work on the temporal relationship between FD and state to support a transition towards trait-based indicators. There is consequently a knowledge gap regarding when functioning changes relative to biodiversity change and where FD change falls in that sequence. We therefore examine the lagged relationship between planktonic FD and abundance-based metrics of system state (e.g. biomass) across five highly monitored lake communities using both correlation and cutting edge non-linear empirical dynamic modelling approaches. Overall, phytoplankton and zooplankton FD display synchrony with lake state but each lake is idiosyncratic in the strength of relationship. It is therefore unlikely that changes in plankton FD are identifiable before changes in more easily collected abundance metrics. These results highlight the power of empirical dynamic modelling in disentangling time lagged relationships in complex multivariate ecosystems, but suggest that FD cannot be generically viable as an early indicator. Individual lakes therefore require consideration of their specific context and any interpretation of FD across systems requires caution. However, FD still retains value as an alternative state measure or a trait representation of biodiversity when considered at the system level.     

Food web manipulation in a small,eutrophic Lake Wirbel,Poland: effect of stocking with juvenile pike on planktivorous fish

In a four year experiment (1988–1991) carried out in a shallow, eutrophic lake, an increase in young-of-the-year pike (Esox lucius) density was used to control planktivorous fish assemblages consisting mainly of small-sized cyprinids: roach (Rutilus rutilus), Leucaspius delineatus, and white bream (Blicca bjoerkna). Stocking with pike fry of ca 30 mm in three successive springs resulted in large-scale mortality among prey of vulnerable sizes. Rotenone treatment at the end of the fourth year showed that roach and white bream populations contained few 0+ to 2+ fish, but very high numbers of 3+ to 6+ fish. By this time, short-lived, slow-growing L. delineatus, with no size refuge from predators, had been driven almost to extinction.     

Stability of grassland production is robust to changes in the consumer food web

Theory predicts that consumers may stabilise or destabilise plant production depending on model assumptions, and tests in aquatic food webs suggest that trophic interactions are stabilising. We quantified the effects of trophic interactions on temporal variability (standard deviation) and temporal stability (mean/standard deviation) of grassland biomass production and the plant diversity–stability relationship by experimentally removing heterotrophs (large vertebrates, arthropods, foliar and soil fungi) from naturally and experimentally assembled grasslands of varying diversity. In both grassland types, trophic interactions proportionately decreased plant community biomass mean and variability over the course of 6 years, leading to no net change in temporal stability or the plant diversity–stability relationship. Heterotrophs also mediated plant coexistence; their removal reduced diversity in naturally assembled grasslands. Thus, herbivores and fungi reduce biomass production, concurrently reducing the temporal variability of energy and material fluxes. Because of this coupling, grassland stability is robust to large food web perturbations.     

消费者多样性对食物网结构和生态系统功能的影响

前所未有的生物多样性丧失使人们越来越关注生物多样性的生态系统功能.现有的绝大多数研究都是局限在单一营养级别上,主要是植物上,但是今天越来越多的证明表明消费者的多样性对生态系统结构和功能具有深刻影响.综述了消费者多样性对相邻或非相邻营养级的种群密度、物种多样性和生产力等方面影响的最新进展,同时也提出了若干研究展望.总体上.消费者多样性,无论是草食动物还是肉食动物,都倾向于增加该消费者所在营养级的养分和能量利用效率,以及生产力.这可能源于取样效应,或者物种之间的互补作用,类似于植物物种多样性影响初级生产力的机制.草食动物可能降低或者提高植物物种多样性,或者没有显著影响,其具体效应取决于生态系统生产力水平和草食动物的大小.捕食者哌能通过直接抑制草食动物而间接提高植物的多样性和生产力,但这种效应的大小差异很大,甚至效应的方向,都可能随团体内捕食者所占的比例而改变.未来的研究,应该考虑应用较大尺度的实验来检测食物网复杂营养关系对生态系统特性的影响,继续探讨消费者对生态系统功能的影响机制.认为异速生长法则和生态化学计量学在食物网组分关系研究中的应用将有利于增强人们对消费者.生态系统功能关系的理解.另外,全球变暖和转基因植物对食物网中消费者结构和生态系统的功能的影响也将是未来的一个重要研究方向.     

Seasonal variation in effects of urea and phosphorus on phytoplankton abundance and community composition in a hypereutrophic hardwater lake

1. Urea accounts for half of global agricultural fertiliser applications, yet little is known of its role in eutrophication of freshwater ecosystems, nor how it interacts with phosphorus (P) in regulating phytoplankton composition, especially during spring and autumn.
2. To identify when and how urea and P inputs interact across the ice-free period, we conducted seven monthly fertilisation experiments in 3,240-L mesocosms from ice-off to ice-formation in a hypereutrophic lake. In addition, we ran bioassays with ammonium (NH₄⁺) to compare the effects of urea with those of NH₄⁺, the immediate product of chemical decomposition of urea.
3. Analysis of water-column chlorophyll a and biomarker pigments by high-performance liquid chromatography revealed that addition of inorganic P alone (100 µg P L^–1 week^–1) had no significant impact on either algal abundance or community composition in hypereutrophic Wascana Lake. Instead, fertilisation with urea (4 mg N L⁻¹ week^–1) alone, or in concert with P, significantly (p < 0.05) increased algal abundance in spring and much of summer, but not prior to ice formation in October. In particular, urea amendment enhanced abundance of cryptophytes, chlorophytes, and non-diazotrophic cyanobacteria during April and May, while fertilisation in summer and early autumn (September) increased only chlorophytes and non-diazotrophic cyanobacteria.
4. Comparison of urea mesocosms with NH₄⁺ bioassays demonstrated that urea lacked the inherent toxicity of NH₄⁺ in cool waters, but that both compounds stimulated production during summer experiments.
5. This study showed that urea pollution can degrade water quality in P-rich lakes across a variety of seasonal conditions, including spring, and underscores the importance of quantifying the timing and form of N inputs when managing P-rich freshwaters.

     

Responses to food web manipulation in a shallow waterfowl lake

The effects of fish stock reduction have been studies in 3 Dutch lakes (Lake Zwemlust, Lake Bleiswijkse Zoom and Lake Noorddiep) and 1 Danish lake (Lake Væng) during 4–5 years. A general response id described. The fish stock reduction led in general to a low fish stock, low chlorophyii-a, high transparency and high abunuance of macrophytes. Large Daphnia became abundant, but their density decreased, due to food limitation and predation by fish. The total nitrogen concentration became low due to N-uptake by macrophytes and enhanced denitrification. In Lake Bleiswijkse Zoom the water transparency deteriorated and the clear water state was not stable. The fish stock increased and the production of young fish in summer was high. lear water occurred only in spring. Large daphnids were absent in summer and the macrophytes decreased.In Lake Zwemlust, Lake Væng and Lake Noorddiep the water remained clear during the first five years. In summer of the sixth year (1992) transparency decreased in Lake Zwemlust (with high P-concentration of 1.0 mg P l^-1). Also in Lake Væng (with a low nutrient concentration of 0.15 mg P.^-1) a short term turbid stage (1.5 month) occurred in summer 1992 after a sudden collapse of the macrophytes. Deterioration of the water quality seems to start in summer and seems related to a collapse in macrophytes. At a low planktivorous fishstock (e.g. Lake Væng)thhe duration of the turbid state is shorter. than in presence of a high planktivorous fish biomass (e.g. Lake Zwemlust, and later years of Lake Bleiswijkse Zoom).