Changes In Food Web Research Articles

Influence of changing food web structure on nutrient limitation of phytoplankton in a highly humic lake

Influence of changing food web structure on nutrient limitation of phytoplankton in a highly humic lake

Changes in the planktonic food web of a new experimental reservoir

Changes in plankton community structure were examined in an experimental reservoir (Lake 979) before and after impoundment. The role of allochthonous organic matter in planktonic food webs is unclear, and reservoir creation can be viewed as an extreme manipulation of terrestrial organic matter inputs. After impoundment of Lake 979, concentrations of phosphorus, nitrogen, and dissolved organic carbon increased as a result of decomposition of flooded terrestrial organic matter. In the first year of impoundment, mean bacterial biomass increased 10 times , and individual bacterial cell volumes increased 2 times over pre-flooding averages. Phytoplankton production and biomass decreased to approximately 25% of pre-flooding levels. Zooplankton biomass and production by Cladocera increased 10 times , and zooplankton community composition changed from dominance by small-sized Bosmina longirostris to dominance by large Daphnia rosea. In the first year of impoundment, production by Cladocera usually exceeded phytoplankton14C productivity, suggesting that the main pathway of carbon flow to secondary producers shifted from an autochthonous to an allochthonous base derived from flooded terrestrial vegetation. In the second year of flooding, bacterial biomass decreased and phytoplankton biomass was higher than in the two previous years of study.

A multivariate analysis of phytoplankton and food web changes in a shallow biomanipulated lake

1. Phytoplankton dynamics, food chain changes and resilience in Lake Zwemlust, a shallow lake in The Netherlands, are described for the period 1986–94.2. After biomanipulation in 1987, the lake moved through two alternative states, while the external nutrient loadings were maintained. A clear‐water phase, mostly dominated by macrophytes, persisted from 1987 to 1991, and a rather turbid state, dominated by algae, occurred in the summers of 1992–94, after several consecutive and sustained perturbations affecting different parts of the food web in the lake. These two periods were characterized by different community structures.3. The phytoplankton assemblage gradually changed in a pattern that reverted in later years towards that of the pre‐biomanipulation stage, although the same species composition was not regained. This agrees with some mathematical models. During the clear‐water phase, nutrient shortage, light climate and zooplankton feeding selected in favour of small, high surface : volume ratio and rapidly reproducing algae. However, in mid‐summer of 1992–94, nutrient availability and cladoceran grazing on edible algae favoured cyanophytes.4. Nutrients were transferred to higher trophic levels or lost from the system at relatively high rates when the lake was in a piscivore–macrophyte‐dominated state, while they tended to accumulate in the algae in a planktivore‐dominated chain without macrophytes. The role of weed beds was central for nutrient competition (mostly nitrogen) with algae, as well as a refuge and a base for alternative food sources to grazers. Weed beds seemed to have a strong effect in increasing connectedness, resilience and stability of the lake community.5. The complete return of Zwemlust to a turbid state dominated by phytoplankton seems to have depended upon turnover of the limiting nutrient, which was retarded by macrophytes and stimulated by planktivorous fish and waterfowl.

Biomonitoring with the BuzzardButeo buteo in the Netherlands: Heavy metals and sources of variation

The feasibility of using Buzzards found dead as indicators of environmental contamination was tested. Liver, kidney, and tibia specimens were examined for the presence of Cd, Cu, Pb, Mn, and Fe, which were used as markers. All buzzards submitted to the Veterinary Institute in 1992 were examined for cause of death and condition. Results showed that more than half had been poisoned or shot. As the condition of the birds worsened, fat reserves were depleted before protein reserves. Concentrations of the heavy metals rose in liver and kidney as nutrient reserves fell. Since the content of heavy metal per organ was not related to body condition, content was considered a better measure for biomonitoring. The concentrations of Cd, Cu, Pb, Mn, or Fe measured in buzzards were not toxic. In several places buzzards had recently ingested relatively large amounts Cd and/or Pb. Buzzards with high contents of Cd, Cu, Pb, and Mn were found in areas having more than average contamination. The North-South gradient in aerosol deposition and soil content of heavy metals (especially Cd & Pb) was reflected in the Buzzard. We concluded that the Buzzard is a suitable and cost-effective biomonitor, to investigate the bioavailability of ecocontaminants in large areas, to signal gross changes in food webs, and to monitor raptor persecution.

Field Estimates of Net Trophic Transfer of PCBs from Prey Fishes to Lake Michigan Salmonids

Dietary exposure is the principal source of polychlorinated biphenlys (PCBs) to Great Lakes salmonids. PCB transfer efficiency has typically been determined with laboratory studies using high food PCB concentrations and clean animals, making extrapolations to the field uncertain. Here, we present estimates of net PCB transfer efficiencies for Lake Michigan coho salmon, chinook salmon, and lake trout based on 15 years of PCB concentrations in these fish and their prey. Between 1975 and 1990, large changes in Lake Michigan's pelagic food web led to changes in coho and chinook salmon diets and chinook salmon growth efficiency. Early in this period, PCB transfer efficiencies increased in lake trout but decreased in chinook and coho salmon. These early trends carry relatively high uncertainty due to high variability in prey and predator PCB concentrations and are probably not ecologically significant. After 1983, PCB transfer efficiencies remained relatively constant at ∼0.55 for lake trout and at ∼0.60 for chinook salmon and increased slightly to ∼0.50 for coho salmon. PCB transfer efficiencies appear to be little affected by changes in prey PCB concentration, shifts in prey type, and shifts in predator gross growth efficiency. Use of a constant PCB transfer efficiency in models that consider PCB accumulation by piscivorous fishes is therefore appropriate.

Cenomanian/Turonian sponge microbialite deep-water hardground community (Liencres, Northern Spain)

A benthic community of sessile metazoans dominated by coralline sponges (e.g.Acanthochaetetes andVaceletia) is found within a Cenomanian-Turonian deep water hardground succession cropping out at the coastal area of the Bay of Biscay near Santander. The characteristic K-strategic community exhibits a very close taxonomic relationship with modern communities from the Pacific realm, which allows for a comparison with Recent environmental conditions. The sponge community was associated with automicrites, microbialites, and thin mineralized limonitic biofilms. This biofacies is typically found in cryptic niches of reefal buildups (“telescoping”). The iron-rich biofilms had a strong electrochemical corrosive ability which explains the distinct submarine dissolution patterns. The hardground conditions are controlled, in part, by strong contour current regimes linked with extremely oligotrophic water masses. This system was established during the drowning of a distal carbonate ramp during the early Middle Cenomanian (A.rhotomagenese zone). In the uppermost portion of the hardground (Late Cenomaian, upperR. cushmani zone) the coralline sponge community was replaced by thick limonitic stromatolites with numerous encrusting foraminifera (Miniacina-type) and by colonies of the problematic iron bacteriumFrutexites. This event is accompanied by an increase of terrigenous influx and detrital glauconite, indicating a fundamental change in food web, and terminates the sponge dominated basal hardground interval. Thehardground was buried by hemipelagic sediments during the Middle Turonian (upperR. kallesi zone).

Benthic algal populations respond to aluminum, acid, and aluminum-acid mixtures in artificial streams

Elevated aluminum (Al) concentrations are often associated with acid-stressed aquatic ecosystems, so it has been unclear whether acidic water or elevated Al is more responsible in changing community composition. Experiments were done to investigate effects of acidification and increased Al on the abundance of benthic algae in artificial streams supplied with natural water and nominal treatments of (a) pH 4.8, (b) 500 µg l-1 Al, or (c) the mixture of pH 4.8 and 500 µg l-1 Al compared to a control without added Al or acid. These treatments are referred to as ‘Acid’, ‘Al-only’, ‘Acid + Al’, and the ‘control’, respectively. In the Acid treatment the abundance of two diatoms, two green algae, dry weight biomass, and chlorophyll a decreased; one diatom and one filamentous blue-green alga increased. In the Al-only treatment, densities of two diatoms, one green alga, one blue-green alga, dry weight biomass, and chlorophyll a increased. In the Acid + Al treatment, abundances of one green alga, two blue-green algae, and concentrations of chlorophyll a decreased below the levels observed in the Acid treatment. Acid and Al concentrations were altered by each other and by chemical and biological processes in the stream system. Species of diatoms, green algae, and blue-green algae responded individually to treatments and mixtures of acid and Al. Shifts in the abundance of species may change food web relationships for higher-level consumers, and algae may be useful biomonitors of ecological stress.

Effects of Lake Acidification and Recovery on the Stability of Zooplankton Food Webs

The effect of food web structure on community stability and resilience has rarely been examined using empirical data. Yet there is a practical application for such studies insofar as resistance stability determines the ability of a system to “absorb” anthropogenic stress and adjustment stability determines the reversibility of resulting damage. The stability of zooplankton food webs in 46 Precambrian Shield lakes was examined using data collected in the 1970s, when pH ranged from 3.8 to 7.0, and in 1990, when pH had increased by up to two units in some lakes. Acidification overcame resistance stability at pH < 5.0, as evidenced by decreases in species richness, numbers of predatory and competitive links, directed connectance, predator generalization, and linkage density, identified by analysis of variance. Adjustment stability was demonstrated by changes in food web attributes in lakes with higher pH in 1990 than in the 1970s. Species richness, numbers of predatory and competitive links, linkage density, and predator generalization all increased relative to the 1970s values. Food web attributes of “recovering” lakes were statistically indistinguishable from those of lakes of similar pH that had not been more acidic in the 1970s. Similar trajectories of food web change were followed during environmental degradation and recovery. Planktonic food webs of anthropogenically acidified lakes may eventually recover to resemble their pre—acidification condition, given sufficient time without acidic inputs. Whether adjustment stability is a general feature of anthropogenically stressed systems remains to be determined.

Ecological aspects, explosive range extension and impact of a mass invader, Corophium curvispinum Sars, 1895 (Crustacea: Amphipoda), in the Lower Rhine (The Netherlands).

A few years after it invaded, the amphipod Corophium curvispinum Sars appeared to be the most numerous macroinvertebrate species in the River Rhine. From 1987 to 1991 the densities of this species on the stones of groins in the Lower Rhine at a depth of 0.5 m increased from 2 to 200000 specimens per m2. In the Lower Rhine and its branches the densities of C. curvispinum increased with increasing current velocities and with increasing water depths. So far, a maximum population density of 750000 specimens per m2 has been found in the Lower Rhine, which is many times the densities recorded elsewhere. Population parameters, densities and distribution of C. curvispinum were studied in the Lower Rhine and its branches, using artificial substrates and sampling stones from groins. The success of this immigrant is related to its competitive strategy, which shows several aspects of a r-strategy. In addition, the heavily eutrophicated Lower Rhine provides abundant food (phytoplankton, suspended organic matter) for this opportunistic filter-feeder. The increased salinity and water temperatures in the Lower Rhine resulting from industrial discharges have contributed to the current success of this southern species originating in brackish waters. The very high densities of C. curvispinum might have an enormous impact on the river ecosystem by changing food webs.

Complex Interactions of Multiple Aquatic Consumers: An Experimental Mesocosm Manipulation

In 7-m3 outdoor tanks filled with lake water, the presence/absence of omnivorous young-of-the-year Micropterus salmoides, zooplanktivorous Menidia beryllina, and herbivorous larval Hyla chrysocelis was experimentally manipulated. A cross-classified design was used to assess the interactive effects of these vertebrate consumers on the experimental food webs. Large zooplankters and large, actively swimming macroinvertebrates (Ceriodaphnia, Simocephalus, Corixidae, Notonectidae, and Dytiscidae) were particularly susceptible to depletion by both fish species. Micropterus tanks contained greater numbers of Keratella quadrata and Hebridae but fewer benthic cyclopoid copepods. Menidia tanks contained more Synedra and algal filaments. Hyla exerted only minor direct effects on the tank communities. When both fish species were in the same tank, both had elevated mortality. Micropterus, in tanks with Menidia and Hyla, were larger and in better condition than in tanks without Hyla. The primary effects of the experimental manipulations on food web components were two- and three-way interactions in which the effect of a given treatment was dependent on the presence of another treatment. Results suggest that the addition or removal of consumers may not cause linear, additive changes in food webs.

Complex biological responses to the experimental acidification of Little Rock Lake, Wisconsin, USA

Acidification can affect aquatic organisms directly through hydrogen ion toxicity, and indirectly through disrupted food web dynamics and altered abiotic conditions. Field populations from selected taxa were studied during the Little Rock Lake whole-basin acidification experiment to illustrate patterns whose timing suggests direct (i.e. immediate) or indirect (i.e. delayed or non-uniform) responses to pH change. As the treatment basin was acidified to pH 5·6, 5·2 and 4·7, immediate changes consistent with a direct pH response were observed for species representing several trophic levels. For other taxa (e.g. littoral invertebrates associated with filamentous algal mats, several species of pelagic zooplankton), indirect mechanisms induced by food web changes were more likely explanations for abundance patterns. The results presented here suggest that the responses of aquatic ecosystems to acidification involve a complex interplay between direct pH effects and subsequent indirect interactions.

Interactions with the Benthos Alter Pelagic Food Web Structure in Coastal Waters

Results from studies in experimental enclosures containing both water column and benthic communities show that the benthos has an important effect on the structure and productivity of pelagic food webs whether the system is nutrient enriched or nutrient limited. Research over a 10-yr period in 13-m3 mesocosms showed that changes in the pelagic food web were correlated with different sediment communities and with the presence or absence of a benthos. The abundance of copepods was inversely correlated with numbers of macrofauna. At both low and high nutrient levels, systems without benthos had greatly enhanced numbers of carnivorous holozooplankton including ctenophores, medusans, chaetognaths, and fish. Our observations indicate that the presence of the benthos shortens the pelagic food web.and inhibits the response of pelagic fauna to nutrient enrichment in well-mixed coastal waters. The strength of benthic–pelagic coupling, which is controlled by the amount of turbulence in the water column, may be more important to food web structure than the rate of nutrient supply and could determine which subsystem responds to eutrophication.

Nature of Dissolved Organic Matter in the Drainage Basin of a Boreal Humic River in Northern Finland

AbstractThe nature of dissolved organic matter in river, brook, and peat mining water in the drainage basin of the humic Kiiminkijoki River, located in an area of minerotrophic peatlands in northern Finland in the boreal region, was studied by gel filtration. Special emphasis was paid to changes in the nature of high molecular weight organic Fe‐P colloids that result from peat mining. The river water is organically colored, with high organic matter and Fe concentrations. High apparent molecular weight (HAMW) organic colloids are important for the transport of Fe and P in the basin. The gel filtration data suggest that the DOM in the water consist mainly of fulvic acids (FA), although humic acids (HA) are also important. The proportion of dissolved organic C (DOC) and Fe in the HAMW organic colloids increases with increasing DOC and iron concentrations in the river water. The apparent molecular weight of the DOM is higher than in the lakes in the area of ombrotrophic peatlands in southern Finland. Color in this humic water increases with DOC and Fe concentrations. The effect of Fe on color values can be seen in filtered (<1.2 µm) river water and in all the gel filtration fractions. The results indicate an increase in the mobilization of HAMW organic Fe‐P colloids in the peatlands of the area following drainage and peat mining. The average ratio of Fe to DOC in the HAMW organic fraction is higher in the peat mining water than in the Keihäsoja brook in a natural peaty urea. It is possible that the Fe to DOC ratio in these colloids transported in the river increases following drainage and peat mining, especially under low flow conditions in summer. This may lead to gradual changes in the detritus food webs of the river ecosystem.

Experimental Perturbations of Whole Lakes as Tests of Hypotheses concerning Ecosystem Structure and Function

Production, respiration, nutrient concentrations and changes in biological communities were monitored for several years in two experimentally eutrophied and two experimentally acidified lakes in the Experimental Lakes Area, Canada. Results were examined to detect changes in ecosystem structure and function that might be sensitive indicators of ecosystem stress. Phytoplankton production in eutrophied lakes was higher than in reference lakes. It decreased quickly to background values when fertilization was terminated. Winter respiration was unchanged, P/R ratios were constant under constant N/P fertilization, but decreased for three years after the N/P ratio in fertilizer was decreased from 14:1 to 5:1 by weight. Phytoplankton production was not affected by acidification to a pH of 5.0. Winter respiration declined at pH values of 5.1 and below, causing P/R to be highest at the lowest pH values. Respiration returned to normal when the pH of the lake was increased to 5.4. Gross periphyton production and respiration both increased with acidification, but P/R ratios declined at pH values below 6.2. Nitrification in both acidified lakes ceased at pH values of 5.4 to 5.7. It recovered when pH was allowed to increase, but with a time lag of one year. No other nutrient cycles appeared to be disrupted. Both eutrophication and acidification caused declines in species diversity among several taxonomic groups. Among phytoplankton, both stresses caused an increase in size of organisms in summer. Among higher taxa, there was not a consistent tendency for either small or large organisms to be favored. Periphyton metabolism was the most sensitive indicator of acidification, followed by taxonomic changes in several groups. Ecosystem-level production and respiration were the most resistant properties to acid stress. The earliest serious changes in ecosystems and food webs occurred when acidification eliminated acid-sensitive organisms that were also the sole occupants of key ecological niches. Such situations occur frequently in northern lakes.

Algal biomass unaltered by food-web changes in Lake Michigan

Bythotrephes cederstroemii Schoedler (Crustacea: Cladocera), a predator previously confined to the Palearctic, has successfully invaded the North American Great Lakes1–3. Bythotrephes is a voracious predator on herbivorous Cladocera, including the dominant grazers in Lake Michigan during the summer. Lake Michigan has been the source of active debate regarding the relative importance of nutrient income versus food-web relations to its trophic state and water quality4–6. The recent species invasion has directly altered the lake's food web at a middle trophic level. During summer 1987 Bythotrephes populations increased rapidly in the offshore regions of Lake Michigan and abundances of herbivorous zooplankton simultaneously declined. Despite the resulting relaxation of herbivory, participate chlorophyll concentrations, an index of algal biomass, did not increase. These results suggest that primary producers are most constrained by abiotic forces in this deep oligotrophic lake.

Colonial Waterbird Management in North America

Colonial waterbirds are an important natural resource highly valued by many people in Canada and the United States. The habit of nesting often in large groups makes these birds especially susceptible to problems, such as human disturbance, predation, severe weather events, and competition for nesting habitat. They, like all birds, also face threats from habitat degradation, loss and contamination of their environments, and changes in food webs. Management strategies to deal with these problems include habitat preservation and restoration, the elimi- nation of toxic chemicals from the environment, reduction of predation, competition, and disturbance at nesting sites, reintroduction of species to nesting sites from which they have been eliminated, and fisheries management from a multispecies ecosystem perspective. Techniques are discussed and examples provided. A few colonial waterbird species have increased greatly in numbers and now pose problems for other bird species or are in conflict with people. Management is also involved in the control of such problem birds. Strategies include habitat modifications and scaring or killing problem birds. There is a continuing need for information and research to allow appropriate management to be applied. Regular surveys and inventories are necessary on a regional basis to detect trends in population status and to minimize and mitigate conflicts with human activities. Studies of population demography are needed for species declining in numbers, and research into species ecology is often necessary before appropriate manage- ment can be applied. Additional techniques to reduce conflicts between birds and humans are also needed. Human activities are likely a major limiting factor for some species, and, conversely, humans are largely responsible for the increases of several species that have become problems. Continued education of the public and of conservation management agencies to the role and importance of colonial waterbirds is important. Many species are likely to continue to suffer from gradual incremental loss and degradation of habitats, and a conservation strategy for the protection of these birds throughout North America is recommended.

Chlorophyll budgets: response to food web manipulation

Budgets for chlorophyll a and selected degradation products were constructed during summer stratification in 1984 and 1985 in two lakes. In Paul Lake, the reference ecosystem, pigment sedimentation showed no significant interannual differences. In Tuesday Lake, fish manipulations in May 1985 changed the zooplankton from an assemblage of Bosmina and small cyclopoids to one of large cladoceran grazers. Sedimentation of pigments, especially the grazing indicator pheophorbide a, increased significantly as the grazer assemblage changed. Mean grazer size was positively related to pheophorbide deposition rate in both lakes. Results of this whole-lake experiment indicate that major changes in lake food webs alter pigment deposition rates.

Recent Changes in the Aquatic Food Web of Eastern Lake Ontario

During the past 10 yr there have been dramatic increases in piscivorous populations of walleye (Stizostedion vitreum) in the Bay of Quinte and of planted lake trout (Salvelinus namaycush) and double-crested cormorants (Phalacrocorax auritus) in the Kingston basin of Lake Ontario. This paper documents changes in the prey stocks shared by these piscivores, including reduction in size and abundance of the slimy sculpin (Cottus cognatus) with subsequent stock equilibration and unabated reduction in the size and abundance of rainbow smelt (Osmerus mordax), both owing to predation pressure from the lake trout. The third primary prey species, alewife (Alosa pseudoharengus), has not yet suffered severely from the depredations of the piscivores. Symptoms of the effects are discernible, however, and the implications of a possible collapse are discussed. Lake trout survival and growth have not yet been affected by the changes in prey availability, but their diet has shifted both in response to their own expanded size composition and relative abundance of the three prey species. Increased utilization of alewife makes the hunting of the trout more pelagic. Problems of obtaining representative samples of the prey are discussed.

Long-Term Ecosystem Stress: The Effects of Years of Experimental Acidification on a Small Lake

Experimental acidification of a small lake from an original pH value of 6.8 to 5.0 over an 8-year period caused a number of dramatic changes in the lake's food web. Changes in phytoplankton species, cessation of fish reproduction, disappearance of the benthic crustaceans, and appearance of filamentous algae in the littoral zone were consistent with deductions from synoptic surveys of lakes in regions of high acid deposition. Contrary to what had been expected from synoptic surveys, acidification of Lake 223 did not cause decreases in primary production, rates of decomposition, or nutrient concentrations. Key organisms in the food web leading to lake trout, including Mysis relicta and Pimephales promelas, were eliminated from the lake at pH values as high as 5.8, an indication that irreversible stresses on aquatic ecosystems occur earlier in the acidification process than was heretofore believed. These changes are caused by hydrogen ion alone, and not by the secondary effect of aluminum toxicity. Since no species of fish reproduced at pH values below 5.4, the lake would become fishless within about a decade on the basis of the natural mortalities of the most long-lived species.