Organisms In Food Webs Research Articles

Bioaccumulation of organochlorines through a remote freshwater food web in the Canadian Arctic

Persistent organochlorines (OCs) were measured in water, sediment and biota from a remote lake in the Canadian Arctic to examine the bioaccumulation of atmospherically deposited OCs through a high-latitude freshwater food web. All major OC groups (polychlorinated biphenyls [PCBs], dichlorodiphenyl trichloroethane and metabolites [DDT], chlordane [CHL]-related compounds, and hexachlorocyclohexane [HCH] isomers) were detectable at sub-ng liter −1 concentrations in lake water and at low ng g −1 levels (dry wt) in surface sediments. Estimates of air–water exchange using a two-film model suggested that lake waters were close to equilibrium with the atmosphere for αHCH, trans-nonachlor and p,p′-DDE but that PCB congeners 52 and 153 were undergoing volatilization. Despite the relatively short open water season, gas absorption was found to be a major input pathway into the lake, when compared to estimated inputs from precipitation. Mean concentrations of ΣHCH, ΣDDT, ΣCHL and ΣPCB ranged from 1 to 10 ng g −1 wet wt in arctic char and from 2 to 82 ng g −1 in lake trout and were comparable to results from other lakes in the Canadian Arctic. Exceptionally high concentrations (six- to 10-fold higher than mean values) of these OCs were observed in a few lake trout and were explained by the larger size, longer lifespans and higher lipid contents of these individuals. Trophic interrelationships of food-web organisms were characterized using their tissue stable carbon and nitrogen isotope ratios. Through the food web, lipid-adjusted concentrations of αHCH, trans-nonachlor, p,p′-DDE, and PCB congeners 52 and 153 in biota were significantly related to their trophic position as determined by measurements of tissue stable nitrogen isotope ratios. In addition, the slopes and coefficients of determination for these relations increased with the lipophilicity and recalcitrance of the OCs, demonstrating that the more lipophilic OCs bioaccumulated to a greater degree through this food web even after accounting for the effects of lipid.

Responses of soil foodweb organisms in the first year following clearcutting and application of chloropicrin to control laminated root rot

Alteration in soil foodweb organism populations (numbers, biomass and activity) were monitored following clearcutting of a mature 70-year old Douglas-fir forest and compared with an uncut mature Douglas-fir stand. Changes in soil foodweb organisms were assessed following application of chloropicrin to stumps in the clearcut to reduce infection by Phellinus weirii, which causes laminated root rot. The effect of chloropicrin was measured using two different spatial scales: (1) around treated stumps, and (2) within 0.2 ha areas where all stumps within the area were treated with chloropicrin. A 10-fold decrease in total fungal biomass, between 2- and 3-fold reductions in total bacterial biomass, a 2-fold reduction in nematode numbers and a nearly 10-fold reduction in the ratio of total fungal to total bacterial biomass were observed between 9 months to a year following clearcutting. Active fungal biomass in the 20% chloropicrin treatment was reduced 1 year (October) after application. Reduction in amoebae numbers were observed between April and June in the 100% chloropicrin treatments, while no effects of chloropicrin on total fungal biomass, total bacterial biomass, flagellate, ciliate or nematode numbers were observed. The ratio of total fungal to total bacterial biomass was reduced as compared to the control in the 100% chloropicrin-treated infected areas in April following chloropicrin treatment. Chloropicrin decreased soil foodweb organisms in only one out of 45 random soil samples from 0.2 ha areas. In samples collected from around stumps, three points out of 180 had significant decreases in all organism numbers as compared to non-chloropicrin treated stumps. Chloropicrin-sensitive tomato and alfalfa seedlings, planted at 2 m from stumps to monitor the release of chloropicrin from stumps, indicated there was no significant release of chloropicrin in the first year following clearcutting and chloropicrin application.

Effects of the Zebra Mussel, Dreissena polymorpha, on Community Nitrogen Dynamics in Saginaw Bay, Lake Huron

The effects of the zebra mussel, Dreissena polymorpha, on chlorophyll and nutrient concentration changes and community ammonium uptake and regeneration rates were determined in bottle experiments on waters collected from a eutrophic site and an oligotrophic site in Saginaw Bay, Lake Huron in 1992. Our objectives were to estimate nitrogen cycling rates and to determine the direct (excretion) and indirect (foodweb) effects of the zebra mussel on these rates. Isotope labeling experiments with added 15NH4+ were conducted on waters collected on five sampling dates between April and October. Direct effects of zebra mussels on ammonium regeneration and potential uptake were examined by comparing results from bottles incubated with (15 individuals in 4 L lake water) and without added zebra mussels. Indirect foodweb effects were examined by measuring regeneration and potential uptake rates in subsamples of water that had previously been incubated in the presence or absence of zebra mussels.Zebra mussels removed a large fraction of chlorophyll from the oligotrophic site on all sampling dates and from the eutrophic site in October, but had a negligible effect on chlorophyll levels in waters from the eutrophic site in June, July, August, and September when cyanophytes were abundant. Community ammonium regeneration rates and uptake rates both followed seasonal patterns resembling those for chlorophyll concentrations in control treatments at the eutrophic site. Rates for water from the oligotrophic site were low (usually not significantly different from zero) and are not reported here. Community ammonium regeneration rates were consistently enhanced in the presence of zebra mussels, indicating that zebra mussel excretion could have a dominant effect on nitrogen regeneration in regions where it is abundant. Zebra mussels appeared to decrease community uptake rates of ammonium in August and September but did not predictably affect nitrogen remineralization rates by other lower foodweb organisms (e.g. bacteria, protozoans, zooplankton).

Effects of anoxia on the viability of subitaneous eggs of planktonic copepods

The effect of anoxia on the viability (as evidenced by hatching) of newly spawned and fully developed subitaneous eggs of three species of copepods, Acartia tonsa (Dana), Centropages hamatus (Lilljeborg), and Labidocera aestiva (Wheeler) was determined for eggs produced in the laboratory by adults collected from inshore waters of the northeastern Gulf of Mexico between January and August 1992. Hatching success decreased to 50% or less when eggs were exposed to oxygen concentrations <0.06 mll-1 for more than 12 d, except for newly spawned eggs of L. aestiva, which still showed 50% survival after 32 d of exposure to anoxia. For all three species, newly spawned eggs survived exposure to anoxia longer than fully developed eggs. The results indicate that the increasing occurrence of anoxia in estuarine and coastal waters could have a major impact on the population growth of these important food web organisms.

Benthic trophic dynamics in California coastal basin and continental slope communities inferred using inverse analysis

The isolation of the deep-sea benthos makes it difficult to measure more than a few elemental transformation rates in a given ecosystem. The measurements usually made include the transfer rates of such biologically important materials as oxygen, particulate carbon, and ammonia between water and sediment. We have developed inverse analysis techniques that produce descriptions which include material flows between trophic groups within the sediments. The techniques use a limited set of process rate and biomass measurements and known physiological and chemical information. Estimated material flows include rates of consun~ption, production, respiration, excretion, and egestion of the major trophic groups in benthic food webs. We analyzed benthic communities in a lowoxygen environment (Santa Monica Basin) and in a higher-oxygen environment on the conhnental slope (Patton Escarpment), both bordering southern Cahfornia (USA). The inverse analyses suggested that the low oxygen community was dominated by anaerobic protozoa and bactena and supported only a small populations of grazers. The dominance of bacteria with high growth rates caused sedimentary detrital carbon and nitrogen there to decompose in a few days. The Patton Escarpment community, overlain with oxygen-rich water, had a more complex food web dominated by the higher trophic level protozoa, meiofauna, and macrofauna. We suggest that carbon and nitrogen were retained in the biomass of these larger grazers for months. The analyses showed that grazers and microbial organisms speciahze in using resources of different nutritional quality. Specialization of this type may have led to a more complete oxidation of sedimenting detritus at Patton Escarpment than at Santa Monica Basin. Organisms in both food webs had low gross production efficiencies that averaged 10 % at the Patton Escarpment and 7 % at the Santa Monica Basin sites. These results suggest that inverse analysis can be a powerful tool to analyze benthic communities.

Epilithon Patch Structure in a Boreal River

Epilithon patch structure was investigated in a 4th-order boreal river in northern New York State, USA. Benthic and suspended sampling units, consisting of 120-cm long strands of 5-mm glass beads oriented parallel to the current, were placed in a fast-flowing riffle and an adjacent slow-flowing peripheral channel 2 mo prior to sampling. During three sampling periods, spring, summer, and autumn, 42 units were collected. The substratum contour beneath each benthic sampling unit was mapped, and epilithon patch development was examined at 1 cm resolution by assaying bead pairs for either chlorophyll or alkaline phosphatase activity. Epilithon patch length, estimated by spatial autocorrelation analysis, was 10 ± 8 (SD) cm, approximately half that of the cobble and boulder substratum (22 ± 9 cm), with no apparent differences between sites or seasons. The magnitude of between-patch variation (patch amplitude), quantified by analysis of semivariance, did show spatial and temporal patterns that were dependent on the parameter measured. Between-patch variation in alkaline phosphatase activity was consistently higher at the fast site, with an average amplitude of about 24 μ mol h^-1 cm^-2 vs. 12 for the slow site. Chlorophyll data showed a seasonal pattern with the highest between-patch variation occurring during the summer. The average fractal dimension of the epilithon (D = 1.81) was much greater than that of the substratum (D = 1.25), suggesting that epilithon patch structure emerged primarily from small scale processes such as turbulence and macroinvertebrate activity. The complexity of the patch structure has implications for food web organization, patch stability, and nutrient spiralling.

The role of the microbial loop in Antarctic pelagic ecosystems

The Paradigm pelagic food web organization in Antarctic waters is undergoing fundamental revision evidence that large fractions of material and energy flow through the microbial food web. because of the unique Antarctic ecosystem conditions, the microbial food web performs some roles that are fundamentally different from those in oligotrophic temperate and tropical waters: 1) during winter, bacterial production, at the expense of slow-turnover DOM (dissolved organic matter) from the previous summer, could be a significant factor in the survival of over wintering animal populations; 2) microbial regeneration of ammonium in nitrate-replete Antarctic waters may spare the reductants necessary for nitrate assimilation and thus enhance primary productivity of deep-mixed light-limited phytoplankton; and 3) the small diatoms and phytoflagellates which dominate the Antarctic pelagic primary production are apparently directly digestible by the metazoan herbivores, whereas cyanobacteria which dominate the primary productivity in lower latitude oligotrophic waters are not digestible by the metazoan herbivores. These roles performed by the microbial loop may, in part, explain why Antarctic waters, in contrast to the lower latitude oligotrophic waters, have high levels of tertiary productivity despite low primary productivity.

Lipid composition of the toxic marine diatom, Nitzschia pungens

A bloom of Nitzschia pungens f. multiseries was the source of domoic acid in mussels which caused an outbreak of amnesic shellfish poisoning in Canada. Because algal lipid and fatty acid production is known to vary during blooms we examined the lipid content, the lipid class composition and the fatty acid composition of N. pungens grown under nutrient-replete and silicon-limited conditions. The total lipid content ranged from 13 to 30 pg per cell; the major lipid classes were polar lipids (including phospholipids) and free fatty acids. GC analysis of total fatty acid methyl esters showed that the proportion of total dienes, trienes and tetraenes increased with silicon limitation at the expense of saturated fatty acids. N. pungens had unusually high contents of 16:4n1 which increased with silicon limitation to 11±1% of the total fatty acids. High proportions of this polyunsaturated fatty acid may thus be a useful marker of N. pungens in natural plankton blooms and associated food web organisms. Lipids extracted from cultured N. pungens may also be a useful source of 16:4n1 for biomedical studies.

Long-Term Development of Biological Communities

I shall define biological community as the species comprising a temporally and spatially localized assemblage. Although members of a community may interact, I do not wish to imply that the community behaves as a functional unit or that interactions among members determine its structure. When discussing community structure in this paper, I refer exclusively to the diversity of species because of the general availability of comparative data and because most of community theory makes predictions concerning number of coexisting species. Other important community attributes, such as relative abundance patterns, trophic level structure, food web organization, resistance to and recovery from perturbation, and physical architecture, will not be considered.

Some lower food web organisms in the nutrition of marine harpacticoid copepods: an experimental study

Some lower food web organisms from the marine littoral environment were studied as food for harpacticoid copepods. In laboratory experiments, it could be shown that, among the ciliates, the slow-movingUronema sp. was taken up while the fast-movingEuplotes sp. was not.Asterionella glacialis, a pennate diatom with spiny projections, was unsuitable as food. The centric diatomSkeletonema costatum was ingested by all harpacticoid species tested, includingTisbe holothuriae, Paramphiascella vararensis, Amphiascoides debilis andDactylopodia vulgaris. All are epibenthic and phytal species occurring in the shallow waters of Helgoland (North Sea). The amount of ciliate and algal carbon taken up was less than that provided by bacteria under laboratory conditions. However, some diatom food may be essential for the development ofD. vulgaris.

Trophic links of community food webs.

This report describes and explains regularities in the numbers and kinds of trophic links in community food webs. To a first approximation, the mean number of trophic links in a community food web is proportional to the total number of trophic species. The mean number of trophic links between any two categories of trophic species (basal, intermediate, and top) is proportional to the geometric mean number of species in the categories joined. These linear relationships, and scale-invariance in the proportions of basal, intermediate, and top species, make it possible to predict with remarkable precision the proportions of each kind of trophic link among all community food webs. The differences between food webs in constant and in fluctuating environments reflect apparently greater constraints on the trophic organization of food webs in fluctuating environments.

The Dynamics of Crayfish and Their Role in Ecosystems

An understanding of the population dynamics of crayfish and their trophic position is critical to understanding energy transformation in lakes. In many lakes crayfish dominate the annual production of the benthic biomass, at times reaching mean biomass values of 1000 1400 kg/ha and numerical densities of 15/m2. Density determines the rate of growth of various populations and determines reproductive capacity, age at maturity and life span. For Orconectes virilis, within the lakes we studied, population stability was mostly due to the short lag between a change in population density and an adjustment in fecundity. As a result, these populations appear to be self-regulated through density-dependent control of brood stock size. In Orconectes virilis, annual production was a function of mean annual biomass. Examination of available data on the ratio between other species showed them to fall within the same range as O. virilis. It seems that the evolutionary strategy governing crayfish biomass replacement and energy turnover has remained essentially unchanged. As regards their role in the community, crayfish do not fit the trophic level concept. Attempts to quantify their food habits show them to be polytrophic and a key energy transformer between various trophic links through utilization of all the trophic levels in lakes. Because of the multiple trophic roles of the crayfish, a comparison with the bioenergetics of various freshwater crust?cea is difficult. Crayfish may be most important in lakes with low phytoplankton production. In such lakes detritus and benthic algae, both of which are ingested by the crayfish, are important sources of overall energy flow. Since crayfish also function as major predators on the benthos, they most likely contribute to the stability of lake communities by interacting with a whole subweb of species ranging from large benthic invertebrates and submerged higher aquatics to the periphytic microcommunity. Hence, crayfish act as an important mechanism contributing to the regulation of most production processes and most specifically to the regulation of benthic production available to fish. In low nutrient lakes, because of their biomass dominance and functional role, crayfish greatly surpass the usual role assigned to them from their occurrence in fish diets. Manipulation of the benthic food chain would seem to be the most appropriate strategy to increase the fish production of such lakes. However, a greater appreciation of the complexity of the trophic organization of such food webs is necessary before such modifications can be attempted. 1 A contribution under Dingell-Johnson Project F-30-R, Michigan. 2 Present address: Department of Biology, Lakehead University, Thunder Bay, Ontario, Canada.

Model ecosystem studies of the environmental fate of six organochlorine pesticides.

Despite more than 20 years of intensive use there are major uncertainties about the environmental distribution and degradative fate of the various organochlorine pesticides in food web organisms. The problem is both controversial and momentous, as 88,641,000 pounds of cyclodienes and toxaphene and 59,316,000 pounds of DDT were produced in the United States in 1970 (1). The laboratory model ecosystem devised in this laboratory (2) has been used to estimate the comparative environmental properties of DDT, methoxychlor, and other DDT analogs (3-5). These investigations have demonstrated environmental degradative pathways and have provided quantitative data on ecological magnification and biodegradability index (6). This paper presents data on six additional organochlorine pesticides which should be of value in judging their overall effects on environmental quality. Furthermore, the data provide a realistic background of the environmental toxicology of standard substances against which screening data for new candidate pesticides can be weighed.