Grazing Food Chain Research Articles

Acidification Effects on the Algal–Zooplankton Interface

An in situ mesocosm experiment was done to assess the impacts of acidification on algal and zooplankton biomass and C assimilation, and on the efficiency of energy transfer in the grazing food chain. Triplicate mesocosms were left untreated (pH 8.5), or were acidified to pH 6.5, 5.5, or 4.5 with H2SO4 over 9 d. Algal biomass was reduced at pH 6.5 and lower, but showed no further decline across the gradient of pH 6.5–4.5. Algal C assimilation rates consistently declined with decreasing pH, reflecting a shift in dominance to larger, less productive cells at pH 5.5 and 4.5. Zooplankton biomass and productivity also declined with decreasing pH, as nearly all crustacean herbivores became extinct. In the low-pH treatments, only cyclopoids and rotifers persisted. Overall, there were significant reductions in the ratios of zooplankton/algal biomass and zooplankton/algal C assimilation (ecological transfer efficiency) with declining pH. The latter was a result of reduced grazer biomass, rather than reduced grazing efficiency; the mean zooplankton P/B ratio at pH 4.5 exceeded that measured in the higher pH treatments.

Loose and firmly attached epiphyton: Their relative contributions to algal and bacterial carbon productivity in a Phragmites marsh

The relative contributions of the loose and firmly attached epiphytic components to biomass, primary productivity (Na 2H 14CO 3 assimilation) and bacterial secondary productivity ( 3H methyl-thymidine incorporation) were studied on one occasion during spring in a Phragmites australis (Cav.) Steud. marsh on the highveld of South Africa (27° 53′E; 26° 19′S). The loose epiphyton contributed orders of magnitude more to algal and bacterial production than the firmly attached component and thus its exclusion or loss during sampling could result in a significant underestimate of total epiphyton production. Conversion of dissolved organic matter to a particulate form during bacterial production may be an important input to the grazing food chain of aquatic macrophyte communities.

Production, Predation, and Decomposition in a Low-Salinity Hibiscus Marsh

The rates of production, leaf grazing, and stem decomposition associated with Hibiscus moscheutos were quantified to contrast the ecology of this brackish herbaceous perennial with fresh and salt marsh species. Average net aerial primary production (NAPP) during 1978 and 1979 was estimated to be slightly over 1200 g°m—2°yr—1, which was calculated by summing: peak standing crop (49%); leaf mortality and litter production (36%); and losses to herbivores (15%). Three species of herbivorous insects were found to be important grazers: Atomacera decepta, Chionodes hibiscella, and Althaeus hibisci. Herbivore consumption was far greater than previously reported in salt marsh systems and rivals that of terrestrial communities. The root: shoot ratio (2.3) is comparable to many fresh marsh species but lower than most high—salinity species. The decomposition rate of Hibiscus stem material was estimated to be on the order of 7—8 yr, or up to five times slower than stems and leaves of other typical brackish marsh species (Typha, Scirpus, and Phragmites). The export potential of detritus from decaying stems is apparently minor because of this slow rate of decomposition combined with the diminished tidal action typical of Hibiscus—dominated marshes. We hypothesize that this unusual marsh dominant differs from Spartina in salt marsh systems by nourishing a more robust grazing food chain and by not exporting significant quantities of detrital material.

The importance of grazing food chain for energy flow and production in three intertidal sand bottom communities of the northern Wadden Sea

In three intertidal sand bottom communities of the “Konigshafen” (Island of Sylt, North Sea), the biomass production and respiration of phytobenthos, phytoplankton, macrozoobenthos, and in situ community metabolism were measured monthly during 1980. The study sites were characterized by different communities (Nereis-Corophium-belt, seagrass-bed,Arenicola-flat) and by a high abundance of the molluscHydrobia ulvae. Benthic diatoms are the major constituents of plant biomass in theArenicola-flat. In this community, gross primary productivity amounts to 148 g C m−2 a−1. 82 % of this productivity is caused by microbenthos, whereas phytoplankton constitutes only 18 %. In the seagrass-bed, gross primary productivity amounts to 473 g C m−2 a−1. 79 % of this is generated by seagrass and its epiphytes, whereas microphytobenthos contributes 19 %. In theNereis-Corophium-belt, only microphytobenthos is important for biomass and primary productivity (gross: 152 g C m−2 a−1). Annual production of macrofauna proved to be similar in theArenicola-flat (30 g C m−2 a−1) to that in the seagrass-bed (29 g C m−2 a−1). Only one third of this amount is produced in theNereis-Corophium-belt (10 g C m−2 a−1). The main part of secondary production and animal respiration is contributed by grazingH. ulvae. In the seagrass-bed, 83 % of the energy used for production is obtained from the grazing food chain. In theArenicola-flat and theNereis-Corophium-belt, the importance of non-grazing species is greater. A synchrony of seasonal development of plant biomass and monthly secondary production was observed. In theArenicola-flat and the seagrass-bed, where density and production of macrofauna are high, a conspicuous decrease in biomass of microbenthos occurs during the warmer season, whereas in theNereis-Corophium-belt primary production causes an increase in microphytobenthic biomass in summer and autumn. Energy flow through the macrofauna amounts to 69 g C m−2 a−1 in theArenicola-flat, 85 g C m−2 a−1 in the seagrass-bed and 35 g C m−2 a−1 in theNereis-Corophium-belt. Based on the assumption that sources of food are used in proportion to their availability, 49 g C m−2 a−1 (Arenicola-flat), 72 g C m−2 a−1 (seagrass-bed) and 26 g C m−2 a−1 (Nereis-Corophium-belt) are estimated as taken up by the grazing food chain. All three subsystems are able to support the energy requirements from their own primary production and are not dependent on energy import from adjacent ecosystems.

Resource Quality, Mutualism, and Energy Partitioning in Food Chains

Energy of net primary production (NPP) flows to heterotrophs in the form of living tissue, the grazing food chain, or as nonliving organic matter, the detritus food chain. When resource quality and the nature of control of transfer at the autotroph-heterotroph interface are considered, it is evident that both grazing and detritus flow routes involve pathways that are sufficiently different to justify recognition as distinct food chains. Accordingly, we propose a multichannel food chain model with six major pathways as follows (fig. 1): (1) direct grazing; (2) granivorous; (3) detrital POM (particulate organic matter); (4) detrital DOM (dissolved organic matter extruded); (5) active extraction (of photosynthate); and (6) nectarivorous. The active extraction pathway (5) can be subdivided into mutualistic and parasitic. Over the short term, paths 3 or 4 are donor controlled, and 1 and 5 are recipient controlled, while 2, 5A, and 6 involve both. Over evolutionary time, most, if not all, autotroph-heterotroph transfers appear to be mutualistic since numerous positive feedbacks have been documented that enhance the welfare of primary producers and contribute to homeostasis at the ecosystem level. A picture that seems to be emerging is that homeostasis at the ecosystem level can be mediated through autotroph-heterotroph mutualism that involves a few highly specialized, coevolved species (the "keystone mutualists" of Gilbert [1980]). Accordingly, the food web network may be an important part of the diffuse subsystem cybernetics that distinguishes ecosystems from organisms and man-made servomechanisms (as recently pointed out by Patten and Odum [1981]). The granivorous and nectarivorous food chains are absent in aquatic ecosystems that lack flowering plants, but in shallow water ecosystems (wetlands, estuaries, etc.) a secondary detritus-based carbon and energy recovery anaerobic route involving reduced inorganic compounds and chemolithotrophic bacteria may be important. Conventional methods of measuring NPP often fail to include, or grossly under-estimate, the high resource quality flows such as those involving DOM exudates from roots and algal cells, and active extraction of photosynthate by mutualistic microorganisms. Data from Biever's study of sporocarp production in pine plantations are presented to show that more than 15% of NPP may flow along this mycorrhizal path. Since feedback response is rapid, that is, nutrients flow back to the plant very soon after the outflow of photosynthate, mycorrhizal networks could very well be a major cybernetic control subsystem, especially in terrestrial communities on nutrient-poor substrate.

Structure and function of a warm monomictic lake

Abstract Construction and simulation of a model of Lake Conway, Florida, U.S.A., provided a framework for defining major characteristics of this ecosystem. The relationships that are formalized in this model comprise a set of hypotheses about the nature of a warm monomictic lake. The data that were used to parameterize the model came primarily from literature estimates, although approximations of biomass levels were available from associated research conducted on the lake. Submersed macrophytes are a major biomass component in Lake Conway; simulation suggested that their role in nutrient recycling overshadows their importance in the grazing food chain. Phytoplankton biomass and degree of fluctuation are considerably lower than are observed in most cool temperate lakes, although simulated respiration and herbivory rates are closer to temperate values than tropical values. Simulated epipelic algae biomass varies an order of magnitude during the year, and this group appears to be a significant part of the food chain. Simulated zooplankton consumption and turnover rates are very high, in part because of the relatively small biomass per individual. Simulation of the model suggests that slightly more carbon is processed through the grazing food chain in Lake Conway than through the detritus food chain.

WETLANDS AS ACCRETING SYSTEMS: ORGANIC CARBON

SUMMARY Accretion of organic carbon in a wetland occurs when inputs of allochthonous organic matter and primary production exceed losses by heterotrophic respiration and export via the outflow. Although both dissolved and particulate organic matter (DOM, POM) are produced, most accretion is of POM and occurs in the sediments. In general, rates of accretion are highest in permanently flooded swamps and marshes where emergent plant production dominates and decomposition is reduced by a combination of low pH, low redox potential and a reduced detritivore component. Wetlands which experience frequent wetting and drying such as floodplains and tidal basins show least accretion as losses via the grazing food chain and exports are large, and decomposition is rapid under the more oxidised conditions. Methods for measuring the fluxes of organic carbon between different pools in a wetland are poorly developed and present day accretion rates have only been estimated in a few instances. Improved methods are particula...

Adaptive Strategies at the Ecosystem Level: Examples from Two Benthic Marine Systems

. Adaptive strategies on the ecosystem level are demonstrated using the properties of systems (community biomass, trophic level energy transfer). Development of stabilizing storages versus rapid turnover and energy transfer through the grazing food chain versus detritus food chain are shown to be adaptive processes leading towards an optimal performance of the system. A model of incorporation and processing of information in ecosystems is proposed which contains the essential features of evolution. The persistence of “interspecific altruism” is discussed.

Length and evolutionary stability of food chains

Grazing food chains in natural ecosystems tend to be relatively short, usually involving no more than four or five trophic levels 1–3. Cohen4 describes 19 food webs with a total of 102 top predators. Recent independent analyses of these data by Cohen4 and Pimm (see ref. 5) show that most top predators occupy the third trophic level. However, as Slobodkin3 has observed, “It is possible to conceive of predators that are small enough and ferocious enough to attempt to live off the top of a trophic pyramid, and thereby become a seventh, or even an eighth, trophic level. It is by no means clear why this situation has not occurred.” Our answer, which we discuss here, incorporates energy transfer calculations introduced by Hutchinson1 and Odum and Odum2 within the framework of the evolutionarily stable strategy (ESS) developed by Maynard-Smith and Price 6,7. We argue that food chains should collapse to the shortest possible length compatible with biochemical and physiological constraints. This minimal length is frequently three, and is largely independent of primary productivity.

The Lake Washington Ecosystem: The Perspective from the Fish Community Production and Forage Base

In Lake Washington, fish production through detritus-based food chains is substantially greater than fish production through the grazing food chain. The lack of significant grazing by fish on the zooplankton is a consequence of both piscivore predation and conditions in the planktivore spawning environment. At low planktivore abundance, squawfish may switch to benthos feeding, exploiting the abundant prickly sculpin. At high planktivore abundance, squawfish feed more heavily on planktivores. Thus, even when reproductive success of planktivores is good, swamping of the squawfish population does not occur and depensatory mortality due to squawfish predation prevents planktivore abundance from increasing to the point where zooplankton resource depletion would occur. Benthic–littoral species are vulnerable to predation essentially only as larva and juveniles. They avoid predation by occupying littoral and epibenthic refugia. Recruitment to the adult population from these refugia may be sufficient to account for the greater rate of benthos exploitation by fish relative to the rate of zooplankton exploitation by fish. Neomysis is an important component of the Lake Washington fish production, since potentially Neomysis is a regulating agent on the zooplankton, and reduction in Neomysis predation on zooplankton, due to decreasing abundance and a deeper vertical distribution, may be partly responsible for the recent reappearance of Daphnia. The response of the fish community to trophic changes in Lake Washington has been slight. No consistent trends in the growth of fish utilizing zooplankton were observed. However, annual growth increments of consumers utilizing the benthic detrital food chain have declined with sewage diversion. The insights gained from analyzing the Lake Washington fish community structure and the Lake Washington carbon budget corroborate the above response of the fish community to trophic changes. Planktivores are predator-controlled and not able to deplete zooplankton resources, and thus would be insensitive to alterations in standing crop of zooplankton. On the other hand, benthic–littoral fish are more resource-limited and would be expected to respond to alterations in their forage base. Key words: ecosystem, fish, production, Lake Washington, eutrophication, predation

Detritus in the Lake Ecosystem

Present views of the roles of detritus are not consistent with the ecosystem concept, and result in an underestimation of the importance of carbon and energy pathways involving detritus. Nonpredatory losses from a plant-herbivore transfer, particularly as dissolved organic matter and egestion, may be greater than the amount of material and energy transmitted up the grazer food chain. Ecosystem efficiencies are significantly higher than Lindeman (food chain) efficiencies. Benthic anaerobic metabolism (CO2 production) is greater than benthic oxygen uptake estimates indicate, and the energy-rich intermediate products which undergo deferred respiration (O2 uptake) far from their origin may create redox gradients, drive chemosynthesis, and subsidize bacterial photosynthesis in the water column. Thus, aquatic detrital material, particulate and dissolved, and its metabolism create a dynamic structure in lake sediments and water somewhat analogous to the trophic-dynamic aspect of the biota. However, detrital-dynamic structure in lakes is not simply an alternative mode of support for biota, but is a major factor in the entrainment of abiotic substances which subsidizes the biota.

A Radionuclide Tracer Study of Arthropod Food Chains in a Spartina Salt Marsh Ecosystem

The arthropods of the Spartina marsh obtain their energy either by grazing on the marsh grass or by eating the microbial—rich organic detritus that is largely derived from the dead Spartina. The two energy sources were labeled with P32 in separate quadrats and the subsequent buildup of radioactivity followed in the arthropod populations. Four species of insects were dominant grazing organisms (one Orthoptera, two Hemiptera and one Homoptera) while two families of Diptera, (Dolichopodidae and Ephydridae) included the important insects associated with the detritus complex. The spiders were the important carnivores and obtained their energy from both the detritus and grazing food chains.

A "Stem Well" Method of Introducing Radioisotopes into Plants to Study Food Chains

To study the transfer to material through grazing food chains in herbaceous vegetation, a well surrounding the lower stem was used to introduce the radionuclides Fe59, Sr85, and Zn65 into the vascular tissue. In most cases after 1 week the radionuclides from the wells were distributed throughout the aboveground part of the plants. Iron—59 was uniformly distributed in the folige of Sorghum and Solidago. Strontium—85 was distributed uniformly in Sorghum but remained concentrated in the stem of Solidago and did not move into the leaves. Zinc—65 moved laterally into the leaves near the point of application but its distribution in the upper portions of the plants was spotty. In Rumex, however, zinc—65 moved to all portions of the plant above the well. The same radionuclides applied as spots to the leaves did not move throughout the plant but remained concentrated at the points of application. See full-text article at JSTOR