Subsidies of Aquatic Resources in Terrestrial Ecosystems

Ecosystems - Floods, spatially complex water flows, and organism movements all generate important fluxes of aquatic-derived materials into terrestrial habitats, counteracting the gravity-driven...     

Respiration of Aquatic and Terrestrial Amphibian Embryos

Respiratory constraints on the structure of single eggs andegg masses have affected the mode of amphibian reproductionin water and in air. Aquatic eggs generally require less oxygen,develop faster, and hatch earlier, but these characteristicsare related to small ovum size. A comparison of two speciesof aquatic and terrestrial breeding frogs with similarly sizedova shows no differences in hatching stage, maximum rate ofoxygen uptake, oxygen conductance of the egg capsule, or Po₂difference across the capsule. However, the aquatic speciesdevelops about 2.4 times faster and tolerates lower environmentalPo₂, suggesting adaptation for development in ephemeral water.Modelling of diffusive oxygen transport into a single aquaticegg shows that a large amount of jelly (or a boundary layer)around the capsule may not greatly restrict gas exchange, ifthe inner radius of the capsule is large. However, gelatinousegg masses that contain other embryos that compete for oxygenare therefore limited in size, unless the eggs are ventilatedby convection of water among them. Aquatic egg are often suspendedin masses above the substrate, promoting oxygen movement intothe mass from all directions. Terrestrial egg masses are morediffusion limited, because gravity and surface tension collapsethem, preventing convection between the eggs, and restrictingthe source for oxygen diffusion. Terrestrial embryos are oftenlarger than their aquatic counterparts and have higher demandsfor oxygen. Terrestrial conditions have selected for adaptationsthat reduce respiratory competition between embryos, for example,separating of embryos by large volumes of jelly or reducingthe number of eggs in a clutch. The size of foam nests is unlimited,because oxygen for each embryo is supplied directly from thefoam.     

Bimodal Respiration in Aquatic and Terrestrial Apodan Amphibians

SYNOPSIS. New data on resting metabolism, aerobic capacity,respiratory morphology and bimodal breathing capacities offourcaecilian species were combined with previously known data oncaecilian and amphibian biology to discern differing or unifyingthemes of gas exchange within the Gymnophiona and among theAmphibia. Discoveries include 1) a uniform resting metabolismamong caecilians that is distinctly lower than that of anuransand urodeles, 2) an aerobic capacity following activity thatrivals or exceeds that of other amphibians, and 3) despite previoussuggestions to the contrary, a substantial contribution of theskin to respiratory gas exchange, apparently sufficient to sustainthe low resting metabolism. In contrast, pulmonary gas transportappears to dominate during metabolic recovery from exercise,which is comparatively fast. The contributing roles of the skinand lungs to this extreme scope of respiratory needs in caeciliansare further discussed in light of the structural nature of andcirculatory supply to each respiratory bed, and the respiratoryproperties of caecilian blood.     

Intrinsic Mechanics and Control of Fast Cranio-Cervical Movements in Aquatic Feeding Turtles

Aquatic feeding strikes on agile prey in snake-necked turtlesinvolve fast neck extension, bucco-pharyngo-oesophageal expansion,and head retraction. The ultimate, rectilinear accelerationof the head towards the prey requires complex vertebral rotations,that vary widely from strike to strike. This poses complex motorcontrol issues for the numerous intrinsic neck-muscles, whichare the sole neck extensors. Mathematical modelling revealsthat extensor activity might be superfluous for this phase ofthe strike. The ultimate acceleration of the head at the endof the strike always coincides with forceful oropharyngeal expansion.The momentum of the induced flow of water is sufficient to pullthe head (and the neck) straight towards the prey. This buccalexpansion proceeds identically to that observed in primary aquaticfeeders: a rostro-caudal expansion sequence characterized byan optimal timing of the functional components supporting theexpansion wave. Yet distinct structural solutions, both at theskeletal, and muscular level, are involved. This points towardsprominent hydrodynamic constraints. Head and neck are retractedby extrinsic neck muscles. Given the high number of degreesof freedom, this musculo-skeletal system is obviously under-determined,which compromises control. We propose that erroneous foldingof the neck (i.e., diverging from the highly persistent retractedconfiguration) might be avoided through the presence of a subtleclick system at the level of the joint between cervical vertebrae5 and 6.     

Herbivores Enforce Sharp Boundaries Between Terrestrial and Aquatic Ecosystems

The transitions between ecosystems (ecotones) are often biodiversity hotspots, but we know little about the forces that shape them. Today, often sharp boundaries with low diversity are found between terrestrial and aquatic ecosystems. This has been attributed to environmental factors that hamper succession. However, ecosystem properties are often controlled by both bottom-up and top-down forces, but their relative importance in shaping riparian boundaries is not known. We hypothesize that (1) herbivores may enforce sharp transitions between terrestrial and aquatic ecosystems by inhibiting emergent vegetation expansion and reducing the width of the transition zone and (2) the vegetation expansion, diversity, and species turnover are related to abiotic factors in the absence of herbivores, but not in their presence. We tested these hypotheses in 50 paired grazed and ungrazed plots spread over ten wetlands, during two years. Excluding grazers increased vegetation expansion, cover, biomass, and species richness. In ungrazed plots, vegetation cover was negatively related to water depth, whereas plant species richness was negatively related to the vegetation N:P ratio. The presence of (mainly aquatic) herbivores overruled the effect of water depth on vegetation cover increase but did not interact with vegetation N:P ratio. Increased local extinction in the presence of herbivores explained the negative effect of herbivores on species richness, as local colonization rates were unaffected by grazing. We conclude that (aquatic) herbivores can strongly inhibit expansion of the riparian vegetation and reduce vegetation diversity over a range of environmental conditions. Consequently, herbivores enforce sharp boundaries between terrestrial and aquatic ecosystems.     

Physiological Diversity in Metabolism in Marine and Terrestrial Crustacea

Two aspects of metabolic adaptation to increased terrestrialismare considered: (1) respiratory adaptations as reflected bycomparative cytochrome c oxidase activity in tissues of crabsfrom aquatic and terrestrial habitats, and (2) thermal acclimationpatterns in cytochrome c oxidase activity in tissues from thesecrabs. Enzymatic assays were done spectrophotometrically ongill, muscle, and mid-gut gland tissues from two aquatic species,Libinia emarginata and Callinectes sapidus, and the terrestrialOcypode quadrata. Cytochrome c oxidase was chosen for this studysince it is generally believed that the more aerobic the cellsor tissues become, the more fully developed the cytochrome systemwill be. This enzyme is also thought to have a role in thermalacclimation. In gill tissue the activity of cytochrome c oxidase is enhancedwith the advent of aerial respiration. Enzymatic activity ofgill tissue from Ocypode quadrata was significantly greaterthan it was in tissue from the aquatic species. No correlationwas observed with increased terrestrialism and enzymatic activityof muscle or mid-gut gland tissue. The thermal acclimation patternsof tissues of these three species of crabs indicate a clear-cuttendency for less enzymatic adaptation to temperature at thetissue level as these crabs evolve toward a land habitat.     

The American Alligator Detects Food Chemicals in Aquatic and Terrestrial Environments

American alligators (Alligator mississippiensis) were offered raw meat or animal carcasses under water or on land in experiments where only chemoreception could be used to discriminate between these and control materials. Alligators also were presented with aqueous extracts and airborne chemicals from meat. Juvenile alligators, tested in indoor tanks, opened more cheesecloth packets containing meat than control packets when these materials were placed under water and on platforms above the water surface. Adult alligators, tested in outdoor semi-natural enclosures, removed more cheesecloth-wrapped meat presented under water, and more of both meat and raccoon (Procyon lotor) carcasses placed under perforated baskets on land, than control materials. Juvenile alligators, tested in tanks partially filled with water, exhibited more lateral head movements and mouth-openings to an aqueous beef extract than to water alone. Juvenile alligators, tested in an olfactometer, exhibited more gular pumps to airborne chemicals from beef than to distilled water. These experiments indicate that alligators may use chemical cues to locate food both on land and under water, and that they detect both water- and airborne chemicals from meat.     

Terrestrial Versus Aquatic Biology: The Medium and Its Message

Many basic biological functions are constrained by the physicalproperties of the fluids in which organisms live. Here I explorefour selected examples in which physical differences betweenair and water have contributed to the functional divergenceof terrestrial and aquatic organisms. 1. Water is about 800times as dense as air. As a result, while the cost of locomotionis generally less for aquatic organisms, the hydrodynamic forcesthey encounter are larger. 2. The combined effects of densityand viscosity insure that the capture of suspended particlesis mechanically more effective on land than in water. 3. Thespeed of sound is four times greater in water than in air, requiringaquatic organisms to use higher frequency sounds in echolocationsystems. 4. The resistivity of air is 16 orders of magnitudelarger than that of seawater, which might explain why aquaticanimals use electrical sensing organs to detect prey but terrestrialanimals do not.     

Physiological and Ecological Correlates of Preferred Temperature in Fish

Fishes released from constraints of their previous thermal historytend to limit thermal exposure to a narrow range of temperatures.Thermoregulatory behavior (final preferendum) is a stable characteristicof each species studied, suggesting strong regulation by naturalselection. However, the adaptive significance of the final preferendumhas not been documented for most species. Species with widegeographic distributions, such as bluegill sunfish and largemouthbass exhibit constant laboratory final preferenda regardlessof their geographic origin or thermal histories. This apparentevolutionary anomaly of the final preferendum requires furtherstudy. It is no longer adequate to describe responses of fishto thermal gradients, rather falsifiable hypotheses concerningadaptive relations between thermoselection, and physiological,biochemical, life-history and genetic correlates must be developedand tested experimentally.     

Cardiovascular Function in Turtles During Anoxia and Acidosis: In vivo and in vitro Studies

During prolonged experimental submergence, freshwater turtlesbecome anoxic and develop a combined respiratory and non-respiratoryacidosis. Anoxia and acidosis are known to depress cardiac functionin turtles and other vertebrate species. In vitro studies ofventricular and atrial tissue of turtles indicate that increasedextracellular Ca⁺⁺ concentration can reverse these depressantactions. Intact turtlesutilize this compensatory mechanism duringanoxia and other conditions leading to acidosis byincreasingplasma Ca⁺⁺ concentration. In addition, cardiac cells may releaseCa⁺⁺ from cell organelles to compensate for induced respiratoryacidosis. Both mechanisms presumably improve cardiac contractilityby elevating the level of sarcoplasmicCa⁺⁺.     

Physiological and Ecological Correlates of Prolonged Incubation in Sea Birds

Sea birds with long incubation periods are identified, togetherwith the features of their incubation physiology which distinguishthem from birds in general. Most sea birds with prolonged incubationare members of the order Procellariformes. The majority of Pelecaniformesand Charadniformes with long incubation periods are tropicalspecies. The total amount of water lost from the egg duringincubation is a similar fraction of the initial egg weight insea birds with prolonged incubation as in other birds. The oxygenconsumption of the newly hatched chick is similarly relatedto the chick weight, regardless of the duration of incubation.Within the constraints imposed by these similarities, sea birdswith prolonged incubation display a number of adaptations. Thedaily rate of water loss from the egg, the water vapor conductanceof the egg shell, and the total functional pore area of theegg are all relatively low in sea birds with prolonged incubation.The eggs of sea birds with long incubation times are large inrelation to the size of the adult bird in the two species thathave been studied, the high energy content of the egg is paralleledby the greater total amount of oxygen consumed during incubation.However, the growth of the embryo is relatively slow in at leastone sea bird with a long incubation time so that prolonged incubationprobably entails a comparatively high allocation of energy resourcesto maintenance requirements. In some species with prolongedincubation, the interval between pipping and hatching is alsolong and it appears to be a period of great physiological importance.Ecologically prolonged incubation is associated with eitherpelagic feeding habits or a tropical environment, both factorsmay be related to food supply.     

Terrestrial Contributions to the Aquatic Food Web in the Middle Yangtze River

Understanding the carbon sources supporting aquatic consumers in large rivers is essential for the protection of ecological integrity and for wildlife management. The relative importance of terrestrial and algal carbon to the aquatic food webs is still under intensive debate. The Yangtze River is the largest river in China and the third longest river in the world. The completion of the Three Gorges Dam (TGD) in 2003 has significantly altered the hydrological regime of the middle Yangtze River, but its immediate impact on carbon sources supporting the river food web is unknown. In this study, potential production sources from riparian and the main river channel, and selected aquatic consumers (invertebrates and fish) at an upstream constricted-channel site (Luoqi), a midstream estuarine site (Huanghua) and a near dam limnetic site (Maoping) of the TGD were collected for stable isotope (δ¹³C and δ¹⁵N) and IsoSource analyses. Model estimates indicated that terrestrial plants were the dominant carbon sources supporting the consumer taxa at the three study sites. Algal production appeared to play a supplemental role in supporting consumer production. The contribution from C₄ plants was more important than that of C₃ plants at the upstream site while C₃ plants were the more important carbon source to the consumers at the two impacted sites (Huanghua and Maoping), particularly at the midstream site. There was no trend of increase in the contribution of autochthonous production from the upstream to the downstream sites as the flow rate decreased dramatically along the main river channel due to the construction of TGD. Our findings, along with recent studies in rivers and lakes, are contradictory to studies that demonstrate the importance of algal carbon in the aquatic food web. Differences in system geomorphology, hydrology, habitat heterogeneity, and land use may account for these contradictory findings reported in various studies.     

Bacterial Growth on Photochemically Transformed Leachates from Aquatic and Terrestrial Primary Producers

Abstract We measured bacterial growth on phototransformed dissolved organic matter (DOM) leached from eight different primary producers. Leachates (10 mg C liter⁻¹) were exposed to artificial UVA + UVB radiation, or kept in darkness, for 20 h. DOM solutions were subsequently inoculated with lake water bacteria. Photoproduction of dissolved inorganic carbon (DIC), ranging from 3 to 16 μg C liter⁻¹ h⁻¹, and changes in the absorptive characteristics of the DOM were observed for all leachates upon UV irradiation. The effects of irradiation exposure on DOM bioavailability varied greatly, depending on leachate and type of bacterial growth criterion. Bacterial carbon utilization (biomass production plus respiration) over the entire incubation period (120 h) was enhanced by UV radiation of leachate from the terrestrial leaves, relative to carbon utilization in non-irradiated leachates. Conversely, carbon utilization was reduced by radiation of the leachates from aquatic macrophytes. In a separate experiment, the stable C and N isotope composition of bacteria grown on irradiated and non-irradiated DOM was estimated. Bacterial growth on UV-irradiated DOM was enriched in ¹³C relative to the bacteria in the non-irradiated treatments; this result may be explained by selective assimilation of photochemically produced, isotopically enriched labile compounds. Received: 17 February 2000; Accepted: 1 May 2000; Online Publication: 28 August 2000     

Physiological Correlates of Emotion-Regulation During Prolonged Cycling Performance

We proposed that experiencing unpleasant emotions during performance represents unsuccessful emotion-regulatory efforts, and that such effort concurrently tax physiological resources. We used data from 2-h cycling trials (N = 28) at a power output equivalent to lactate threshold. Emotions were calculated before and during cycling with ongoing assessments of ventilation, respiratory quotient, heart rate, and oxygen uptake. Emotion data indicated significant changes over time with all participants reporting decreases in vigour and increases in fatigue, with 14 cases of concurrent increases in anger, depression, and tension. After grouping participants into positive and negative emotion groups, a time x unpleasant emotion group ANOVA indicated a significant interaction effect for changes in ventilation (F _6,21 = 3.09, P = .03, Partial Eta² = .47) over time, with no significant difference in other physiological variables or perceived exertion. Among athletes reporting negative emotions, ventilation increased during the middle section, whereas among athletes reporting positive emotions, ventilation increased shortly before completion. Findings suggest that regulating negative emotion is an effortful process taxing physiological substrates.     

Trophic Transfer of Arsenic from an Aquatic Insect to Terrestrial Insect Predators

The movement of energy and nutrients from aquatic to terrestrial ecosystems can be substantial, and emergent aquatic insects can serve as biovectors not only for nutrients, but also for contaminants present in the aquatic environment. The terrestrial predators Tenodera aridifolia sinensis (Mantodea: Mantidae) and Tidarren haemorrhoidale (Araneae: Theridiidae) and the aquatic predator Buenoa scimitra (Hemiptera: Notonectidae) were chosen to evaluate the efficacy of arsenic transfer between aquatic and terrestrial environments. Culex tarsalis larvae were reared in either control water or water containing 1000 µg l⁻¹ arsenic. Adults that emerged from the control and arsenic treatments were fed to the terrestrial predators, and fourth instar larvae were fed to the aquatic predator reared in control or arsenic contaminated water. Tenodera a. sinensis fed arsenic-treated Cx. tarsalis accumulated 658±130 ng g⁻¹ of arsenic. There was no significant difference between control and arsenic-fed T. haemorrhoidale (range 142–290 ng g⁻¹). Buenoa scimitra accumulated 5120±406 ng g⁻¹ of arsenic when exposed to arsenic-fed Cx. tarsalis and reared in water containing 1000 µg l⁻¹ arsenic. There was no significant difference between controls or arsenic-fed B. scimitra that were not exposed to water-borne arsenic, indicating that for this species environmental exposure was more important in accumulation than strictly dietary arsenic. These results indicate that transfer to terrestrial predators may play an important role in arsenic cycling, which would be particularly true during periods of mass emergence of potential insect biovectors. Trophic transfer within the aquatic environment may still occur with secondary predation, or in predators with different feeding strategies.     

A Source of Terrestrial Organic Carbon to Investigate the Browning of Aquatic Ecosystems

There is growing evidence that terrestrial ecosystems are exporting more dissolved organic carbon (DOC) to aquatic ecosystems than they did just a few decades ago. This “browning” phenomenon will alter the chemistry, physics, and biology of inland water bodies in complex and difficult-to-predict ways. Experiments provide an opportunity to elucidate how browning will affect the stability and functioning of aquatic ecosystems. However, it is challenging to obtain sources of DOC that can be used for manipulations at ecologically relevant scales. In this study, we evaluated a commercially available source of humic substances (“Super Hume”) as an analog for natural sources of terrestrial DOC. Based on chemical characterizations, comparative surveys, and whole-ecosystem manipulations, we found that the physical and chemical properties of Super Hume are similar to those of natural DOC in aquatic and terrestrial ecosystems. For example, Super Hume attenuated solar radiation in ways that will not only influence the physiology of aquatic taxa but also the metabolism of entire ecosystems. Based on its chemical properties (high lignin content, high quinone content, and low C:N and C:P ratios), Super Hume is a fairly recalcitrant, low-quality resource for aquatic consumers. Nevertheless, we demonstrate that Super Hume can subsidize aquatic food webs through 1) the uptake of dissolved organic constituents by microorganisms, and 2) the consumption of particulate fractions by larger organisms (i.e., Daphnia). After discussing some of the caveats of Super Hume, we conclude that commercial sources of humic substances can be used to help address pressing ecological questions concerning the increased export of terrestrial DOC to aquatic ecosystems.