THERMAL SENSITIVITIES OF MALATE DEHYDROGENASE ISOZYMES IN TYPHA

Two species of cattail. Typha latifolia L. and T. domingensis Pers., occur naturally along the shoreline of Par Pond, an 1,128 ha impoundment subjected to intermittent thermal stress at the Savannah River Plant in South Carolina. T. domingensis is not tolerant of the stresses imposed at the hottest section of this lake. Typha latifolia is established throughout the lake but at the hottest section displays an altered morphology compared with stands from ambient areas. Both T. latifolia and T. domingensis are electrophoretically monomorphic, and no variation in isozyme expression in ten enzyme systems was recorded in populations from Par Pond. The malate dehydrogenase (MDH) isozyme patterns for T. latifolia and T. domingensis are identical, but their thermal sensitivities differ sharply. All six major T. domingensis MDH isozymes are denatured by heating at 50 C; whereas, T. latifolia has three isozymes which are stable at 50 C and three isozymes which are denatured at this temperature. Tests of thermal stability are more useful than measurements of electrophoretic mobility in detecting enzyme differences and a possible biochemical basis for differences in thermal tolerance in these natural populations.     

A thermal effluent as a sporadic cornucopia: effects on fish and zooplankton

Synopsis We report effects of a thermal effluent on fish and zooplankton in an impoundment in South Carolina. The effects are not directly due to heated effluent but are indirect effects on the trophic dynamics of the system and depend on the physical structure of the system, season, and geography. As effluent from a nuclear reactor cooling pond (Pond C) enters the larger Par Pond it sporadically carries with it zooplankton and/or dead fish. Pond C produces high zooplankton densities in two situations: (1) high and hot effluent flow during cold winter weather; and (2) low (cool) effluent flow during warmer seasons. High zooplankton densities attract Blueback herring, Alosa aestivalis, which attract largemouth bass, Micropterus salmoides. Dying and dead bluegills, Lepomis macrochirus from Pond C (heat killed) are eaten by Par Pond bass which swim into effluent temperatures as high as 46°C to take these easy prey. Blueback herring and largemouth bass distribution are affected by the presence of food rather than by temperature. Par Pond blueback herring appear to be available to bass mainly when herring are near the effluent from Pond C and when they spawn in spring in the littoral zone. Because Blueback herring in Par Pond live only one year and because their presence in bass habitat is seasonal there is a strong seasonal component to bass food abundance. A previously reported annual oscillation in bass condition (K), with a peak in winter, occurs throughout Par Pond but is extreme in the vicinity of the effluent. The strong seasonality of food abundance for bass at the effluent correlates the winter peak in bass condition.     

BIOCHEMICAL GENETICS OF MOSQUITOFISH. IV. CHANGES OF ALLELE FREQUENCIES THROUGH TIME AND SPACE

Gene frequency data from samples of Gambusia affinis populations at 76 localities across the Savannah River drainage were used to investigate temporal and spatial patterns in population genetic structure. Localities in the Par Pond system on the Savannah River Plant were sampled in 1971, 1977, and 1979. Allelic frequencies in these populations were generally stable through time, although significant temporal changes were observed among samples from Pond C, an impoundment receiving thermal effluent. Significant spatial heterogeneity in allele frequencies was observed on both microgeographic and regional scales. Populations within the Par Pond system were spatially subdivided at four of the five loci surveyed (mean F_ST = 0.051). Subdivision was even more pronounced when samples from across the Savannah River drainage were compared (mean F_ST = 0.196). A hierarchial analysis of gene diversity (G_ST) demonstrated that most of the genic diversity across the drainage exists as within-subdivision diversity. Even when populations from such contrasting habitats as rivers, creeks, ponds, and reservoirs are compared, an average of only 13% of the total gene diversity was attributed to between-group diversity. Greatest between-group gene diversity was observed when reservoirs were compared with one another. This general pattern of low between-habitat diversity suggests that differential selection pressures are not playing a major role in producing the observed levels of subdivision. In the Par Pond system, neither single locus nor multilocus genetic distances were significantly associated with geographic distance or with its reciprocal. For samples from over the Savannah River drainage, significant correlations between genetic and geographic distance were observed only for the Gpi-2 and Pgm-2 loci. Thus, there was a general lack of concordance between genetic and geographic distances. Spatial autocorrelation demonstrated patterns consistent with Wright's isolation by distance model. Significant positive correlations in allelic frequencies among neighboring populations were observed for five of six alleles; allelic frequencies in more distantly separated populations were typically not correlated.     

Lipid and reproductive cycles of bluegills (Lepomis macrochirus) exposed to 35 years of elevated environmental temperatures

1. We studied bluegills to compare how thermal perturbation affected lipid dynamics and reproductive cycles of fish from heated (Pond C) and normothermic (Par Pond) sites.

2. Bodies of bluegills contained over 90%, and gonads and livers 10% of the non-polar lipids.

3.Overall, the percent body lipids of bluegills from the heated pond was twice as high (12.4%) as that of Par Pond bluegills (6.6%).

4. During a prolonged reactor down period, when both ponds were normothermic, body lipids of bluegills from Pond C and Par Pond increased at a rate of approximately 3 and 1% of body mass per week, respectively.

5. Juveniles from Pond C had higher percent stored lipids than did adult males or females.

6. Lipid cycling in Par Pond bluegills was seasonal, whereas Pond C bluegill lipid cycles corresponded to long reactor down periods.     

Thermal Inactivation Properties of Enzymes from Typha latifolia L. Ecotypes

Irreversible thermal denaturation experiments with 3 enzymes from Typha latifolia populations native to distinct thermal climates produced 3 different responses: (1) malate dehydrogenase was much more resistant to high temperature inactivation when obtained from plants native to a hot climate, (2) glutamate-oxaloacetate transaminase was quite resistant to thermal denaturation regardless of origin, and (3) aldolase was rapidly inactivated by heat regardless of origin.     

Regeneration of Enteromorpha

Young plants of E. intestinalis have been grown in culture. Cutting off the thalli of attached plants just above the basal rhizoids leads to the regeneration of new branches. After a short time the point of wounding is indistinguishable. The process can be repeated many times; each regeneration resulting in a more branched thallus. Segments of unattached plants kept in culture may give rise to rhizoids from the basal cut ends and to papillae from the upper ends. They may also produce “bottle brush” forms similar to those found amongst ship-fouling algae. Such forms seem to arise when “swarmers” are retained in the parent cell and germinate in situ to give aggregations of juvenile plants. Their formation can be stimulated by a temperature shock.     

The influence of outdoor thermal environment on young Japanese females

The influence of short wave solar radiation appears to be strong outdoors in summer, and the influence of airflow appears to be strong outdoors in winter. The purpose of this paper was to clarify the influence of the outdoor environment on young Japanese females. This research shows the relationship between the physiological and psychological responses of humans and the enhanced conduction-corrected modified effective temperature (ETFe). Subjective experiments were conducted in an outdoor environment. Subjects were exposed to the thermal environment in a standing posture. Air temperature, humidity, air velocity, short wave solar radiation, long wave radiation, ground surface temperature, sky factor, and the green solid angle were measured. The temperatures of skin exposed to the atmosphere and in contact with the ground were measured. Thermal sensation and thermal comfort were measured by means of rating the whole-body thermal sensation (cold–hot) and the whole body thermal comfort (comfortable–uncomfortable) on a linear scale. Linear rating scales are given for the hot (100) and cold (0), and comfortable (100) and uncomfortable (0) directions only. Arbitrary values of 0 and 100 were assigned to each endpoint, the reported values read in, and the entire length converted into a numerical value with an arbitrary scale of 100 to give a linear rating scale. The ETFe considered to report a neither hot nor cold, thermally neutral sensation of 50 was 35.9 °C, with 32.3 °C and 42.9 °C, respectively, corresponding to the low and high temperature ends of the ETFe considered to report a neither comfortable nor uncomfortable comfort value of 50. The mean skin temperature considered to report a neither hot nor cold, thermally neutral sensation of 50 was 33.3 °C, with 31.0 °C and 34.3 °C, respectively, corresponding to the low and high temperature ends of the mean skin temperature considered to report a neither comfortable nor uncomfortable comfort value of 50. The acceptability raised the mean skin temperature even for thermal environment conditions in which ETFe was high.     

Field experiments on interactions between vertebrate predators and larval midges (Diptera: Chironomidae) in the littoral zone of a reservoir

Summary A field experiment was designed to test a frequent assumption in the literature that vertebrate predators (in this case, fish and turtles) are capable of regulating the seasonal abundance and diversity of benthic communities in the littoral zone of lentic environments. Effects of thermal effluents from a nuclear reactor on predator-prey relationships were also examined. Benthic samples were removed after each of three, 3-month test periods from 36 predator exclusion cages (4m²) and 36 control plots located along a thermal gradient in Par Pond, an 1,100 ha freshwater reservoir in the southeastern United States.Results of our field experiments provide little evidence to suggest that either a single keystone species or vertebrate predators as a group were capable of regulating the abundance, diversity or productivity of chironomids in Par Pond. The relationship between predator treatment and community response (changes in density and species richness) was generally unaffected by either plot location or temperature fluctuations. When data from caged and control plots were pooled, however, both location and water temperature individually had significant impacts on the chironomid community. Alternative hypotheses are proposed to explain the lack of regulatory control of the benthic community by individual species or guilds of predators.     

CYTOGENETIC STUDIES IN CLARKIA,SECTION PRIMIGENIA. IV. A CYTOLOGICAL SURVEY OF CLARKIA GRACILIS

Clarkia gracilis (2n = 28) is an allotetraploid which combines genomes from two subsections of section Primigenia. Natural populations consist of plants homozygous for a single chromosome arrangement, which may differ from the arrangements in other populations by one or more reciprocal translocations. One arrangement, the “standard,” was widespread. Cytological observations on plants derived from crosses between populations of C. gracilis, and on triploid plants derived from crosses with C. amoena subsp. huntiana, one of the parents of the tetraploid, were used to determine the end arrangements present. Ten arrangements were identified, and it was found that the standard amoena subgenome of C. gracilis is identical in end arrangement to the previously defined standard arrangement of the diploid C. amoena. Hence a race of C. amoena with this arrangement was involved in the hybridization which gave rise to C. gracilis. Evidence was found that other arrangements of C. amoena have probably been introduced into C. gracilis by subsequent introgressive hybridization. Cytological differences, coupled with differences in morphology, ecology, and distribution indicate that C. gracilis should be subdivided into four subspecies instead of the three presently recognized.     

Deep body and surface temperature responses to hot and cold environments in the zebra finch

Global warming increasingly challenges thermoregulation in endothermic animals, particularly in hot and dry environments where low water availability and high temperature increase the risk of hyperthermia. In birds, un-feathered body parts such as the head and bill work as ‘thermal windows’, because heat flux is higher compared to more insulated body regions. We studied how such structures were used in different thermal environments, and if heat flux properties change with time in a given temperature. We acclimated zebra finches (Taeniopygia guttata) to two different ambient temperatures, ‘cold’ (5 °C) and ‘hot’ (35 °C), and measured the response in core body temperature using a thermometer, and head surface temperature using thermal imaging. Birds in the hot treatment had 10.3 °C higher head temperature than those in the cold treatment. Thermal acclimation also resulted in heat storage in the hot group: core body temperature was 1.1 °C higher in the 35 °C group compared to the 5 °C group. Hence, the thermal gradient from core to shell was 9.03 °C smaller in the hot treatment. Dry heat transfer rate from the head was significantly lower in the hot compared to the cold treatment after four weeks of thermal acclimation. This reflects constraints on changes to peripheral circulation and maximum body temperature. Heat dissipation capacity from the head region increased with acclimation time in the hot treatment, perhaps because angiogenesis was required to reach peak heat transfer rate. We have shown that zebra finches meet high environmental temperature by heat storage, which saves water and energy, and by peripheral vasodilation in the head, which facilitates dry heat loss. These responses will not exclude the need for evaporative cooling, but will lessen the amount of energy expend on body temperature reduction in hot environments.     

Seasonal responses of lentic midge assemblages to environmental gradients

We examined biotic and abiotic factors associated with density and diversity of larval midges (Diptera: Chironomidae) in the littoral zone of Par Pond, an 1100 ha cooling reservoir located near Aiken, South Carolina, USA. A total of 144 core samples were removed from 12 areas during each of 3 sample periods. Principal component analysis. Mantel analysis, and various univariate statistical tests were used to detect seasonal differences in (1) associations among areas and genera of Chironomidae, and (2) responses of midges to a multi-faceted environmental gradient. Water temperature, which in this case was a function of both season and thermal effluents, was the factor most strongly associated with relative changes in density and diversity within the chironomid assemblages. Plant biomass and/or plant diversity were significant factors correlated with midge density and diversity. Both particle size and organic content of the substratum, which varied little across stations, were relatively unimportant factors. Despite the presence of a multifaceted environmental gradient, the composition and relationships within the midge assemblages were remarkably similar across areas although total abundance and diversity varied.
On the basis of their similarity in structure and function, we classify the midge assemblages within the reservoir as members of the same community. However, in response to dissimilar or unequal selective forces (principally the area-specific effects of thermal effluents), these assemblages appear to behave environmentally as multiple units rather than as a single component of the benthic community in Par Pond.     

Lactose variability of Escherichia coli in thermally stressed reactor effluent waters.

Lactose-utilizing and nalidixic acid-resistant populations of Escherichia coli, having an optimum growth temperature of 37 degrees C, were placed in modified diffusion chambers. The chambers were submerged in the epilimnion and hypolimnion of a 1,100-hectare lake (Par Pond) which receives cooling water from a nuclear production reactor. Control chambers were placed in a deep-water reservoir and a Flowing-Streams Laboratory, both of which had comparable temperatures to Par Pond. The populations of E. coli were sampled regularly for up to 3 weeks. Viability of the bacteria was determined by dilution plating to nutrient agar followed by replicate plating onto selective medium to determine lactose utilization and nalidixic acid sensitivity. Initial populations of E. coli were lactose positive but changed to lactose negative in Par Pond when the reactor was operating (i.e., cooling water from the heat exchangers was being discharged to the lake). This alteration occurred most rapidly in the chambers closest to the cooling-water discharge point. Such changes did not occur in a deep-water reservoir, in Par Pond when the reactor was not operating, or in the Flowing-Streams Laboratory. The nalidixic acid-resistant characteristic remained stable regardless of the chambers' placement or reactor operations. Although the reasons for such alterations are unclear, it appears that lactose-negative populations of E. coli are selected for in these reactor effluent waters. The loss of the lactose characteristic prevents the recognition and identification of E. coli in this cooling lake (when the reactor is operating) and may prevent the assessment of water quality based on coliform recognition.     

THREE SELECTIONS ARE BETTER THAN ONE: CLINAL VARIATION OF THERMAL QTL FROM INDEPENDENT SELECTION EXPERIMENTS IN DROSOPHILA

We report the results of two independent selection experiments that have exposed distinct populations of Drosophila melanogaster to different forms of thermal selection. A recombinant population derived from Arvin California and Zimbabwe isofemale lines was exposed to laboratory natural selection at two temperatures (T_AZ: 18°C and 28°C). Microsatellite mapping identified quantitative trait loci (QTL) on the X‐chromosome between the replicate “Hot” and “Cold” populations. In a separate experiment, disruptive selection was imposed on an outbred California population for the “knockdown” temperature (T_KD) in a thermal column. Microsatellite mapping of the “High” and “Low” populations also uncovered primarily X‐linked QTL. Notably, a marker in the shaggy locus at band 3A was significantly differentiated in both experiments. Finer scale mapping of the 3A region has narrowed the QTL to the shaggy gene region, which contains several candidate genes that function in circadian rhythms. The same allele that was increased in frequency in the High T_KD populations is significantly clinal in North America and is more common at the warm end of the cline (Florida vs. Maine; however, the cline was not apparent in Australia). Together, these studies show that independent selection experiments can uncover the same target of selection and that evolution in the laboratory can recapitulate putatively adaptive clinal variation in nature.     

Physiological effects of heat and cold exposure in the common reef coral Acropora millepora

Reef-forming corals are under threat globally from climate change, leading to changes in sea temperatures with both hot and cold events recorded and projected to increase in frequency and severity in the future. Tolerance to heat and cold exposure has been found to be mutually exclusive in other marine invertebrates, but it is currently unclear whether a trade-off exists between hot and cold thermal tolerance in tropical corals. This study quantified the changes in physiology in Acropora millepora from the central Great Barrier Reef subjected to three temperature treatments; sub-lethal cold, ambient and sub-lethal heat (23.0 °C, 27.0 °C and 29.5 °C, respectively). After 10 weeks, pigment content and Symbiodiniaceae density increased in cold-treated corals but decreased in heat-treated corals relative to corals at ambient conditions. Heat-treated corals gained less mass relative to both ambient and cold-treated corals. These results indicate that the physiological condition of A. millepora corals examined here improved in response to mild cold exposure compared to ambient exposure and decreased under mild heat exposure despite both these temperatures occurring in situ around 15% of the year. The energetic condition of corals in the hotter treatment was reduced compared to both ambient and cooler groups, indicating that corals may be more resilient to mild cold exposure relative to mild heat exposure. The results indicate that the corals shifted their resource allocation in response to temperature treatment, investing more energy into skeletal extension rather than maintenance. No evidence of thermal tolerance trade-offs was found, and cold thermal tolerance was not lost in more heat-tolerant individuals. An enhanced understanding of physiological responses of corals at both ends of the thermal spectrum is important for predicting the resilience of corals under projected climate change conditions.

     

Pollinator response to male floral display size in two Sagittaria (Alismataceae) species

One hypothesis for the evolution of maleness in plants postulates the following: male plants appear that have re-allocated resources from female parts into a larger number of male flowers, creating a larger floral display. Pollinators respond “dramatically” to the increased display, driving the spread of males in the population (Bawa, K. S., 1980, Annual Review of Ecology and Systematics 11:15–40). To test this, we measured the total number of flowers and the size of male displays in dioecious Sagittaria latifolia and monoecious S. australis. We also measured how fast visitors arrived at “target” flowers in the displays. Then we used the Cox Model, a failure time analysis procedure, to analyze the relationship between visitor arrival and display size. We found that male display sizes were somewhat larger in the dioecious than the monoecious plants, but this was due to more compressed blooming rather than to a larger total number of flowers. The visitors to both plants were similar arrays of generalist bees. Visitors did not show the predicted “dramatic” response, however, but rather a plateauing response to larger display sizes, so our results do not support the hypothesis. Reviewing the literature, we found no reports of the “dramatic” response the hypothesis asserts. Instead, relative insensitivity to display size is the rule.     

Effects of temperature gradients resulting from reservoir discharge on Dorosoma cepedianum spawning in the Savannah River

Synopsis The timing and spatial pattern of gizzard shad, Dorosoma cepedianum, spawning were markedly affected by a temperature gradient caused by the release of hypolimnetic water from an upstream reservoir into the middle reaches of the Savannah River. During 1983 and 1984, a distinct thermal gradient occurred with the warmest temperatures at the downstream end of the 257 km study reach and the coolest temperatures at the upstream end. The occurrence of gizzard shad larvae indicated that spawning began at the sample stations farthest downstream and progressed upstream, peaking at each sample station when it warmed to approximately 19° C. The estimated difference in date of peak density of larvae between the upstream and downstream ends of the gradient was 23 days in 1983 and 35 days in 1984. Because of this pattern, densities of larvae were highest at the downstream end of the study area early in the spawning season and highest at the upstream end of the study area late in the spawning season.Photoperiod and daylength appeared unrelated to the patterns under study. Far-reaching effects of anthropogenic temperature changes on fish spawning and the distribution of larvae are probably common in rivers that receive hypolimnetic discharge.     

INTRAPOPULATION DIFFERENTIATION IN ANNUAL PLANTS. III. THE CONTRASTING EFFECTS OF INTRA- AND INTERSPECIFIC COMPETITION

The roles of intraspecific and interspecific competition in producing differentiation within populations of Veronica peregrina were studied in two populations under controlled, greenhouse conditions. In nature, each population spans an environmental gradient across the center and sides of a temporary, vernal pool in California. Individuals at the center are subjected to intense intraspecific competition produced by high densities (to 30 seedlings/cm²) generated by quasi-simultaneous germination (90% of seeds germinate in one week). Individuals at the periphery are subjected to interspecific competition with grasses, which shade out the Veronica 4–6 weeks after the onset of winter growth. I predicted that 1) when grown under immediate intraspecific competition in the greenhouse, offspring of plants from the central subpopulation (C) would perform better (i.e., grow larger and produce more seeds) than those from the periphery (P) and that 2) when grown under delayed interspecific competition provided by Agrostis tenuis and Lollium multiflorum, offspring of plants from the periphery would perform better than those from the center. Both predictions were confirmed. The center-periphery differences were pronounced and statistically significant in an undisturbed population (V-2), while in a population disturbed by yearly plowing (V-3), the differences tended to be consistent with those in V-2 but seldom significant. Distribution of variability tended to be positively skewed and/or leptokurtic in subpopulations grown under “foreign” competition (i.e., intraspecific for P plants and interspecific for C plants) but was normally distributed following exposure to “familiar” competition. Timing of competition affected many results. There were four additional significant differences between the central and peripheral subpopulations. 1) Germination rate: the faster rate in central plants can be advantageous under immediate intraspecific competition. The slower rate in peripheral plants can be advantageous under conditions of erratic and unpredictable soil moisture. 2) Response to nutrient competition: central plants were more sensitive to N-deficiency and peripheral plants were more sensitive to P-deficiency. 3) Allocation of biomass: central plants allocated a greater proportion of biomass to seeds, while peripheral plants allocated a greater proportion of biomass to leaves under all growing conditions. 4) Root elongation: at the seedling stage, central plants have longer roots, while at the adult stage, peripheral plants have longer roots (but not more root mass). Most components of this complex pattern of differentiation are interpretable in an adaptive context. Other results defy simple explanations and underline the importance of phenotypic plasticity, which was pronounced in the competition experiments.     

Aquatic vegetation and trophic condition of Cape Cod (Massachusetts,U.S.A.) kettle ponds

The species composition and relative abundance of aquatic macrophytes was evaluated in five Cape Cod, Massachusetts, freshwater kettle ponds, representing a range of trophic conditions from oligotrophic to eutrophic. At each pond, aquatic vegetation and environmental variables (slope, water depth, sediment bulk density, sediment grain size, sediment organic content and porewater inorganic nutrients) were measured along five transects extending perpendicular to the shoreline from the upland border into the pond. Based on a variety of multivariate methods, including Detrended Correspondence Analysis (DCA), an indirect gradient analysis technique, and Canonical Correspondence Analysis (CCA), a direct gradient approach, it was determined that the eutrophic Herring Pond was dominated by floating aquatic vegetation (Brasenia schreberi, Nymphoides cordata, Nymphaea odorata), and the algal stonewort, Nitella. Partial CCA suggested that high porewater PO₄-P concentrations and fine-grained sediments strongly influenced the vegetation of this eutrophic pond. In contrast, vegetation of the oligotrophic Duck Pond was sparse, contained no floating aquatics, and was dominated by emergent plants. Low porewater nutrients, low sediment organic content, high water clarity and low pH (4.8) best defined the environmental characteristics of this oligotrophic pond. Gull Pond, with inorganic nitrogen-enriched sediments, also exhibited a flora quite different from the oligotrophic Duck Pond. The species composition and relative abundance of aquatic macrophytes provide good indicators of the trophic status of freshwater ponds and should be incorporated into long-term monitoring programs aimed at detecting responses to anthropogenically-derived nutrient loading.     

Lactose Variability of Escherichia coli in Thermally Stressed Reactor Effluent Waters

Lactose-utilizing and nalidixic acid-resistant populations of Escherichia coli, having an optimum growth temperature of 37°C, were placed in modified diffusion chambers. The chambers were submerged in the epilimnion and hypolimnion of a 1,100-hectare lake (Par Pond) which receives cooling water from a nuclear production reactor. Control chambers were placed in a deep-water reservoir and a Flowing-Streams Laboratory, both of which had comparable temperatures to Par Pond. The populations of E. coli were sampled regularly for up to 3 weeks. Viability of the bacteria was determined by dilution plating to nutrient agar followed by replicate plating onto selective medium to determine lactose utilization and nalidixic acid sensitivity. Initial populations of E. coli were lactose positive but changed to lactose negative in Par Pond when the reactor was operating (i.e., cooling water from the heat exchangers was being discharged to the lake). This alteration occurred most rapidly in the chambers closest to the cooling-water discharge point. Such changes did not occur in a deep-water reservoir, in Par Pond when the reactor was not operating, or in the Flowing-Streams Laboratory. The nalidixic acid-resistant characteristic remained stable regardless of the chambers' placement or reactor operations. Although the reasons for such alterations are unclear, it appears that lactose-negative populations of E. coli are selected for in these reactor effluent waters. The loss of the lactose characteristic prevents the recognition and identification of E. coli in this cooling lake (when the reactor is operating) and may prevent the assessment of water quality based on coliform recognition.     

DISTRIBUTION ECOLOGY OF HAPLOPAPPUS AND CHRYSOTHAMNUS IN THE MOJAVE DESERT. I. NICHE POSITION AND NICHE SHIFTS ON NORTH-FACING GRANITIC SLOPES

The close systematic affinities between the composite genera Haplopappus and Chrysothamnus, including different points of contact with “intergeneric” hybrids, are reviewed. The Mohave Desert has several shrubby representatives of each genus, and at census sites from the east-central New York Mountains to buttes at the extreme western end of the desert, north-facing granitic slopes at the foothills of mountain ranges support four Haplopappus species (H. cuneatus. H. laricifolius, H. linearifolias and H. cooperi) and one Chrysothamnus (C. teretifolius). These five species occupy “distributional niches” represented as ranges and areas of dominance on the habitat plane of slope angle and proportion of rocks in the substrate. Central desert sites have four species (excluding C. teretifolius); peripheral desert sites always include C. teretifolius rather than H. laricifolius and support two to four species depending on the size and isolation of the suitable habitat area. Species may occupy different positions on the habitat gradient depending on species number at the site, moisture availability, and in particular competition from other species. C. teretifolius appears to be favored when population turnover rates are high, either in disturbed sites or on natural islands of habitat.