Temperature Manipulation Experiments Research Articles

Impact of temperature and prey shell thickness on feeding of the oyster drill Urosalpinx cinerea Say

Feeding rates and selection of the oyster drill Urosalpinx cinerea were investigated with several long term temperature alteration experiments. Oyster drill prey selection was assessed in terms of prey (oyster) size and shell thickness. Drills fed on oysters with thinner shells 86% of the time and selectively preyed on smaller (1g wet weight) oysters during colder months and larger (10g wet weight) oysters during warmer months. Analysis of U. cinerea seasonal feeding patterns showed virtually no feeding during winter months (January to March) and a significant increase to over 1 gram of prey tissue per week during summer (July to September). A nine month experiment compared feeding rates of U. cinerea at two different temperature levels: ambient seawater (control) and temperatures elevated 4°C above ambient. Oyster drills showed a 60% increase in feeding rate in the warmer treatment, especially during late summer, fall and early winter. Both temperature manipulation and annual feeding experiments revealed a significant temperature dependence of U. cinerea feeding rates. Seasonal or long-term seawater temperature changes related to climate change could strongly affect the predator–prey relationship between U. cinerea and its prey.

Fine root turnover and litter production of Norway spruce in a long-term temperature and nutrient manipulation experiment

Increased soil temperature and nutrient availability enhance soil biological activity. We studied how these affect fine root growth and survival, i.e. below-ground litter production, in relation to above-ground foliage litter production of Norway spruce (Picea abies (L.) Karst.). The treatments, irrigation (I), soil warming + irrigation (WI), fertilization + irrigation (FI) and soil warming + fertilization + irrigation (WFI) were started in 1987 (F, I) and in 1995 (W). The annual production of fine root litter was estimated from minirhizotrons (survival) and soil-cores (biomass) and the annual above-ground litter production from litter traps. The number and elongation of fine roots tended to be higher in WI and I compared to the other treatments, which may indicate nutrient shortage. Fine roots in the WFI treatment had the lowest median longevity and from three to fourfold higher below-ground litter production compared to WI, FI or I - higher soil temperature increased the litter input particularly into the mineral soil. Only fertilization increased the above-ground litter production. As warmer and more nutrient-rich soil significantly shortened the fine root lifespan and increased the litter input, the storage of carbon in boreal forest soil may increase in the future.

The Impact of Winter and Spring Temperatures on Temperate Tree Budburst Dates: Results from an Experimental Climate Manipulation

Budburst phenology is a key driver of ecosystem structure and functioning, and it is sensitive to global change. Both cold winter temperatures (chilling) and spring warming (forcing) are important for budburst. Future climate warming is expected to have a contrasting effect on chilling and forcing, and subsequently to have a non-linear effect on budburst timing. To clarify the different effects of warming during chilling and forcing phases of budburst phenology in deciduous trees, (i) we conducted a temperature manipulation experiment, with separate winter and spring warming treatments on well irrigated and fertilized saplings of beech, birch and oak, and (ii) we analyzed the observations with five temperature-based budburst models (Thermal Time model, Parallel model, Sequential model, Alternating model, and Unified model). The results show that both winter warming and spring warming significantly advanced budburst date, with the combination of winter plus spring warming accelerating budburst most. As expected, all three species were more sensitive to spring warming than to winter warming. Although the different chilling requirement, the warming sensitivity was not significantly different among the studied species. Model evaluation showed that both one- and two- phase models (without and with chilling, respectively) are able to accurately predict budburst. For beech, the Sequential model reproduced budburst dates best. For oak and birch, both Sequential model and the Thermal Time model yielded good fit with the data but the latter was slightly better in case of high parameter uncertainty. However, for late-flushing species, the Sequential model is likely be the most appropriate to predict budburst data in a future warmer climate.

Simple additive effects are rare: a quantitative review of plant biomass and soil process responses to combined manipulations of CO2 and temperature

In recent years, increased awareness of the potential interactions between rising atmospheric CO2 concentrations ([ CO2 ]) and temperature has illustrated the importance of multifactorial ecosystem manipulation experiments for validating Earth System models. To address the urgent need for increased understanding of responses in multifactorial experiments, this article synthesizes how ecosystem productivity and soil processes respond to combined warming and [ CO2 ] manipulation, and compares it with those obtained in single factor [ CO2 ] and temperature manipulation experiments. Across all combined elevated [ CO2 ] and warming experiments, biomass production and soil respiration were typically enhanced. Responses to the combined treatment were more similar to those in the [ CO2 ]-only treatment than to those in the warming-only treatment. In contrast to warming-only experiments, both the combined and the [ CO2 ]-only treatments elicited larger stimulation of fine root biomass than of aboveground biomass, consistently stimulated soil respiration, and decreased foliar nitrogen (N) concentration. Nonetheless, mineral N availability declined less in the combined treatment than in the [ CO2 ]-only treatment, possibly due to the warming-induced acceleration of decomposition, implying that progressive nitrogen limitation (PNL) may not occur as commonly as anticipated from single factor [ CO2 ] treatment studies. Responses of total plant biomass, especially of aboveground biomass, revealed antagonistic interactions between elevated [ CO2 ] and warming, i.e. the response to the combined treatment was usually less-than-additive. This implies that productivity projections might be overestimated when models are parameterized based on single factor responses. Our results highlight the need for more (and especially more long-term) multifactor manipulation experiments. Because single factor CO2 responses often dominated over warming responses in the combined treatments, our results also suggest that projected responses to future global warming in Earth System models should not be parameterized using single factor warming experiments.

ABIOTIC REGULATION OF INVESTMENT IN SEXUAL VERSUS VEGETATIVE REPRODUCTION IN THE CLONAL KELP LAMINARIA SINCLAIRII (LAMINARIALES, PHAEOPHYCEAE)1.

Clonal kelp taxa may reproduce both sexually and vegetatively resulting in a potential trade-off in the allocation of acquired carbon and nitrogen resources. Such trade-offs may dictate a different response of clonal kelps to varying environmental conditions relative to aclonal kelp taxa. Laboratory temperature and nutrient manipulation experiments demonstrated that investment in sexual and vegetative reproduction in Laminaria sinclairii (Harv. ex Hook. f. et Harv.) Farl., C. L. Anderson et D. C. Eaton was regulated by different abiotic factors. Sorus production (investment in sexual reproduction) and blade growth were significantly higher at 12°C compared to 17°C, regardless of nutrient concentration. Net carbon storage and depletion in rhizomes were observed in the low- and high-temperature treatments, respectively, suggesting that carbon stores were not responsible for increased growth. Rhizome elongation (investment in vegetative reproduction), on the other hand, was significantly higher in 12 μM NO3- than in 2 μM NO3- , irrespective of temperature. This increase in rhizome growth was concurrent with elevated rhizome percent tissue nitrogen levels also observed in treatments with higher nutrients, again indicating a growth response to treatment independent of previous nutrient stores. These results suggest that regulation of growth and investment in sexual reproduction in L. sinclairii is similar to that in aclonal kelps (i.e., warmer temperatures result in decreased reproductive output). Additionally, depletion of carbon and nitrogen from rhizomes in suboptimal conditions confirms the role of clonal kelp rhizomes in carbon and nutrient storage.

Mechanisms of habitat segregation between corallimorpharians: photosynthetic parameters and Symbiodinium types

Corallimorpharians are evolutionarily important relatives to reef-building corals, yet lit- tle is known about their ecophysiology. We demonstrate that physiological mechanisms determine, in part, the light-dependent distributional patterns of 2 corallimorpharians, Rhodactis rhodostoma and Discosoma unguja, on coral reefs in the northern Red Sea. Field measurements of the physiological parameters related to photosynthetic activity revealed that zooxanthellae abundance, chlorophyll a concentration, maximum quantum yield (Fv/Fm), and excitation pressure of Photosytem II (Qm) varied equally for both species among 3 depths. In contrast, laboratory measurements in 3 experimental light treatments — low light (LL), medium light (ML) and high light (HL) — revealed that D. unguja was more sensitive to HL than R. rhodostoma. Genetic characterization of their endosymbiotic algae using a PCR for the internal transcribed spacer region (ITS2) of nuclear ribosomal DNA showed that both R. rhodostoma and D. unguja contain Symbiodinium ITS2-type C1 symbionts in shallow areas, but that the algal assemblage were either Type C1 or Type D1a in both species in deeper areas. In reciprocal depth, light, and temperature manipulation experiments, bleached R. rhodostoma never shuffled Symbiodinium ITS2 types, however, some R. rhodostoma in the control treatment did. In contrast, some individuals of D. unguja shuffled Symbiodinium ITS2 types both in field-reciprocal experiments and laboratory-control treatments. Both species contained UV radiation absorbing com- pounds over a wide-depth range. We concluded that the photoacclimation mechanisms of R. rhodos- toma and D. unguja were influenced by both host- and symbiont-mediated factors. We hypothesize that a low abundance of D. unguja in shallow water was due to decreased tolerance to the high irra- diance found in host factors.

Temperature regulation of bacterial production, respiration, and growth efficiency in a temperate salt-marsh estuary

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 43:243-254 (2006) - doi:10.3354/ame043243 Temperature regulation of bacterial production, respiration, and growth efficiency in a temperate salt-marsh estuary Jude K. Apple1,2,*, P. A. del Giorgio3, W. Michael Kemp1 1Horn Point Laboratory, University of Maryland Center for Environmental Science, Cambridge, Maryland 21613, USA 2US Naval Research Laboratory, Washington, DC 20375, USA 3Département des sciences biologiques, Université du Québec à Montréal (UQÀM), Montréal H3C 3P8, Canada *Email: jude.apple@nrl.navy.mil ABSTRACT: There is consensus that temperature plays a major role in shaping microbial activity, but there are still questions as to how temperature influences different aspects of bacterioplankton carbon metabolism under different environmental conditions. We examined the temperature dependence of bacterioplankton carbon metabolism, whether this temperature dependence changes at different temperatures, and whether the relationship between temperature and carbon metabolism varies among estuarine sub-systems differing in their degree of enrichment. Two years of intensive sampling in a temperate estuary (Monie Bay, Chesapeake Bay, USA) revealed significant differences in the temperature dependence of bacterial production (BP) and respiration (BR), which drove a strong negative temperature response of bacterial growth efficiency (BGE). Accordingly, BGE was lower in summer (<0.2) and higher in winter (>0.5). For all measured metabolic processes, the most pronounced temperature response was observed at lower temperatures, with Q10 values generally 2-fold greater than in warmer waters. Despite significant differences in resource availability, both the temperature dependence and magnitude of BR and bacterioplankton carbon consumption (BCC) were remarkably similar among the 4 estuarine sub-systems. Although temperature dependencies of BP and BGE were also similar, their magnitude differed significantly, with highest values in the nutrient-enriched sub-system and lowest in the open bay. This pattern in carbon metabolism among sub-systems was present throughout the year and was confirmed by temperature manipulation experiments, suggesting the temperature effects on BP and BGE did not override the influence of resource availability. We conclude that temperature is the dominant factor regulating seasonality of BR and BCC in this system, whereas BP and BGE are influenced by both temperature and organic matter quality, with variation in the relative importance of each of these factors throughout the year. KEY WORDS: Bacterioplankton · Temperature · Growth efficiency · Estuary Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 43, No. 3. Online publication date: July 19, 2006 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2006 Inter-Research.

Plant performance in a warmer world: general responses of plants from cold, northern biomes and the importance of winter and spring events

During the past three decades the Earth has warmed with a rate unprecedented during the past 1000 years. There is already ample evidence that this fast climate warming has affected a broad range of organisms, including plants. Plants from high-latitude and high-altitude sites (‘cold biomes’) are especially sensitive to climate warming. In this paper we (1) review the response in the phenology of plants, changes in their range and distribution, soil nutrient availability, and the effects on the structure and dynamics of plant communities for cold, northern biomes; and (2) we show, by using data from an ongoing snow and temperature manipulation experiment in northern Sweden, that also winter and spring events have a profound influence on plant performance. Both long-term phenological data sets, experimental warming studies (performed in summer or year-round), natural gradient studies and satellite images show that key phenological events are responsive to temperature increases and that recent climate warming does indeed lead to changes in plant phenology. However, data from a warming and snow manipulation study that we are conducting in northern Sweden show that plants respond differently to the various climatic scenarios that we had imposed on these species and that especially winter and spring events have a profound impact. This indicates that it is necessary to include several scenarios of both summer and winter climate change in experimental climate change studies, and that we need detailed projections of future climate at a regional scale to be able to assess their impacts on natural ecosystems. There is also ample evidence that the range shift of herbs and shrubs to more northern regions is for the vast majority of species mainly caused by changes in the climate. This is in line with the observed ‘up-greening’ of northern tundra sites. These rapid northern shifts in distribution of plants as a result of climate warming may have substantial consequences for the structure and dynamics of high-latitude ecosystems. An analysis of warming studies at 9 tundra sites shows that heating during at least 3 years increased net N-mineralization from 0.32±0.31 (SE) g N m−2 yr−1 in the controls to 0.53±0.31 (SE) g N m−2 yr−1 in the heated plots (p<0.05), an increase of about 70%. Thus, warming leads to higher N availability in high-latitude northern tundra sites, but the variability is substantial. Higher nutrient availability affects in turn the species composition of high-latitude sites, which has important consequences for the carbon and water balance of these systems.

Peptidergic Contribution to Posttetanic Potentiation at a Central Synapse of Aplysia

Posttetanic potentiation (PTP)-like phenomena appear to be mediated by a variety of mechanisms. Although neuropeptides are located in a large number of neurons and many neuropeptides, like PTP, can enhance synaptic transmission, there is a paucity of studies indicating that peptides may actually participate in PTP. Here, we utilize a single central synapse in the feeding circuit of Aplysia to investigate a possible peptidergic contribution to PTP in the CNS. The cholinergic command-like interneuron, cerebral-buccal interneuron 2 (CBI-2), contains two neuropeptides, feeding circuit activating peptide (FCAP) and cerebral peptide 2 (CP2). Previous studies showed that tetanic prestimulation or repeated stimulation of CBI-2, as well as perfusion of FCAP and CP2, increase the size of the cholinergic excitatory postsynaptic potentials (EPSPs) that CBI-2 evokes in the motoneurons B61/62 and shorten the latency to initiate B61/62 firing in response to CBI-2 stimulation. We used temperature-dependent suppression of peptide release and occlusion experiments to examine the possible contribution of FCAP and CP2 to PTP at the CBI-2 to B61/62 synapse. When peptide release was suppressed, perfusion of exogenous peptides increased the size of posttetanic EPSPs. In contrast, when peptide release was not suppressed, exogenous peptides did not enhance the size of posttetanic EPSPs, thus indicating occlusion. Temperature manipulation and occlusion experiments also indicated that peptides extend PTP duration. This peptide-dependent prolongation of PTP has functional consequences in that it extends the duration of time during which the latency to initiate B61/62 firing in response to CBI-2 stimulation is shortened.

Interactions between NH + 4 and NO − 3 uptake and assimilation: comparison of diatoms and dinoflagellates at several growth temperatures

Ammonium concentrations of ∼1 M are commonly cited as being the threshold for inhibition of NO3 − uptake, but the applicability of this threshold to phytoplankton from different taxonomic classes has rarely been examined. Additionally, little is known about the influence of environmental variables (e.g. growth temperature) on the interaction between ambient NH4 + and NO3 − uptake. Four species of estuarine phytoplankton, two diatom [Chaetoceros sp., and Thalassiosira weissflogii (Grunow) Fryxell et Hasle] and two dinoflagellate [Prorocentrum minimum (Pavillard) Schiller, and Gyrodinium uncatenum Hulburt], were grown on NO3 − at several different temperatures (4, 10, 15, or 20 °C), and the impact of NH4 + additions on NO3 − uptake/assimilation (non-TCA-extracted) and assimilation (TCA-extracted) was assessed. For all species at all temperatures, NO3 − uptake/assimilation and assimilation rates decreased in a roughly exponential manner with increasing NH4 + concentrations but were not completely inhibited even at elevated NH4 + concentrations of 200 μM. Estimated half-inhibition concentrations (K i) were significantly greater in the diatom species (mean ± SE; 2.70 ± 0.67 μM) than in the dinoflagellate species (1.26 ± 0.55 μM). Half-inhibition constants were positively related to temperature-limited relative growth rate although not significantly. The observed inhibition of NO3 − uptake and assimilation, as a percentage of NO3 − uptake in the absence of NH4 +, averaged about 80% and ranged from 49 to 100%. For all species, a significant (P < 0.001) positive correlation was found between percent inhibition of NO3 − assimilation and temperature-limited relative growth rate. Two experiments on Chesapeake Bay phytoplankton during an April 1998 diatom bloom showed that in short-term (∼1 h) temperature manipulation experiments, percent inhibition of NO3 − uptake/assimilation was also positively related (P = 0.05) to experimental temperature. The observed relationships between temperature-limited relative growth rate and percent inhibition of NO3 − assimilation rates for the species tested suggest that at the enzyme level, the inhibitory mechanism of NO3 − assimilation is similar among species, but at the whole cell level may be regulated by species-specific differences in the accumulation of internal metabolites. These findings add not only to our understanding of species-specific variability and the role of growth temperature, but also provide additional data with which to evaluate current models of NH4 + and NO3 − interactions.

Global Change and Arctic Ecosystems: Conclusions and Predictions from Experiments with Terrestrial Invertebrates on Spitsbergen

Extensive studies on invertebrates from Ny-Ålesund, Spitsbergen, Svalbard and more limited data on aphids from Abisko, Sweden, produced the following main conclusions: (1) The population response to raised summer temperatures differed between the above and the below ground species, both in terms of speed and magnitude. (2) Similar animal communities responded differently to similar temperature manipulations on sites with different vegetation cover and composition. (3) For soil animals the between-year and between-site variations in population densities, were greater than the differences produced by the temperature manipulation experiments at any one site in any year. (4) Infrequent extreme climatic events strongly influence long-term trends in population density and community composition. (5) The population response of invertebrates to climate warming is greatest and most rapid at the coldest sites. (6) The spatial distribution of the above ground insect herbivores on their host plant is temperature limited. (7) The numerical abundance of flying predators/parasitoids of the above-ground herbivores is low. (8) The spatial distribution of some predators may be thermally restricted and less extensive than that of their prey. (9) Habitat temperature is the driving variable determining the flight activity patterns of insects. (10) Increased summer temperatures may alter or disrupt the seasonal patterns of insect emergence, particularly in species where the life cycle is cued into the seasonal rhythm. (11) The common species of arctic soil mites and Collembola are well adapted to survive enhanced summer temperatures, providing that moisture is not limited. (12) Water availability during the summer growing period is probably of greater significance than temperature in determining the survival and success of many arctic soil invertebrate groups. (13) Arctic soil microarthropod species are well adapted to survive and operate at subzero and low positive summer temperatures. (14) Freeze-thaw events represent critical points in the life history of the microarthropods. (15) Supercooling points are sometimes poor indicators of the capacity of arctic soil microarthopods to survive low temperatures. From these findings predictions are made as to how high arctic communities will respond to predicted changes in climate.

Global Change and Arctic Ecosystems: Conclusions and Predictions from Experiments with Terrestrial Invertebrates on Spitsbergen

Extensive studies on invertebrates from Ny-Alesund, Spitsbergen, Svalbard and more limited data on aphids from Abisko, Sweden, produced the following main conclusions: (1) The population response to raised summer temperatures differed between the above and the below ground species, both in terms of speed and magnitude. (2) Similar animal communities responded differently to similar temperature manipulations on sites with different vegetation cover and composition. (3) For soil animals the between-year and between-site variations in population densities, were greater than the differences produced by the temperature manipulation experiments at any one site in any year. (4) Infrequent extreme climatic events strongly influence long-term trends in population density and community composition. (5) The population response of invertebrates to climate warming is greatest and most rapid at the coldest sites. (6) The spatial distribution of the above ground insect herbivores on their host plant is temperature limited. (7) The numerical abundance of flying predators/parasitoids of the above-ground herbivores is low. (8) The spatial distribution of some predators may be thermally restricted and less extensive than that of their prey. (9) Habitat temperature is the driving variable determining the flight activity patterns of insects. (10) Increased summer temperatures may alter or disrupt the seasonal patterns of insect emergence, particularly in species where the life cycle is cued into the seasonal rhythm. (11) The common species of arctic soil mites and Collembola are well adapted to survive enhanced summer temperatures, providing that moisture is not limited. (12) Water availability during the summer growing period is probably of greater significance than temperature in determining the survival and success of many arctic soil invertebrate groups. (13) Arctic soil microarthropod species are well adapted to survive and operate at subzero and low positive summer temperatures. (14) Freeze-thaw events represent critical points in the life history of the microarthropods. (15) Supercooling points are sometimes poor indicators of the capacity of arctic soil microarthopods to survive low temperatures. From these findings predictions are made as to how high arctic communities will respond to predicted changes in climate.

Can High Arctic Soil Microarthropods Survive Eleveated Summer Temperatures?

1. Tolerance of high summer temperatures was investigated in seven common species of oribatid mite (Acarina) and springtail (Collembola) found in soils associated with dominant vegetation types in West Spitsbergen, Svalbard. 2. Laboratory survival experiments are linked with field data from a 3-year temperature manipulation experiment to characterize the response of these animals to elevated field temperatures. 3. A 1-h exposure to 30⚬C under moist conditions showed that none of the species studied suffered significant mortality. The temperature required to kill all individuals was between 35-40 and 40-45⚬C for springtails and mites respectively. Under dry conditions at equivalent temperatures, differences in survival between mites and Collembola were strongly accentuated. Oribatid mites were little affected by the additional drought stress but the lethal temperature for springtails was shifted downwards by several degrees. 4. In parallel 3-h exposure experiments under moist conditions, the thermal death point was shifted downwards by c. 2.5⚬C in all the species, compared with 1-h exposures. Under dry conditions most Collembola died but mite survival differed little from that in the corresponding wet treatment over 3 h. 5. In longer-term experiments some mature Onychiurus arcticus survived more than 196 and 68 days at 5 and 25⚬C respectively. Above 30⚬C individuals survived for <24 h. 6. Regression relationships were established between maximum microhabitat temperatures and screen temperatures over 3 years. These were used to predict past maximum vegetation/litter mat and soil temperatures from historical screen data and to estimate the screen temperatures necessary to raise microhabitat temperatures to the thermal death point of the soil-dwelling arthropods. 7. The results are used to interpret the findings of a 3-year field experiment which examined the response of mite and springtail populations to temperature enhancement on two tundra sites, using small polythene tents. 8. High Arctic soil microarthropods appear able to cope with the higher summer temperatures they may experience as a result of climate change. It is the associated changes in soil moisture status that will probably produce the more significant changes in the microarthropod community. Species differ in their susceptibilities, but a decrease in soil moisture appears to favour oribatid mites at the expense of Collembola.

Effects of temperature on the incorporation of leucine and thymidine by bacterioplankton and bacterial isolates

The effect of temperature on the incorporation of 3H-leucine (W-Leu) and 3H-thymidine (3H-TdR) by bacterioplankton and bacterial isolates from marine, estuarine, and freshwater habitats was examlned. There was a significant, positive correlation between in situ temperature and molar Leu:TdR incorporation by bacterioplankton in oceanic and lagoonal environments. Temperature manipulation experiments showed that Q,,, values for Leu incorporation by bacterial assemblages and isolates were significantly higher than corresponding Q, values for TdR incorporation. Evidence indicated that the temperature-dependence of Leu:TdR ratios was not a funct~on of community compositlon, unbalanced growth, non-specific labelling of macromolecules, or substrate limitation. Thus, Leu and TdR incorporation appear to be affected by temperature differently. These experiments indicate that Leu and/or TdR conversion factors, for estimating bacterial production, are not constant with respect to temperature.

Multiscale variability in bacterioplankton abundance, production, and specific growth rate in a temperate salt‐marsh tidal creek

Heterotrophic bacterioplankton abundance, production, and specific growth rate in a salt‐marsh tidal creek were measured weekly from April 1991 to September 1992. During the same period, tidal and diel sampling studies were performed in May, June, and October 1991 and May and August 1992. Seasonal variability of bacterial abundance, production, and specific growth rate was regulated by temperature during nonsummer seasons when temperature was &lt;20°C. During summer, bacterial variables were not limited by temperature. Daily variability of bacterial abundance, production, and specific growth rate was regulated interactively by tidal mixing, substrate supply, and temperature over several tidal cycles. Higher bacterial abundance, production, and specific growth rate observed at low tide indicated that bacterial growth rate in the tidal creek was higher than in the adjacent river waters. This pattern might be due to larger nutrient fluxes originating in the tidal creek. Occasionally, the tidal effect was overridden by temperature during study periods when temperature changed dramatically over several tidal cycles. Bacterial diel patterns in production and specific growth rate in the tidal creek occurred only when day‐night temperature differences exceeded 10°C, with maximal values during daytime. Short‐term temperature manipulation experiments suggested that diel patterns in bacterial production and specific growth rate were probably caused by temperature, not light intensity. Thus, temperature regulated both seasonal and diel variations in bacterial production.

Prevalence of Chemical Interference Competition in Natural Populations of Wood Frogs, Rana sylvatica

When wood frog tadpoles are crowded in the laboratory, their intestines become packed with a unicellular alga (Prototheca) that inhibits growth. We conducted a series of experiments to determine the importance of this phenomenon in natural breeding ponds where local densities often exceed 15,000 wood frog tadpoles/m3 of water. Water from three of 10 breeding sites inhibited the growth of assay tadpoles relative to controls. Although Prototheca was found in the intestines of tadpoles from all sites, several lines of evidence suggest that it was not the causative agent of inhibition. First, Prototheca densities in wild-caught animals were extremely low compared to those that develop in laboratory stock and did not appear to be sufficiently dense to stunt growth. Second, tadpoles from sites where inhibitors were detected did not have disproportionately large numbers of Prototheca cells in their intestines. Third, wild-caught tadpoles which were crowded for three days in the laboratory almost always produced growth inhibitors; however, the production of inhibitors was not associated with a proliferation of Prototheca in the intestines. Tadpoles crowded in small containers placed outdoors did not produce inhibitors. However, tadpoles grown indoors under otherwise similar conditions did. The results of a temperature manipulation experiment suggest that inhibitors are most likely to develop when tadpoles are maintained at room temperature. Tadpoles did not produce inhibitors when maintained at relatively cool temperatures which approximate those of natural breeding sites. Collectively, our data suggest that growth inhibition involvingPrototheca is primarily a laboratory artifact that has little relevance to the ecology of natural populations of wood frogs.