Colonization dynamics of river benthos in response to local changes in bed characteristics

1. The effects of small-scale variation in the nature of a river bed upon the colonization dynamics of benthic invertebrates were explored through a series of microhabitat implants placed in a small river in southern Ontario. The implants presented variations in the levels of four microhabitat features: substrate particle size, current regime, added food and light. The sixteen possible combinations of these variables were replicated five times, for a total of eighty implants.
2. After 14 days of exposure, some of the controlled variables had been modified and a number of other variables (sand, fine (FPOM) and coarse (CPOM) particulate organic matter) had been added by the river.
3. Benthic diversity differed among the implants, with sand and CPOM accounting for 15.5% of the variation. Benthic density was more uniform and was weakly related only to added food. Application of the CANOCO program to the data set suggested that, in general, the environmental variables manipulated in this study were not the key factors influencing colonization, although they may have been important in non-linear combination.
4. A number of important positive associations were evident among colonizing species, some of which are thought to reflect those of predator and prey.
5. Control of colonization appears to be through multiple factors (abiotic, biotic, autochthonous, allochthonous) that differ in nature according to species, and which underline the complexity of selection responses of individual taxa to local variations in bed structure. Researchers must acknowledge this multifactorial organization and dynamic nature if workable models of lotic community structure and development are to be forthcoming.     

Muscle protein loss in laying House Sparrows Passer domesticus

In a study of female House Sparrows Passer domesticus , it was established that almost all of the lean dry material lost from the flight muscles during the period of egg formation and laying was protein. The two large fractions of muscle protein, the sarcoplasmic and myofibrillar fractions, were measured biochemically. Protein in the myofibrillar fraction, which formed the greater part of the total protein, decreased during the study period. There was no evidence to support the proposal that sarcoplasm in muscle acts as the main reserve of proteins.     

Evergreen leaf respiration acclimates to long-term nocturnal warming under field conditions

Acclimation of plant respiration rates (R) to climate warming is highly variable and many results appear contradictory. We tested the recently suggested hypotheses that pre‐existing, long‐lived leaves should exhibit a relatively limited ability for R to acclimate to climate warming, and that acclimation would occur via changes in the short‐term temperature sensitivity of respiration. Seedlings of a subalpine, evergreen tree species (Eucalyptus pauciflora) were grown under naturally fluctuating conditions within its natural distribution. We used a free air temperature increase (FATI) system of infra‐red ceramic lamps to raise night‐time leaf temperatures by 0.3±0.1, 1.3±0.1, and 2.2±0.1 °C above ambient for 1 year. Light‐saturated assimilation rates and plant growth did not change with nocturnal FATI treatments. Leaf R measured at prevailing temperatures did not differ between FATI treatments. Within each FATI treatment, nocturnal leaf R was highly sensitive to artificial temperature changes within minutes, and also correlated strongly with natural nocturnal and seasonal temperature variation. The corresponding values of Q₁₀ of R varied according to time scale of measurements, but did not vary between FATI treatments. Instead, acclimation of R to nocturnal FATI occurred through changes in the base rate of respiration.     

Nitrogen fertilization enhances water‐use efficiency in a saline environment

Effects of salinity and nutrients on carbon gain in relation to water use were studied in the grey mangrove, Avicennia marina, growing along a natural salinity gradient in south‐eastern Australia. Tall trees characterized areas of seawater salinities (fringe zone) and stunted trees dominated landward hypersaline areas (scrub zone). Trees were fertilized with nitrogen (+N) or phosphorus (+P) or unfertilized. There was no significant effect of +P on shoot growth, whereas +N enhanced canopy development, particularly in scrub trees. Scrub trees maintained greater CO₂ assimilation per unit water transpired (water‐use efficiency, WUE) and had lower nitrogen‐use efficiency (NUE; CO₂ assimilation rate per unit leaf nitrogen) than fringe trees. The CO₂ assimilation rates of +N trees were similar to those in other treatments, but were achieved at lower transpiration rates, stomatal conductance and intercellular CO₂ concentrations. Maintaining comparable assimilation rates at lower stomatal conductance requires greater ribulose 1·5‐bisphosphate carboxylase/oxygenase activity, consistent with greater N content per unit leaf area in +N trees. Hence, +N enhanced WUE at the expense of NUE. Instantaneous WUE estimates were supported by less negative foliar δ¹³C values for +N trees and scrub control trees. Thus, nutrient enrichment may alter the structure and function of mangrove forests along salinity gradients.