Molecular Forms of Acetylcholinesterase: Regulation in a Testosterone-Sensitive Nerve-Muscle Axis

We measured the distribution of molecular forms of acetylcholinesterase (AChE) in muscles of a song bird, the zebra finch, and found a pattern similar to those reported in other vertebrates. As in other species, the most rapidly sedimenting form of the enzyme decreases to barely detectable levels following denervation. In the muscles of the syrinx, castration causes a large decrease in AChE activity, but has little or no effect on the relative abundance of AChE forms. This suggests that the number of AChE catalytic sites is changing without affecting the distribution of catalytic sites among the molecular forms. This is in marked contrast with the effect of denervation in the syrinx, which causes changes in the distribution of activity, as well as in total activity.     

Models to predict lake annual mean total phosphorus

A lake is a product of processes in its watershed, and these relationships should be empirically quantifiable. Yet few studies have made that attempt. This study quantifies and ranks variables of significance to predict annual mean values of total phosphorus (TP) in small glacial lakes. Several new empirical models based on water chemistry variables, on map parameters of the lake and its catchment, and combinations of such variables are presented. Each variable provides only a limited (statistical) explanation of the variation in annual mean values of TP among lakes. The models are markedly improved by accounting for the distribution of the characteristics (e.g., the mires) in the watershed. The most important map parameters were the proportion of the watershed lying close to the lake covered by rocks and open land (as determined with the drainage area zonation method), relief of the drainage area, lake area and mean depth. These empirical models can be used to predict annual mean TP but only for lakes of the same type. The model based on map parameters (r ²=0.56) appears stable. The effects of other factors/variables not accounted for in the model (like redox-induced internal loading and anthropogenic sources) on the variation in annual mean TP may then be estimated quantitatively by residual analysis. A new mixed model (which combines a dynamic mass-balance approach with empirical knowledge) was also developed. The basic objective was to put the empirical results into a dynamic framework, thereby increasing predictive accuracy. Sensitivity tests of the mixed model indicate that it works as intended. However, comparisons against independent data for annual mean TP show that the predictive power of the mixed model is low, likely because crucial model variables, like sedimentation rate, runoff rate, diffusion rate and precipitation factor, cannot be accurately predicted. These model variables vary among lakes, but this mixed model, like most dynamic models, assumed that they are constants.     

The impact of zooplankton size structure on phosphorus cycling in field enclosures

Plastic bag field enclosures (1570 1) were used to investigate the relationship between zooplankton size distributions and the turnover rate of pelagic phosphorus. Water within the enclosures was filtered to simulate either low (200 µm mesh net) or high (80 µm) planktivory. The bags were then spiked with radiolabelled phosphorus and sampled 7 times during the following 24 h period. Zooplankton communities dominated by smaller size classes cycled more phosphorus than those composed of larger species. These experiments reveal that higher trophic level interactions, such as size selective predation, may have a significant impact upon nutrient regeneration rates and, hence, primary production.