Algal bioassay of the water from two linked but contrasting Welsh lakes

SUMMARY. A comparison of the potential water fertility of two linked lakes, Llyn Peris and Llyn Padarn, was carried out over a year using Asterionella formosa as the test organism in batch cultures. In unenriched lake water, growth was more rapid in Padarn water during autumn-winter-spring, but there was no significant difference between growth in water from the two lakes during the period of thermal stratification. Mean growth rates in treatments with additions of nitrogen and chelated iron were not significantly different from unenriched water samples, both between treatments and lakes during thermal stratification. Growth in water from both lakes was increased greatly by addition of phosphate. Maximum growth rates occurred on addition of nitrogen, phosphate and chelated iron, and there was no significant difference between growth rates in this treatment for the two lakes throughout the year. Comparisons of mean growth rates in bioassays by Tukey's interval estimate showed significant differences between other treatments and the two lakes. The potential fertility of water samples from the two lakes differs more during mixed water conditions than in the period of thermal stratification. Inhibition of A. formosa growth was recorded four times in unenriched filtered water from Peris, once on addition of nitrogen and once on addition of iron. Inhibition was alleviated by the addition of nutrients. The addition of diluted sewage effluent which enters the drainage system between the two lakes, resulted in similar rates of growth of A. formosa in water from both lakes. Results of the bioassays are discussed in relation to differences in phytoplankton between the two lakes. Although A. formosa will grow in samples of unenriched lake water from Peris, other features, including a shorter retention time of water, lower level of incident radiation and, occasionally, inhibitory substances in the lake water, contribute to the rare occurrence of this diatom in Peris. Padarn is enriched by sewage effluent which aids the growth of A. formosa.     

Experience‐ and age‐mediated oviposition behaviour in the yellow fever mosquito Stegomyia aegypti (=Aedes aegypti)

In repeated behaviours such as those of feeding and reproduction, past experiences can inform future behaviour. By altering their behaviour in response to environmental stimuli, insects in highly variable landscapes can tailor their behaviour to their particular environment. In particular, female mosquitoes may benefit from plasticity in their choice of egg‐laying site as these sites are often temporally variable and clustered. The opportunity to adapt egg‐laying behaviour to past experience also exists for mosquito populations as females typically lay eggs multiple times throughout their lives. Whether experience and age affect egg‐laying (or oviposition) behaviour in the mosquito Stegomyia aegypti (=Aedes aegypti) (Diptera: Culicidae) was assessed using a wind tunnel. Initially, gravid mosquitoes were provided with a cup containing either repellent or well water. After ovipositing in these cups, the mosquitoes were blood‐fed and introduced into a wind tunnel. In this wind tunnel, an oviposition cup containing repellent was placed in the immediate vicinity of the gravid mosquitoes. A cup containing well water was placed at the opposite end of the tunnel so that if the females flew across the chamber, they encountered the well water cup, in which they readily laid eggs. Mosquitoes previously exposed to repellent cups became significantly more likely to later lay eggs in repellent cups, suggesting that previous experience with suboptimal oviposition sites informs mosquitoes of the characteristics of nearby oviposition sites. These results provide further evidence that mosquitoes modify behaviour in response to environmental information and are demonstrated in a vector species in which behavioural plasticity may be ecologically and epidemiologically meaningful.     

Cellular and Environmental Variables Determining Numbers of Flagella in Temperature-shocked Naegleria*

SYNOPSIS Naegleria gruberi amebae normally transform into biflagellated cells. When subjected to high temperatures during flagellate differentiation, populations develop an average of 4–5 flagella/flagellate. Attempts to maximize this phenomenon by altering cellular and environmental variables revealed that: (a) few Naegleria isolates become multiflagellated: strain NB-1 gives the greatest response to heat shocks: (b) temperature is the most critical variable: highest numbers of flagella are obtained only if cells are temperature-shocked at precisely 38.2 ± 0.1 C, then returned to 19–22 C to complete differentiation; (c) although pH alone does not affect numbers of flagella. a pH optimum of 5.5–7.0 exists for temperature-shocked cells; and (d) single cells in microdrops become multiflagellated, but the population response is density-dependent. Optimal conditions are described for growing, washing, and transforming amebae to generate reproducibly highest numbers of flagella.