No abstract

1972. Distribution of aquatic gastropods across an interface between the Canadian Shield and limestone formations. Can. J. Zool. 50: 1433J. Zool. 50: -1445 This quantitative study gives a convincing demonstration in situ of the direct relationship which exists between the distribution and density of aquatic gastropods and the calcium concentration of their natural waters. The area chosen, in the Gravenhurst-Barrie district of southern Ontario, north of Lake Simcoe, provided a well-demarcated natural gradient across the interface between limestone formations and the calcium-poor rocks of the Pre-Cambrian Shield. Monthly quantitative samples over a 14-month period at 25 stations showed that most pulmonates, notably Physa gyrina and Helisoma anceps, had high densities in hard and medium water, whereas the prosobranchs Amnicola limosa and Campeloma decisum showed a reverse pattern, with highest densities in soft and medium water and very low densities in hard. Most pulmonates showed a slower individual growth rate in soft water and a lower proportion of large individuals. MCKILLOP, W. B., et A. D. HARRISON. 1972. Distribution of aquatic gastropods across an interface between the Canadian Shield and limestone formations. Can. J. Zool. 50: 1433-1445.Des Btudes quantitatives ont permis de demontrer in situ que la rkpartition et la densite des gasteropodes aquatiques sont fonction directe de la concentration de calcium dans l'eau du milieu. La region choisie, dans le district Gravenhurst-Barrie, en Ontario du Sud, au nord du lac Simcoe, est un gradient nature1 bien dktermine, a la jonction des formations calcaires et des roches pauvres en calcium du Bouclier Canadien. Des khantillons quantitatifs, prelevBs A tous les mois durant 14 mois, en 25 stations diffBrentes, ont montrB que la plupart des pulmones, notamment Physa gyrina et Helisoma anceps, ont des densitBs plus grandes en eau a duretk Blevke ou moyenne, alors que l'inverse se produit chez les prosobranches Amnicola limosa et Campeloma decisum, dont les densitBs sont BlevBes, en eau douce ou A duretC moyenne, et trbs basses en eau dure. En eau douce, la croissance individuelle est plus lente chez la plupart des pulmones et l'importance relative des gros individus est moindre.[Traduit par le journal]

No abstract

The community structures of the benthic and weed-bed faunas of Lake Awasa, Ethiopia Rift Valley, are described and related to the various types of substratum present. L. Awasa is non-saline (conductivity 860 p S cm -'), and lies in a shallow basin with maximum depths of 2 l-22 m. It supports an important commercial fishery mainly based on the tilapia Oreochromis niloticus.

Sixty-six genera'of nematodes representing 37 families were collected over a 13-month period from untreated and treated water from three water treatment plants in southern Ontario. Two plants receive water from the Grand River which drains agricultural, residential and industrial regions, the third from a small stream in an agricultural district. Specimens were isolated by filtration from weekly samples of untreated and treated water, most were identified to genus but a few to species.Estimated mean density in the organic drift of the river ranged from 0.58 individuals per litre in winter, when it was under ice, to 10.57 per litre during the spring thaw. Most of the 66 genera found passed through the treatment plants and over 50% of these were still motile.Eleven genera were abundant at some time of the year, six known only from soil-dwelling forms and five from aquatic or soil/ aquatic species. There was no indication that sewage purification works were a major source of any genus. There were a few plant parasites.Aquatic nematodes occurred during most of the year although they were more abundant during the warmer months, high densities of soil nematodes were a feature of the spring thaw and rains and during heavy autumn rains. More than 50% of the specimens were juveniles and some common genera lacked males.