Arni H. Litt

Conochilus in Lake Washington

The rotifer Conochilus unicornis appeared in Lake Washington sporadically and usually in small numbers during a total of 28 years of observation since 1933. Conochilus hippocrepis was present even less frequently until the 3 year period 1977–1979, when it became extraordinarily abundant. The abundances of food organisms and known predators have been examined.

Nutrients and Plankton Biomass in the Rift Lake Sources of the White Nile: Lakes Albert and Edward

The rift lakes Albert and Edward, as well as Lake George, Uganda, were sampled at nearshore and offshore sites during March 1995 with attention to water column chemistry, plankton biomass, and production rates. Biomass of phytoplankton exceeds that of zooplankton in both rift lakes, and the predictive relationship between light attenuation and particulate chlorophyll is consistent with that of Lake Victoria. Cyclopoid copepods dominate Albert and Edward; calanoids are rare (<1 m−3) and Cladocera represent less than ten percent of zooplankton biomass. Although previously reported to be present in other lakes in the Nile system, Thermodiaptomus galebi was not confirmed in Lake Albert before this study. Adult males and ovigerous females of the species were collected in Lake Albert and immature females were sampled in Lake Edward. Mesocyclops ogunnus was collected from Lake Albert, which is a new record for this species from East African Rift Valley lakes. Present water temperatures of Lake Albert are elevated by about 0.5 °C compared with historical records, a circumstance which parallels changes recorded for Lake Victoria. Both Albert and Edward have lower levels of Total P and nitrate than historical accounts have reported. In shallow Lake George, heterocyst-forming cyanobacteria and centric diatoms were rare compared with historical records, a recent condition that may be the result of metal contamination, especially by copper from mining activities in the Ruwenzoris.

A classic example of congeneric occurrence of diaptomus species

Congeneric occurrences of Diaptomus species are quite common. Cole (r96i) lists 34 well-documented congeneric associations for Diaptomus species. However, of these 34 cases only in Io instances were the species of the same subgenus . On the basis of several papers and personal observation, Cole (1961) concluded that congeneric occurrences of Diaptomus would be limited to habitats with a variety of niches and there would be a difference in size of the associated species. This size difference was especially clear-cut in the to cases where 2 members of the same subgenus occurred simultaneously . Another factor (Cole, 1961) that may allow coexistence of species is spatial and temporal isolation. It is the role of isolation that will be the main subject of this note . As a portion of a larger study (Pederson, 1974) vertical net hauls for plankton were routinely made over a 2-year period in Findley Lake . Findley Lake is a small, subalpine t i ha. lake at 1131 m elevation in the Cascade Mountains of Western Washington that contains no fish . In addition to the routine net hauls, two 24-hour vertical distribution studies were performed . Examination of the routine vertical net hauls revealed 2 species of Diaptomus of quite different sizes and with temporally dissimilar life cycles that allowed them to coexist in their rather restricted habitat . Contrarily, the two 24-hour vertical distribution studies revealed no significant spatial isolation ; although a greater sampling

Morphological variation in Kellicottia longispina

The lengths of the body, the posterior spine and the three longest anterior spines were measured for 25 specimens of Kellicottia longispina from each of the eight lakes distributed from Imikpuk at Point Barrow, Alaska (latitude 71° 15′) to Lake Washington (latitude 47° 38′). Collections were available for more than two dates from six of the lakes. Temperature ranged from 1.2° to 18 °C. Mean lengths and ratios were examined in relation to latitude and temperature. Each population differed from the others in some aspect of absolute size, variability, or shape as expressed by the ratios of the dimensions. The population from Point Barrow is similar but not identical to Olofssons var. heterospina.