Studies on Daphnia pulex Leydig in sewage oxidation ponds

Abstract

Populations of Daphnia pulex Leydig inhabiting two aerated sewage treatment ponds at Canning, Nova Scotia, were studied from May to September 1975 and October 1976 to January 1977. Very large females (< 4.4-mm total length) present in May exhibited high fecundity, producing up to 70 parthenogenetic eggs per brood (average, 29.5). These nonephippial eggs hatched immediately, giving rise to population densities as high as 932 ℓ−1 in mid-June. Maximum body size and fecundity decreased during summer 1975 to 2.5 mm and two eggs per brood respectively. Two periods of parthenogenetic ephippial egg production were recorded, one in May to June and a second in September to October. Males were not found at any time. Seasonal changes in abundance, an inverse relationship between numerical abundance and oxygen saturation levels, and changes in fecundity are considered to indicate that the summer populations were at times food limited. During the winter, mean densities declined from 473 and 178 ℓ−1 in October 1976 to zero by 21 January 1977. A complete shift from nonephippial to ephippial egg production occurred in October, but less than 25% of ephippia collected in January contained eggs. The role of Daphnia in sewage oxidation ponds is discussed.