A population dynamics analysis of the cladoceran disappearance from Lake Tahoe, California‐Nevada1

Abstract

A comparison of the population dynamics of Daphnia pulicaria and Daphnia rosea in 1967–1969 and during their decline from Lake Tahoe in 1970 showed that the decline resulted from a combination of increased death rates and decreased birth rates. The remaining cladoceran, Bosmina longirostris, disappeared from the plankton in 1971. The elimination of cladocerans coincided with high densities of the opossum shrimp, Mysis relicta, and the kokanee salmon, Oncorhynchus nerka. Predation by these two introduced species is believed to have increased cladoceran death rates. Changes in the timing of the peaks of primary productivity are a possible cause for the decline in birth rates. A brief resurgence of Bosmina in late 1974 was associated with a dramatic decline of the mysid population and the continued decline of kokanee which began in 1970. The mysids recovered by late 1975 but Bosmina again disappeared and has remained absent since. The failure of Daphnia to reappear suggests that factors in addition to predation by kokanee and Mysis exclude them from Lake Tahoe. Changes in the phytoplankton community composition may have altered the ability of the cladocerans to maintain birth rates sufficiently high to offset increased losses due to predation.